“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.