While glyphosate-resistant Palmer amaranth pigweed has gotten a lot of attention lately in farming circles, other herbicide resistance issues have been confirmed in growers’ fields and could present challenges in the future.

“When we talk about herbicide resistance, we’re talking about weeds that you could at one time kill with a given herbicide, but now you can’t. These are weeds that you could control before some type of mutation occurred,” says Scott McElroy, associate professor-weed science, at Auburn University.

McElroy discussed resistance issues during the recent Alabama Sprayer Clinic, held in Shorter.

He and other researchers are testing thousands of plants from throughout the state for signs of resistance, he says, focusing on glyphosate and/or dicamba resistance to horseweed, common ragweed, annual ryegrass and annual bluegrass.

“Every state surrounding Alabama is getting hammered with herbicide-resistant weeds,” says McElroy. “Do we have problems we’re not aware of, is there something unique about Alabama agriculture, or are we just that good that we’re not having problems?”

It has been confirmed, he says, that herbicide-resistant horseweed is an issue in Alabama fields.

“Ten to 15 years ago, we typically would not have thought of horseweed as a summer, row-crop weed. It was a weed that occurred during the winter, and we burned it down. But with the popularity of no-till production and glyphosate-resistant crops, we’re getting a weed that wasn’t a problem several years ago, but it’s now a major issue,” he says.

After glyphosate-resistant horseweed was confirmed, says McElroy, growers began moving towards glyphosate/dicamba mixtures for controlling the weed.

Control failures

“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.

Testing available

“If you think you’ve got resistance, we can do some testing for you, especially if you’ve got a new population. If you’ve got glyphosate-resistant horseweed, welcome to the club. If you have potential resistance to new herbicides, please contact us, and we’ll take a look at it.”

It should be considered, he says, whether or not the problem is resistance or another factor.

“Is it application timing, environmental conditions, sprayer error or something else? Normally, herbicide resistance will pop up in one species at a time. You’re not going to see two or three species that suddenly are not being controlled by one herbicide. It’ll normally occur one species at a time, and then multiple ones may occur after that.”

A pattern for herbicide resistance normally will be widespread — it won’t be along the field edge, and it won’t be a streak throughout the field, he says. It’s usually a disperse pattern of uncontrolled plants.

“We also need to look at the crop characteristics. How well is the crop itself growing?

“Sometimes, when you get into weather extremes, herbicides might not work as well and crops won’t grow as well. This gives you an indication that the control was not maximized.

“Is it resistance or is it something else, and is it occurring two years in a row? In farming, anything can happen from one year to the next. But did it consistently occur over two consecutive years, or have you made multiple applications of a herbicide and you’ve still seen no response? That gives you an idea that a resistant population might have developed.”

With more wheat being planted in Alabama, McElroy says growers should be aware of the possibility of herbicide-resistant annual ryegrass or Italian ryegrass.

“We’ve seen smooth pigweed in a lot of fields, which is amaranth that has hybridized. These amaranths can hybridize, so you can get gene flow from one resistant species to another. If one amaranth has become resistant, we might start to see more.”

Other potential resistance problems include giant ragweed in north Alabama and johnsongrass in Arkansas, he says.

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