Effect of Level and Source of Shade on the Biomass of Speargrass in Nigeria

Field studies were conducted in Oyo, Nigeria, to determine the effect of level and source of shade (polyethylene plastic fabric and velvetbean canopy) on the dry matter of speargrass (Imperata cylindrical (L.) Raeuschel). Shade levels provided by the plastic fabric were 0 (full solar radiation), 50, 75 and 88% of the incident solar radiation. Velvetbean shade levels of 40.5 to 97% were provided by densities of 4, 8, and 16 plants m-1. In 1999, the velvetbean canopy intercepted 40.5% of solar radiation at 3 weeks after treatment (WAT) and 82% between 5 and 11 WAT. At 11 WAT, velvetbean shading reduced the speargrass biomass by 48% and shade from plastic fabric by 41%. In 2000, radiation interception by the velvetbean canopy was 86.7-97.2% between 3 and 11 WAT. Shade from the velvetbean canopy reduced speargrass biomass by 90% and shade from plastic fabric by 44% at 11 WAT. Velvetbean biomass and LA1 were negatively correlated with speargrass biomass (r = -0.85 to -0.91, P<0.05) and incident solar radiation (r = 0.46 to 0.79, P = 0.05). The similarity of the effects on speargrass biomass of shading by plastic fabric and velvetbean indicated that shading was the principal mechanism by which velvetbean suppresses speargrass.