In 2005, the existence of glyphosate-resistance in Palmer amaranth was
confirmed at a single 250 ha field site in Macon County, Georgia. Currently,
all cotton producing counties in Georgia are infested, to some degree, with
glyphosate-resistant Palmer amaranth. In 2010 and 2011, surveys were
administered to Georgia growers and extension agents to determine how the
development of glyphosate-resistance has affected weed management in cotton.
According to respondents, the numbers of cotton acres that were treated with
paraquat, glufosinate and residual herbicides effective against Palmer
amaranth more than doubled between 2000 to 2005 and 2006 to 2010. Glyphosate
use declined between 2000 to 2005 and 2006 to 2010 although, on average, the
active ingredient was still applied to a majority of cotton acres. Although
grower herbicide input costs have more than doubled following the evolution
and spread of glyphosate resistance, chemically-based control of Palmer
amaranth is still not adequate. As a consequence, Georgia cotton growers
hand weeded 52% of the crop at an average cost of $57 per hand-weeded ha;
this represents a cost increase of at least 475% as compared to the years
prior to resistance. In addition to increased herbicide use and hand
weeding, growers in Georgia are also using mechanical, in-crop cultivation
(44% of acres), tillage for the incorporation of preplant herbicides (20% of
the acres), and post-harvest deep-turning (19% of the acres every three
years) for weed control. Current weed management systems are more diverse,
complex and expensive than those employed only a decade ago, but are
effective at controlling glyphosate-resistant Palmer amaranth in
glyphosate-resistant cotton. The success of these programs may be related to
producers improved knowledge about herbicide resistance, and the biological
attributes that make Palmer amaranth so challenging, as well as their
ability to implement their management programs in a timely manner.