Even with all the attention directed toward weed resistance to various herbicides, and especially glyphosate, there are still too many farmers ignoring the messages or making only minor changes to their farming practices, especially in the northern Midwest.
Farmers need to plan for keeping resistant weeds at bay and not attempt to react to resistance when the first herbicide resistant weed is identified, explained Jeff Stachler, Ph.D., Extension agronomist for weed science and assistant professor at North Dakota State University and the University of Minnesota.
Stachler defines resistance more from a farmer angle. He said, “I now define resistance as the inactivity to respond to a changing weed population.”
He continued, “We don’t scout enough to know when the population is changing. We don’t understand the impact that one plant will have, and, therefore, we let it go because we’ll get it next time.”
Stachler explained his point of view on the current build up of weed resistance to herbicides, mainly glyphosate. He noted that if two weeds are resistant to nine-tenths of a full rate of herbicide and are only hit with nine-tenths of a full rate and survive, then the likelihood is that crossbreeding in the field of those two weeds will result in plants growing from the crossbred seed that are resistant to a full rate or higher rate of herbicide. Increased resistance will continue to balloon in each subsequent year’s batch of weeds.
“This is why I’ve come to the conclusion that zero tolerance is the only thing that you can accept. Can we get to zero tolerance all the time? No, but if we don’t try or talk about it, we can’t even get close to it,” Stachler said. His zero tolerance is zero weeds left in a field so that it is pristinely clean, as was the case when many farmers would hand hoe the last few weeds from their fields in the decades prior to the 1970s.
Weed resistance to herbicides in North Dakota and Minnesota where Stachler works is similar to reports across most of the upper Midwest. “It is very bad in some areas, and you can go to other areas and we have very little weed resistance to one or more herbicides. But it is pretty hard to not find a field in a county in all of Minnesota and at least the eastern two-thirds of North Dakota that doesn’t have glyphosate resistant weeds at this time,” he said.
Mike Jones, Bayer technical service representative in eastern Iowa, concurs with Stachler’s observations. “In the 20 counties in which I work, I’ve seen glyphosate resistance issues in each of them.”
Jones has watched as weed resistance has become obvious because of overuse of glyphosate. “Any herbicide that is overused will encourage weed resistance. You could say the weeds will adapt,” he said.
“Bayer is trying to point out the need for herbicide diversity as well as trait and crop diversity,” Jones said. “Respect the Rotation field days and materials are tools we have been using for the last three years to tell the message about weed resistance. We have also been working with universities—in my area it is Iowa State University—supporting their messages.”
Confirmation of weed resistance usually comes late in the cycle of resistance buildup in a field, noted Stachler. Although not officially confirmed in all cases, it appears there is probably glyphosate resistance to giant ragweed, common ragweed, waterhemp, marestail and kochia in his northern climate area. In other words, weed resistance isn’t being contained to the southern U.S. It has spread from border to border.