Abstract
In transplanted rice (variety BR 11) grown during July to November in Bangladesh, weed infestation in terms of absolute density, weed dry matter weight and intensity of weed infestation was studied 25 (W1) and 45 (W2) days after transplanting at five N levels (0, 40, 60, 80 and 100 kg/ha) and three spacings (20×10 cm, 25×15 cm and 30×20 cm) in a split-plot design with four replications. Nine species of weeds belonging to six families were identified and found to infest the transplanted rice field. Weed growth increased significantly with the increase in spacing and linearly with N level, but the weed growth rate was higher at W1 than W2. It was revealed that, with the increase in every kg of N/ha, the absolute density (AD) of weeds increased by 0.422 and 0.200 at W1 and W2; the weed dry matter wight (DM) was 0.068 and 0.061 g/m2 at W1 and W2 and the intensity of infestation (II) was 0.018 and 0.010 at W1 and W2. Nitrogen level was correlated with AD, DM and II, and several equations were obtained as follows: AD (AD1=0.422N+159.602, r2=0.937 and AD2=0.200N+96.626, r2=0.872), DM (DM1=0.068N+16.92, r2=0.907 and DM2=0.061N+16.115, r2=0.968) and II (II1=0.018N+6.586, r2=0.852 and II2=0.010N+3.962, r2=0.928), where AD is expressed as no. per m2, DM as g per m2 and II as no. per rice hill. These results indicate the effect of spacing and N level on weed infestation in transplanted rice field in similar agro-environmental areas. However, depending on the climatic and edaphic factors, the results may vary with the area.
