CHAPTER 6 PASPALUM PANICULATUM 6.1 Introduction

CHAPTER 6 PASPALUM PANICULATUM 6.1  Introduction
CHAPTER 6
ALLELOPATHIC EFFECT OF PASPALUM PANICULATUM AND PASPALUM
URVILLEI ON GROWTH OF SUGAR CANE
6.1 Introduction
Sugar cane is a very important crop in Mauritius and occupies approximately 80% of the arable land.
This perennial plant of the grass family is grown over a period of 12 to 18 months during the first year,
followed by a 12-month ratoon crop for another six to eight years. As the growing period is relatively
long, taking between 4 to 8 months for complete canopy closure, weeds need to be controlled
efficiently (Rochecouste, 1967). The traditional practice has been to target 100% control of all weeds
from sugar cane fields irrespective of the amount and species of the weeds and stage of growth of the
cane, by use of large amounts of pre- and post-emergence herbicides. The average amount of
herbicides used annually has varied between 8 to 10 kg a.i. ha-1 during the last three decades (MSIRI,
2004). The costs for weed control have increased significantly during the last ten years; the average
cost for herbicides exceeds MUR 4 000 ha-1 (120 US $ ha-1) annually (see Chapter 1).
Increasing pressure on farmers to optimise their use of pesticides to reduce environmental
effects and to minimize costs has led to the development of strategies for integrated weed management
(IWM) and use of alternative methods to herbicides for weed control. IWM has also become the basis
of all FAO plant protection activities because it contributes directly towards the achievement of
sustainable agriculture in developing countries (Labrada & Parker, 1994). Development of such
strategies in the Mauritian sugar industry became even more urgent with the announcement and
implementation of a price reduction of 37% by 2009 by the EU, the main importer of Mauritian sugar.
Several projects have been initiated since 1998 to develop weed management strategies in sugar cane.
Firstly, trials studying critical periods of weed competition under the worst agroclimatic conditions of
the island have revealed that weed competition started 12 WAH and ended 26 WAH in ratoon cane,
and control measures may need to be maintained up to 29 WAP to keep yield losses below 5% in plant
cane (Chapter 2; Seeruttun & Lutman, 2004). These studies to compare relative competitiveness of
various weed species commonly present in sugar cane fields have revealed sugar cane as a stronger
competitor than most of the weeds tested; the time of emergence and rate of development of the weed
species influencing the effect. The mechanism of the aboveground competition in sugar cane has been
studied by comparing the competitive ability of two Paspalum species with different morphological
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traits; P. urvillei being a tussocky mostly erect perennial reaching 150-200 cm in height and leaves 1250 cm long while P. paniculatum reaches a maximum height of 100-150 cm with lanceolate leaves 2040 cm long and 1.0-2.5 cm broad, with a more planophile arrangement (Mc Intyre, 1991). Paspalum
paniculatum has been found to be relatively more competitive than P. urvillei despite the latter
growing taller and having higher relative leaf areas. This difference led to investigations on
mechanisms of competition occurring between sugar cane and the two Paspalum species (Chapter 5).
Shoot versus root competition trials showed that root (underground) competition was important in
sugar cane. However, the trials were not able to elucidate the cause of the difference in
competitiveness between the two Paspalum species.
Weed interference is a term used to express competition by both indirect interaction (e.g. crop
and weeds competing for limited resources such as light, mineral nutrients, water, or volume of space)
and direct interactions/interference (e.g. suppression of growth of one individual by the other releasing
phytotoxic chemicals). Allelopathy is a phenomenon observed in many plants that release chemicals
into their near environment either from their aerial or underground parts in the form of root exudates
(Rice, 1984). The chemical compounds released into the environment act on the other organisms, such
as weeds, plants, animals and microorganisms, by inhibitory or excitatory ways. These chemicals
accumulate and persist for a considerable time, thereby imparting significant interference on the
growth and development of neighbouring weeds and plants (Putman & Duke, 1974). Literature
reviews by Putnam (1988) and Williamson (1990) have described allelopathy caused by substances
from a number of cultivated plants and weeds. Allelopathic potential of many gramineous weeds have
been reported including that of extracts of Paspalum notatum Flueggé (bahiagrass) and other warmseason grasses on alfalfa and Italian ryegrass (Martin & Smith, 1994), interference between
bermudagrass [Cynodon dactylon (L.) Pers] or johnsongrass [Sorghum halepense (L.) Pers] and cotton
or corn (Vasilakoglou et al., 2005) and nutgrass (Cyperus rotundus) on rice seedlings (Quayyum et al.,
2000). Ishmine et al. (1987) studied the potential of some dominant weeds of sugar cane on the
Ryukyu Islands and reported that exudates of P. urvillei caused an adverse effect on growth of
Phaseolus vulgaris in greenhouse trials. Root exudates of P. notatum have also been reported to
reduce soybean and okra (Hibiscus esculentus) height increments (Pope et al., 1984). Mc Intyre (1998)
reported an allelopathic effect of C. rotundus on sugar cane.
Considerable current research on allelopathy is focused on its use in weed management
strategies, either by identifying allelochemicals for production of bioherbicides or to serve as leads for
synthetic herbicides. Much research effort is also spent on identification of crop cultivars having
allelopathic properties which can suppress weeds. One means of exploiting allelopathy for weed
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control is through the use of decaying crop residues, for example, the release of allelochemicals from
rice straw (Fujii, 1992; Chou, 1999; Ahn & Chung, 2000). In sugar cane, evidence of allelochemical
substances continually being leached from trash that suppressed weeds has been reported by Lorenzi et
al. (1988). The leachates from sugar cane trash have also been reported to cause autotoxicity; Viator et
al. (2006) contended that benzoic acid in leachates from trash blanket impairs cane ratooning and
growth.
One concern often voiced by researchers of allelopathic interactions is that many laboratory
bioassays do not adequately predict the responses observed in field situations. Inderjit and Weston
(2000) concluded that a laboratory bioassay could not demonstrate that allelopathy is operational in
natural settings. Current research is addressing this issue and many new methodologies and techniques
for identification, assessment, etc. are being developed. Recent examples include a ‘sandwich method’
for elucidating allelopathic effect of leaf litter leachates under laboratory conditions (Fujii et al., 2004)
and use of dose-response curves with known standard allelochemicals in bioassay based on
hydroponic culture to screen cultivars for allelopathic traits (Belz & Hurle, 2004).
Benzoxazolin-2(3H)-one (BOA) or hydroxamic acids are commonly occurring secondary
metabolites in cultivated and wild Gramineae (Zuniga et al., 1983; Niemeyer, 1988 and Friebe et al.,
1998) and have been shown to have an effect on radicle growth and elongation (Aiupova et al., 1979)
or causing abnormal growth (Wolf et al., 1985). BOA has been reported by Barnes and Putnam
(1987), and Belz (2004) as the responsible agent for the inhibitory activity of rye residues.
In the present study root exudates (leachates) from P. urvillei and P. paniculatum have been
tested for allelopathic properties in four glasshouse experiments between December 2005 and July
2007. The main objectives of the trials were (i) to determine if root exudates from the two Paspalum
species exert allelopathic effects on sugar cane and, if yes, (ii) would there be different varietal
responses to such chemicals, and (iii) to compare the two Paspalum species with respect to their
allelopathic properties.
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6.2 Materials and methods
Methodology for collection and application of leachates
The methodology used in classical allelopathy trials, i.e. laboratory bioassays with extracts applied on
seeds in Petri dishes or other techniques such as the “sandwich method” for leaf litter, could not be
used with sugar cane as the plant is vegetatively propagated using cuttings from the stem and the
growth period is relatively long. Furthermore, the collection of leachates from the donor plant was
more difficult and the approach for continuous trapping of chemicals from an undisturbed root system
as developed by Tang and Young (1982) was not possible for practical reasons. The methodology used
by Mc Intyre (1998) for transferring leachates from Cyperus rotundus to young sugar cane shoots was
also physically limiting, as the Paspalum species would grow much taller than C. rotundus.
For this study, the methodology consisted of applying leachates collected from the donor plant
grown in a relatively ‘inert’ medium to young pre-germinated cane setts of four sugar cane varieties
grown in a similar medium.
Trial site and plant material
The experiments were carried out in an unheated glasshouse with no supplementary lighting at Réduit
(MSIRI) experiment station. Seedlings or young plants of the donor plants, i.e. P. paniculatum and P.
urvillei, were uprooted/collected from sugar cane fields or abandoned lands in the Belle Rive area
where it is more humid and these two Paspalum species are common weeds. The recipient plant in the
four experiments consisted of young sugar cane plants of four widely grown varieties namely M
3035/66, R 570, R 579 and M 695/69. They were selected on the basis of the total area cultivated with
them and their tolerance to post-emergence herbicide treatments (MSIRI, 2003). M 3035/66,
cultivated on approximately 5% of the area cultivated by Miller-Planters (growers possessing a mill
and owning approximately 45% of total land under sugar cane) in 2005, is classified as a tolerant
variety (MSIRI, 2006). R 570, occupying more than 23% of the area grown by that group of growers,
is very susceptible to herbicide treatments. R 579 and M 695/69, respectively covering 10% and 8% of
the acreage by Miller-Planters, are classified as moderately susceptible varieties.
Sugar cane was planted using two-eyed cuttings (cane setts with two buds each) obtained by
cutting cane stalks 9 to 12 months old (plant cane) from fields on the station or nearby nursery. They
were allowed to germinate in filter mud before transplanting in the buckets.
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Containers and growing medium
Plastic containers with a diameter of 20 cm and 15 cm deep (10 litres capacity) were used for the
weeds. These were perforated at the bottom to allow excess irrigation water (leachates) to collect in
plastic bowls/trays placed 10 cm below each container. The clearance between the growing container
and the collecting device was assured by placing the container on wooden frames (Fig. 6.1). When the
pre-germinated sugar cane setts were at the 2-leaf stage they were uprooted from the filter mud
medium, cleaned to remove most of the filling medium before being transplanted in larger plastic
containers (buckets) of 20 L capacity. These buckets also had perforations at the bottom but were
placed directly on the collecting bowls to enable excess water to be absorbed back into the medium
though capillarity movement.
Fig. 6.1 Paspalum paniculatum (left) and P. urvillei (right) transplanted in trays filled with
mixture of rocksand and filter mud (left), and containers and collecting bowls arrangement for
leachates collection from weeds (right)
The growing medium used for both cane and the weeds was a mixture of ‘rocksand’ and filter
mud at a ratio of 2:1. The ‘rocksand’ consists of small size (max. 4 mm) particles obtained by crushing
basaltic rocks (volcanic origin); this material is usually used in construction. The inert property of the
rocksand was assured by washing it with clean water prior to mixing with filter mud. The latter is a
cake which is produced after filtration of the precipitated cane juice and also contains much of the
colloidal organic matter anions that precipitate during clarification. The filter mud consists mainly of
moisture (>60%) and has approximately 1% by weight of phosphate (P2O5) (Paturau, 1989).
The medium used was analysed by the Agricultural Chemistry department of MSIRI for pH,
CEC, total N, P & K, and dry matter content. Pre-experimentation analysis of the filling medium in
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Trial IV had revealed the presence of total N at 1.17%, total P at 0.83% and total K at 0.11%; the soil
pH was 6.7 with a CEC of 19.3 cmol kg-1. At the end of the experiment, analysis showed the presence
of total N at 0.75%, total P at 0.46% and total K at 0.07%, with a pH of 7.0 and a CEC of 14.0 cmol
kg-1.
Planting weeds and transplanting of sugar cane
The collected weeds were transplanted at a density of four stools per container after their leaves were
pruned to reduce transpiration. In all trials, 15 containers were planted with each weed species for
leachate collection while 10 others were kept unplanted to act as a control.
The two-eyed cane setts for each variety were treated (cold dip) against ‘pineapple’ disease
(caused by Ceratocystis paradoxa) with a solution of benomyl at 0.3 g per litre before being planted in
large trays filled with rocksand and filter mud (50:50) for germination. Once the setts had germinated,
they were uprooted and transplanted in the buckets – one pre-germinated sett per bucket (Fig. 6.2).
