L Weeds
A weed can be loosely defined as any plant that is growing in a place where it is
interfering with the growth, harvesting or marketing of other plants. Cowpeas for
example can be a serious weed in soybean and maize crops that are mechanically
harvested, since the long cowpea stems can block the harvester; cowpea seed is also not
easily removed from soybean seed.
Weeds can be the limiting factor that determine the area of land that a farmer can
cultivate; in these cases, correctly applied herbicides may enable more land to be
cultivated.
Losses due to Weeds
The percentage of crop yield lost due to the growth of weeds has been estimated to be
about 16% in Africa, 11% in Asia, 8% in North, South & Central America and 7% in
Europe
Excessive weed growth damages crops by competing with them for moisture,
nutrients and light, and may also have other negative effects such as increasing the
damage caused by pests and diseases. However a moderate weed infestation often has
little or no effect on crop yields.
Some weeds, such as Couch Grass (“Twitch”, “Quick Grass” or “ Devil s Grass”
Agropyron repens), produce exudates that retard the growth of other plants.
Weed seed that is present in harvested grain may spoil it for consumption or sale, or
make the grain unusable as seed.
Weed Control
This is an important factor in crop management, especially when the crop is young.
There are many different techniques available, including crop rotation, mulching and
mixed cropping, as well as mechanical and chemical methods. There is a great deal of
recent interest in the use of natural enemies of weeds (and insects) in a branch of
agriculture referred to as biological control or biocontrol, outside the scope of this
handbook. In general, strong and healthy crops will tend to outgrow and dominate
weeds while weaker crops will succumb to them.
Hand weeding
Pulling up weeds by hand, or using various types of hoes, is the normal method for
small scale and subsistence crop producers to deal with weeds. Herbicides and the
equipment to apply them are expensive, and often not available. Hand tools enormously
Circle weeding
Fruit trees and bushes generally benefit if weeds are removed from under them,
especially when the plants are young and/or the growing conditions are not favourable.
In these cases, recently planted trees and bushes should have weeds removed in a circle
of about one metre around them. In poor land, or with very old or weak trees, weeds
should also be removed in a circle about the same size as the crown of the tree (the
spread of the branches). Take care not to damage the tree or bush roots by hoeing too
88 TONY WINCH
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increase the efficiency of weeding by hand, as discussed on page 92, 1M. “Tools”.