Competitiveness of Four Rice Cultivars against Barnyardgrass,Echinochloa oryzicolaVasing, with Reference to Root and Shoot Competition

Four cultivars of rice (Oryza sativa L.) different in plant stature were grown on a paddy field, fairly fertile without nitrogen application, with or without barnyard grass, Echinochloa oryzicola Vasing, in the middle of the rice rows. The reduction of the growth variables of the four cultivars by competition with E. oryzicola was examined. The effect of a veneer board inserted in the soil between the rice and barnyardgrass rows was also examined to evaluate the growth reduction by shoot competition (WCS) and root competition (WCr) separately. On the average, the reduction of top dry weight (DWtop) by competition with weed (full competition, WCf) and by WCr increased with the time during the early growth period. Thereafter, the reduction of DWtop by WCr decreased steeply and instead the reduction by WCS increased and became a primary factor of the reduction by WCf in the late growth period. Nitrogen accumulation in the top (Ntop) tended to be reduced at a higher rate than DWtop by WCr. The cultivar difference in the rate of DWt0P reduction by WCf was largest at maturity ranging from 0.22 in Ch86 (very tall indica) to 0.45 in Taichung65d47 (short japonica), and was intermediate in Taichung65 (moderately tall japonica) and Takanari (moderately short indica). The reduction of DWtop and Ntop by WCS in all cultivars almost fully accounted for that by WCf at maturity and closely correlated with the height of rice plant. Although the rate of DWt0P reduction by WCr before heading significantly differed among cultivars (0.13 - 0.28), it did not contribute to the rate of DWtop reduction by WCf at maturity (0.01 - 0.06). These results indicated that shoot competition was a more important factor to cause cultivar difference in the reduction of final biomass by competition with E. oryzicola. Considering its great contribution to WCf during the early growth period especially for Ntop, WCr might be an important factor in the competition with weed of rice cultivars under crucially nitrogen-limited conditions.