This step was done to guarantee homogeneity of having two well-developing primary shoots per
bucket. For Trial III, due to a poor and erratic germination, the two-eyed cuttings were cut into
planting material with only one primary shoot before transplanting into the buckets.
Fig 6.2 Pre-germinated two-eyed cuttings planted in buckets to receive
leachates from P. paniculatum and P. urvillei.
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Leachate collection and application to recipient plant
Distilled water was used to irrigate all weeded containers on a daily basis as from establishment; the
containers without weeds also received the same amount of water. All excess water percolating
through the containers were collected from the bowls every morning and were bulked together into
three treatments: leachate from P. paniculatum, leachate from P. urvillei and leachate from unplanted
containers. The containers with the collected leachates were covered and stored under the bench to
avoid direct sunlight.
Cane setts were irrigated with distilled water for one or two weeks after transplanting before
treatments commenced. Once treatment started the cane received only leachates collected from the
donor containers or control. The onset of treatments varied across trials as the establishment of the
weeds differed (Table 6.1). The volume of water used to irrigate the weeds varied between 300 and
750 ml depending on the stage of growth and rate of evapotranspiration. This was monitored closely
and adjustments were made according to volume of water left in collecting bowls and physiological
state of the weeds – water-stress conditions or the presence of too much (diluted) leachates were
avoided. All cane buckets received the same volume of leachates from the treatments; the volume
applied again varied with water requirements of cane plant with respect to evapotranspiration and its
stage of growth. In Trial I, distilled water was applied directly in the control buckets whereas in the
other experiments the control received water collected through the similar containers without weeds.
Table 6.1 Treatment dates in trials assessing allelopathic potential of two Paspalum species on sugar
cane
Dates
Trial
Weeds
Cane transplanted
transplanted
Start irrigating with
End of trial
leachates
Trial I
14 December 2005
28 December 2005
12 January 2006
23 March 2006
Trial II
14 April 2006
2 May 2006
15 May 2006
7 October 2006
Trial III
20 October 2006
4 November 2006
11 November 2006
12 February 2007
Trial IV
3 February 2007
23 February 2007
5 March 2007
7 July 2007
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Experimental layout and data collection
The buckets with cane plants were placed on a bench (1 m above floor) on one side of the glasshouse
while the weeds were placed in a similar manner on the opposite side. The temperature in the
glasshouse was slightly higher than the outside temperatures during the day; all window/opening were
left open with a fine mesh wire gauze screen to prevent any insects, etc. Natural day-light was used
and the main advantage of placing the trays indoors were to control water regimes by preventing the
effect of rainfall.
Data collection consisted of measuring dewlap heights of the primary shoots in each bucket at
regular intervals. For Trial I, the first measurement of cane shoot height (dewlap height) was made on
11 January 2006 and was followed by a second one on 6 February 2006. On 23 February 2006 (12
weeks after transplanting), all cane shoots were cut and measurements were taken for stalk height.
The harvested material was sorted into primary tillers and new tillers from each bucket and weighed.
Sub-samples from the harvested material were weighed before and after being oven-dried at 105oC for
48 hours. The buckets were emptied on 25 February 2006 for dry weight analysis of root biomass.
For Trial II, cane measurements started on 15 May 2006 and subsequently were taken on 2 June
2006, 19 June 2006, 3 July 2006, 17 July 2006, 1 August 2006, 14 August 2006, 29 August 2006, 13
September 2006 and 28 September 2006. On 6 October 2006, all shoots were measured for the last
time before being cut at ground level and the roots excavated. All harvested samples were weighed
before and after oven-drying at 105oC for 48 hours.
For Trial III, dewlap height was measured for each primary shoot on 24 November 2006, 4
December 2006, 14 December 2006, 26 December 2006, 10 January 2007, 19 January 2007 and 29
January 2007. Aboveground and root biomass (dry weights) of cane were measured for each treatment
at the end of the trial, as described above.
Cane measurements in Trial IV started on 6 March 2007 and were also carried out on 21 March
2007, 6 April 2007, 26 April 2007, 10 May 2007, 25 May 2007, 20 June 2007 and 5 July 2007. All
cane shoots were chopped and roots excavated on 5 July 2007. They were weighed before and after
being oven-dried at 105 oC for 48 hours; dry weight of cane stalks, cane leaves and biomass of cane
root per bucket were recorded.
For determining root biomass, the buckets were emptied and all roots separated from the filling
material before being washed to remove all filter mud. The roots were separated from cane setts and
oven-dried for 48 hours before being weighed.
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Statistical design and analysis
Genstat (Discovery Edition 2) was the statistical package used for all the statistical analyses. All data
recorded from cane measurements (dewlap heights), aboveground and root biomasses were subjected
to analysis of variance (ANOVA) by using a split-plot design, and main effects and interactions were
tested for significance. The four cane varieties were the main-plots and the three sub-plot treatments
consisted of leachates from P. paniculatum, P. urvillei and control; each treatment was replicated three
times. Treatment means obtained by ANOVA were compared using LSD procedures at P = 0.05 level
of significance.
Chemical analysis of leachates from P. paniculatum and P. urvillei
Leachates from the two grass weeds were collected from Trial IV and brought to the Agricultural
Chemistry department of MSIRI for analysis for the presence (and quantification if present) of 2(3H)benzoxazalinone, commonly called BOA, and for identification of other allelopathic substances
present using gas chromatography-mass spectrometry (GC-MS).
Test for BOA
Analysis of BOA in the leachates was conducted using an HPLC equipped with a DAD detector (HP
1050). A polar C-18 reversed phase column was used, and eluted with a gradient of 5% acetonitrile
and 95% Na2HPO4–buffer (1 mM, pH 2.4, 10% acetonitrile) at 0.35 ml min-1 flow rate. Quantitative
analysis was done by the external calibration method using certified BOA (2-Benzoxazolinone)
standards (Sigma-Aldrich, Germany; CAS 59-49-4). Identification of BOA peaks was based on
retention time window of pure standards; the retention time was 6.5 ± 0.05 minutes.
Identification of allelopathic substances by GC-MSD
Leachate aliquots of 100 ml from both Paspalum species plus samples collected from bowls without
weeds and irrigated with distilled water were extracted twice with dichloromethane and once with
hexane. The combined organic extract was rotary evaporated to 1-2 ml, followed by reconstitution into
7-8 ml hexane. The hexane extracts were evaporated under a gentle N2 stream, followed by
reconstitution in 1 ml hexane (US EPA, 1996). An aliquot of 1 ml was injected (splitless) into the
GCMSD (GC HP 6890, MSD 5973). The chromatographic data were obtained on an HP 5mS column
(30 m x 0.25 mm I.D., 0.25 µm film thickness) and were screened for allelochemicals using the NIST
2002 Mass Spectral library, inbuilt in the software.
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6.3 Results
6.3.1 Trial I
6.3.1.1 Effect of leachates on shoot elongation and cane growth
Pre-treatment cane measurement
Cane measurement made on 11 January 2006 showed a difference in mean stalk height among the
main factors (varieties) and no difference between leachate treatments and control (distilled water),
thus confirming that all shoot heights were similar before irrigation with leachates started (Table 6.2).
Table 6.2 Mean dewlap height at start of experimentation (before irrigating with leachates) in
Trial I
Mean dewlap height (cm shoot-1)
Cane variety
Distilled
P. paniculatum
P. urvillei
water
Mean
(varieties)
M 3035/66
8.8
12.0
12.2
11.0
R 570
8.5
8.2
8.7
8.4
R 579
10.3
8.0
9.3
9.2
M 695/69
11.3
13.5
12.8
12.6
Mean (leachates)
9.8
10.4
10.8
Values are means of three replications. Standard error of difference (s.e.d.) of means for variety
(d.f.=6) = 1.10 and s.e.d. of means for leachate treatments (d.f. = 16) = 0.92. S.e.d. for comparing
between individual varieties x leachate treatments = 1.85 (d.f. = 16).
Second cane measurement
Dewlap height of cane stalks in the all buckets was again measured on 6 February 2006 (3.5 weeks
after start of irrigation with leachates). A few new shoots (tillering) were observed in some of the
buckets. Statistical analysis carried out separately on the mean height of primary shoots alone or the
latter together with the new tillers revealed no significant difference between the leachate treatments
(Table 6.3). Irrespective of the effect of the leachate treatments, variety M 695/69 produced taller cane
shoots than the other three varieties.
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Table 6.3 Effect of leachates from P. paniculatum and P. urvillei on mean dewlap height of four
cane varieties 3 weeks after start of treatments
Mean dewlap height (cm shoot-1)
Variety
Distilled
P. paniculatum
P. urvillei
water
Mean
(varieties)
M 3035/66
23.7
26.3
24.8
24.4
R 570
26.5
22.5
23.5
24.2
R 579
22.7
23.0
23.2
22.9
M 695/69
32.7
32.5
31.3
32.2
Mean (leachates)
26.4
26.1
25.7
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f. = 6) = 1.82 and s.e.d. of means for subplot treatments (d.f. = 16) = 1.47.
S.e.d. for comparing between individual varieties x leachate treatments = 2.95 (d.f. = 16).
Final cane measurement
The experiment was stopped 10 weeks after the start of irrigation with leachates (on 23 March 2006).
There were significant differences (P< 0.01) in total dewlap heights between the cane varieties (main
plots). For the leachate treatments, a significant difference in the dewlap height of all shoots (primary
+ tillers) for variety M 695/69 was noted with leachates of P. paniculatum. This difference was also
observed in the means of all four varieties (Table 6.4). Paspalum urvillei did not cause a significant
decrease in shoot height.
Table 6.4 Effect of leachates on total dewlap height (primary shoots + tillers) 10 weeks after
start of leachate application in Trial I
Mean dewlap height (cm bucket-1)
Variety
Distilled water
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
139.7 a
115.0 a
113.3 a
122.7
R 570
129.0 a
94.7 a
113.0 a
112.2
R 579
177.3 a
118.7 a
155.3 a
150.4
M 695/69
340.3 a
227.0 b
270.7 ab
279.3
Mean (leachates)
196.6 a
138.8 b
163.1 ab
Values are means of three replications. Standard error of difference (s.e.d.) of means for variety
(d.f.=6) = 19.44 and s.e.d. of means for leachate (d.f. = 16) = 21.33. S.e.d. for comparing between
individual varieties x leachate treatments= 42.67 (d.f.=16). Mean values in the same row not
sharing the same lower-case letter are significantly different at P < 0.05 (LSD test).
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However, measurements of the individual primary shoot (two per bucket) showed a highly
significant (P< 0.01) difference between both the leachate treatments and the control (Table 6.5). The
decrease in mean dewlap height with leachates from P. paniculatum was highly significant (P< 0.01)
while that from P. urvillei was significant at P< 0.05. Irrespective of the data set analysed, the
difference in dewlap heights between the four varieties was highly significant, and no interaction
between the main-plot factors (variety) and the sub-plot treatments (leachates) was recorded.
However, the response of the leachates was mainly due to that observed on cane variety M 695/69.
Table 6.5 Effect of leachates on mean shoot dewlap height of primary shoots 10 weeks after
start of leachate application in Trial I
Mean dewlap height (cm shoot-1)
Variety
Distilled water
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
55.2 a
39.8 a
44.2 a
46.4
R 570
64.5 a
41.0 a
47.7 a
51.1
R 579
65.2 a
52.2 a
49.7 a
55.7
M 695/69
125.0 a
73.2 b
89.2 b
95.8
Mean (leachates)
77.5 a
51.5 b**
57.7 b*
Values are means of three replications. Standard error of difference (s.e.d.) of means for main plot
– variety (d.f.=6) = 5.23 and s.e.d of means for subplot treatments – leachate (d.f. = 16) = 6.0.
S.e.d. for comparing between individual varieties x leachate treatments= 12.0 (d.f.=16). Mean
values in the same row not sharing the same lower-case letter are significantly different at P < 0.05
(LSD test). Treatment significant at * P < 0.05 and ** P < 0.01.
