• Chemical control : the place of herbicides in the weed control

Pre-emergence herbicides

The pre-emergence herbicides have been the first to be distributed in tropical zones, specially in the cultivations of cotton and industrial crops (sugar cane). These products are easy to market, since their efficacy spectrum is very often quite broad and they are used at a very definite period, just after the sowing.
However, these herbicides are very dependent upon the physical status of the soil : they cannot be applied on soils with too much clods or covered by a thick mulch. Their availability in the soil depends on texture : the product is absorbed by thin layers of clay or colloidal organic matter. Inversely, in sandy soil, the risks of toxicity are increased.
Rain, before or after the application, generally favours the surface diffusion of these herbicides which penetrate by the root system; however, a heavy downpour after the application may wash away the product by surface run-off.

Post-emergence products

Frequently used in rice or sugarcane, these chemicals are chosen according to the flora of weeds present. They are often specific : anti-dicotyledonous action in maize, rice or sugar cane cultivation, graminicide (seed killing) in cotton or leguminous plants. They are independent of the soil type or status. Rain diminishes the efficacy of these herbicides penetrating via the leaves when spread on the foliage, by washing away the chemical spray. The necessary interval between the spray and rain depends on the product and the rain’s intensity. Furthermore, deciding upon the specific date for the spray is sometimes difficult.

Broad-based herbicides

Those herbicides are the most widespread for post-emergence of weeds. They can be used at different periods of the crop cycle in full or split application if the crop is not fully developed. installed, in specialised application during the crop development. The choice of the product depends on the weed species to be destroyed.

• in case of infestation by perennial species like Cynodon dactylon, Imperata cylindrica, Cyperus esculentus, Cyperus rotundus ou Launaea sp., the products to be used are systemic products like glyphosate or sulfosate;
• if the flora comprises only annual species (Digitaria horizontalis, Tridax procumbens, etc...), contact products like paraquat or ammonium glufosinate would be sufficient.

Rotation of herbicides

The continuous use of the same herbicide inevitably leads to selections of flora often made of monospecific plant population, on which these active ingredients are not efficient. These new populations can only be controlled if the weeding techniques are modified or, at least, if the products used are diversified by choosing other chemical families that have other modes of action.

Among these populations, two types of behaviour must be distinguished:

• the species doesn’t form part of the efficacy spectrum of the product used and its selection by the herbicide treatment is foreseeable. The species is called tolerant;
• it concerns a population on which the product is usually active, but certain individuals are not affected ; the plants, not destroyed, multiply themselves, thus creating a new population qualified as resistant

Genetically modified varieties resistant to an herbicide

The selection of resistant varieties or the introduction of genes resisting to an herbicide in cultivated varieties offer new avenues in the control of weediness. It’s the case of rice weed control. The product, usually a broad-based herbicide but perfectly selective of the genetically modified variety, could be applied with efficacy on weeds and without risks for the crop. However, the problem of the transfer of the resistant gene to an herbicide arises in the case of weeds species very close to the crop, as in the case of rice weeds. Crossed pollination would be possible between the rice weed and the cultivated rice. An invasion by resistant rice weeds could happen, more rapidly since a strong herbicide would be applied in successive crop cycles. Thus, a genetic pollution risk exists if varieties of rice derived from genetic transformations are developed.

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