“Do not rely on a single method for weed control. We may need to consider cultivation again.”

Many farmers have gotten away from tillage with reduced till systems, but Baughman said plowing does a good job on a weed like horseweed/marestail. “Also, use a product with an alternate site of action.”

Palmer amaranth (pigweed) poses problems as well. “Pigweed has both male and female plants so a lot of crossing in the environment is possible.” That increases the chances of transferring resistance, especially since pigweed produces a large number of seed (as many as 500,000 per plant).

Baughman said resistance doesn’t just develop in a field. Resistant weeds are already there; farmers simply select for them by using the same herbicide over and over and killing off the susceptible weeds and leaving the resistant ones with little competition.

 “Depending on one herbicide is a big factor. A single site of action can select for resistant weeds.”

Weeds that survive can explode in a short time, growing from one pigweed plant per 100 feet of row to as many as one every three square feet in about two years. Controlling 99 percent of the weed population still leaves enough to continue building damaging infestations if the same herbicide is used alone. But adding a second herbicide, one with a different mode of action, can soon take the population back to where it started.

“We have to be prepared to control pigweed,” Baughman said.

The prospect should not be overly daunting. “We were controlling weeds before we had resistance,” Baughman said. “We can still control weeds if we don’t make the solution worse than the problem. Not every control issue is a resistance issue. Some weeds are just hard to kill. The climate also plays a role, as was the case the past two years.

“But we have seen a dramatic increase in weed resistance over the past 25 to 30 years. That increase is a result, he said, of too much reliance on a specific herbicide with a single mode of action. He noted products such as Glean, Ally and Cadre as herbicides that “work on a single enzyme. A slight modification of the plant population could affect control.”

He said farmers are using more herbicides than they did in the 1970s but are using much lower rates. “We have reduced the number of pounds of herbicides in the environment.”

Allowing a serious infestation of resistant weeds to develop could make weed control much more challenging, Baughman said. “We have to go to an integrated weed management program that includes reduced reliance on one mode of action, use of a number of different control techniques, multi-year management systems, timely applications and use of residual herbicides.”

Prevention, he said, is the most sensible approach to resistant weed management.


Also of interest:

Go back to yellows for resistant weed control in cotton

Consider resistance in weed management strategies

Across the Sunbelt, herbicide resistance changes production options