6.3.1.2 Effect of leachates on cane biomass
Total aboveground biomass
The dry weights of the ‘aboveground’ biomass of each treatment are shown in Table 6.6. Irrespective
of cane variety, leachates from both weed species caused a reduction in aboveground biomass
compared to cane shoots receiving distilled water; the decrease was more pronounced with leachates
from P. paniculatum. Cane variety M 695/69 did not show any sensitivity to leachates from the P.
urvillei.
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Table 6.6 Effect of leachates on total aboveground biomass (dry wt) in Trial I
Variety
Total biomass (g)
Distilled water
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
99.2 a
67.9 b
73.3 b
80.1
R 570
102.5 a
64.1 c
80.3 b
82.3
R 579
105.5 a
65.3 b
76.1 b
82.3
M 695/69
104.3 a
77.7 b
96.2 a
92.7
Mean (leachates)
102.9 a
68.8 c
81.4 b
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 6.48 and s.e.d. of means for subplot treatments - leachate (d.f. = 16) =
3.81. S.e.d. for comparing between individual varieties x leachate treatments= 7.63 (d.f.=16).
Mean values in the same row not sharing the same lower-case letter are significantly different
at P < 0.05 (LSD test).
6.3.1.3 Effect of leachates on root development
Root biomass
The root biomass was easily removed and washed from the filling mixture used. The effect of the
leachates on development of cane roots was visible, particularly for those being receiving leachates
from P. paniculatum (Fig. 6.3).
Dry weight of roots
The dry weight analysis of root biomass showed that leachates from P. paniculatum had an adverse
effect on root formation of sugar cane (main-plot - mean of four varieties) (Table 6.7). Among the four
varieties, leachates applied to M 3036/66 and M 695/69 caused a significant reduction. Irrespective of
leachates/distilled water treatment, R 579 had a higher biomass of roots compared to the other three
varieties.
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M 3035/66
R 570
R 579
M 695/69
Fig. 6.3 Effect of leachates from Paspalum species on root biomass of sugar cane. For each
variety, roots on left are from distilled water, in centre for P. paniculatum and right for P. urvillei.
(For M 3035/66 and R 570, roots from two repetitions (top & bottom) are shown)
Table 6.7 Effect of leachates from P. paniculatum and P. urvillei on root biomass (dry wt) of
sugar cane in Trial I
Total root biomass (g bucket-1)
Variety
Distilled water
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
12.8 a
6.8 b
11.2 ab
10.3
R 570
12.1 a
8.6 a
10.4 a
10.3
R 579
14.8 a
12.9 a
15.3 a
14.3
M 695/69
12.3 a
7.2 b
11.7 a
10.4
Mean (leachates)
13.0 a
8.9 b
12.2 a
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 1.57 and s.e.d. of means for subplot treatments - leachate (d.f. = 16) =
1.05. S.e.d. for comparing between individual varieties x leachate treatments= 2.09 (d.f.=16).
Mean values in the same row not sharing the same lower-case letter are significantly different
at P < 0.05 (LSD test).
154
6.3.2 Trial II
6.3.2.1 Effect of leachates on shoot elongation and cane growth
Pre-treatment cane measurement
The first cane measurement made on 5 May 2006 showed a slightly lower germination and initial
development of variety R 579 compared to the others but showed no difference between treatments
(leachates v/s control) for the same level of variety (Table 6.8). The latter confirmed that all shoot
heights were similar before start of irrigation with leachates.
Table 6.8 Mean dewlap height at start of experimentation (before irrigating with leachates) in
Trial II
Mean dewlap height (cm shoot-1)
Cane variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
13.2
11.2
15.0
13.1
R 570
11.7
13.2
11.7
12.2
R 579
8.3
7.7
8.7
8.2
M 695/69
11.5
11.3
10.3
11.1
Mean (leachates)
11.2
10.8
11.4
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot - variety (d.f.= 6) = 0.78 and s.e.d. of means for subplot treatments - leachate (d.f.= 16) =
0.72. S.e.d. for comparing between individual varieties x leachate treatments= 1.44 (d.f.= 16).
Cane elongation
Dewlap height measurements over a 20 weeks period showed that stalk elongation varied for each
variety, but were in general relatively slow, particularly as from end of June. Variety
M 3035/66 grew 5 cm during the first month but slowed down almost completely later on (Fig 6.4)
and no difference between the respective treatments was observed.
The elongation rate for variety R 570 was relatively higher during the first six weeks after start
where a 15 cm increase was recorded (Fig. 6.4). The rate of growth slowed down later and no
difference between the various treatments was recorded.
The early growth of variety R 579 was similar to R 570 but had a slightly higher rate of growth
as from the end of August for the ‘control’ and P. urvillei treatments (Fig. 6.4). Cane shoots irrigated
with water collected from the P. paniculatum containers seemed to reduce stalk elongation.
155
mean dew lap height
(cm/stalk)
M ean st alk height (cm)
M 3035/66
35
30
60
25
50
20
40
15
30
P .paniculatum
P .paniculatum
10
R 570
20
P . urvillei
P . urvillei
Co ntro l
5
Co ntro l
10
0
15-M ay
14-Jun
14-Jul
Mean dewlap
height
(cm/stalk)
50
13-A ug
0
15-M ay
12-Sep
14-Jun
mean dew lap height
(cm/stalk)
R 579
14-Jul
13-Aug
12-Sep
M 695/69
40
40
30
30
20
20
P.paniculatum
P .paniculatum
P. urvillei
10
P . urvillei
10
Co ntro l
D. water
0
15-M ay
14-Jun
14-Jul
13-A ug
0
15-M ay
12-Sep
14-Jun
14-Jul
13-Aug
12-Sep
Fig 6.4 Effect of leachates from P. paniculatum and P. urvillei on stalk elongation of variety M
3035/66 (top left), R 570 (top right), R 579 (bottom left) and M 695/69 (bottom right) in Trial II.
The vertical error bars indicate 2 x s.e.d. at each observation date.
The stalk elongation for variety M 695/69 was also slowed down as from the month of August
and no difference between the three treatments was observed for each date of measurement (Fig 6. 4).
Final cane measurement
The experiment was stopped 20 weeks after start of irrigation with leachates (on 6 October 2006).
Cane measurements showed a mean increase in dewlap height of shoots of 10 cm, 26 cm, 18 cm and
18 cm for varieties M 3035/66, R 570, R 579 and M 695/69 respectively. The final dewlap height for
variety R 570 was significantly higher than R 579 and M 695/69, which were themselves higher than
156
M 3035/66. The final dewlap measurement also revealed that there was no significant difference
among the treatments (means of four varieties). However, a decrease in the dewlap height was
confirmed for variety R 579, the mean dewlap height of shoots receiving leachates from P.
paniculatum was significantly reduced (Table 6.9).
Table 6.9 Effect of leachates on final mean dewlap height (primary shoots) 20 weeks after start
of leachates application in Trial II
Mean dewlap height (cm shoot-1)
Variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
21.5 a
20.3 a
22.2 a
21.3
R 570
37.2 a
39.8 a
38.8 a
38.6
R 579
33.3 a
26.2 b
33.3 a
30.9
M 695/69
29.5 a
28.5 a
29.5 a
29.2
Mean (leachates)
30.4 a
28.7 a
31.0 a
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 2.06 and s.e.d. of means for subplot treatments (d.f. = 16) = 1.65.
S.e.d. for comparing between individual varieties x leachate treatments= 3.31 (d.f.=16). Mean
values in the same row not sharing the same lower-case letter are significantly different at P <
0.05 (LSD test).
6.3.2.2 Effect of leachates on shoot biomass
Dry weight of stalks and leaves
The dry weight of cane stalks for variety R 570 were found to be higher than for R 579 and
M 3035/66. No difference in weight of stalks was found between treatments for the same variety
(Table 6.10). The higher dewlap heights for R 579 with the control and leachates from P. urvillei did
not result in higher biomass of stalk compared to those receiving leachates from P. paniculatum
though the difference approached significance.
For each variety, the total aboveground biomass (stalk + leaves) was also found to be similar for
all treatments (Table 6.10).
157
Table 6.10 Effect of leachates on aboveground biomass (dry weight) 20 weeks after start of
application in Trial II
Mean dry weight (g bucket-1)
Variety
Control
P. paniculatum
P. urvillei
Mean
Stalk stk+lvs
Stalk stk+lvs
Stalk stk+lvs
Stalk stk+lvs
M 3035/66
5.5
17.1
9.5
19.1
3.7
11.5
6.2 15.9
R 570
29.6
70.7
27.2
71.2
29.9
68.6
28.9 70.2
R 579
12.3
30.6
8.8
28.3
13.7
35.6
11.6 70.2
M 695/69
12.8
28.6
14.1
28.6
12.2
26.7
13.1 28.0
Mean (leachates)
15.1
36.7
14.9
36.8
14.9
35.6
Stk+lvs = stalk + leaves. Values are means of three replications. For stalk dry weight, standard
error of difference (s.e.d.) of means for main plot – variety (d.f.= 6) = 6.54; s.e.d. of means for
subplot treatments (d.f.=16) = 1.60 and s.e.d. for comparing between individual varieties x
leachate treatments= 3.21 (d.f.=16). For total aboveground biomass, s.e.d. of means for main
plot – variety (d.f.= 6) = 12.88, s.e.d. of means for subplot treatments (d.f.= 16) = 2.54 and
s.e.d. for comparing between individual varieties x leachate treatments= 5.08 (d.f.=16).
6.3.2.3 Effect of leachates on root development
The dry weight analysis showed no difference in root biomass between the various treatments; i.e.
leachates from the two weed species had no effect of root biomass (Table 6.11). The difference
between the main-plot factor (variety) was significant; variety R 570 which produced higher
aboveground biomass also had more roots.
Table 6.11 Effect of leachates from P. paniculatum and P. urvillei on root biomass (dry wt) of
sugar cane in Trial II
Mean dry weight (g bucket-1)
Variety
Control
P. paniculatum
4.3
3.7
2.3
3.4
R 570
38.8
38.0
47.4
41.4
R 579
15.9
12.4
15.1
14.5
M 695/69
5.1
3.5
6.0
4.9
14.4
17.7
M 3035/66
Mean (leachates)
16.1
P. urvillei
Mean (varieties)
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 1.94 and s.e.d. of means for subplot treatments (d.f. = 16) = 2.49. S.e.d.
for comparing between individual varieties x leachate treatments= 4.97 (d.f.=16).
158
6.3.3 Trial III
6.3.3.1 Effect of leachates on shoot elongation and cane growth
Pre-treatment cane measurement
The first cane measurement made on 24 November 2006 showed a slightly lower germination and
initial development with varieties R 579 and M 695/69 compared to the others but no difference
between treatments (leachates v/s control) was obtained for the same level of variety (Table 6.12). The
data confirmed that all shoot heights were similar before start of irrigation with leachates.
Table 6.12 Mean dewlap height at start of experimentation (before applying leachates) in Trial III
Mean dewlap height (cm shoot-1)
Cane variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
14.7
9.7
16.7
13.7
R 570
10.3
9.7
10.3
10.1
R 579
5.7
7.7
7.3
6.9
M 695/69
8.3
7.7
7.2
7.7
Mean (leachates)
9.8
8.7
10.4
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 0.96 and s.e.d. of means for subplot treatments (d.f. = 16) = 1.19. S.e.d.
for comparing between individual varieties x leachate treatments= 2.38 (d.f.=16).
Cane elongation
Dewlap height measurements made over a 12 weeks period showed that stalk elongation was quite
satisfactory; each stalk gained an average of 70 cm over that period. The elongation was quite similar
for all varieties, irrespective of the treatments, for the first four to six weeks before some differences
started to occur. For variety M 3035/66, stalk elongation for the three treatments were similar till the
first weeks of January 2007 when a slowing down in the P. urvillei treatment was observed
(Fig. 6.5). Similarly, a more rapid growth was recorded in the control treatment compared to P.
paniculatum; the difference was, however, not significant.
Stalk elongation of variety R 570 was similar for the three treatments for the first seven weeks
(Fig. 6.5). After that, the cane shoots receiving leachate from containers with no Paspalum plants
(control) elongated at a higher rate than the two treatments receiving leachates from the weeds.
