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.