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• Scott McElroy and other researchers are testing thousands of plants from throughout Alabama for signs of resistance, focusing on glyphosate and/or dicamba resistance to horseweed, common ragweed, annual ryegrass and annual bluegrass.
“But we weren’t getting effective control with those mixtures. We have to ask ourselves do we have resistance, do we have something that actually has changed within the plant population that makes it impossible to control, or do we just have much larger plants at the time of application?”
It’s difficult to find horseweed in Alabama that’s not resistant to glyphosate, he adds.
“There’s more out there right now that’s resistant than non-resistant. When we’re dealing with horseweed, we’re dealing with situations in the burndown prior to planting.
“Plants will sit in a stagnated state for weeks and they won’t grow, but they’ve held onto their green leaf material, and maybe four to six weeks later — eight weeks after treatment — they’ll eventually start to put on new buds. You might think the plants will die, but they eventually recover, and we’re seeing dicamba resistance as well.”
McElroy says herbicide resistance research is very complex. “It takes a long time to test these things, because we’re spraying at different rates and at different growth stages of the plant, trying to get a sense of the type of resistance,” he says.
Glyphosate resistance also has been found in common ragweed in Alabama cotton fields, he says.
“We went out in the late winter of 2012, and saw common ragweed that germinated much earlier than we expected. We see odd growth characteristics on these resistant ragweed plants. Ragweed normally produces a finely divided leaf, but after we treat these, we’re seeing more solid leaves and tillering along the main stems. You’ll see these odd growth characteristics with resistant common ragweed.”
Horseweed, says McElroy, was mostly a burndown problem, but common ragweed has proven to be different.
“We couldn’t control horseweed at burndown, so it crept into the crop itself and began to survive throughout the summer. Common ragweed is different in that it has risen in the crop itself.”
Researchers also are looking at annual ryegrass that’s potentially resistant to glyphosate, he says.
“We’re seeing situations in test plots where people go out and make applications and plants survive from high rates of glyphosate. We collect the plants and collect the seed, and then we spray them and kill them.
“We can’t figure it out. It seems like the plant is just getting so big we can’t kill it. Is that resistance? It’s borderline.
“Typically, when we talk about herbicide resistance, we talk about a genetic change within the plant that makes it to where the herbicide doesn’t work anymore. In this case, it may be that due to our management practices, ryegrass is germinating earlier, growing faster, and surviving longer, so it’s just harder to kill.”
In the turfgrass arena, perennial annual bluegrass and annual bluegrass are proving resistant to certain herbicides, says McElroy.
“We’re seeing all types of resistance in these grasses throughout the state, to prodiamene and trifloxysulfuron. It’s a pre-emergence resistance.”
Herbicide resistance — whether in crops or turf — is being dealt with throughout Alabama, he says.