159
M ean dewlap height
cm/ st alk
M ean dewlap height
cm/st alk
M 3035/66
R 570
120
120
100
100
80
80
60
60
40
40
P . paniculatum
P. paniculatum
P urvillei
Control
P urvillei
20
20
Co ntro l
0
24-Nov-06 15-Dec-06
5-Jan-07
26-Jan-07
M ean dewlap height
cm/stalk
0
24-Nov-
15-Dec-06
M ean dewlap height
cm/stalk
5-Jan-07
26-Jan-07
M 695/69
R 579
140
140
120
120
100
100
80
80
60
60
P . paniculatum
40
40
P . paniculatum
P urvillei
Co ntro l
P urvillei
20
20
Co ntro l
0
0
24-No v-
15-Dec-06
5-Jan-07
26-Jan-07
24-No v-
15-Dec-06
5-Jan-07
26-Jan-07
Fig 6.5 Effect of leachates from P. paniculatum and P. urvillei on stalk elongation of variety
M 3035/66 (top left), R 570 (top right), R 579 (bottom left) and M 695/69 (bottom right) in Trial
III. The vertical error bars indicate 2 x s.e.d. at each observation date.
Cane shoots in the three subplot treatments with variety R 579 elongated at the same rate for the
initial five weeks (Fig. 6.5). It seemed that leachates from P. urvillei caused a reduction in the
elongation rate of R 579 as from the third week of January 2007. No significant difference was,
however, noted.
Variety M 695/69 did not seem to be affected by the leachates treatments (Fig. 6.5).
160
Final cane measurement
As some of the cane shoots had reached more than 80 cm in dewlap height, the experiment was
stopped 12 weeks after start of leachates application. Cane measurements showed the mean dewlap
height of M 695/69 to be slightly higher than the other varieties, the difference, however, was not
significant. Compared to the control, a tendency for P. urvillei causing a reduction in dewlap heights
of all the varieties was observed; the differences were, however, not significant.
6.3.3.2 Effect of leachates on shoot biomass
Dry weight of stalks and leaves
The aboveground biomass (dry weight) of cane shoots was found to vary with variety; R 570
producing higher biomass and M 695/69 the least (Table 6.13). Irrespective of cane variety, the effect
of leachates on mean (main-plot means) aboveground biomass was significant, P. urvillei caused a
reduction in shoot development. Paspalum urvillei adversely affected shoot development of varieties
M 3035/66 and R 570. Leachates from P. paniculatum caused no adverse effect on weight of
aboveground biomass, thus confirming effect on dewlap height.
Table 6.13 Effect of leachates on aboveground biomass (dry weight) 12 weeks after start
of application in Trial III
Mean dry weight (g bucket-1)
Variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
83.0 a
71.4 ab
52.8 b
69.1
R 570
100.4 a
101.6 a
81.1 b
94.4
R 579
83.1 a
83.2 a
67.2 a
77.8
M 695/69
51.3 a
66.7 a
49.4 a
55.8
Mean (leachates)
79.5 a
80.7 a
62.6 b
Values are means of three replications. For stalk dry weight, standard error of difference (s.e.d.) of
means for main plot – variety (d.f.= 6)= 3.35 and s.e.d. of means for subplot treatments (d.f.= 16)=
4.54. S.e.d. for comparing between individual varieties x leachate treatments= 9.08 (d.f.= 16).
Mean values in the same row not sharing the same lower-case letter are significantly different at P
< 0.05 (LSD test).
6.3.3.3 Effect of leachates on root development
A significant reduction in root biomass between the control and the two leachates treatments was
obtained for the main-plot treatments (four varieties). Significant effects on individual cultivars were
161
less evident, though the data showed similar trends for all cultivars. Leachates from P. urvillei were
found to cause a significant reduction only in variety M 695/69 (Table 6.14).
Table 6.14 Effect of leachates on root biomass (dry weight) 12 weeks after start in Trial III
Mean dry weight (g bucket-1)
Variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
17.1 a
14.8 a
14.5 a
15.4
R 570
18.8 a
18.3 a
20.9 a
19.4
R 579
15.7 a
13.1 a
13.5 a
14.1
M 695/69
12.9 a
10.6 a
8.2 b
10.5
Mean (leachates)
16.1 a
14.2 b
14.3 b
Values are means of three replications. For stalk dry weight, standard error of difference
(s.e.d.) of means for main plot – variety (d.f.= 6)= 0.48 and s.e.d. of means for subplot
treatments (d.f.= 16)= 0.83. S.e.d. for comparing between individual varieties x leachate
treatments= 1.67 (d.f.=16). Mean values in the same row not sharing the same lower-case letter
are significantly different at P < 0.05 (LSD test).
162
6.3.4 Trial IV
6.3.4.1 Effect of leachates on shoot elongation and cane growth
Pre-treatment cane measurement
The first cane measurement made on 6 March 2007 showed a lower germination and initial
development with variety R 579 (Table 6.15). No difference between treatments (leachates v/s
control) was obtained for the same level of variety, thus confirming that all shoot heights were similar
before start of irrigation with leachates.
Table 6.15 Mean dewlap height at start of experimentation (before applying leachates) in Trial IV
Mean dewlap height (cm shoot-1)
Cane variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
11.8
11.3
10.8
11.3
R 570
11.3
10.7
12.0
11.3
R 579
8.7
8.0
8.2
8.3
M 695/69
11.8
11.0
10.7
11.2
Mean (leachates)
10.9
10.3
10.4
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 0.73 and s.e.d. of means for subplot treatments (d.f. = 16) = 0.80. S.e.d.
for comparing between individual varieties x leachate treatments= 1.60 (d.f.=16).
Cane elongation
Dewlap height measurements made over a 17 weeks period showed that stalk elongation was quite
steady; stalk elongation gained between 55 and 70 cm over that period. Variety R 579 had the highest
gain in dewlap and M 3035/66 the least. The elongation was quite similar for all varieties, irrespective
of the treatments, during the first six weeks before some differences started to occur as from the end of
April 2007.
For variety M 3035/66, stalk elongation for the three treatments were similar till the end of
April 2007. As from early May, the rate of elongation recorded in the P. urvillei treatment was slower
than the other two treatments, the gap increasing with time (Fig. 6.6). No difference in rate of
elongation was noted between the P. paniculatum treatment and the control.
163
For R 570, a similar tendency as for M 3035/66 was observed but this time, the rate of
elongation of both leachates treatments was lower than the control (Fig 6.6). The effect of the
leachates seemed to increase with time.
M ean dewlap height
(cm/stalk)
M ean dewlap height
(cm/ st alk)
M 3035/66
120
120
100
100
80
80
60
60
40
40
R 570
P. paniculatum
P. urvillei
P . paniculatum
20
D water
20
P . urvillei
D water
0
6-M ar
5-A pr
M ean dewlap height
(cm/st alk)
5-M ay
4-Jun
0
6-M ar
4-Jul
R 579
5-Apr
M ean dewlap height
(cm/ st alk)
5-M ay
4-Jun
4-Jul
M 695/69
140
120
120
100
100
80
80
60
60
40
P . paniculatum
40
P. paniculatum
P . urvillei
P. urvillei
20
D water
20
D water
0
0
6-M ar
5-A pr
5-M ay
4-Jun
6-M ar
4-Jul
5-A pr
5-M ay
4-Jun
4-Jul
Fig 6.6 Effect of leachates from P. paniculatum and P. urvillei on stalk elongation of cane variety
M 3035/66 (top left), R 570 (top right), R 579 (bottom left) and M 695/69 (bottom right) in Trial
IV. The vertical error bars indicate 2 x s.e.d. at each observation date.
The difference between the control and the P. urvillei treatment was observed later in variety
R 579 (Fig. 6.6); the gap was more visible during the last two measurements. Paspalum paniculatum
did not seem to reduce elongation rate of this variety.
164
Variety M 695/69 behaved similarly to R 570 and the mean dewlap height with the control
seemed to increase faster than those treated with leachates from the two grasses at the later observation
dates (Fig. 6.6).
Final cane measurement
The experiment was stopped 17 weeks after start of leachates application as cane was relatively tall in
the buckets. Cane measurements showed the mean dewlap height of M 695/69 to be significantly
higher than M 3035/66 but they were not different to R 570 and R 579. Leachates from P. urvillei
caused a reduction in the mean dewlap height of cane shoots of main-plot treatments (varieties) though
no significant effects were recorded for the individual varieties (Table 6.16). Paspalum paniculatum
also appeared to cause a reduction compared to the control but the difference was not significant.
Table 6.16 Effect of leachates on final mean dewlap height (primary shoots) 17 weeks after
start of application of treatments in Trial IV
Mean dewlap height (cm shoot-1)
Variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
69.5
68.5
56.5
64.8
R 570
83.3
68.0
73.2
74.8
R 579
80.5
75.8
72.2
76.2
M 695/69
91.2
83.2
79.7
84.7
81.1 a
73.9 a
70.4 b
Mean (leachates)
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 5.21 and s.e.d. of means for subplot treatments (d.f. = 16) = 4.61. S.e.d.
for comparing between individual varieties x leachate treatments = 9.22 (d.f.=16). Mean values
in the same row not sharing the same lower-case letter are significantly different at P < 0.05
(LSD test).
6.3.4.2 Effect of leachates on shoot biomass
Aboveground biomass
The total biomass of leaves and stalks were reduced by the P. urvillei treatment (mean of mainplot treatments) (Table 6.17). For individual varieties, the response was again not significant as for the
total dewlap and weight of leaves.
165
Table 6.17 Effect of leachates on aboveground biomass (dry weight) 17 weeks after start of
application in Trial IV
Mean dry weight (g bucket-1)
Variety
Control
P. paniculatum
M 3035/66
137.6
124.5
109.7
123.9
R 570
139.7
129.0
128.9
132.6
R 579
124.2
131.0
117.0
124.0
M 695/69
114.9
90.9
91.4
99.0
129.1 a
118.8 a
111.7 b
Mean (leachates)
P. urvillei
Mean (varieties)
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot – variety (d.f.=6) = 6.20 and s.e.d. of means for subplot treatments (d.f. = 16) = 7.49. S.e.d.
for comparing between individual varieties x leachate treatments = 14.99 (d.f.=16). Mean
values of four varieties not sharing the same lower-case letter are significantly different at P <
0.05 (LSD test).
6.3.4.3 Effect of leachates on root development
As seen in earlier trials, M 695/69 produced less roots irrespective of treatments. For the main-plot
treatments, a reduction in root biomass was observed between the control and treatments consisting of
leachates from P. paniculatum; the latter was not different from P. urvillei (Table 6.18).
Table 6.18 Effect of leachates on root biomass (dry weight) 17 weeks after start in Trial IV
Mean dry weight (g bucket-1)
Variety
Control
P. paniculatum
P. urvillei
Mean (varieties)
M 3035/66
33.4
29.4
33.2
32.0
R 570
50.6
41.4
40.5
44.1
R 579
28.7
17.6
20.9
22.4
M 695/69
19.3
12.3
17.8
16.5
33.0 a
25.2 b
28.1 ab
Mean (leachates)
Values are means of three replications. Standard error of difference (s.e.d.) of means for main
plot (variety) = 3.53 (d.f.= 6) and s.e.d. of means for subplot treatments= 2.89 (d.f.= 16). S.e.d.
for comparing between individual varieties x leachate treatments= 5.78 (d.f.=16). For mean of
four varieties, values not sharing the same lower-case letter are significantly different at P <
0.05 (LSD test).
166
6.3.5 Chemical analysis of leachates from P. paniculatum and P. urvillei
6.3.5.1 Presence of BOA (2-benzoxazolinone)
The samples analysed did not show any presence of BOA in the leachate samples collected. Although
BOA often exists or is converted to other derivatives such as DIBOA, MBOA, etc, any trace of BOA
should have been detected by the analysis. These preliminary analyses therefore excluded detectable
levels of BOA in leachates from the two Paspalum species.
