DIFFERENTIAL WEED SUPPRESSION ABILITY IN UPLAND RICE CULTIVARS

Twenty-five upland rice (Oryza sativa L.) cultivars were evaluated in 2 yr of field experiments in the Philippines. The research was conducted with low and moderate levels of handweeding to determine the extent of cultivar differences in weed suppression ability. We sought the plant characters that may confer superior competitiveness, and determined their practical significance in contributing to weed management. A large significant variation was observed in the dry weight of weeds (0.84 to 3.89 Mg/ha) associated with the different cultivars. Several cultivars had consistently low weed weights in both weeding treatments across years. Weed weights for the five most competitive cultivars were 75% lower than for those of the five least competitive cultivars. Grain yields were negatively correlated with weed weights, in both weeding treatments in both years (r = -0.48** to -0.59**-significant at P = 0.01). Plant height, which ranged from 0.73 to 1.35 m, was the character most strongly related to low weed weight, with negative correlations that varied from -0.76** to -0.88** among weeding levels and years. Leaf area index (determined at 60 d after seeding) was also negatively correlated with weed weight, but tiller number was not significantly related to weed weight. The minimum plant height necessary to adequately suppress weeds was approximately 1.00 to 1.15 m. Cultivars with superior yield potential were identified that had a minimum height of 1.0 m, with weed suppression ability similar to the tall traditional cultivars.