When weeds begin showing resistance, it’s not a case of the herbicide changing the weed, it’s a simple “survival of the fittest” case, according to a Texas A&M AgriLife Extension Service specialist.
Scott Nolte, Ph.D., AgriLife Extension weed specialist, College Station, said the common misconception is the herbicides cause changes. Instead, it’s mainly the inherited ability of a species to survive.
Building resistant weeds
Target-site mutation is the most common cause of herbicide resistance, he said. This is where a change at the target site prevents the herbicide from binding or otherwise disabling the action, thus preventing herbicidal activity.
Repeated use of herbicides with a single site of action promotes selection for resistant weeds. It kills out the susceptible biotypes and leaves only the resistant biotypes. And a single resistant weed multiplies over and over.
Resistant weeds multiply
“I know the weeds are still growing. And, sometimes it doesn’t pencil out to keep treating,” Nolte said. “But I can’t tell you to do nothing. We have to use what we do have to slow them down.”
The problem often occurs when producers use herbicides that have a single mode of action, are very effective and have a long soil residual, high-use rate and high-use frequency.
Weed management plan
Application rates and weed size calculations can be off, Nolte said. Applying herbicides to weeds larger than what the label specifies, or at a rate below the recommendation, can cause unacceptable control. Soil conditions, weather conditions, stressed plants and other factors can result in poor herbicide control.
Confirming herbicide resistance
If resistance is confirmed, Nolte said, immediate steps to take are:
- Eliminate the resistant weed population to limit or prevent seed set and shed.
- Don’t use the same herbicides and don’t let the plants go to seed.
- Prevent movement of the resistant population to other fields by cleaning all equipment.
- Implement a weed-management strategy to prevent future occurrences of resistant weeds.
Start clean, utilize all tools
Cultural practices can be used to eliminate the resistant weeds, including delaying planting or using a non-selective herbicide, promoting crop competitiveness, scouting fields for weed population shifts, using certified seed or possibly employing crop rotation.
Through the application of science-based knowledge, AgriLife Extension creates high-quality, relevant continuing education that encourages lasting and effective change.