Cover crops provide opportunity for enhanced weed control

SYCAMORE, Ill. — Weed resistance is a problem for many corn and soybean farmers that is likely to continue.

“We have some definite problems with weed resistance to herbicides and there is nothing on the chemists’ shelf that is going to be a miracle,” said Bryan Young, professor of weed science in the Department of Botany and Plant Pathology at Purdue University.

“There will be little components for weed management in the future, but nothing like Roundup, so we need something else that can help out and that’s where cover crops come in,” he said during a presentation in Sycamore at the 2026 Conservation Cropping Seminar, coordinated by the Illinois Department of Agriculture, Illinois Environmental Protection Agency, U.S. Department of Agriculture’s Natural Resources Conservation Service, and Champaign County Soil and Water Conservation District.

The best time to manage weeds, Young said, is when they are small.

“If you pull a plant and from the soil line down it’s about the size and color of white thread, that is the best time to manage,” he said. “Beyond that, we lose control practices.”

Some weeds have learned to adapt to many consecutive years of corn-soybean rotations.

“We have weeds that can germinate all season long,” Young said. “Giant ragweed and lambsquarter used to be early spring weeds, but we can find them germinating over long periods of time now because they have adapted to our production.”

Marestail was considered a fall-emerging or winter annual weed, the university professor said.

“That weed now comes up anytime the ground is not frozen from March until November, so that takes a lot of weed management,” he said. “Diversity in terms of our management practices is key and we don’t have a lot of diversity at times.”

One option for soybean farmers, Young said, is to reduce row spacing from 30 inches down to 15 inches to improve weed control.

“But I have had very little luck in getting farmers to do that,” he noted.

Waterhemp emergence starts early, from about June until early July with 90% of the weeds emerged.

“The other 10% occurs in July and August, so we need some non-chemical means to control that and hopefully that is canopy closure,” Young said.

“But with 30-inch rows and if we have late planting, sometimes we don’t get that,” he said. “So, narrow row spacing and cover crops can help us get there.”

The key to waterhemp control is early season weed management.

“We used to be able to get six weeks of residual control, but now with Group 15 and Group 14 resistance I am happy if we get to four weeks,” the professor admitted.

By applying a tank mix of herbicides, Young said, sometimes the weed control will be five and maybe six weeks.

“But that is probably going to cost in the low $20s per acre for the residual herbicides and that is typically more money than people want to pay early season,” he said.

Young talked about a research project that was conducted over numerous states — including Arkansas, Illinois, Indiana, Kansas, North Dakota, Ohio and Wisconsin — to look at the effects of cover crops on weed suppression.

“Cereal rye is our favorite because it grows well and survives the winter,” he said.

The secret for weed control is a good stand of cereal rye.

“You get 50% reduction in the waterhemp population when we had 4,500 pounds per acre of biomass,” Young said. “We achieved that level of biomass when the cereal rye was about 30 inches tall.”

The fastest way to get herbicide resistance is relying on post-emergence herbicides, the professor said.

“We cannot trust post-emergence herbicides anymore,” he said. “We need to invest in early season weed management and cover crops are a part of that.”

Cover crops are not simple, consistent or as easy as spraying a herbicide. However, Young said, “we are not going to have any more simple and effective solutions to weeds, so you have to be willing to get more complicated than what you have been and maybe cover crop management is part of that.”

“There is no sledgehammer for weed management, it has to be integrated,” he said. “Right now, cover crops are a good complement to the herbicide programs that we have.”