Total acres of row crop agriculture that have been infested with glyphosate-resistant weeds include about 90 to 95 million acres and more than 50 percent of the 82 to 83 million soybean acres.
Jason Norsworthy, professor and Endowed Chair of Weed Science at the University of Arkansas
"The two major weeds at the forefront of glyphosate resistance would be Palmer amaranth and waterhemp," said Jason Norsworthy, professor and Endowed Chair of Weed Science at the University of Arkansas. "There are a few others of importance like marestail and giant ragweed. In the U.S., we are up to 16 glyphosate-resistant weeds in soybeans."
Glyphosate isn’t the only resistance problem – widespread ALS resistance began in the 1990s and contributed to the rapid adoption of Roundup Ready herbicide. In the last three to four years, PPO resistance in waterhemp has taken off in the Midwest and is now in the Mid-South. Also in the South, dinitroaniline (DNA) resistance is common in Palmer amaranth.
"Glyphosate used to be the easy button – you didn’t have to know your weeds because it killed every weed in your field," said Don Porter, Syngenta herbicide technical lead. "Glyphosate also had a wider window of time that it was effective, so it would kill weeds that were much taller – even 12" to 18" – than herbicides can handle today."
Nature Finds a Way
"Today, we have fields with four-way weed resistance, which puts us in a position where only one commercial product is available post-emergence, and that’s glufosinate," Norsworthy said. "Rotating Roundup Ready soybeans to Roundup Ready corn while continuing to use glyphosate as the foundation, is not effective crop rotation. It doesn’t work because this is about the continual use of the same tool over and over."
Resistant weeds are smart and can evolve, so we’ll never spray our way out of this problem, he said. Growers need an integrated approach using diversity in chemical strategies along with non-chemical strategies to create a long-term plan.
"Another major component to resistance management is trying to keep the soil seed bank as low as possible. Four or five Palmer amaranth plants on a 500-acre farm may not seem concerning, but it’s important to make sure those four or five plants do not produce seeds," he said. "Once they do, they’ll add millions of seed to the field’s soil seed bank, and that will continue to expand exponentially across the farm."
Resistant Seed Movement
Resistant weeds are moving across soybean-growing states at a surprising rate.
"We moved a lot of seed around, and it’s well beyond a farmer moving it from field to field. Waterhemp can shed 500,000 seeds per plant, and those seeds are the size of a pinhead. Just think how many places a seed that size could get in a combine," said Kevin Bradley, associate professor at the University of Missouri.
Palmer amaranth is quickly moving across a larger geography than we’ve seen with any other resistant weed. Bradley said this movement is occurring through equipment, feed, seed and even waterfowl. Resistant Palmer amaranth is surprisingly showing up in places like Michigan, Indiana, Iowa, Illinois, Ohio, Delaware, Wisconsin, and parts of Missouri where it’s never been seen before.
Plan Now or Pay Later
Planning isn’t easy, but it can serve as a map for the future, giving everyone a chance to see which herbicides really contain different sites of action.
"It’s hard to get growers to plan long-term, but it doesn’t mean we shouldn’t keep trying," said Bradley. "I recommend developing a herbicide program and integrating cultural practices like crop rotation, tillage and cover crops possible. Growers who come up to me at the meetings and start talking through their plan for next year or two years from now – I just start smiling because I know they get it. They’re ahead of the game; they’re going to be fine."