Every weed every time
When weeds begin showing resistance, it’s not a case of the herbicide changing the weed, it’s a simple case of “survival of the fittest,” according to a Texas A&M AgriLife specialist.

Dr. Scott Nolte said the common misconception is that herbicides cause changes. Instead, it’s mainly the inherited ability of a species to survive.

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, thus reducing or preventing herbicidal activity.

Repeated use of herbicides with a single mode of action promotes selection for resistant weeds. It kills out the susceptible biotypes and leaves only the resistant ones. Then, a single resistant weed multiplies over and over.

“If the weeds are still growing, sometimes it doesn’t pencil out to keep treating,” Nolte said. “But we have to use what we have to slow them down.”

The problem often occurs when producers use herbicides that have a single mode of action, have a long soil residual, high use rate, and high use frequency. 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 also result in poor herbicide control.

If resistance is confirmed, immediate steps to take include elimination of the resistant weed population and herbicide rotation.

Cultural practices can be used to eliminate the resistant weeds, including delaying planting or using a non-selective herbicide, or possibly employing crop rotation.

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