6.3.5.2 Chemical composition of leachates from P. paniculatum and P. urvillei
The GC-MSD revealed the presence of 2-Propenoic acid, 3-(4-methoxyphenyl)- (CAS number: 546677-3) in leachates from both weeds but not from the control treatment. The retention time was
26.94/26.95 minutes. 2-Propenoic acids form part of the family commonly known as cinnamic acids
which include cinnamic acid (2-propenoic acid, 3-phenyl), ferulic acid (2-propenoic acid, 3-(4-hydrxy3-methoxyphenyl)-), p-coumaric acid (2-propenoic acid, 3-(4-hydroxyphenyl)-), isoferulic acid (2propenoic acid, 3-(3-hydroxy-4-methoxyphenyl)-) and caffeic acid (2-propenoic acid, 3-(3,4dihydroxyphenyl)-). All these compounds are known to have allelopathic properties (Fernandez et al.,
2006). P-Coumaric acid, in particular, has been proven to cause a significant effect on the growth of
roots and aboveground organs of Linum usitatissimum (Ray & Hastings, 1992).
The chromatograph area (%) covered by the 2-propenoic acid, 3-(4-methoxyphenyl) was three
to four times higher in the P. paniculatum samples than in leachates from P. urvillei (Appendix 1).
This may suggest that P. paniculatum produced more of that allelopathic chemical, but this needs to be
studied further as the amount of chemicals released from the roots would vary with time and several
other factors. However, the presence of this chemical in the two Paspalums confirms the potential
interference from allelopathic substances released by weeds over and above the other mechanisms of
competition between sugar cane and weeds.
167
6.4 Discussion and conclusions
Stalk elongation and cane growth
This study has shown that leachates from both Paspalum species can cause an adverse effect on cane
growth. Irrespective of varieties (mean of main-plot treatments), leachates from P. urvillei caused a
significant reduction in mean dewlap of primary shoots in three trials (Trial I, III and IV). A
significant reduction was obtained by leachates of P. paniculatum in Trial I where the effect was more
pronounced to that of P. urvillei. No difference between the three treatments was observed in Trial II
where cane growth was much slower than the other trials; this may be attributed to the lower mean
temperatures which prevailed during the respective trials (Table 6.19).
Table 6.19 Effect of temperature on rate of cane stalk elongation
Trial
Mean daily temperatures (oC)
Rate of stalk elongation
Max.
Min.
cm/week
Trial I
28.4
21.0
6.7
Trial II
23.9
15.3
1.0
Trial III
28.7
20.6
6.0
Trial IV
26.0
18.2
4.2
The lower cane growth in Trial II resulted in maximum dewlap heights not exceeding 35 cm per
shoot except for variety R 570. A reduction in stalk elongation with P. paniculatum leachates on
R 579 was also recorded in this trial.
Cane growth and initiation of allelopathic effect
Cane measurements showed that the difference between the control and the ‘leachate’ treatments was
not apparent during the early weeks after start of experimentation. The difference was in general
visible after the cane shoots had reached a mean dewlap height of 40 cm or more. This may also
explain why no difference was noted in Trial II. In general, the differences between the control and the
leachate treatments increased with time; it is possible that more significant differences would have
been observed if the trials were prolonged for a few weeks more. Increasing growth-inhibiting or
168
phytotoxic effects from the weeds on sugar cane with time could have been due to increased
allelochemical release from weed roots as the plants matured.
Variety response to leachate treatments
There was no interaction between variety and treatments; all varieties showed susceptibility to the
leachates. The order of susceptibility of the cane varieties to leachates differed from their known
relative tolerances towards herbicides or herbicide mixtures. M 3035/66 is known to be a more
resistant variety towards herbicides than M 695/69 and R 579, with R 570 classified as a susceptible
variety. As the leachates were applied as irrigation water underneath the leaves (not applied on cane
leaves), it means that water and allelochemical uptake were solely by the roots. Variability in the
tolerance of sugar cane varieties to herbicides is mostly associated with foliar-applied herbicides.
Effect of leachates on root biomass
The leachates from both weed species were found to have a growth-inhibiting effect on root
development in all the trials except in Trial II where a slight (non-significant) reduction was caused by
P. paniculatum leachate on roots of varieties M 3035/66 and M 695/69. The difference in root biomass
observed in Trials I and III seems to explain the difference caused by the leachates; a correlation
between reduction in root biomass and effect on dewlap height indicated that the primary effect of the
allelochemicals was on root development. An adverse effect on root development also impacted
negatively on aboveground biomass development, although M 695/69 with the least root biomass
produced the tallest stalks (dewlap height).
P. paniculatum vs P. urvillei
On basis of results from the four trials, P. urvillei was found to cause more allelopathic (phytotoxic)
effects than P. paniculatum, although the reverse occurred in Trial I. Although both weeds were
transplanted at the same initial density, growth of P. urvillei was more vigorous and it produced more
leaves and biomass; suggesting that more root exudates may have been released. Both weeds had a
quick and similar development in Trial I and this may have influenced the allelochemical production
of P. paniculatum to the extent that the latter species seemed to cause more reduction in root biomass
than P. urvillei in that trial. The implication of this finding is that, on a unit mass basis, P. paniculatum
may be more allelopathic than P. urvillei.
169
The effect on root growth may have been due to the presence of 2-propenoic acid, 3-(4methoxyphenyl) found in root exudates from both weeds. Cinnamic acids are known for their
allelopathic properties, in particular for impairing root development (Rice, 1984; Fernandez et al.,
2006). The presence of a higher concentration of 2-propenoic acid in P. paniculatum leachates may
partly explain the greater reduction in root biomass of sugar cane that was observed at this treatment.
These results confirm the allelopathic potential of weeds on sugar cane; the effect of leachates from
Cyperus rotundus has been reported by Mc Intyre (1998). However, the results presented in this study
are only preliminary ones as there may be other allelochemicals involved and the exact effects of
cinnamic acids need to be confirmed by simulating effects using pure chemicals. With the same
approach, dose-response curves may be used to estimate the minimum concentrations required for any
effect on cane. Allelopathic effects also need to be verified under natural conditions.
Although this study proved some interference due to allelopathic effects from the two
Paspalums on sugar cane, the results cannot completely explain the higher interference
(competitiveness) reported earlier for P. paniculatum, as both weeds seemed to cause similar
allelopathic effects. Their relative rates of development and competitiveness under field conditions
need to be studied more closely together with the identification and quantification of the major
allelochemicals involved as well as their effects on sugar cane. Further studies are also required to
ascertain whether allelochemical production in the live weeds and their release from live plants or
from decomposing plant material is governed by growth stage, plant part (leaves or roots), or by stage
of decomposition of residual plant material.
170
CHAPTER 7
A NEW HERBICIDE TANK-MIX OF TRIFLOXYSULFURON + AMETRYN AND
AMICARBAZONE TO PROVIDE A COST-EFFECTIVE BROAD-SPECTRUM
PRE- AND POST-EMERGENCE TREATMENT FOR MANAGING WEEDS
IN SUGAR CANE
7.1 Introduction
Traditionally, weed control in sugar cane in Mauritius was geared towards eradication of all weeds
from planting or harvest up to complete canopy closure. In the humid and superhumid areas, canopy
closure may take between 20 to 30 weeks; consequently, two or three herbicide applications had to be
made, often complemented by manual weeding (MSIRI, 2004). The work presented in earlier Chapters
have shown that it is possible to reduce costs of weed control by developing, new weed management
strategies based on critical periods of weed competition. The research presented in Chapter 2 showed
that critical periods varied between 6 and 27 weeks after planting or 12 and 26 weeks after harvest in
the humid areas where cane growth is slower and weed infestations are higher (Chapter 2; Seeruttun &
Lutman, 2004). The new strategies proposed included delaying of the first herbicide application to
coincide with onset of the critical periods of weed competition. The success of such an approach
would rely on the efficacy of the herbicide treatment in knocking down all emerged weeds present on
the day of spraying and providing a relatively long residual activity against a broad spectrum of
weeds.
A mixture of trifloxysulfuron 1.85% + ametryn 73.15% (Krismat® - WDG 75), developed by
Syngenta Crop Protection AG has been tested in Brazil where all the key sugar cane weed species
including the most economically important grass species such as Brachiaria spp., had been controlled
(Howard et al., 2001). The efficacy of this mixture on many grass species including Rottboellia
cochinchinensis (Lour.) Claiton and some broad-leaved weeds such as Euphorbia heterophylla L. has
also been reported in Cuba (Diaz et al., 2004). At rates of 1.5 kg a.i. ha-1, the new herbicide was well
tolerated by sugar cane. Amicarbazone (triazolinone) (Dinamic® WDG 70), from Arysta LifeScience
has also been reported to provide excellent control of many major annual dicotyledonous weeds and
grasses in sugarcane (Philbrook et al., 1999).
The current standard herbicide treatments available in Mauritius have limited effectiveness on
some grasses and sedges and are not fully effective if control is delayed until after early weed
171
emergence. A tank-mixture of trifloxysulfuron + ametryn and amicarbazone appeared from research
elsewhere to have the potential to provide broad-spectrum pre- and post-emergence control. The new
management strategies proposed would imply the application of early post-emergence treatments at
timings which differ from the traditional approach where the selectivity of the herbicides was achieved
by applying either pre-emergence of the cane, or when the latter had reached at least a growth stage of
12 to 14 weeks after planting. At 12 or 14 weeks, the crop better tolerates some of the herbicide
treatments. According to the new strategies, herbicide treatments would be applied post-emergence of
the crop (and weeds) and most probably at a stage of growth between four to eight weeks after
planting when risks of herbicide phytotoxicity would be higher. Consequently, this set of experiments
was done to assess the performance of these two new products and to investigate the feasibility of
developing new weed control approaches based on the critical period research. The objectives of the
trials were to:
1.
Evaluate the pre-emergence potential of the two products and their tank-mixes against the
weeds present in sugar cane in Mauritius, and to compare the length of residual activities
obtained to that of other currently available herbicides.
2.
Assess the potential and spectrum of control of the new herbicides and their tank-mixes
applied post-emergence to weeds in both plant and ratoon cane.
3.
Determine any phytotoxicity of the new products or tank-mixes on the crop when applied both
pre- and post-emergence of cane.
172
7.2 Materials and methods
Trial characteristics and treatments
Eleven trials were conducted in plant and ratoon cane between March and December 2005. Details and
characteristics of the trial sites are given in Table 7.1. In the first four trials, treatments were applied
pre-emergence of plant cane and weeds. Amicarbazone at 0.7, 0.875, 1.05 and 1.4 kg a.i. ha-1,
trifloxysulfuron+ametryn at 0.0263 + 1.097 and 0.0315 + 1.317 kg a.i. ha-1, and amicarbazone at 0.875
and 1.05 kg a.i. ha-1 tank-mixed with trifloxysulfuron + ametryn at 0.0263 + 1.097 kg a.i. ha-1 were
compared to two standards, namely, oxyfluorfen + diuron (0.5 + 2.0 kg a.i. ha-1) and tebuthiuron +
atrazine (1.6 + 2.0 kg a.i. ha-1). An untreated control was also included.
In the second series of four trials (Trials V - VIII), treatments were applied post-emergence at
the same corresponding sites between 10 and 12 weeks after planting. Treatments comprised of
amicarbazone at 0.875, 1.05, 1.25 and 1.4 kg a.i. ha-1, trifloxysulfuron + ametryn at 0.0263 + 1.097
and 0.0315 + 1.317 and amicarbazone at 0.875 and 1.05 kg a.i. ha-1 tank-mixed with trifloxysulfuron +
ametryn at 0.0263 + 1.097 kg a.i. ha-1. A standard treatment consisting of the tank-mix tebuthiuron +
atrazine + 2,4-D amine salt (1.3 + 2.0 + 2.0 kg a.i. ha-1) and an untreated control were also included.
The last three trials (Trials IX, X and XI) were conducted in ratoon cane and post-emergence of
the weeds. The rates of amicarbazone, trifloxysulfuron + ametryn and amicarbazone +
trifloxysulfuron+ametryn were similar to those used post-emergence of plant cane, except that
amicarbazone alone at 1.25 kg a.i. ha-1 was excluded. A tank-mix of hexazinone + atrazine + 2,4-D
amine salt (0.6 + 2.0 + 2.0 kg a.i. ha-1) was included as an additional standard.
