Weeds in slash-and-burn rice fields in northern Laos

Slash-and-burn farmers in northern Laos consider weeds, insufficient rainfall and rodent damage as the most important constraints to upland rice ( Oryza sativa L.) production. Labour inputs of 140-190 days ha^-1 for weed control result in low labour productivity. Average weed cover observed in rice fields was 5.6, 4.1, 2.1, 1.7, 0.7 and 0.7 cm m^-1 (transect) for Chromolaena odorata (L.) King and Robinson, Ageratum conyzoides L., Commelina spp., Lygodium flexuosum (L.) Sw., Panicum trichoides Sw. and Corchorus spp, respectively. Chromoiaena odorata , which was introduced in the 1930s, has become the main fallow species and is considered a desirable fallow plant by most farmers. Average fallow periods reported for the 1950s, 1970s and 1992 were 38, 20 and 5 years respectively. Reduced fallow periods in the last decades have re-suited in a marked increase in weeding requirements. Above-ground biomass for rice stem, herbaceous plants and trees after rice harvest was 168, 67 and 60gm^-2 in 1991 and 115, 43 and 24 gm^-2 in 1992. Weeding at 14-day intervals did not increase rice grain yield.