Response of Setaria faberi demographic processes to herbicide rates

Traditionally, herbicide efficacy has been evaluated by visual ratings, but these data provide little insight to the biological response of weeds to h erbicides. Field studies were conducted in 1995 and 1996 to determine the r ate response of Setaria faberi seedling survival, seed production, and biom ass to postemergence herbicides in Zea mays and Glycine max. Nicosulfuron a nd sethoxydim were applied to Z. mays and G. max, respectively, at 1x, 1/2x , 1/4x, 1/8x, 1/16x, 1/32x, and Ox the label rate. Mature plant density of S. faberi was linearly related to seedling density, indicating that seedlin g survival was not density dependent. Based on a nonlinear dose-response an alysis, maximum S. faberi survival was 55% in Z. mays across years and 60 a nd 45% in G. max in 1995 and 1996, respectively. Minimum survival was 0% ex cept for Z. mays in 1996 when it was 13%. The minimum survival was greater in Z. mays in 1996 due to greater survival of late cohorts than in 1995. Se taria faberi seedling survival was greater in 1/2x than 1x herbicide treatm ents in Z. mays and G, max each year. Setaria faberi seed production was re lated to mature plant density with a negative exponential function. Seed pr oduction per plant was similar between Ix and 1/2x rates in Z. mays and amo ng 1x, 1/2x and 1/4x rat es in G. max each year. However, seed production p er square meter was greater in 1/2x than Ix treatments due to greater seedl ing survival. Regardless, seed production per square meter was 95% less in the 1/2x herbicide treatment compared to seed production by untreated plant s in Z. mays and G. max.