Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

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HORSEWEED RESPONSE TO NO-TILL PRODUCTION SYSTEMS. M.J. VanGessel,
B.A. Scott, Q.R. Johnson and S.E. White, Univ. of Delaware, Georgetown.
ABSTRACT
Horseweed (Conyza canadensis) has been a common weed in no-tillage
production systems in Delaware since the adoption of no-till. In 2000, horseweed plants
from multiple fields in DE were identified as resistant to glyphosate. Since then
glyphosate-resistant biotypes have become wide-spread and have forced farmers to
find an alternative to glyphosate for its control. Fields planted with no-till corn in DE are
often treated with paraquat and simazine during the early spring and this provides
effective control. Thus, research has focused on horseweed management in soybeans.
A series of studies were conducted at the University of Delaware to examine horseweed
response to various management practices and the environmental impacts on
horseweed growth and development.
Horseweed plants at three stages of growth (seedling, large rosettes, and
bolting) were treated in the greenhouse with various rates of glyphosate. Glyphosatesusceptible
plants were effectively controlled, regardless of stage of growth, while a
growth stage by glyphosate interaction was observed with the glyphosate-resistant
biotype. Plants in the large rosette stage were not as sensitive to glyphosate as the
other two stages of growth.
Horseweed seed buried at 1 or 10 cm of depth in the field did not respond the
same way at two locations. However, in general, seed viability was greater over a 36
month period at the 10 cm depth than at the 1 cm depth.
Seeding a rye (Secale cereale) cover crop in the fall reduced the size and density
of horseweed plants when evaluate the following spring, but not to commercially
acceptable levels. In addition, although the individual horseweed plants were smaller,
their susceptibility to paraquat and 2,4-D was not different from larger horseweed plants
grown without the rye competition.
Research to date at the University of Delaware has shown little to no biological or
ecological differences between glyphosate-resistant and glyphosate-susceptible
horseweed biotypes. Future research needs to focus on management practices that
reduce the selection pressure for development of multiple-resistant biotypes. Continued
research on horseweed ecology and biology is critical to lessen this resistance
pressure.
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