Glyphosate Resistant Weeds

Chris Boerboom

A glyphosate-resistant horseweed (marestail) was recently reported by weed scientist Mark VanGessel to occur in a few Delaware fields. The fields all had herbicide histories where glyphosate (Roundup) had been used for burndown applications for many years and had glyphosate applications to Roundup Ready soybeans in recent years. Resistance was first suspected in 2000 when some horseweed plants were escaping control while others were controlled in the same field. Seeds from these suspected resistant plants were grown in the greenhouse and tested for resistance by comparing them to susceptible horseweed seedlings. In these tests, it took six times more glyphosate to kill the resistant horseweed than the susceptible horseweed in one test and ten times more glyphosate in a second test. These tests clearly demonstrate that the horseweed had greater resistance to glyphosate. This is the first case of confirmed glyphosate resistance where soybeans were part of the cropping system. Previously reported glyphosate-resistant ryegrass and goosegrass occurred in plantation situations where glyphosate was applied several times each year for many years.

While the glyphosate-resistant horseweed was a clear confirmation of a field scale problem, there have been several concerns of potential glyphosate-resistant waterhemp in Iowa and Missouri. Weed scientists from both states have been studying a couple fields where waterhemp has not been controlled as expected with one or more glyphosate applications. Pat Tranel, a University of Illinois weed scientist, has also evaluated 63 waterhemp populations from 36 Illinois counties for glyphosate sensitivity. He found that three of the populations had greater tolerance to glyphosate compared to the other populations.

Although the story on potential glyphosate-resistant waterhemp is still evolving, it and the other examples of resistance should raise flag that glyphosate-resistant weeds should be a concern. Just like herbicides with other modes of actions, if we abuse a product through over-use and do not practice tactics to delay resistance, resistance will happen. If the Roundup Ready technology has a value that you want to extend many years into the future, consider which resistance delaying strategies that you can work into your program. Strategies to consider include: 1) avoid respraying unless truly necessary, 2) rotating herbicide modes of action between years, 3) tankmixing a herbicide with a different mode of action (preferably one that controls many of the same weeds), and 4) using cultivation to supplement your weed control.

March 2001