CRITICAL PERIODS OF WEED COMPETITION IN SUGAR CANE IN MAURITIUS

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Trial II – Competition from Bidens pilosa, Digitaria horizontalis and Paspalum urvillei on

sugar cane grown in trays Plant material A glasshouse trial was established to study competition of B. pilosa, D. horizontalis and P. urvillei on young sugar cane shoots. Conditions inside the glasshouse were similar to those outside as all openings (wire mesh to prevent insects, etc.) were left opened to maintain almost the same temperatures. Sugar cane was planted on 14 April 2001 in trays 1 m x 0.4 m x 0.3 m and filled with soil (Humic Latosols according to Parish and Feillafé, 1965) collected in fields at Réduit Experiment station. All trays were irrigated by applying manually the same amount of water to keep the soil humid and avoid any water stress on the crop. No extra fertilizer was added as soil analysis had shown a sufficient amount of NPK was present. Ten two-budded cane setts of variety R 570 were planted in each tray, in double rows in the centre of the tray. Eight weed densities (7 + 1 weed-free control) were established for each weed. Seeds of B. pilosa were sown directly in the trays on 10 May 2001 at increasing densities and some thinning was carried out after germination. For D. horizontalis and P. urvillei, young plants were uprooted from abandoned fields and leaves were pruned to reduce transpiration at transplanting.

Trial III – Weed competition from Paspalum paniculatum and Paspalum urvillei on sugar

cane under field conditions Plant material A field experiment was initiated in October 2001 at Réduit, L soil group (Parish & Feillafé, 1965), to study competition on sugar cane from P. paniculatum and P. urvillei. Sugar cane, variety R 570, was planted on 7 September 2001 using three-eyed cuttings obtained from a plant cane field on the station at a row spacing of 1.5 m. Young plants of the two weeds were collected from abandoned fields in the Belle-Rive regions and were transplanted after pruning of the upper part of the leaves to reduce transpiration. Weed control in the plots was achieved by applying a selective treatment consisting of atrazine at 2.0 kg a.i. ha-1 after transplanting of weeds and by regular manual weeding of emerged weeds (mostly grasses). Other agronomic practices were the same as in commercial sugar cane crops. Treatments and experimental layout Paspalum urvillei and P. paniculatum were both transplanted at densities of 6.7, 10, 15, 20 and 33.3 plants m-2 on 13 October 2001 (5 weeks after planting cane). A weed-free plot was also included. Each plot consisted of three cane rows of 1.4 m long with a row spacing of 1.6 m. The statistical design was a split-plot; main plots consisted of the two weeds, sub-plots were six weed densities. Each treatment was replicated three times.

Discussion and conclusions

Time interval between start of infestation and effect on cane growth The trials have demonstrated that the effect of competition from the different weed species on sugar cane was visible only several weeks (e.g. 12-14 weeks) after introducing the infestations although a few effects were observed earlier in some of the trials at the higher weed densities. This implied that there should be a minimum level of weed infestation and duration of interference to cause any adverse effect on cane growth; the higher densities reached that level of infestation earlier. The latter may be reached even earlier for quick growing species such as B. pilosa (in Trial II) where some effects were detected at the highest densities as from 3 WAT. The relative rate of growth of the same weed with respect to the crop may differ with growth conditions; B. pilosa was found to show an adverse effect on the total dewlap height as from 9 WAT in Trial IV. All the grasses tested took, more or less, the same time (between 13 and 16 WAT) to show their competitive effect.

Trial I – Effect of time of observation and two transplanting dates on the relative

competitiveness (q value) of Paspalum paniculatum and Paspalum urvillei in competition with sugar cane Trial site and plant material A field experiment was initiated in November 2003 at Réduit Experiment Station, L soil group (Parish & Feillafé, 1965), to compare competition from P. paniculatum and P. urvillei on sugar cane. The field was planted on 24 November 2003 using three-eyed cuttings of cane variety R 575 obtained from a plant cane field on the station and adopting all other cultural practices as per normal recommendations. Young plants of the two weeds were collected from abandoned fields in the BelleRive region and transplanted after pruning of the upper part of the leaves to reduce transpiration. Treatments and experimental layout The weeds were transplanted at two dates, the first on 23 and 24 January 2004 (9 WAP) and the second on 17-19 March 2004 (17 WAP). At each date, P. urvillei and P. paniculatum were both manually transplanted at densities of 6, 10, 15, 20 and 33 plants m-2; a weed-free plot was also included. Each plot consisted of three cane rows of 1.5 m long and cane planted at a row spacing of 1.5 m; the effective competition area was 1.2 x 1.5 (1.8 m2 ) for each row of cane, with a walking path of 0.3 m in the centre of the interrows. The statistical design used was a split-split plot with the two transplanting dates as main-plots, weed species (P. paniculatum v/s P. urvillei) as sub-plot and six weed densities as sub-sub-plot treatments. Each treatment was replicated three times. The middle row within each plot was kept for cane measurement at end of the treatment period whereas the two border rows were used for destructive sampling for cane and weed dry weight and leaf area data. The field was irrigated regularly and all emerging weeds other than those transplanted were hand-weeded..

