Research Highlights

Controlling Weeds in Virginia: Cover Crops and Herbicide Management are Making Strides

By Carol Brown

Virginia farmers who are looking for better weed control measures in their soybean crop can turn to Michael Flessner and his research. The weed extension specialist at Virginia Tech has been exploring cover crop usage and post-emergence herbicides and their effectiveness on weeds in soybeans. His research is supported by the Virginia Soybean Board and the Virginia Natural Resources Conservation Service (NCRS).

Cover crop research

One objective of Flessner’s project focused on the effectiveness of wheat or cereal rye cover crops for suppression of Palmer amaranth and common ragweed. The project examined how well the cover crops performed as a weed suppressor after they were chemically terminated and either left standing or rolled.

“We came down to the basic result that biomass is king,” Flessner said. “As long as you can get sufficient biomass, other issues become less important.”

Sufficient biomass, according to Flessner’s research results, is around 7,500 lbs/acre, which achieved approximately 75 percent weed control for about six weeks. Research results revealed that as biomass increased so did weed control, and both cover crops drew similar results.

“Cover crops are terminated typically two weeks ahead of planting, then about four weeks after planting, most farmers go back over the field with a post-emergence herbicide. The cover crop that has sufficient biomass can get you through those six weeks between termination and herbicide application,” Flessner said.

There was a difference in termination methods in regard to weed suppression. For ragweed, Flessner’s team found that rolling the cover crop was not as effective as leaving it standing.

But with enough biomass, both termination methods were still more effective in weed control than no cover crop.

“This offers the farmer some management options,” he said. “If they have the biomass levels, they could terminate either way and still get the weed control benefits.”

“We measured the amount of light getting to the soil surface under the cover crop residue,” Flessner said. “In fields where the cover crop was left standing, the sunlight was more filtered, reducing the light quantity getting to the weed seed, inhibiting weed growth.”

Alternative herbicide programs

The other objectives in Flessner’s research looked at controlling Palmer amaranth and common ragweed through herbicide programs beyond glyphosate.

“In Virginia, we don’t have PPO resistance and we need to preserve PPOs if we can,” Flessner said. “We also have these new technologies that allow us to combine herbicides we couldn’t combine before in soybeans. We know that multiple effective sites of action are key to maximizing our weed control strategy.”

He and his research team are comparing test plots with several herbicide treatments, pre- and post-emergence, alone and in combinations such as: Reflex (fomesafen) and Xtendimax (dicamba); Liberty (glyphosate) and Reflex, and others, each compared with a nontreated check plot. They tested the treatments on weeds at three different growth stages: 2 to 4 inches, 4 to 8 inches, and 8 to 12 inches tall.

Not surprisingly, Flessner said, weed control decreased as weed plant size increased for all herbicide treatments. And the combinations generally performed better than the herbicides alone.

“But an interesting result was for Palmer amaranth. It really didn’t matter what herbicide trait we used, there wasn’t a better treatment by size,” he said. “Weed control all got worse at the same rate with size.”

In addition to finding the best herbicide for weed control, Flessner and his team also measured herbicide effectiveness on weed seed viability and seed production. They were able to completely eliminate seed production at the 2 to 4-inch plant size, but some weeds produced seeds at the 8 to 12-inch size, which Flessner says is very concerning for herbicide resistance.

The next step is to look closely at some of these combinations to see if they equate good resistance management or not.

“It is best to kill the weeds earlier in the season, when they are small (2 to 4 inches),” Flessner said. “Now we want to find how effective these combinations are in soybean plots and what we can do later in the season to control weeds that have escaped all other management methods.”

Published: Nov 23, 2020