“We’re sure about this,” says Ken Smith, Arkansas Extension weed scientist. “We collected samples from up there, put them in the greenhouse and have done everything that needs to be done to confirm resistance.
Glyphosate, which was developed and sold under the trade name Roundup by Monsanto, is now available under a number of brand names. It is one of the most widely used herbicides in no-till and other conservation tillage applications, according to weed scientists.
“(May 19) was our magic day on this marestail,” he said. “We spoke with Monsanto personnel, University of Arkansas scientists and everyone else who needed to be contacted. If it hadn’t shown a proper response to spraying by (May 19), we were all in agreement that it would be pronounced resistant.”
Smith doesn’t know the marestail’s level of resistance. “Is it resistant at eight times the normal amount of Roundup? At 10 times the normal rate? We do know it’s definitely resistant at a 3X rate, which is far too high.”
This finding confirmed Smith’s suspicions last winter that the resistant plant would be found before this growing season was up.
“We’ve now got to start managing for it. I see marestail all over the Delta side of Arkansas – and even in the southern part of the state – that has the characteristic yellow bloom right on the top. I hope I’m just seeing it wrong. But when you spray resistant marestail, that yellow bloom tends to appear right at the top and then turns back to green and continues growing. That’s very typical of resistance.”
Smith says a Mississippi County crop consultant reported the problem to a “very astute” county agent, Susan Matthews. Matthews, who first worked in Tennessee and had already seen resistant marestail first hand, then called Smith. “She was on that weed like a chicken on a June bug,” he says.
“Well, the consultant wanted to know how to deal with this weed. At that point, there weren’t many alternatives. We can go in with high rates of MSMA or a couple of other things. But this particular farmer rotates rice and cotton and doesn’t want any MSMA used because of his cotton.”
Smith says he and his research colleagues will follow through and see what level of resistance the marestail expresses.
“It’s sad, but it was inevitable. We’ve now got the same problems our neighbors have.”
For all the problems that could be caused by glyphosate resistance in marestail, Smith is even more concerned about herbicide-resistant pigweed.
“It will be a real inconvenience and will cost extra money to manage resistant marestail. But we know that if we get in early enough with burndown with the right chemicals, marestail can be dealt with. It’s inconvenient to get in 30 days ahead of planting because of weather, but we can do it.”
But if pigweed becomes resistant, it will revolutionize how people farm, says Smith.
“That scares me terribly. We haven’t seen resistant Palmer pigweed yet. But just like with marestail, I think it’s a matter of ‘when’ not ‘if.’”
Smith says the kissing cousin to Palmer pigweed, water hemp, has already been found to be resistant to some herbicides in the Midwest.
“I wouldn’t be surprised if water hemp and pigweed cross-pollinate and it shows up (in the Delta),” says Smith. “That scenario is frightening, and we’re trying to scramble and get some alternative control measures available when it is found.”
He said researchers will be meeting to brainstorm their options.
“We need to know what the options are,” he said. “We don’t want to get out of conservation tillage or no-till practices. Those practices are cost saving, are beneficial to the environment and so they need to be maintained as much as possible. But we still may have to go to some kind of dirt scratching to get herbicides where they need to be. I’m just not sure. Hopefully, we’ll have time – years – to get plot tests out and a plan devised.”
If resistant pigweed showed up this year, “we’d be in a world of hurt,” says Smith. “I don’t know what we’d do. Whatever we ended up doing wouldn’t be good. So we’re trying to be proactive and stay ahead of this coming problem.”