Effect of root and shoot competition on shoot elongation and cane growth

Paspalum paniculatum Cane shoot elongation, measured from the end of December (seven weeks after planting), revealed no differences between the various combinations of root and shoot competition treatments and the control, i.e., no root or shoot competition until the first week of April 2007 (21 WAP) when a significant reduction in mean dewlap height from root competition between sugar cane and P. paniculatum was observed (Fig. 5.6). This difference was maintained until the end of the trial, i.e., for another two months. Shoot competition did not seem to affect cane elongation and the adverse effect of root competition on cane elongation was not apparent when sugar cane was exposed to both root and shoot competition from P. paniculatum.

TABLE OF CONTENTS :

• DECLARATION
• ACKNOWLEDGEMENTS
• LIST OF ABBREVIATIONS
• ABSTRACT
• CHAPTER 1 INTRODUCTION
  • 1.1 Sugar Cane: a brief description
  • 1.2 The island of Mauritius
  • 1.3 Introduction of sugar cane to Mauritius
  • 1.4 Cultural practices of sugar cane
  • 1.5 Weeds of sugar cane
  • 1.5.1 Major weeds of sugar cane in Mauritius
  • 1.5.2 Paspalum paniculatum
  • 1.5.3 Paspalum urvillei
  • 1.6 Weed control in sugar cane
  • 1.7 Sugar cane in the Mauritian economy
  • 1.8 Development of weed management strategies
  • 1.9 Objectives of thesis
• CHAPTER 2 CRITICAL PERIODS OF WEED COMPETITION IN SUGAR CANE IN MAURITIUS
  • 2.1 Introduction
  • 2.2 Materials and methods
  • 2.3 Results and discussion
  • 2.4 Conclusion
• CHAPTER 3 WEED COMPETITION IN SUGARCANE: THE RELATIVE COMPETITIVENESS OF DIFFERENT WEED SPECIES
  • 3.1 Introduction
  • 3.2 Materials and methods
  • 3.3 Results
  • 3.4 Discussion and conclusions
• CHAPTER 4 EFFECT OF TIME AND LEAF AREA DISTRIBUTION ON WEED COMPETITION BETWEEN SUGAR CANE AND PASPALUM PANICULATUM OR PASPALUM URVILLEI
  • 4.1 Introduction
  • 4.2 Materials and methods
  • 4.2.1 Trial I – Effect of time of observation and two transplanting dates on the
  • 4.3 Results
  • 4.3.1 Trial I – Effect of time of observation and two transplanting dates on the relative competitiveness of P. paniculatum and P. urvillei in competition with sugar cane
  • 4.3.2 Trial II – Relative competitiveness of P. paniculatum and P. urvillei on sugar cane at two observation dates and effect of leaf area distribution on competition
  • 4.3.3 Trial III – Relative competitiveness of P. paniculatum and P. urvillei at two transplanting dates in sugar cane and effect of leaf area distribution on competition
  • 4.4 Discussion and conclusions
• CHAPTER 5 COMPARISON OF ROOT AND SHOOT COMPETITION BETWEEN
  • SUGAR CANE AND PASPALUM PANICULATUM OR P. URVILLEI
• CHAPTER 6 ALLELOPATHIC EFFECT OF PASPALUM PANICULATUM AND P
  • URVILLEI ON GROWTH OF SUGAR CANE
• CHAPTER 7 A NEW HERBICIDE TANK-MIX OF TRIFLOXYSULFURON +
  • AMETRYN AND AMICARBAZONE TO PROVIDE COST-EFFECTIVE BROAD SPECTRUM PRE- AND POST-EMERGENCE TREATMENT FOR MANAGING WEEDS IN SUGAR CANE
• CHAPTER 8 GENERAL DISCUSSIONS & CONCLUSIONS

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Weed management in sugar cane: critical periods of weed competition and mechanisms of interference from Paspalum paniculatum and P. urvillei