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

The repeated use of a limited number of herbicides has led to the developme nt of resistant meed species in California rice (Oryza sativa L.). Competit ive rice cultivars could help reduce herbicide dependency and decrease sele ctive pressure for resistance. We conducted field experiments to determine if differences in competitive ability existed between two semidwarf cultiva rs of water-seeded rice. Cultivars M-202 and A-301 were grown on Stockton c lay soil at two seed rates (84 and 168 kg ha(-1)) in 1996 and four seed rat es (56, 112, 168, and 224 kg ha(-1)) in 1997. Molinate (S-ethyl hexahydro-1 H-azepine-1-carbothioate) and propanil [N-(3,4-dichlorophenyl)propanamide] were used to control a mixed infestation of watergrass [Echinochloa oryzoid es (Ard.) Fritsch, E. phyllopogon (Stapf) Koss] in 1996 and 1997, respectiv ely: 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, produ ced more leaf dry weight, had greater light interception, and reduced water grass biomass more than A-301 in both years of the study. The more competit ive M-202 also had higher yields than A-301 under weedy and weed-free condi tions. Watergrass growth,vas not affected by rice seed rates in either year . This study suggests that herbicide rates could be reduced and weed contro l could be improved if more competitive cultivars were developed for waters eeded rice.