Experimental layout and treatment application
In all post-emergence trials, a non-ionic surfactant at 0.025% v/v was added to all treatments. At all
sites, the experimental design was a randomized complete block with three replicates and a plot size of
64 m2 (4 rows of 10 m length at a spacing of 1.6 m). Treatments were applied with hand-operated
knapsack sprayers with double hollow cone jet nozzles delivering 350 L ha-1 of spray mixture at a
working pressure of 300 kPa.
173
Table 7.1. Characteristics and details of trial sites
Trial
no.
I
Site
Soil group
*
Humic
Ferruginous
Sans Souci
Latosol
II
Deux Bras
Latosolic
Brown
Forest
III
Belle Mare
Lithosol
IV
V
VI
VII
VIII
IX
X
XI
Latosolic
Brown
Forest
Humic
Sans Souci Ferruginous
Latosol
Latosolic
Deux Bras
Brown
Forest
Valetta
Belle Mare
Valetta
Gros-Bois
Combo
Lithosol
Latosolic
Brown
Forest
Latosolic
Brown
Forest
Humic
Ferruginous
Latosol
Humic
Côte D’Or Ferruginous
Latosol
Mean
annual Altitude Date of
Date of
Cane variety
rainfall
(m)
planting
spraying
(mm)
3800
290
28.02.05 M 1400/86
02.03.05
2350
140
10.03.05 M 1394/86
16.03.05
1500
40
12.04.05 M 2024/88
15.04.05
3200
430
07.04.05
13.04.05
3800
290
28.02.05 M 1400/86
05.05.05
2350
140
10.03.05 M 1394/86
20.05.05
1500
40
12.04.05 M 2024/88
11.07.05
3200
430
07.04.05
M 52/78
27.07.05
2950
245
04.07.05
R 575
18.08.05
3300
410
13.07.05
M 52/78
02.09.05
2800
450
19.07.05
M 52/78
22.09.05
M 52/78
* According to Parish & Feillafé (1965). Soil groups are described in Chapter 1.
174
Data collection and statistical analysis
For pre-emergence trials in plant cane, data collection comprised regular observations on weed
infestation and cane growth. Visual observations were made at 4 and 8 weeks after spraying (WAS),
whereas weed surveys were carried out twice between 12 and 19 WAS using the ‘Frequency
Abundance Method’ (Rochecouste, 1967). The latter method consists of, firstly, a listing of all weeds
present in the treatment plots, and then assigning their relative presence/cover on a scale varying
between 0 and 8. Stalk height was measured from ground level to the first visible dewlap at 12 WAS.
For the post-emergence trials in plant cane, a weed survey was carried out prior to spraying in
each individual plot to identify and quantify all weeds present. The first post-treatment weed survey
was conducted between 4 and 6 weeks after spraying to assess the post-emergence potential of each
treatment. Results were expressed in % weed kill for each plot by dividing the difference in weed
infestation (Frequency Abundance Method) between the two surveys by the initial infestation. The
second survey carried out between 10 and 13 WAS was mainly geared towards assessing the residual
activity following early post-emergence application.
For the post-emergence trials in ratoon cane, a weed survey was conducted to record weed
species and infestation levels in all plots a few days prior to spraying of treatments. Two formal weed
surveys were carried out between 6 and 12 WAS using the ‘Frequency Abundance Method’ to
calculate the % weed kill. Regular visual observations were made to assess any phytotoxicity on the
different cane varieties.
Data for weed control (expressed as % of the untreated control) and % weed kill were
transformed using the arcsine square root before statistical analysis was performed. Likewise, the %
increase in stalk height (x) for effect of the treatments on cane elongation was transformed using (x +
0.5)0.5 (Steel et al., 1997).
175
7.3 Results and discussion
7.3.1 Potential of amicarbazone and trifloxysulfuron + ametryn for pre-emergence weed control
Efficacy on weeds
Both trifloxysulfuron + ametryn and amicarbazone provided good pre-emergence control compared to
the two standards. The efficacy of amicarbazone improved with increasing rates as opposed to
trifloxysulfuron + ametryn where the two rates tested provided a similar level of control (Table 7.2). In
general, trifloxysulfuron + ametryn was superior to amicarbazone as the former proved more effective
on sedges (C. rotundus and Kyllinga spp.) and some grasses (Table 7.3). Amicarbazone showed a
higher efficacy on broad-leaved weeds which explains its better efficacy in Trial III. Amicarbazone
also provided good control of Digitaria horizontalis which was poorly controlled by trifloxysulfuron +
ametryn (Table 7.3). Tank-mixing amicarbazone with trifloxysulfuron + ametryn improved the level
and spectrum of control (Table 7.2). The residual activity of the tank-mix trifloxysulfuron + ametryn +
amicarbazone was comparable to the two standards. Weed surveys at 16 or 19 WAS showed a
satisfactory level of control in Trials I, III and IV. Cane growth was faster at Deux Bras (Trial II) and
the cane canopy had almost closed before 16 WAS.
Observations made during the first eight weeks showed that all the treatments were safe towards
the four cane varieties tested. These observations were confirmed when cane measurements taken
between 12 and 16 WAS revealed no significant differences in stalk height and number of shoots. The
tank-mix trifloxysulfuron + ametryn and amicarbazone showed no adverse effect on the mean dewlap
height (Fig. 7.1) compared to the standard treatment. There were very few weeds left uncontrolled in
the plots treated with either the standard herbicides or the new tank-mixes, so these could not have
caused any additional adverse effect on the cane due to weed competition.
176
Table 7.2. Pre-emergence control of weeds presented as % of weed infestation on the untreated treatment (detransformed arcsine data) by
trifloxysulfuron+ametryn and amicarbazone in plant cane. Values in parentheses represent transformed (arcsine) data
Weed control (expressed as % of untreated control *)
kg a.i. ha-1
Treatments
Trial I
Trial II
Trial III
Trial IV
12 WAS
16 WAS
12 WAS
12 WAS
19 WAS
12 WAS
16 WAS
Amicarbazone
0.7
78 (1.08)
81 (1.11)
64 (0.92)
24 (0.51)
44 (0.72)
57 (0.86)
91 (1.26)
Amicarbazone
0.875
63 (0.912)
73 (1.02)
63 (0.92)
24 (0.51)
35 (0.64)
51 (0.80)
83 (1.15)
Amicarbazone
1.05
71 (1.00)
71 (1.00)
58 (0.87)
11 (0.34)
27 (0.54)
33 (0.61)
73 (1.02)
Amicarbazone
1.4
66 (0.95)
70 (0.99)
51 (0.79)
9 (0.30)
20 (0.46)
25 (0.52)
67 (0.96)
Trifloxysulfuron+ametryn
0.0263+1.097
46 (0.75)
59 (0.88)
44 (0.73)
16 (0.42)
34 (0.62)
44 (0.72)
68 (0.97)
Trifloxysulfuron+ametryn
0.0315+1.317
55 (0.84)
60 (0.89)
33 (0.61)
19 (0.45)
37 (0.67)
49 (0.78)
68 (0.97)
33 (0.61)
47 (0.76)
54 (0.83)
12 (0.36)
42 (0.70)
32 (0.60)
69 (0.98)
45 (0.74)
57 (0.85)
49 (0.77)
13 (0.37)
32 (0.60)
30 (0.58)
73 (1.03)
Amicarbazone + trifloxysulfuron+ametryn
Amicarbazone + trifloxysulfuron+ametryn
0.875 +
0.0263+1.097
1.05 +
0.0263+1.097
Oxyfluorfen + diuron
0.5 + 2.0
48 (0.76)
67 (0.96)
45 (0.74)
23 (0.37)
37 (0.66)
28 (0.56)
66 (0.94)
Tebuthiuron + atrazine
1.6 + 2.0
52 (0.80)
56 (0.84)
42 (0.71)
9 (0.31)
18 (0.44)
26 (0.53)
59 (0.87)
0.20
0.19
0.12
0.18
0.25
0.13
0.12
Standard error of transformed data
WAS = weeks after spraying
* values represent detransformed (arcsine) data
177
cm/plant
35
30
25
20
15
10
5
0
Trial I
12 WAS
Trial II
14 WAS
amicarbazone+
trifloxysulfuron+ametryn
(0.875 + 0.0263+1.097
kg a.i. ha-1)
Trial III
14 WAS
amicarbazone+
trifloxysulfuron+ametryn
(1.05 + 0.0263+1.097
kg a.i. ha-1)
Trial IV
16 WAS
tebuthiuron+atrazine
(1.6+2.0 kg a.i. ha-1)
Fig. 7.1 Effect of trifloxysulfuron+ametryn and amicarbazone on cane growth.
Error bars represent 2 x s.e.d.
Table 7.3 Relative efficacy of amicarbazone, trifloxysulfuron + ametryn and the tank-mix
amicarbazone + trifloxysulfuron+ametryn for the pre-emergence control of some common weeds in
sugar cane
Amic
Herbicide treatments (kg a.i. ha-1)
trif+amet
trif+amet+amic
oxyf+diur
teb+atraz
(1.4)
(0.0315+1.317)
(1.05+0.0263+1.097)
(0.5+2.0)
(1.6+2.0)
+++
++++
++
+++
++++
++++
+++
++++
+++
++++
+
++
++
+
+
Digitaria horizontalis
+++
+
++++
++++
++++
Digitaria timorensis
+++
+
++++
++++
++++
Drymaria cordata
+++
+++
++++
++++
++++
Kyllinga bulbosa
+
+++
+++
+++
+++
+++
++
+++
+++
+++
+
+++
+++
+++
+++
+++
+++
+++
+
+++
Ageratum conyzoides
Amaranthus dubuis
Cyperus rotundus
Oxalis corniculata
Paspalum paniculatum
Phyllanthus sp.
+ Poor ++ Fair +++ Good ++++ very good
amic = amicarbazone, trif+amet = trifloxysulfuron+ametryn, oxyf+diuron= oxyfluorfen + diuron,
teb+atraz= tebuthiuron + atrazine
178
7.3.2 Potential of amicarbazone and trifloxysulfuron + ametryn for post-emergence control
Efficacy on weeds
With the exception of Trial V, the efficacy of the two new herbicides applied alone was comparable or
superior to the standard (tebuthiuron + atrazine + 2,4-D amine salt). In general, both herbicides
effectively controlled most broad-leaved weeds. Trifloxysulfuron + ametryn provided good
knockdown of C. rotundus, Kyllinga spp. and Paspalum spp. that were poorly controlled by
amicarbazone (Table 7.4). Likewise, amicarbazone was effective on D. horizontalis, which was not
controlled by trifloxysulfuron + ametryn (Table 7.4). Tank-mixing the two products improved
significantly their level of control (Table 7.5); the combination controlled Digitaria timorensis
(Kunth.) Balans, which was found to be weakly controlled by both products applied alone (Table 7.4).
For post-emergence control, increasing the rate of amicarbazone within the new tank-mix did not
improve the level of efficacy (Table 7.5).
Table 7.4 Level of post-emergence control on weed species by amicarbazone, trifloxusulfuron +
ametryn and the tank-mix amicarbazone + trifloxusulfuron + ametryn
Weeds
Herbicide treatments
amicarbazone
trifloxysulfuron+ametryn
amicarbazone +
trifloxysulfuron+ametryn
Ageratum conyzoides
+
+
+
Cyperus rotundus
-
+
+
Digitaria horizontalis
+
-
+
D. timorensis
-
-
+
Eleusine indica
-
+
+
Kyllinga sp.
