REDUCED BIOMASS AND FECUNDITY IN HERBICIDE RESISTANT ECHINOCHLOA PHYLLOPOGON BIOTYPES DOES NOT TRANSLATE TO REDUCED COMPETITIVENESS AGAINST RICE

Echinochloa phyllopogon (Stapf) Koss. is one of the most important weeds of rice in California, Japan and other temperate regions, and has evolved resistance in California to most available herbicides, thus severely limiting control options. Resistance to a wide variety of herbicide modes of action presents a need for non-chemical control measures. A series of experiments were conducted to explore possible differences in E. phyllopogon growth, fecundity and competitive ability with rice. Growing at five densities within a rice stand, resistant (R) E. phyllopogon biotypes had lower biomass, relative leaf area and fecundity when compared to susceptible (S) biotypes. R-biotypes are morphologically and genetically homogeneous in California. However, rice biomass, grain weight, plant height and leaf area were not affected differently by R and S biotypes. Thus R plants’ ability to interfere with rice was not disadvantaged by their inferior ability for light capture and biomass accumulation, suggesting other factors besides competition for light may be at play. These results also suggest the lower fecundity of the R biotypes could result in lower ecological fitness in the absence of herbicide selection pressure