Weeds - friend or foe?
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CTA. 1997. Weeds - friend or foe?. Spore 72. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/48906
Internet URL: http://spore.cta.int/images/stories/pdf/old/spore72.pdf
Farmers throughout the world look upon weeds as pests and generally speaking they are right to do so. With neither sufficient time nor money to control their weed problems, farmers feel justified in holding this opinion. But there are those who...
Farmers throughout the world look upon weeds as pests and generally speaking they are right to do so. With neither sufficient time nor money to control their weed problems, farmers feel justified in holding this opinion. But there are those who recognize that there may, after all, be something to gain from weeds, both in terms of time and money. For the majority of African farmers, weeds represent the main threat to their crops because the work involved in getting rid of them is arduous, time-consuming and therefore requires considerable manpower. This work adds to the final production costs. Many farmers in developing countries spend more time on weeding than farmers elsewhere, for the simple reason that they have no means of control other than to dig or hoe weeds out manually. It has been estimated that it takes between 15 and 20 days to clean a hectare of heavily weed-infested land, or half that time if the land is less badly infested. If the life cycle of the weed species is properly understood, the number of times that weeding has to be carried out can be reduced by ensuring that it is done at the optimum time. Herbicides can undoubtedly alleviate the workload considerably and have also contributed significantly to improved yields, but they have done no more than partially solve the problem. For many people herbicides are not an option, either because they are too expensive or because, as happens in some more remote regions, they are difficult to obtain. Chemical control does, however, pose problems of its own. The toxicity of the chemicals used may directly affect the health of the user. If applied at an incorrect dosage rate, or at the wrong time, herbicides can damage the crops they are designed to protect. Their use is not simple, and may require a level of education that many farmers in developing countries lack. Furthermore, repeated use of herbicides gives an advantage to resistant species which then proliferate to the detriment of the growing crops. Some weed species cover the soil and reduce the growth of other weeds. If a selective herbicide is then used, the long-term effect may be infestation by a weed species that is more invasive and more difficult to control than the original weed cover. Nevertheless, weeds must be controlled if good crop yields are to be obtained. A piece of land that has not been kept free of weeds may lose 85% of its yield potential. Digging up weeds and hoeing are still the most effective methods of control and the timing of the task makes all the difference to its success. Draught power can help to reduce the labour required. Deep ploughing will be useful for getting rid of some weed species, for example Cyperus rotundus, which dries out when brought to the surface. Other, more drought resistant, species will be positively encouraged by such practices and each little piece thrown up to the surface will grow into a new plant. Integrated weed control offers a variety of solutions. In Sri Lanka, for example, there has been some success in controlling Chromolaena odorata by introducing a predatory caterpillar. Could the same method be adapted for use in Africa? Weeds as mulch Weeds are certainly a major nuisance for farmers, but ground cover also has a beneficial effect. It protects land from rain and wind erosion while at the same time providing a useful quantity of biomass which can be incorporated to enrich the soil. Here, too, an understanding of the species involved will determine the efficacy of the operation, because some will need to be dug in at a specific time of the year in order to avoid a new infestation. Even on cultivated land, well-controlled ground cover can sometimes benefit crops by forming a living mulch to help keep humidity levels up. Weeds can be cut down before they seed, again to form a green mulch, which can be spread around crops in order to preserve moisture and inhibit other weeds. If fed to animals or used as bedding, for example for draught animals or rabbits, weeds can be usefully recycled and their value enhanced. Some weed species should be allowed to grow close to cultivated crops because they attract crop pests, such as insects or caterpillars, away from the crop itself. Others may have the effect of discouraging the influx of pests. Local knowledge is invaluable in these instances Apart from such uses as mulch, green manure or forage, some weeds have an intrinsic value which is often unrecognized. Members of the Malvaceae family (which includes hibiscus and abutilon) have considerable economic potential in the production of fibres, notably for sound and heat insulation, for which outlets are rapidly expanding. Other potential niche markets are also worth exploring. The presence of certain species will reveal the composition of the soil and how it has been used in the past. They may also indicate precisely which areas need to be irrigated or drained, which crops may be the most suitable and what needs to be done to improve the soil. Such information can replace costly laboratory analysis to which farmers in ACP countries are, in any case, unlikely to have access. Farm production costs can be reduced if weeds are properly understood and, in some cases, weeds may even be a source of gain. Farmers have every interest in turning weeds to their advantage, at least in so far as they are able. In any case, the cost of controlling weeds should never be more than what is gained by the effort. Knowledge, and therefore information, to say nothing of imagination, is of great importance to farmers if they wish to engage with the enemy and, perhaps, seize the opportunity of recruiting future allies.