-
+
+
Oxalis debilis
-
+
+
Paspalum paniculatum
-
+
+
P. urvillei
-
+
+
Setaria barbata
-
+
+
Youngia japonica
+
-
+
+
Good control
-
179
Poor control
Table 7.5 Post-emergence control by trifloxysulfuron+ametryn and amicarbazone in plant cane expressed as % kill (detransformed arcsine data) by
trifloxysulfuron+ametryn and amicarbazone in plant cane. Values in parentheses represent transformed (arcsine) data
Treatments
kg a.i. ha-1
Weed control (% kill*)
Trial V
Trial VI
Trial VII
Trial VIII
5 WAS
5 WAS
6 WAS
6 WAS
Amicarbazone
0.875
55 (0.83)
52 (0.80)
83 (1.14)
74 (1.04)
Amicarbazone
1.05
69 (0.98)
76 (1.05)
79 (1.09)
76 (1.05)
Amicarbazone
1.25
73 (1.02)
72 (1.01)
83 (1.14)
78 (1.08)
Amicarbazone
1.4
68 (0.97)
69 (0.98)
75 (1.05)
87 (1.21)
Trifloxysulfuron+ametryn
0.0263+1.097
73 (1.03)
63 (0.91)
66 (0.95)
77 (1.07)
Trifloxysulfuron+ametryn
0.0315+1.317
74 (1.03)
62 (0.91)
81 (1.12)
78 (1.08)
Amicarbazone + trifloxysulfuron+ametryn
0.875+
0.0263+1.097
89 (1.23)
82(1.14)
84 (1.16)
96 (1.38)
Amicarbazone + trifloxysulfuron+ametryn
1.05+
0.0236+1.097
96 (1.36)
77 (1.07)
86 (1.19)
98 (1.42)
Tebuthiuron + atrazine + 2,4-D
1.3 + 2.0 + 2.0
73 (1.02)
57 (0.85)
87 (1.21)
73 (1.03)
0.092
0.133
0.105
0.086
Standard error of transformed data
* values represent detransformed (arcsine) data
180
Effect on cane growth
Cane measurements made prior to spraying and 6 weeks later revealed that neither of the two new
herbicides nor their tank-mixes caused a reduction in tillers or lower cane dewlap heights when
compared to the standard (tebuthiuron + atrazine + 2,4 D amine salts). As the latter is known to be safe
for post-emergence application in sugar cane, the new tank-mix should therefore be relatively safe for
such application. As the level of post-emergence weed control by the new tank-mixes was superior to
that obtained in the standard plots, the few weeds left uncontrolled in the latter plots may suggest some
weed competition which would mask the effect of crop damage by the new herbicides. The possibility
of the latter occurring was minimised by also comparing the cane growth parameters with the
measurements recorded in the plots from the pre-emergence trials, which were initiated in the same
field at each locality (same variety and planting dates).
7.3.3 Potential of amicarbazone and trifloxysulfuron+ametryn for early post-emergence weed
control in ratoon cane
Post-emergence control of weeds
The three trials conducted in ratoon cane were sprayed 6 to 8 weeks after harvest to assess
trifloxysulfuron+ametryn and amicarbazone for use within the newly developed weed management
strategy. The two new herbicides, applied alone, were again found to be as effective as the two
standards for their knockdown effect. Higher rates of amicarbazone resulted in increased efficacy
(Table 7.6). The tank-mix of trifloxysulfuron+ametryn + amicarbazone once more tended to show
higher level of control than the two standards. Thus, superiority was achieved as a result of a more
effective control of species such as D. horizontalis, P. paniculatum, P. urvillei, S. barbata, Kyllinga
spp. and C. rotundus (see Table 7.4).
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Table 7.6 Post-emergence control and residual activity following application of
trifloxysulfuron+ametryn and amicarbazone in ratoon cane expressed as % kill (detransformed arcsine
data) and % of untreated control (detransformed data) respectively. Values in parentheses represent
transformed (arcsine) data.
Treatments
kg a.i. ha-1
Trial IX
% of
% killa untreated
controlb
Trial X
% of
% kill untreated
control
Trial XI
7 WAS
12 WAS
7 WAS
11 WAS
6 WAS
% kill
Amicarbazone
0.875
64 (0.92)
27
54 (0.82)
53 (0.81)
57 (0.86)
Amicarbazone
1.05
57 (0.86)
30
59 (0.87)
42 (0.71)
68 (0.97)
Amicarbazone
1.4
79 (1.10)
27
66 (0.66)
38 (0.66)
83 (1.155)
Trifloxysulfuron+ametryn
0.0263+1.097
68 (0.97)
20
67 (0.96)
16 (0.42)
84 (1.15)
Trifloxysulfuron+ametryn
0.0315+1.317
61 (0.90)
24
58 (0.86)
22 (0.49)
85 (1.17)
Amicarbazone+
trifloxysulfuron+ametryn
0.875+
0.0263+1.097
87 (1.21)
20
82 (1.13)
16 (0.41)
90 (1.25)
Amicarbazone+
trifloxysulfuron+ametryn
1.05+
0.0263+1.097
65 (0.94)
10
80 (1.11)
19 (0.45)
87 (1.20)
Tebuthiuron+atrazine+2,4-D
1.6+2.0+2.0
57 (0.86)
23
56 (0.84)
39 (0.68)
73 (1.03)
Hexazinone+atrazine+2,4-D
0.6+2.0+2.0
49 (0.77)
49
63 (0.91) 43 (0.71)
71 (1.00)
(0.192)
n/a+
Standard error of transformed data
(0.071)
(0.092)
(0.037)
a
– post-emergence control; b – residual activity= recovery of weeds + new emergence
+ data from only one rep – no statistics
Residual herbicide activity on weeds
The residual activity of the new tank-mix following the knockdown of weeds was significantly
superior to the two standards (Table 7.6), particularly to the one containing tebuthiuron which is
known to provide fairly long pre-emergence control (approx. 14 WAS). It seemed that the higher rate
of amicarbazone within the tank-mix extended the residual activity.
Visual observations made throughout the duration of the trials did not show any phytotoxic
effects of the tank-mix on the different cane varieties.
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7.4 Discussion and conclusions
The good potential of herbicides trifloxysulfuron + ametryn and amicarbazone as both pre- and postemergence treatments was demonstrated in plant and ratoon cane. Applied pre-emergence of weeds,
both herbicides were effective on most broad-leaved weeds and some annual grasses. Trifloxysulfuron
+ ametryn was less effective on Digitaria horizontalis and D. timorensis, and amicarbazone did not
control Cyperus rotundus, Paspalum spp. and Kyllinga spp (Table 7.3). Tank-mix at lower rates of
both herbicides overcame their individual weaknesses while maintaining a residual activity of over 14
to 16 weeks. When applied early post-emergence of weeds, both trifloxysulfuron + ametryn and
amicarbazone were effective on most broad-leaved weeds and some grasses. The efficacy of
trifloxysulfuron + ametryn on Paspalum spp., C. rotundus and other sedges, and that of amicarbazone
on Digitaria horizontalis compensated for their individual inefficacies when they were tank-mixed
(Table 7.4). As far as could be ascertained from the trials, which were not set up to specifically assess
crop tolerance, the tank-mixes trifloxysulfuron + ametryn + amicarbazone were well tolerated by both
young plant and ratoon cane.
The efficacy (pre- and post-emergence) of the new tank-mix offers a new perspective for
managing weeds in sugarcane by delaying of the first herbicide application which will result in
savings of at least one herbicide treatment per season. The tank-mix trifloxysulfuron + ametryn +
amicarbazone (0.0263 + 1.097 + 0.875-1.05 kg a.i. ha-1) has been registered and recommended for use
in Mauritius; the higher rate of amicarbazone would be useful where a relatively longer residual
activity is required. At these rates, the cost of the new tank-mix is comparable to the conventional
treatments, but the possibility of saving one treatment per season renders the new tank-mix more costeffective.
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CHAPTER 8
GENERAL DISCUSSIONS & CONCLUSIONS
8.1 Weed competition in sugar cane
Competition between sugar cane and the major weeds
This study has shown that sugar cane is affected by competition from weeds just like other crops but
the effect is often relatively small. Under the worst scenarios assessing the critical period of weed
competition in sugar cane, the maximum reduction in cane yield was recorded in plant cane and was
53% of the weed-free treatments after weeds were left in competition with sugar cane for nearly 30
weeks. This reduction is lower than that reported by Suwanarak (1990) who found cane yields to be
lowered by more than 70% after no weeding during the first four months after planting in the wet
season in Thailand. In ratoon cane, the maximum losses in cane yields varied between 20% and 30%.
Similarly, in the trials evaluating competition from individual species (Chapters 3 & 4), competition
on the total dewlap height or biomass from very high weed densities rarely exceeded 50%. In other
crops, some yield losses due to weed competition have been reported by Naylor (2002); a summary of
51 experiments carried out in UK and involving wild oats densities ranging 8 to 662 plants m-2 caused
yields of spring barley to decrease by 0 to 72% while canary grass (Phalaris minor) and black grass
(Alopecurus myosuroides) reduced yields of winter wheat by 26% and 45% at densities of 300 and 500
plants m-2 respectively.
Relative competitiveness ‘q’ values of eight weed species commonly found in sugar fields,
determined by model developed by Kropff and Spitters (1991), showed that sugar cane was a stronger
competitor than most of the weeds tested. Although use of this model, based on the relative leaf areas
of the weed and crop, showed similar trends when the same weeds were compared, their q values were
found to vary across trials. However, the variations in q values found for weeds in sugar cane are
smaller than those reported for competition between Sinapis alba L. (white mustard) and sugar beet
(Beta vulgaris L.) or spring wheat (Triticum aestivum L.) (Lotz et al., 1996). The varying q values
may limit the use of this model for predicting yield losses in sugar cane and comparisons between
various species would only be possible if all the weeds were tested under a range of similar conditions.
Despite these limitations, it was, however, possible to identify some of the weeds as being more
competitive, i.e. A. conyzoides, P. paniculatum, D. horizontalis and S. barbata, compared to a lesser
competitive group including B. pilosa, P. urvillei, P. conjugatum and P. commersonii. The latter
184
information conflicts with the perception of many growers that grasses are more competitive than
broad-leaved weeds. The difficulty of achieving control of all grasses with selective herbicides in
sugar cane may have created this belief.
Timing of competition
The critical periods of weed competition determined in Chapter 2 revealed that the adverse effect of
weed competition in sugar cane was not experienced before several weeks following cane and weed
emergence. This was also confirmed in the different trials, both under glasshouse and field conditions,
assessing competition from one weed species at a time; the adverse effects on cane growth were
measurable only 10-12 weeks after imposing weed infestations. In some of the trials with the broadleaved weeds, some treatments at higher densities showed the adverse effects earlier due to the quicker
rate of growth of the weeds. This lag period between weed emergence and competition explains why
the onset of the critical periods of weed competition is several weeks later in ratoon cane. Competition
started earlier (6 WAP) in the critical period trial carried out in plant cane and this may be explained
by the presence of more broad-leaved weeds at that site, the period of the year and the relatively
slower cane growth.
The relative competitiveness based on ‘q’ values of both P. paniculatum and P. urvillei was
found to remain unchanged with time within the first nine weeks after establishment of weed
infestations. A reduction in their competitiveness was recorded after 13 WAT (in Trial III, chapter 4),
mainly explained by the distribution of the leaves within the canopy though they had similar relative
leaf areas (Lw).
The timing of weed emergence on the final cane yield was illustrated in Chapter 4 (Trials 1 &
III). Both trials revealed that the second transplanting of weeds tested caused no significant difference
on cane yield. The physiological difference between the two dates of transplanting included both mean
height of shoots and the stage of tillering. The results indicate that weed infestations, occurring when
the cane approaches peak tiller density for that variety and when shoot heights are more than 40 cm,
would be less prone to weed competition.
Measurements of the total cane dewlap height at the different observation dates had shown some
significant reduction although the same treatments did not show any difference at harvest. It is
believed that due to its long growing period after the cane leaves are less exposed to the competition
for light till harvest, sugar cane has an ability to recover and compensate for earlier losses. Apparent
185
effects of weed competition observed before canopy closure do not necessarily translate into yield
losses.
Effect of weed density on weed competition
Although it was difficult to maintain the ‘original’ densities as at transplanting, increasing weed
density was found to influence weed competition and result in earlier weed competition. However,
there was often little difference between the higher weed densities, as a result of high level of intraspecific competition between the weeds. Broad-leaved weeds such as A. conyzoides and B. pilosa have
a more prostrate growth and hence were subjected to more intra-specific competition as compared to
the grasses with a more upright growth of the leaves. This may explain the lack of a major difference
between the two infestation levels studied in some of the critical period trials; the 50% infestation
level was most probably not so different to the natural infestations.
