Response of Setaria faberi Demographic
Processed to Herbicide Rates

Alvin J. Bussan, Chris M. Boerboom, and David E. Stoltenberg

Traditionally, herbicide efficacy has been evaluated by visual ratings, but these data provide little insight to the biological response of weeds to herbicides. Field studies were conducted in 1995 arid 1996 to determine the rate response of Setaria faberi seedling survival, seed production, and biomass to postemergence herbicides in Zea mays and Glycine max. Nicosulfuron and sethoxydim were applied to Z. mays and G. max, respectively, at 1X, 1/2X, 1/4X, 1/8X, 1/16X, 1/32X, and 0X the label rate. Mature plant density of S. faberi was linearly related to seedling density, indicating that seedling survival was not density dependent. Based on a nonlinear dose-response analysis, maximum S. faberi survival was 55% in Z. mays across years and 60 and 45% in G. max in 1995 and 1996, respectively. Minimum survival was 0% except 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. Setaria faberi seedling survival was greater in l/2X than 1X herbicide treatments in Z. mays and G. max each year. Setara faberii seed production was related to mature plant density with a negative exponential function. Seed production per plant was similar between lX and 1/2X rates in Z. mays and among lX, 1/2X, and 1/4X rates in G. max each year. However, seed production per square meter was greater in l/2X than 1X treatments due to greater seedling survival. Regardless, seed production per square meter was 95% less in the 1/2X herbicide treatment compared to seed production by untreated plants in Z. mays and G. max.

Weed Science, 48:445-453. 2000