Farmers know that rotating crops is an effective strategy for controlling weeds. Rotating herbicides also helps fight weeds, says agronomist Denise McWilliams of the University of Minnesota Extension Service.

Rotating herbicides is particularly beneficial in hindering the development of herbicide-resistant weeds, she adds.

McWilliams cites research from a four-year study of crop rotations and herbicides by Ohio and Virginia researchers. The study looked at population trends for weeds such as common lambsquarters, ragweed, several amaranthus species and jimsonweed at two sites. Cropping systems included continuous corn, continuous soybeans and corn-soybean rotations.

"For both common lambsquarter and ragweed, populations increased over four years when the same herbicide was used every year," says McWilliams. "Amaranthus populations decreased over time except with one continous herbicide treatment. Jimsonweed control was variable based on the treatment."

McWilliams says the researchers found that weed shifts were less severe when herbicides were rotated annually. Shifts to tolerant species occurred where the same herbicide or herbicides with similar activities were used continuously. Many other studies have also shown this, McWilliams points out.

Source: Denise McWilliams, (701) 231-8160