Resistance to herbicides and the lack of viable control options have led to an interest in increasing the role of crop competition as a weed management tool in water-seeded rice production. Weed-suppressive rice cultivars have been suggested as a tool that could improve weed control and reduce the reliance of growers on herbicides. Field studies were conducted at Biggs, CA, in 1999 and 2000 with six to eight semidwarf rice cultivars to identify water-seeded rice traits related to the suppression of watergrass growth. Cultivars S-201 and M-302 were the most suppressive in both years. The dry weight (DW) of watergrass grown with the most suppressive cultivar was only 16% in 1999 and 57% in 2000 of the DW of watergrass grown with the least suppressive cultivar. Rice leaf area and root DW in weed-free plots were linearly related to watergrass DW in both years. Weed-suppressive traits were not inversely correlated with rice yields in monoculture; competitive cultivars also had high yields. This study suggests that an indirect selection program, based on traits that can be identified early in the season under weed-free conditions, has great potential for developing more competitive cultivars for water-seeded rice.

Nomenclature: Early watergrass, Echinochloa oryzoides (Ard.) Fritsch ECHOR; late watergrass, Echinochloa phyllopogon (Stapf) Koss. ECHPH; rice, Oryza sativa L.