There still aren't any Extension-confirmed cases of glyphosate-resistant horseweed (or marestail) in Louisiana or Arkansas. But such cases are on their way.
“I fully expect to see it this year. When producers begin their burndown programs, I fully anticipate a phone call,” says Ken Smith, Arkansas Extension weed scientist at the Southeast Research and Extension Center in Monticello. “This isn't a matter of if, but when.
“I was at a Lee County meeting in Marianna recently and made the comment that we've yet to document resistant marestail. A couple of farmers there swore they had it. If they end up having it, we need to come out and check and make sure.”
Once he gets the call, Smith says he or a colleague will visit the farm and make sure the marestail is sprayed thoroughly and then checked.
“If the marestail isn't dying, we'll take a sample and take it to the greenhouse. Once the suspected plant is grown, we'll collect the seeds and subject them to further tests.”
Last summer, Smith said resistant marestail would show up in Arkansas two or three years into the future. He has since revised his timetable down and thinks it'll show up this spring.
“Just in the last week, I've gone to a map of Tennessee (where glyphosate resistant marestail was first found) and checked the movement of the plants. If you track the movement and draw circles around the infested areas, it would be foolish to assume such plants aren't already in Arkansas.”
Smith points out the seeds of marestail have a little plume that aids it in movement. Such seeds are wind-blown over a wide area very quickly.
Louisiana may have a little longer to breath easy.
“We had a couple of scares last year but I think they had something to do with application timings. Larger weeds were left too long and growers had problems,” says Donnie Miller, Louisiana weed scientist at the Northeast Research Station at St. Joseph.
“I'm praying we don't get the resistant marestail in, but it could come in. Right now, glyphosate is still effective on Louisiana marestail so we're using it. If some of this stuff makes it inside our borders, I think we'll hear about it rather quickly.”
Miller's Louisiana colleague, Roy Vidrine, says there have been resistant weeds in the past.
“We've had cockleburs resistant to DSMA and MSMA,” says Vidrine, a weed scientist at the Dean Lee Research Station in Alexandria. “We've dealt with johnsongrass resistant to Fusilade and Poast.
“We haven't seen problems with horseweed yet. But it may be a bit difficult to define because of the weed already being harder to control with glyphosate. If there's a weed-size problem, an environmental problem, or spray coverage trouble, it may appear to be resistance. But we've got to see it on the genetic level to tell for sure — and that takes time.”
Along the same lines, Vidrine says Louisiana has problems controlling the winter weed Carolina geranium. Glyphosate, he says, is “fairly ineffective” in controlling the plant. Luckily, Gramoxone and several other products are fairly effective.
“In the past, we've used more glyphosate than Gramoxone and now we're seeing more Carolina geranium. I think we're likely to see a lot more weed shifts — not necessarily weed resistance — due to glyphosate use. Weeds that glyphosate isn't very effective on, we're liable to see more of because spraying glyphosate is just so standard now.”
When resistant marestail is found, Smith says, “we'll probably ride on the coat-tails of Tennessee and Missouri. We're likely to latch on to their educational programs and do some of the same here. When we're talking resistant marestail, clean seedbeds become even more imperative. If these plants are left to grow, there's nothing to come back with to combat them.”