Horseweed Resistance to Glyphosate
This is a brief update on the status of glyphosate-resistant
horseweed. Horseweed, also known as marestail, is a common
winter annual weed. It can be a serious problem in no-till
soybeans, where it needs to be controlled by a burndown
treatment. Its leafy stalk grows to a height of three or four
feet.
Resistant horseweed was first confirmed in Delaware in 2000.
Resistant populations also exist in nearby Maryland and New
Jersey. Last year, University of Tennessee weed scientists
confirmed resistant horseweed in their state. Resistance has
also been recently confirmed in Kentucky. So, it is known that
this resistance already affects five states. Perhaps more
interestingly, comments from University of Tennessee weed
scientists indicate that the resistant horseweed affects about
200,000 acres of cotton and 200,000 acres of soybeans in their
state.
Glyphosate-resistant horseweed may not act like some of the
other resistant weeds that we have in Wisconsin. Most of the
resistant weeds like triazine-resistant lambsquarters or ALS-resistant
nightshade are practically unaffected by the herbicide
application. The main shoot keeps right on growing after
application. However, the glyphosate-resistant horseweed biotype
studied in Kentucky was stunted and the terminal bud killed with
glyphosate, but the plant branched out and survived the
application. These horseweed plants may only be 8- or 10-fold
resistant, but they are resistant enough to survive, produce
seed, and be a problem.
The occurrence of glyphosate-resistant horseweed does not
mean that we should stop using glyphosate. Rather, the continued
expansion of glyphosate-resistant horseweed means that we should
keep our eyes open for resistance in Wisconsin. Plus, we should
continue to use good stewardship practices such as tank mixing
or alternating the use of different modes of action. For
example, we could tank mix 2,4-D with glyphosate before no-till
soybeans or use a non-glyphosate product postemergence in
no-till corn to control any horseweed escaping a glyphosate
burndown treatment.
August 2001 |
