Weeds are plants, normally of an indigenous type, which become established on land where they are not wanted. Many weed plants are well adapted for seed dispersal and the efficiency of this mechanism makes them effective in becoming rapidly established on land which is not fully occupied with a crop.
Many weed seeds also have a fairly long period during which they can remain dormant in the soil, so that they may remain ungerminated until conditions favour their activity, Weeds often have efficient root systems so that they can obtain water, even from relatively dry soils. Where weeds have become well established, their roots may dry out the upper layers of soil to such an extent that a following crop may yield considerably less than it would have done if the ground had remained fallow. Weeds are therefore not generally recommended as a soil cover crop, much better results can be obtained by establishing a leguminous cover crop. Mulching, which has already been discussed in, is also an effective means of checking weed germination and growth. Weeds also compete with plants for soil nutrients.
As with all cultivated crops which are harvested at maturity and removed from the land where they have been grown, the removal of established weeds from land which is required for cultivation deprives the soil of nutrients which the weeds have absorbed. In addition to competing with plants for water and nutrients, weed roots also require oxygen for their respiration. If they are growing close to crop seedlings, the more vigorous growth of the weed roots can result in depriving crop seedling roots of their required supply of oxygen. The result of this competition may not be immediately obvious but may become so later in the growth of the plant. Young cocoa seedlings, for example, are particularly sensitive to weed competitor rains begin and may compete seriously with annual crops sown at this time. Crops particularly sensitive to early weed competition are maize, groundnuts and cotton; later weeding, when the crops are established does not compensate for this early check to crop growth. Early soil preparation, therefore, is recommended so that the soil can be prepared and cultivated to suppress the early growth of weeds before a crop is sown. Excessive soil cultivation, however, should be avoided, since this could lead to a loss of structure, particularly as this loss is also promoted by heavy rainfall. which may not become apparent until the young trees produce yields which are lower than those from trees which had no weed competition. An additional factor of weed competition is their ability to grow more rapidly than many crop plants, resulting in a shading of the crop seedlings which reduces yields due to a lack of sunlight.
Weed seedlings usually germinate rapidly as soon as their ins begin and may compete seriously with annual crops sown at this time. Crops particularly sensitive to early weed competition are maize, groundnuts and cotton; later weeding, when the crops are established does not compensate for this early check to crop growth. Early soil preparation, therefore, is recommended so that the soil can be prepared and cultivated to suppress the early growth of weeds before a crop is sown. Excessive soil cultivation, however, should be avoided, since this could lead to a loss of structure, particularly as this loss is also promoted by heavy rainfall.
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Weed control
Bush burning is a very popular method of weed control in Nigeria. This method is easy to carry out but has both advantages and disadvantages. If a good burn is obtained, most grasses and herbs and a portion of their fallen seeds are destroyed, and the farmer may gain considerably by starting his cultivation on a comparatively weed-free area. On the whole, bush burning may improve the physical condition of some soils, reduce weed competition, partially sterilise the soil, increase the pH of the soil, increase the microbial population as well as the rate of nitrogen mineralisation.
It may, however, have an adverse effect on the fertility of the land. By baking the soil and leaving the surface exposed to rainfall, erosion and leaching possibilities are increased.
Weed control by hand measures usually involves hoeing the surface soil to either uproot shallow-rooted weeds or to disturb or bury seedlings so that they cease to grow. For larger areas, weed control by hand hoeing is not possible and it is in this situation that the implements, drawn by either oxen or tractors and which are described in are most useful.
Cultivators, spring tine cultivators, disc harrows, ox-drawn hoes and cultivator or hoe blades attached to a toolbar are most effective in weed control prior to sowing and also for inter row cultivation to kill weed growth. In addition to annual weeds, many perennial grass weeds can be controlled by dry season or even early wet season cultivation. These include Lalang or Spear Grass (Imperata cylindrica) and Bermuda, Bahama or Dhub grass (Cynodon dactylon). The use of machinery makes it possible for the large-scale farmer to carry out most of his operations at the most appropriate time, this is particularly important for the control of weeds. Most perennial weeds have to be uprooted and left on the surface of the soil to be desiccated or dried in the sun. If they are merely cut into pieces by a hoe or disc plough and are only partly buried, some perennial weeds will produce new plants since the cut pieces can produce new roots and shoots.
Rotary cultivation to control some of those creeping perennial weeds may be unsuccessful due to this capacity of weeds for regeneration. Alternate ploughing and harrowing during the dry season has been found to be an effective method of controlling many perennial grass and broad leaved weeds. Early weed control also prevents flowering and seeding and this reduces the amount of weeding required during the period of crop growth.
Herbicides
These are chemicals which kill broad-leaved dicotyledonous weeds but which do not normally affect monocotyledons such as cereal crops, e.g. maize, They are not widely used in West Africa, since they are expensive and require spraying equipment, sometimes of a particular type, for effective application. In addition, a great deal of care and skill is required to apply them in the most efficient manner. These chemicals are therefore mainly used by large-scale farmers, where the weed problem is a serious one, and by commercial plantations growing tree crops. Labour for plantation work is becoming increasingly expensive and for some tree crops, the spraying of chemicals such as paraquat and simazine around the base of individual trees to check weed growth is less expensive than paying workers the wages to ring weed the trees by hand hoe.
A great deal of information is still required concerning the effective use of herbicides in the tropics.
Some crops appear to be more sensitive to herbicides such as 2, 4-D than many temperate crops; the effects of the weather, soil constitution and condition also affect the efficiency of many herbicides. Cotton and tobacco, for example, are very sensitive to 2, 4-D and many other herbicides. Weeds in maize crops are best controlled with pre-emergence herbicides which kill weed seedlings as they germinate, applications of 2, 4-D to maize after it has reached a height of 15 cm are likely to distort the leaves and reduce yields.
Sorghum is even more sensitive to most herbicides than maize, and rice is sensitive up to the tiller inf stage; later applications have no effect on the rice crop and can be used at concentrations which kill most broad-leaved weeds. Perennial grasses are very difficult to control since they are mostly tolerant to the commonly used selective herbicides.
It is difficult, at the present time, to recommend any particular herbicide or combination of herbicides as being effective in controlling certain types of weed.
This is mainly due to the variation in sensitivity of both weed and crop which may occur under changing weather and soil conditions. More information is therefore required on this subject, also on the strength or concentration of spray which should be used on crops at different stages of growth.
