New research out of the University of Illinois shows just how bad a problem herbicide resistance has become. And with tillage out of the equation for no-tillers, it’s a reminder that they must be especially proactive in keeping weeds under control.
In greenhouse and field trials, the university looked at the response of a waterhemp population from McLean County, Ill., to HPPD inhibitors and six other herbicide classes. Researchers tested whether they could control the waterhemp at higher application rates, but found that the plants were recovering after 2 weeks, even when they applied twice the label rate of some herbicides.
They also looked at timing of post-emergence herbicide application, and whether they could get more effective control when the plant was at a smaller size. Their results concluded that timing alone couldn’t overcome the resistance.
Aaron Hager, University of Illinois weed scientist, says the key to winning this battle is to attack the weed at its most vulnerable stage — when it’s a seed. If growers do so, “we could beat this thing in 5-7 years,” he says.
Growers can reduce the weed seedbank by waiting as long as possible to plant, which allows the waterhemp seeds to germinate.
“Any emerged plants then are controlled either mechanically or chemically before planting,” he explains. “Effective burndowns are certainly an option in no-till scenarios.”
He notes there are several herbicides that can effectively control emerged waterhemp prior to soybean planting, but only if the population is sensitive to the herbicides used.
“Herbicides such as glyphosate, 2,4-D, saflufenacil, glufosinate, etc., can be used for this type of application, but the level of control achieved will largely depend upon the sensitivity of the population,” Hager says.
If no-tillers see waterhemp emerging after planting, they’ll want to pull those plants by hand in the summer before they go to seed.
“You can’t let female plants go and make hundreds of thousands of seeds and then run a combine through at the end of the year,” he says. “You’re going to reseed that whole field.”
Cover crops can be a component of a waterhemp control strategy as well. But Hager notes the key to effective weed suppression is to allow the cover crop, usually cereal rye, to reach about 90% anthesis before terminating the cover by mechanical rolling.
“Unfortunately, our contemporary time of terminating cover crops greatly reduces the amount of biomass and lessens its ability to suppress emergence of many summer annual weed species,” he says.
But some growers, such as Pennsylvania no-tillers Lucas Criswell and Jim Hershey, are able to achieve high cover crop biomass by planting directly into their living cover crops. For more information, see the article “The Next Step in No-Till: Planting into Living Covers,” from the February 2016 issue of Conservation Tillage Guide.
The important takeaway here is that growers can no longer rely on chemicals to effectively control their herbicide-resistant waterhemp.
“We’ve got to get people off this idea that we’ve got a chemical solution for waterhemp,” Hager says, “because in some cases, we don’t.