Late-season weed escape survey in Wisconsin identifies a second county with a glyphosate-resistant horseweed population

Ross Recker (Graduate Research Assistant), John Buol (Undergraduate Research Assistant), and Vince Davis (Assistant Professor) Department of Agronomy, UW-Madison

Glyphosate-resistant weeds continue to be a major threat to corn and soybean production across the Nation. A pro-active survey of late-season weed escapes in corn and soybean fields was conducted throughout Wisconsin in 2012 and 2013. One objective of the survey was to identify areas where populations of glyphosate-resistant weeds may exist.

In 2012, a population of horseweed (Conyza Canadensis) collected from Jefferson County, and glyphosate screening in the greenhouse confirmed it was glyphosate-resistant with a 6-fold tolerance compared to a susceptible population. This announcement and more information about horseweed biology and management were previously reported and can be read here.

During the 2013 late-season survey, another putative glyphosate-resistant horseweed population was collected from Columbia County. Again whole-plant dose response experiments were recently conducted in the greenhouse and results confirm this population is also nearly six-fold glyphosate-resistant (Figure 1).


Figure 1. Shoot dry biomass of Columbia County horseweed and susceptible horseweed following treatment with glyphosate at doses up to 3.48 kg ae ha-1 as estimated by a four-parameter log-logistic regression function.

Figure 1. Shoot dry biomass of Columbia County horseweed and susceptible horseweed following treatment with glyphosate at doses up to 3.48 kg ae ha-1 as estimated by a four-parameter log-logistic regression function.

The horseweed seeds were collected from mature plants in a soybean field with a history of no-till, corn/soybean rotation, and glyphosate use. These plants displayed symptomology at the time of collection including shortened internodes (Figure 2) and were scattered randomly at low densities in about a 2 to 3 acre patch. If you have horseweed or other weeds that survive postemergence applications, you should have concern about herbicide resistance. Contact your local county Ag Extension Agent who can help you further evaluate the situation and plan a pro-active resistant management program so you can take action against herbicide resistance.


Figure 2. Horseweed plant late in the 2013 growing season that was not controlled with a previous postemergence glyphosate application in a no-till soybean field in Columbia County, Wisconsin.

Figure 2. Horseweed plant late in the 2013 growing season that was not controlled with a previous postemergence glyphosate application in a no-till soybean field in Columbia County, Wisconsin.

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