We conducted experiments in 1999 to determine the critical period of competition between rice and watergrass, to examine the effect of light and N on watergrass growth, and to identify cultivar traits related to competitive ability. Delaying watergrass emergence by 15 days relative to the crop reduced watergrass density by 97% and aboveground DW by 75% relative to plots where watergrass emerged with the crop. The weed was eliminated when its emergence was delayed by 30 days. However, applying herbicides after 30 DAS resulted in substantial yield losses. The critical period of competition and optimal period for watergrass control is between 15 and 30 DAS. |
Low light and low N both substantially reduced watergrass growth. However, the effect of intermediate levels of shade on watergrass growth depended on the rate of nitrogen. Higher N rates compensated, to some degree, for reduced light. The results suggest that reducing the quantity of N absorbed by watergrass may make watergrass less competitive with rice.
In 1997 and 1998 we found significant differences in competitive ability among two commercially available cultivars suggesting that more competitive cultivars could be used to improve weed control. We expanded our work in 1999 to identify traits related to competitive ability to be used in a breeding program. We found highly significant differences among 10 cultivars in 1999. However, these differences were not related to yields under weed-free conditions suggesting that more competitive cultivars can be developed without reducing yields. We are in the process of analyzing the data to identify competitive traits.