Screening for weed competitiveness among selections of rice in West Africa

Weed competition causes serious losses of rice yield in West Africa, partic ularly in the rainfed uplands and lowlands. The use of herbicides is limite d and the majority of rice farmers have access to few resources. It is esti mated that weeding accounts for between 28-40% of the total labour used in growing rice, and labour is the main constraint to the productivity of the systems. Competitiveness with weeds is a selection criterion for rice cultivars suit ed to the rainfed areas. Conventional field experiments to examine the effe cts of weed competition on rice have the limitation that they require relat ively large plot sizes and hence the number of cultivars is limited. To fac ilitate selection from a large number of cultivars, using smaller plot size s, methods were reported which used sown competitors including O. glaberrim a, maize, cowpeas, other rice cultivars and weeds to measure the competitiv eness of test cultivars. Maize and the O.glaberrima were particularly compe titive against the test lines, and there were good correlations between the se and previous results.