Doctor of Philosophy / Department of Agronomy / Johanna A. Dille / Challenges in weed management include occurrence of multiple weed species in the
field, variable emergence among weed species, different spatial distribution and weed densities,
which leads to the persistence of weed patches. The overall objective of this research was to
understand the interaction of weed emergence, weed density, herbicide choice, and herbicide rate
in soybean. Specific objectives were 1) to characterize the seedbank and emergence patterns of
shattercane (Sorghum bicolor L.), prickly sida (Sida spinosa L.), and ivyleaf morningglory
(Ipomoea hederacea Jacq.) including initial, peak, end, and duration of emergence in response to
crop and herbicide treatments in soybean, and 2) to evaluate large crabgrass (Digitaria
sanguinalis L.), shattercane, Palmer amaranth (Amaranthus palmeri S.), and velvetleaf (Abutilon
theophrasti Medik.) mortality and dry weight reduction in response to herbicide rates across
varying weed densities as well as to determine the influence of velvetleaf growth stage and
density on herbicide efficacy. In the emergence study of 2006 to 2008, four treatments were nocrop,
no-residual herbicide, half-rate of residual herbicide and full-rate of residual herbicide.
Reduction in weed emergence was observed over the years in the same species patch. Species
emerged in mid-May in both years, coinciding with soybean planting. Extended emergence was
observed for shattercane when moisture was low and temperature high, while for prickly sida
and ivyleaf morningglory, extended emergence was observed when moisture was high and
temperature low. Applying residual herbicide decreased weed emergence. Herbicide choice was
the whole plot, herbicide rates were subplots and weed densities were sub-subplots in field
experiments conducted in 2006 and 2007. Shattercane was more susceptible to both glyphosate
and clethodim than large crabgrass. Increasing large crabgrass density reduced percent mortality
with clethodim, while with glyphosate, density did not affect both species mortality. Shattercane
dry weight was reduced to 0 g per plot with 0.1X labeled rate of clethodim or glyphosate while
0.5X of the labeled rate reduced dry weight of large crabgrass to 0 g per plot. For broadleaf
weeds, higher percent mortality was observed with glyphosate than with lactofen at high
densities. Palmer amaranth was more susceptible than velvetleaf. Velvetleaf response was
density dependent, such that increasing density did not increase dry weight. Velvetleaf growth
stage was of importance, as stage affected herbicide efficacy, with higher mortality achieved at
the two-leaf stage than the four- and six-leaf stages. For glyphosate, 0.125X of labeled rate on
velvetleaf density of 5 seedlings per pot achieved more than 90% mortality when applied at the
two-leaf stage, but dropped to 60 and 50% mortality when applied at the four- and six-leaf stage,
respectively. The trend was the same for velvetleaf at a density of 30 seedlings per pot, which
had 80, 60, and 55% mortality for the two-, four-, and six-leaf stages, respectively. Weed
managers and farmers have the opportunity to better select herbicide choice and rate based on
weed species, weed emergence patterns, and weed density.
| Identifer | oai:union.ndltd.org:KSU/oai:krex.k-state.edu:2097/1668 |
| Date | January 1900 |
| Creators | Ndou, Aifheli Meshack |
| Publisher | Kansas State University |
| Source Sets | K-State Research Exchange |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
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