Resistant list keeps growing
By James D. Ritchie

Weeds that tolerate glyphosate keep popping up.

What man can do, Nature eventually can un-do. That’s happening as more and more weeds develop resistance to glyphosate, the active ingredient in Roundup and similar herbicides.

In southern Missouri and Arkansas, there first appeared glyphosate-resistant horseweed (or mare’s tail). A year or so later, Reid Smeda, University of Missouri weed scientist, confirmed resistance in a field of common ragweed in central Missouri.

And now…“I have been working with some populations of common waterhemp in northwest Missouri that are not controlled by glyphosate,” said Kevin Bradley, MU weed scientist. “I want to look at this waterhemp in the field this season before I confirm glyphosate resistance, but I fully expect resistance to be passed along to succeeding generations of the waterhemp.”

The implications of glyphosate-resistant common waterhemp are significant for Missouri and most of the rest of the Midwest, where waterhemp is one of the more troublesome weeds, added Bradley.

“So far, we have confirmed resistance only in horseweed in Arkansas,” said Andy Vangilder, with University of Arkansas Extension. “But that adds to weed control costs where resistant horseweed exists—there’s no cheap way to control weeds, especially in soybeans, where glyphosate resistance is present. The cheapest option is to do a good burndown and hope you don’t get a big second flush of horseweed.”

With soybeans, resistant weeds can add $5 to $10 per acre to the costs of controlling them, he added.

In Arkansas, as in Missouri, glyphosate resistance is the result of intensive selection pressure: where Roundup Ready soybeans are grown year after year in the same field, or where Roundup Ready soybeans are rotated with Roundup Ready cotton. However, weed scientists usually are not ready to declare resistance on the basis of only a year’s observation.

“We confirm resistance only after official ‘off-spring’ tests,” said MU weed scientist Andy Kendig. He noted that resistance must show up in the weed’s succeeding generations. “If you think a weed is glyphosate resistant, but then the offspring bursts into flames when you treat it with only a half-label rate of glyphosate, what does that mean? It probably means the weed is not truly resistant.”

Kevin Bradley agrees. That is why he wants to do one more season’s field studies before he publishes glyphosate resistance in the common waterhemp he’s working with.

In the same way, Reid Smeda did not rush to declare the common ragweed he found as resistant. For one thing, the glyphosate-treated ragweed plants were infested with a stem boring insect (the ragweed borer) and Smeda didn’t know if or how much the insect may have affected glyphosate activity in the weed. So, Smeda conducted experiments in the greenhouse and in the field with plants that were not infected with the borer before he confirmed that particular ragweed biotype as glyphosate resistant.

Is the glyphosate resistance observed so far an indication of what might lie ahead? Probably.

“Many weed scientists across the Midwest warn of the potential for additional glyphosate-resistant weeds if high selection pressure is maintained,” Bradley said. “By that, I mean the repeated use of glyphosate without the interruption by herbicides with other modes of action or other weed management practices.

“Glyphosate and Roundup Ready crops still offer a lot to growers,” he added. “But we need to evaluate how we use these technologies to reap their value without increasing the risk of resistance.”

In that regard, Bradley recommends:

1. Tank mixing glyphosate with another mode of action (such as 2,4-D) in burndown treatments, when glyphosate will be applied to the crop.

2. Alternating glyphosate use with other herbicide modes of action between years.

3. Using appropriate integrated weed management practices.

“Where weeds are glyphosate resistant, use the cheapest effective pre-emergence treatment you can use and continue with your Roundup Ready program,” he said. “Glyphosate is still a cost-effective way to control the vast majority of problem weeds.”