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Indirect control methods : how to prevent weediness ?

Written by Administrateur Modified on the

Soil preparation

Ploughing, if it is sufficiently deep, buries weeds and their seeds. It is seldom the case in manual or mechanical practices. The material used plays a decisive part. Thus, equipment with disks favour the multiplication of species from rhizomes or stolons.: Imperata cylindrica, Cyperus rotundus, Cynodon dactylon in rainfed crops, Oryza longistaminata in irrigated rice. Each piece or cutting gives rise to a new plant.

Crop introduction

The use of cleanseeds, free from weeds, avoid their development in the field. It is particularly true for the rice seed production, which follows precise rules as regards the level of rice weeds. A vigourous variety with tall leafy stems, and rapid growth has an advantage in soil competition. The increase in seed density is often advised, under condition of sufficient soil fertility. In rice growing, the planting out as soon as the plot is covered with water does not favour weeds, in particular rice weeds, that are just starting to germinate. It is the reverse in the case of the planting out of direct seeding on a plot which favours the development of weeds.

Water management

In irrigated rice growing, a pre-irrigation followed by a drying out period gives rise to an outburst of weeds, that are then eliminated. This cycle can be repeated if water is available. This method gives good results on Echinochloa colona and Ischaemum rugosum if the plot is perfectly flat and if the water level reaches 10 to 15 cm. The maintenance of canals reduces the source of infestation of irrigation waters, with Scirpusmaritimus or Typhadomingensis for example.

Soil mulching

Mulch gives good results in the case of manioc, yam or sugar-cane. Except for the latter, where it is done by the thinning out of the leaves at harvest time, this method needs at least 7 t/ha of straw to cover a plot. Soil mulching by means of plastic martetial is mainly used in market gardening. It combines the effects of shade and solarization (sterilization of the soil under the effect of the solar rays).

Covering plants

The use of covering plants meets two priorities : controlling erosion and weeds.

A crop cover of graminaceae or leguminosae modify rather deeply the whole of the physical, chemical and biological components of the cultivated eco-system :

  • by protecting the soil from the mechanical action of the rains, severe in tropical environment, and by absorbing water, the cover being in favour of infiltration rather than streaming ; it thus protects the soil structure.
  • A closed and everlasting coverage reduces the proliferation of weeds by the shade effect and the competition for the environmental resources
  • allelopathic effects are frequently observed.

 

Some covering plants, tested currently in different situations in tropical zone, are particularly interesting for their agronomic behaviour :

  • leguminosae: Arachis pintoi, Calopogonium mucunoides, Canavalia ensiformis, Cassia rotundifolia, Centrosema pubescens, Mucuna pruriens var. utilis, Pueraria phaseoloides
  • graminaceae: Paspalum notatum, Pennisetum clandestinum, le mil, Pennisetum glaucum ou le sorgho, Sorghum bicolor.

 

Those species are known for their good behaviour and seeds are easily available. However, by definition, covering plants are species with a high growth and multiplication potential. To avoid any invasion problem owing to exotic cover plants, specially in insular contexts, it is preferable to focus on species that can be found locally.

Crop rotation

Crop rotation on a same plot gives the opportunity to break the infestation of species difficult to handle, diversifying at the same time the cultural conditions and the control measures adapted to each crop. For example, in irrigated rice growing, the proliferation of rice weeds is stopped by a rotation with a rainfed crop.

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