Effect of residue management and fallow length on weeds and rice yield

Abstract

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 mulching 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 & 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.