The introduction of selective weed killers or herbicides in recent years has made it possible to maintain an
effective control of broad leaved weeds in many parts of the world, Most of these herbicides are forms of
paraquat or simazine, In West Africa, however, these herbicides are expensive and require skilled application if they are to be fully effective; all should be applied in strict accordance with manufacturer's
instructions. There is, therefore, a continuing need to maintain control over weed growth since up to 12 00 of
crop yields may be lost due to weed competition.
Weeds compete with crops not only for water and minerals but also soil, oxygen and sunlight; newly
established seedlings or recently transplanted plantation crops can be severely checked in growth due to
early competition from weeds.
Hand weeding on a garden scale is very effective, young weed seedlings can be cut through at the base or uprooted by a short handled hoe, larger weeds
should be pulled out by hand, including the root system.
On farms and large holdings, where machinery is available, various implements may be used for weed
control. In the young stages of growth, spring tine cultivators are effective, also chain or seed harrows.
For larger weeds, disc ploughs or disc harrows may have to be used to partially bury the weed growth.
Where crops are grown in rows, a toolbar fitted with L-shaped blades is necessary. For intensively grown
crops, small 2-wheeled rotary cultivators can be used to bury weed growth; some of the smaller models are
suitable for inter row cultivations and are most effective when fitted with L-shaped blades.
Cover Crops
The term cover crop is widely used in West Africa to describe the crops which are established with the objective
of either protecting the surface soil from being washed away or eroded, or preventing the growth of weeds, particularly when sown between rows of plantation crops.
They are often used to occupy land which is lying fallow before being prepared for the next crop. Many cover crops are legumes, but some grasses can also be included in this category. Many plants used as cover crops are annuals or short lived perennials and almost all are established from seeds sown broadcast, generally at the beginning of the wet season.
Since many cover crops are legumes, they contribute to the fertility of the soil by replenishing it with nitrogenous compounds, particularly if the whole plant is hoed or ploughed into the soil after it has completed its growth period.
Some legumes used as cover crops also produce edible seeds which can be used as food crops, livestock can also be fed on the leaves of most legumes used as cover crops; this is an important advantage when a farmer also has livestock, since leguminous Leaves are nutritious, being high in protein.
Many of the annual cover crops do not have extensive root systems and they therefore grow well soils which have a relatively fertile topsoil. However when used mainly for erosion control, they may be sown on soils which have been partially eroded or exhausted from previous overcropping. In these Conditions, cover crops which will survive in relatively infertile soils and which also have some resistance to drought conditions should be selected.
For effective erosion control, rapidly growing species which produce a fairly large leaf area but which also have a vigorous root system are most suitable, since their main function is to cover the soil rapidly at the beginning of the wet season, protecting the surface from the impact of heavy rainfall. Examples of cove crops include pigeon pea, Pueraria spp. and Tephrosic•
