Figure 1. Plants from a glyphosate-resistant (left) and susceptible population (right) three weeks after application of 44 oz/acre of Roundup PowerMax.
Figure 1. Plants from a glyphosate-resistant (left) and susceptible population (right) three weeks after application of 44 oz/acre of Roundup PowerMax.

Glyphosate dose response studies have confirmed a glyphosate-resistant population of common ragweed (Ambrosia artimisiifolia) from southeast Nebraska. This population survived doses up to eight times the suggested labeled use rate. While this would not be considered a high level of resistance, it is clear that labeled use rates would not provide acceptable control.

This is the first confirmation in Nebraska, however glyphosate-resistant common ragweed populations have been reported in 14 other states dating back to 2004. Thirteen of the 14 listed cases were reported in soybean environments. Nebraska producers with common ragweed in their fields should be able to identify this weed and closely monitor the performance of their herbicide programs where common ragweed is present. In Figure 1, extra plants from the resistant (left) and susceptible (right) populations used in the dose response studies are shown three weeks after application of 44 oz/ac of Roundup PowerMax.

Glyphosate-resistant common ragweed confirmed in NebraskaIn Nebraska common ragweed (Figure 2) traditionally has not been one of the most problematic species in corn and soybean production fields. However, when present and left uncontrolled it can have a significant impact on soybean stands, growth, and yield (Figure 3).

Common ragweed is a summer annual broadleaf plant and is found in many environments including urban areas, roadsides, ditches, gardens, landscapes, unmanaged areas, and row crops.

Glyphosate-resistant common ragweed confirmed in NebraskaGlyphosate, HPPD, and auxin herbicide-resistant weeds have been confirmed over the last six years in Nebraska. This continued confirmation of herbicide-resistant weeds indicates not enough producers have diversified their weed management approach to prevent or delay the evolution of herbicide-resistant weed populations.

Herbicide-Resistant Weed Management Field Days July 9, 10

Herbicide resistance is quickly becoming one of the most prevalent and challenging issues in row crop production.  UNL Extension weed scientists, with support from the Nebraska Soybean Board, will be holding two field days focusing on the prevention and management of herbicide-resistant weeds July 9 near Fremont and July 10 near Lincoln. (See event flyer.)

The Fremont site has glyphosate-resistant waterhemp and the Lincoln site has glyphosate-resistant marestail.  The events are free, but online pre-registration is requested.