Brett Craigmyle snipped off shoots of brown and wilted waterhemp one at a time, weighing the plants to determine how dead they really are.
For Craigmyle, a University of Missouri graduate student, this is just one step in a research project that looks at how to control an ever-escalating challenge in farm fields across the country — herbicide-resistant weeds.
The goal is to find the best chemical mix to control weeds and that works well with newly developed herbicide-resistant seeds.
The increasing resistance of weeds has reduced the effectiveness of go-to herbicides like glyphosate. New combinations of herbicides use different modes of actions, attacking weeds in different ways. For example, one herbicide can hurt a weed by manipulating its hormones while another can hinder photosynthesis.
“We spray weeds in the greenhouse at 6-inch and 12-inch heights, rate how much injury they have and then harvest, weigh and dry them to see how much living tissue remains in their system,” Craigmyle said.
Kevin Bradley, an MU Extension weed scientist and associate professor in the MU College of Agriculture, Food and Natural Resources, said farmers and researchers have to do a better job at managing weed populations and not just controlling weeds with one herbicide.
“Our No. 1 thing is we have to move away from spraying just one herbicide over and over and get a different mode of action out there,” Bradley said.
Current tests focus on the best tank mixes for 2,4-D and dicamba in waterhemp, giant ragweed, cocklebur and a variety of other weed species. By pairing those chemicals with different amounts of other herbicides like glyphosate and glufosinate (known by the brand names Roundup and Liberty, respectively), researchers hope to give farmers better control of the problem weeds in their fields.
Bradley hopes that U.S. agriculture can avoid measures taken by other countries. To tackle problems associated with herbicide-resistant weeds, countries like Australia have been forced to adopt different crops, move towards a greater reliance on tillage for weed control or develop new types of machinery to remove weed seeds from fields.
“We haven’t had to even think about doing those kinds of things—yet,” he said.
New developments will soon make 2,4-D and dicamba tank mixes all the more important.
MU recently partnered with Dow AgroSciences to engineer and field test soybean plants that tolerate 2,4-D. The chemical, developed during World War II, is one of the cheaper herbicides on the market, and 2,4-D-resistant seed would work in much the same way as Roundup Ready soybeans and corn.
Regulated testing started in MU field plots last year, Bradley said. While 2,4-D-resistant seed won’t hit the market for several years, it’s important for farmers to learn now from the mistakes made during the Roundup heyday, he said.
“Growers might look at 2,4-D- or dicamba-resistant soybean technologies and think it will be the next silver bullet, just like Roundup Ready,” Bradley said. “If we have that mindset we’ll lose these technologies to resistance, too.”
He says the days of one chemical solving all weed problems are over, and farmers will need to be smarter and timelier to control problems.
“We’re starting to run out of options to control those species with herbicides we have,” he said, “and while that’s not widespread yet, we’re seeing this as a growing problem.”
The solution is a balanced approach. Farmers should spray fields while weeds are small, use herbicides that have different modes of action and even use traditional control techniques involving crop rotation and tillage.
That’s especially important for farmers looking to eke out a few more bushels per acre.
“Yield loss from weeds can be astonishing if the proper measures aren’t taken to control them,” Craigmyle said. “In the end it’s all about farmers making that dollar to get by and feed the world.”
— Roger Meissen