Weeds as rivals of crops
First of all, weeds can have a direct negative effect by competing with the crop in terms of factors necessary for growth (water, nutriments, light, development space). This competition is more important at the beginning of the cycle because weeds consume nutrients more quickly than cultivated crops.
Allelopathy phenomena also play a part in crop production losses, but it is impossible to make the difference between the mechanism of this phenomenon and competition in the field. Various species are known to have an allelopathic effect on crops. For example, Cyperus esculentus has a depressive effect on maize and soya through the emission of allelopathic substances.
Weediness related to agricultural practices
As regards the cultivation of sugar cane, the loss in production ranges from 400 to 500 kg/ha and for each day late in weeding as from the second month after the plantation of virgin plant cane crop. The loss in cotton cultivation in Africa is estimated to 35 % if the weeds are not controlled during the first month after sowing. It can reach 90% in the case of competition with Cyperus rotundus in irrigated cultivation in Sudan. In Togo, a cultivation of maize weeded twice represents a production loss of 15 % , while the same cultivation not weeded would bear 85% of loss. In the case of rain fed peanuts in Sudan, losses ranging from 63% to 88% can be recorded. In Asia, losses in rain fed rice can vary from 40 to 100% if weediness is not correctly managed between 40 and 60 days after sowing.
Weediness favoured by the limiting factors of the environment
All those values are given as an indication, since the same crop can react differently to the competition with weeds according to the ecological conditions of the plot and the weather conditions of the year. Likewise, the importance of the competition varies in function of the predominating species among the weed flora and the crop concerned. This competition will be more important and more harmful to the crop if the environmental conditions are limiting (low water supply in dry season or low level in nutrients in degraded soils).
Weeds as parasites
One particular case of direct harm is parasitism. The main species in tropical zone is the Genus Striga family, and particularly three species ( S. hermonthica, S. lutea, S. gesnerioides). In Africa, the Striga is present in 40% of sub-saharian arable lands and causes average losses of 48% to cereal production (maize, sorghum, millet). S. asiatica leads to losses of 15 to 65 % in maize, while production losses of sorghum related to S. hermonthica in Nigeria vazries from 10 et 90%. In Africa, direct revenue loss related to Striga was estimated in 1991 to 2,9 billion $US.
Weeds, host to parasites as well as beneficial organisms :
Weeds can be alternative hosts to harmful insects or diseases, but in the same way can also host parasitoids (see glossary). In Sudan, some fifty weeds harbour the Bemisia tabaci, harmful to cotton.
Weediness responsible for the depreciation of the harvest :
Weeds can play an indirect negative part in agricultural production. The presence of seeds or plant debris reduces the quality of the harvest and diminishes its commercial value. The presence of Rottboellia cochinchinensis seeds in a maize or rice harvest reduces its selling price or prevent its use as seeds. The price of the seed of cotton in Cameroon is function of its purity.
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