Arguably, the simplest and most widely accepted definition of weed among those who study it (weed scientists and weed ecologists) is that of “a plant growing where it is not wanted”. In simple terms, we can say a plant qualifies as a weed more based on its location than on its usability or non-usability. Therefore, a yam plant (vine) growing in a field sown to cowpea (beans) will be promptly removed as a weed; same way a carpet grass, though nurtured in a sports field, will be unsightly in a vegetable farm.
To many farmers (and homeowners), weeds are obnoxious plants, growing on lands cultivated for ‘wanted’ plants (crops), competing with crop plants for nutrients, space and light; and reducing the productivity of those tended plants to which the farmer has invested his time, labour and other resources. Besides, weeds reduce crop quality and the market value of crops, especially grains, by contaminating them with their seeds; they harbour and provide refuge for pests, serve as alternate hosts to disease pathogens on fields, interfere with farm operations, and take ample farming time and efforts used in their control.
Research results have shown that uncontrolled weed growth could cause as much as 40% yield reduction in maize, 12-80% in soybean, 50% in peanut, 91% in sweet potato and 77% in pepper, all depending on the type of weed (species) and the density of the crops. In addition to this, the labour requirement for the control of weeds, especially for smallholder African farmers with little or no access to machinery, could be as high as 36% for maize, 25% for cassava and 37% for sorghum.
Because of all these, farmers, yearly, invest heavily in chemical weed killers (herbicides) e.g. Glyphosate, Atrazine, Primextra, Roundup etc, to deal with the weed problem. Some of these herbicides are used as pre-emergent treatment i.e. sprayed before the emergence of the weeds to kill the seeds in the soil and prevent sprouting; while others are post-emergent herbicides i.e. those sprayed after the weeds have germinated. Also, some are selective in the types of plants they are toxic to or kill, while others are non-selective, broad spectrum plant killers. And to many farmers, applying herbicides (many times excessively or indiscriminately) seems the easiest and best way of weed control. But, this often comes at a cost – in fact many costs.
First, herbicides like other pesticides – because weeds are also regarded as plant pests – are made of chemicals which could be, and are indeed many times, toxic to the environment and organisms in it. Herbicides could kill non-target and beneficial organisms (e.g. pollinators or insect pest bio-controls); leach out to pollute waterways and disrupt aquatic ecosystems, build up to extremely toxic level in the food chain (biomagnifications) and the environment (persistence), and can lead to resistance in weed plants – creating plants known as “superweeds” – which become resistant and no longer susceptible to treatment by a particular herbicide and its variants.
Second, for many smallholder farmers (of Africa and other developing countries of the world), the cost of herbicide is one they would rather do without, given that they are resource-poor and that the ill-organized value chains in these countries mean they are poorly remunerated for their efforts. Moreover, with the increasing awareness and willingness to buy more organic produce among sustainability-sensitive consumers and the proliferation of organic certifications to fill the new niche, smallholder farmers may benefit more in keying into the organic/sustainable agriculture niche and increase their incomes by selling to consumers willing to pay more to conserve the environment. Thus, the argument against herbicides use for smallholder farmers is not just environmental or ecological but also economical.
Still, while going the sustainable way, weeds can be controlled culturally or biologically without causing damage to the environment. All it takes is the understanding of their biology and characteristics; the first being that, weeds are opportunistic plants and primary colonizers that take advantage of any opening (vacuum) in the ecosystem (land). Therefore, the solution is to reduce or eliminate open spaces on farmlands.
This can be done culturally through covercropping, mulching (using either dead plants or intercropping live leguminous plants), reducing inter-row spacing between plants, and by adopting the mixed cropping method – which all reduce or cover the space and light needed by weeds seeds to germinate.
Biologically, natural enemies like insects which find the weed plants palatable – but not the crops – can be deployed (entomo-control), or packaged pathogens (bioherbicides) which kill the weed plants by increasing the stress level on them can be sprayed; or improved crop seeds with allelopathic abilities can be sown to control weeds. This way, weeds are controlled without introducing chemicals (herbicides) that disrupt the functions of ecosystems and without harming the environment, but in such manner more profitable to (African) smallholder farmers – hitherto neglected at the bottom of the chain.