Water-seeded rice cultivars differ in ability to interfere with watergrass

The repeated use of a limited number of herbicides has led to the development of resistant meed species in California rice (Oryza sativa L.). Competitive rice cultivars could help reduce herbicide dependency and decrease selective pressure for resistance. We conducted field experiments to determine if differences in competitive ability existed between two semidwarf cultivars of water-seeded rice. Cultivars M-202 and A-301 were grown on Stockton clay soil at two seed rates (84 and 168 kg ha(-1)) in 1996 and four seed rates (56, 112, 168, and 224 kg ha(-1)) in 1997. Molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) and propanil [N-(3,4-dichlorophenyl)propanamide] were used to control a mixed infestation of watergrass [Echinochloa oryzoides (Ard.) Fritsch, E. phyllopogon (Stapf) Koss] in 1996 and 1997, respectively: Molinate (0 and 4.5 kg a.i. ha(-1)) was applied in 1996, and propanil (0, 1, 2 and 4 kg a.i. ha(-1)) was applied in 1997. M-202 was taller, produced more leaf dry weight, had greater light interception, and reduced watergrass biomass more than A-301 in both years of the study. The more competitive M-202 also had higher yields than A-301 under weedy and weed-free conditions. Watergrass growth,vas not affected by rice seed rates in either year. This study suggests that herbicide rates could be reduced and weed control could be improved if more competitive cultivars were developed for waterseeded rice.