Reduced fallow length in slash-and-burn rice (Oryza sativa L.) production systems of northern Laos increased weed pressure, labour requirement and the need for soil and moisture conservation. On-farm and on-station studies and on-farm surveys were used to evaluate the effect of residue management and cropping intensity on weed population, rice yield and nematode density. Residue loads were 2.3-4.4 t ha-1 after a rice crop and 9.5 t ha-1 after 1 year of fallow. Compared with farmers' traditional burning of crop and weed residues, mulching reduced rice yield by 43% in one out of four comparisons and increased weed biomass by 19-100%. Compared with continuous rice treatments (averaged over burning and mutching treatments), treatments with fallow or cowpea [Vigna unguiculata (L.) Walp.] in the previous year had 32% less herbaceous weed biomass, 90% fewer Ageratum conyzoides L. and over 99% fewer Meloidogyne graminicola Golden and Birchfield. Rice yield was negatively associated with A. conyzoides density (-0.62, P < 0.01) and M. graminicola number (-0.42, P < 0.05). Less striking effects of fallow period on A. conyzoides and M. graminicola, observed from on-farm surveys, demonstrate the limitations of on-farm studies because of undocumented effects of farmers' management decisions.
View source
