What You Can’t See Can Hurt You
Weed scientists warn that more resistance to glyphosate is on the way, it’s simply a matter of when
 
        Every grower has seen escaped weeds. However, what could be escaping their attention is an underlying problem of resistance.

        “Based on recent grower surveys, those growers who have identified herbicide resistant weeds on their farms, typically did not identify the weed to be resistant until about 30 percent of their population was resistant,” said Bryan Young, weed scientist at Southern Illinois University.

        According to Young, escapes can be common and often do not mean resistance is an immediate issue. However, in the case of growers who continuously apply glyphosate, those escapes can be the first warning signs of a larger problem.

        “Generally anything less than 90 percent control would be written off as failure due to factors like environmental conditions or rate,” Young said. “It won’t be attributed quickly to the weed being resistant to the herbicide. You want to eliminate all other factors before you even consider resistance as a possibility.”

        Bill Johnson, extension weed scientist at Purdue University, agrees that 30 percent of a weed population can be resistant before a grower recognizes the problem.

        “That resistant population builds year after year following continual glyphosate use because the individuals that are resistant survive and produce seed, and their offspring repopulate the area,” he said.

        “The more applications, the greater the opportunity to get resistance,” Young explained. “Assuming you don’t change how you manage your weeds, you will have exponential increase in the percentage of resistant biotypes in the field.”

        While some states, including Illinois, have not confirmed any weed resistant to glyphosate, Young said that does not mean it won’t be a problem. In fact, horseweed, which is already confirmed in 10 states including neighboring states Missouri and Indiana, could be a future issue for Illinois and other Midwest growers.

        “The last two years we’ve either had really good environmental conditions for control or we didn’t have a lot of horseweed that posed a problem in the spring,” Young said. “So maybe it’s been here for a couple years, we just haven’t had the right conditions for a good selection year.”

        To decrease chances of developing resistant weeds, both Young and Johnson suggest growers incorporate another mode of action into their weed control plan.

        “The main thing farmers need to do is target their biggest weed problems with at least two modes of action,” Johnson said. “What a person does is dependent on the specific weed that they’re after, but we need to move towards using more herbicide modes of action in our soybean systems.

        “Growers are still very dependent on glyphosate because it’s simple. But the ones that are having problems have been readily adopting the use of alternative burndown programs. I think many growers wait until they have a problem before they make a change in their management practices, rather than being a bit more proactive.”

        Young added, “If a grower needs to see resistance to believe it, he may find himself with an annual management problem before he takes action.”

        Rick Austin, a Wisconsin grower who farms about 3,200 acres, warns against that mindset about glyphosate resistance.

        “Anybody who is not concerned about resistance is somebody who doesn’t have the problem,” Austin said. “Anytime you use a product year after year after year, you open yourself up for some sort of resistance to come in.”

        For Austin, weed control is a constant battle because of the ALS-resistant shattercane on his farm.
        “Every year we started getting more breakthroughs on our shattercane,” he said. “All of a sudden one year we couldn’t kill it. We went back in and hit it a second time and the herbicide didn’t even touch it. We knew we had a serious problem.”

        Today, glyphosate is Austin’s best tool for killing shattercane, but he said he is not about to abuse the chemistry. Instead, he uses a residual herbicide to complement the glyphosate.

        “We have an ALS resistance problem, and we’re fighting that now. We definitely don’t want to get into a glyphosate resistant weed,” he said. “We’re rotating crops, but I think maybe more than that, we’re rotating the modes of action that are killing the weeds.  We’re not giving the plants a chance to build up resistance. If one chemistry doesn’t take them out, the other one does.”