Study Shows Herbicide Rotation Alone Can’t Stop Weed Resistance

By: Emily Unglesbee

ROCKVILLE, Md. (DTN) — This bi-monthly column condenses the latest news in the field of crop technology, research and products.

CHEMICAL ROTATION ALONE CAN’T KEEP RESISTANCE AT BAY

Rotating herbicides to avoid weed resistance is not a magic bullet against resistance development, according to a new study from the University of Illinois. In the past, researchers have assumed that many of the random genetic mutations that help weeds survive a herbicide application come with a “fitness cost,” meaning the mutated weeds are not as strong and productive as the susceptible weeds. From a painstaking study done with waterhemp plants, Illinois crop scientist Pat Tranel and his team discovered this was not the case. Over six generations, the waterhemp plants in their test population with herbicide resistance survived and reproduced at the same rate as susceptible waterhemp plants.

This is tough news for any farmers who believe all they need to do to avoid weed resistance is spray an alternate herbicide mode of action every other year. Because no herbicide kills 100% of a weed population, there is a good chance that resistant genes to a certain herbicide will still be in the field even after a year or multiple years of using a different chemical, the Illinois researchers concluded.

“This study tells us that fitness cost isn’t going to help you much in terms of herbicide resistance, so even long rotations aren’t going to work,” Tranel said in a university press release. “I tell farmers, ‘Once you have resistance, you’re stuck with it.’ It gives us that much more incentive to do the right things to avoid resistance in the first place.” That means using multiple herbicide modes of action during single applications, following pre-emergence applications with post-emergence ones and even hand weeding any escapes if necessary, the researchers said.