Mechanisms of weed competition
Weed competition impaired both tillering (shoot density) and stalk elongation (dewlap height of
stalks). In most of the trials, early weed competition resulted more in a reduction of the number of
shoots or stalks. Stalk elongation was reduced when competition occurred after the peak of the
tillering phase or stalks had reached a mean dewlap height of 25 cm or more. The effect of competition
on stalk elongation was also demonstrated in the split-box and allelopathy trials where the effect of
competition was observed only after the stalks had reached a dewlap height of 35 to 40 cm.
One of outcomes of this study has been the identification of the involvement of other
mechanisms of weed competition as well as that for light. In the critical period trials, competition was
still observed with weeds that emerged towards the end of the CPWC or when the cane stalks were
higher than the weeds. This was confirmed with the comparison of P. paniculatum and P. urvillei
where the former proved to be more competitive in some treatments although the latter produced more
leaf area (for similar densities) and grew taller to intercept more light within the canopy. The vertical
distribution of leaf area of cane and weeds (Chapter 4) showed that P. paniculatum was as or more
competitive even though most of the cane leaf area was found higher in the canopy than the weeds.
This indicated that other mechanisms might be involved and that competition for light was more
important during the earlier growth stages where tillering was mostly affected.
Root competition was shown to be as important as shoot competition or more in Chapter 5. Root
development of sugar cane was impaired by both root and shoot competition and the fact that they
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were not resulting in a more severe competition when both occurred simultaneously suggested that
they were not affecting root development in the same manner. The effects of root competition were
observed several weeks after imposing competition when the cane stalks reached more then 35 cm in
dewlap height suggesting that root competition was more important than competition for light after the
post-tillering phase.
Although root competition seemed to cause more reduction in root biomass of P. urvillei
compared to P. paniculatum, the higher competitiveness of the latter was still not completely
explained. Collection of leachates (root exudates) from the two grasses applied daily to sugar cane
confirmed an effect from allelopathic compounds resulting in a reduction of root biomass of sugar
cane. In one trial (Trial I), P. paniculatum developed vigorously and the effect of its leachates on cane
growth was more pronounced than those from P. urvillei. In the other trials, where an adverse effect
from the allelochemicals was observed, P. urvillei was more competitive; P. paniculatum had not
developed so vigorously as in the first trial. One chemical identified from the leachates that may be
responsible for the allelopathic effects was 2-Propenoic acid, 3-(4-methoxyphenyl), from the known
(for their allelopathic properties) family of cinnamic acids. The presence of higher concentration of
this chemical in the leachates (samples taken in Trial IV) from P. paniculatum suggests a link with the
greater reduction in root biomass observed between this treatment and the control (distilled water).
In conclusion, although weeds appear to impact on the growth of sugar cane by competing for
light, there are also effects arising from below ground competition. This may be linked to competition
for water and nutrients but may also involve allelochemicals. The allelopathic potential of the other
weeds, particularly grasses such as D. horizontalis and Panicum species, occurring in sugar cane fields
need to be assessed. The allelopathic properties of C. rotundus on sugar cane had been demonstrated
by Mc Intyre (1998). Further research is needed in the mechanism of below ground competition and its
importance for other weed species apart from the two Paspalum spp.
The mechanisms of weed competition may be summarized by competition for light at the earlier
stages of growth (germination/tillering) and root competition, with or without allelopathic exudates
from the weeds, later within the tillering/elongation phase.
8.2
Applications and recommendations for the Mauritian sugar industry arising
from this research study
The main application of the above findings for the Mauritian sugar industry would be a change in the
timing of application of herbicide treatments. The critical periods study shows that the ‘traditional’
187
approach of applying a pre-emergence treatment immediately after planting or within a few days after
harvest to prevent any weed emergence is not totally justified. Although the trials to determine the
CPWC were established under the most severe agro-climatic conditions, the results can be
extrapolated on the basis of the GDDs to other areas and cane varieties (early v/s late maturing).
Similarly, the CPWC would imply an earlier end of weed control compared to the current approach
where fields are maintained almost weedfree until the complete closure of the crop canopy.
Application of the CPWC will, in general, result in the reduction of at least one herbicide application
per season. This is possible by delaying the first herbicide treatment until onset of the first flush of
weeds and applying an effective herbicide treatment to kill all weeds present and provide a fairly long
residual activity to keep field weed-free until the end of the CPWC (Fig. 8.1).
Traditional approach:
Manual
2nd Tmt
1st Tmt
1
2
3
4
1st Tmt
3rd
Tmt
5
6
2nd Tmt
7
8
9
10
11
12
‘spot’
Manual
Manual
Plant cane
New approach based on CPWC:
Ratoon cane
CPCW in plant cane
2nd
1st Tmt
1
2
3
4
6
5
7
8
9
10
11
12
1st Tmt
‘spot’
CPWC in ratoon cane
Fig. 8.1 Timing of herbicide applications in sugar cane based on CPWC (arrows showing start
and end of control period) (bottom) compared to the conventional method (top). Figures in
boxes represent months after planting or harvest. Treatments for plant cane are represented in
green boxes and ratoon cane in orange boxes.
188
Row spacing influences the critical timing for weed removal (Knezevic et al., 2003). Planting
cane at higher density by changing the row spacing would reduce further the period of control based
on the CPWC. Dual row planting, consisting of pairs of cane rows 0.5 m apart with 1.8 m between
their centres, has been tested successfully and recommended to the producers in 2006 (MSIRI, 2006;
Ismael et al., 2007). The new row spacing also has the potential of increasing cane yield with the same
amount of planting material and with no increase in fertilizer used compared to conventional planting
(1.62 m spacing). It also reduces costs of production by improving weed management and the
efficiency of chopper-harvesters. This improvement in weed management results from earlier canopy
closure and consequently the end of the CPWC is reached four to eight weeks earlier (Ismael et al.,
2007).
The success of such a weed management strategy as above would only be possible if the
herbicide treatments are able to kill all the weeds present at the time of application and provide
effective residual control of most of the weeds present for the duration of the CPWC. Traditional
herbicide treatments did not have that potential and the evaluation and the recommendation of the new
tank-mix amicarbazone + trifloxysulfuron+ametryn (Chapter 7) has satisfied this requirement. The
new tank-mix consisting of trifloxysulfuron+ametryn (0.0263+1.097 kg a.i. ha-1) and amicarbazone
(0.875 to 1.05 kg a.i./ha) has a residual activity varying between 14 to 16 weeks and, has postemergence activity. It is able to control almost all weeds found in sugar cane in Mauritius including
D. horizontalis, D. timorensis, C. rotundus, Paspalum spp. and Kyllinga spp. Moreover,
trifloxysulfuron+ametryn has the potential of controlling partly C. rotundus pre-emergence. The tankmix, amicarbazone + trifloxysulfuron+ametryn (0.875-1.05 + 0.0263+1.097 kg a.i. ha-1) did not cause
crop injury in young plant or ratoon cane. The efficacy (pre- and post-emergence) of this new tankmix has offered a new opportunity for managing weeds in sugar cane, as delaying of the first herbicide
application will result in savings of at least one herbicide treatment per season.
New weed management strategies based on the CPWC include the exploitation of control
methods other than use of herbicides. The use of mechanical weeding during the first two or three
months after planting has also been tested successfully (MSIRI, 2006). Two or three passes of duck’s
foot cultivators have proved to be sufficient to control weeds up to the end of the critical periods. This
method of weed control has been recommended in plant cane and where fields are either in rock-free
soils or have been derocked for mechanized harvest; this approach would be possible on some 50% of
the replanted area every year.
The concept of limiting weed control during the CPWC period, particularly that of leaving
weeds uncontrolled after the end of the CPWC, has been discussed by many growers in the past. They
189
were concerned about the production of seeds from the ‘residual’ weeds and its consequences on the
seedbank in the mid- or long-term. Trials (not reported in this study) initiated in parallel to the above
development have shown that there was no significant increase in the seedbank between the same
plots where weed control had been stopped 16 weeks after harvest for three consecutive years and
plots which were kept weed-free. This study is being pursued but as the new weed management
strategies are geared towards weed control until 20 to 26 WAH, the risks of increasing the seedbank is
minimised. Riemens et al. (2007) has shown that appropriate weed management practices in organic
farming resulted in no increase in the weed seedbank after seven years. Weed control strategies based
on density thresholds were found more cost-effective than spraying every year after modelling seed
production of Alopecurus myosuroides and Poa annua (Munier-Jolain et al., 2002). Similarly, Smith
et al. (1999) reported a reduction in the population of Anisantha sterilis in winter wheat through
changes in patterns of management. In sugar cane, Witharama et al. (1997) reported that the similarity
between species in the seed bank and emerged seedling population in the field was low. This may
imply that all the seeds produced do not necessarily pose a threat of more competition later on.
Green cane trash blanketing (GCTB) is practised on approximately 25% of the area harvested
and is expected to increase as more fields are harvested mechanically in the near future. The trash
blanket controls the weeds effectively until it decays; in humid areas this may happen before end of
the CPWC and a herbicide treatment may be required. Similarly under some agro-climatic conditions,
especially in plant cane, a second treatment, over and above the new tank-mix applied before the onset
of the CPWC, may be justified. Under these conditions, the use of models to predict the weed
competition expected from the different infestation levels and weed species present would be
beneficial and would suggest further savings of herbicides. However, the findings of this study have
revealed varying relative competitiveness (q) values across trials and standardization of the results
needs more work. Furthermore, the use of such parameters in sugar cane would be more difficult due
to the length of the growing season; the q values changes with time of weed emergence and
assessment date.
The allelopathic potential of the other weeds needs to be determined before making any decision
on leaving such weeds in the fields after the end of the CPWC. As root competition seems to be
important and sugar cane roots do not exploit the cane interrows entirely, weed management could be
envisaged that was focused in the vicinity of the stubble or cane roots. This is supported by work
carried out by Witharama et al. (2007) who found that more weeds emerged in the cane furrows than
on the ridges and the difference was influenced by the soil moisture. The latter may imply a herbicide
treatment on a localised band nearer to the cane stools in situations where a second post-emergence
190
treatment would be required to reach the end of the CPWC. As the soil moisture varies within the
three agroclimatic zones of Mauritius, such approaches would require more research and development.
8.3 Suggestions for future research
Relative competitiveness (q value) for more weeds
This study has indicated two groups of weeds according to their relative competitiveness. More trials
should be conducted to evaluate the relative competitiveness (q values) of more weed species
occurring in sugar cane fields; the data would be useful for prediction of yield losses for management
purposes or Decision Support Systems. The q values could be used to regroup weeds into two or three
categories. The results of this study would assist in redefining the various densities for estimating q
values. New technologies using Multi Spectral Reflectance (MSR) or radiospectrometry are being
successfully tested and calibrated in sugar cane. The use of such technology would give quicker leaf
areas estimations.
Threshold for sugar cane and testing of herbicides
The variability within the weed infestations and cane measurements or leaf area estimates observed in
this study may restrict the use of threshold infestation levels in sugar cane under the Mauritian
conditions. However, with a reliable estimation of leaf areas with the new or forthcoming
technologies, prediction of yield losses near the end of the critical periods may assist in the necessity
of a second or ‘spot’ application.
The q values of the different weed species will certainly be useful in the choice of the herbicide
treatments. DSS using the relative competitiveness (or any other index) together with information on
the level of infestation of each species (e.g. frequency abundance method) will certainly enable more
precise selection of herbicide treatments and their rates for cost-effective management of weeds.
The current methods for evaluating herbicides for sugar cane do not provide information on the
interaction between weed infestation level or size of weeds and rates of treatments. The efficacy of
lower rates on weaker weed infestation levels or smaller weeds or less competitive ones would permit
further savings of herbicides.
191
Allelopathic potentials of more weeds
The screening of more allelopathic compounds from the weeds occurring in sugar cane fields will not
only enable a better understanding of the mechanisms of weed competition, but could be used to
identify some potential bio-herbicides, for use in other crops.
Future work would be necessary to identify, using dose-response curves with known amounts of
the chemicals, the minimum dose of the allelochemicals (e.g. cinnamic acids) required to cause
adverse effects on sugar cane. The release of the various chemicals with time and the amounts
released will also enable a more complete understanding of the mechanism of weed competition in
sugar cane.
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