Outsmarting resistant weeds

From mechanical control to cover cropping, Professor Michael Flessner helps farmers adopt more sustainable weed management strategies.

By Julie Shlisky

Herbicide resistance is the inherited ability of a plant to withstand exposure to an herbicide that would have previously killed it. Repeated use of the same herbicide — or herbicides with similar function — favors the survival of resistant weeds, which can eventually dominate the population. This process is accelerated by practices like monoculture and over-reliance on a single herbicide. 

The widespread adoption of genetically engineered crops, such as Roundup Ready varieties, initially made weed control cheaper, easier, and more effective. But overuse caused what scientists call the "herbicide resistant treadmill," where newly herbicide-resistant weeds necessitate use of more herbicides, in higher doses.

This cycle drives up costs as growers need to spray additional herbicides. As treatments become less effective and weed growth persists, weeds compete with cash crops, reducing yields for farmers.

Michael Flessner, a professor in the School of Plant and Environmental Sciences, develops proactive, non-chemical strategies to combat herbicide-resistant weeds. While these approaches require more investment than herbicides alone, they offer a more sustainable solution and help reduce the weed seed bank.

One method his team examines is harvest weed seed control (HWSC), which targets and destroys weed seeds during harvest. HWSC adds a weed-control component to existing harvest operations, at a time of year when farmers may not focus on weed management.

“An impact mill attaches to a combine to mechanically destroy weed seeds during harvest,” Flessner explained.

The majority of seeds exiting the machine are no longer viable.

“Using a combine without HWSC can make the problem worse,” Flessner said. “Seeds stay intact, and the combine disperses them back into the field.”

For smaller combines, chaff lining is a HWSC technique that uses a funnel to collect weed seeds. With every pass of the combine, the collected weed seeds are laid down in narrow piles along with chaff — small pieces of leaves, flowers, and other crop residues — to help suppress weed growth.

Although HWSC reduces the amount of weed seeds returned to the soil, it may take a few seasons to see the benefits. Flessner says a 20 to 30 percent reduction between seasons is typical, but for weeds like common ragweed, a 50 percent reduction in emergence the following season is possible.

Another alternative to herbicide use is planting cover crops that suppress weed growth. Cover crops are planted in the fall and grow through winter and spring. When it is time to plant cash crops, the cover crops are terminated with herbicides, leaving their biomass residue on fields.

Cover crop residue blocks sunlight from reaching weed seeds, hindering germination and allowing cash crops to grow through. For example, cereal rye leaves considerable residue that can suppress herbicide-resistant common ragweed and Palmer amaranth. Fewer, smaller weeds emerge, but they are easier to control with less herbicide.

Flessner’s work has directly benefited local growers by informing regulatory planning and state guidelines for the cover crop cost-share program, making it easier and more affordable for farmers to access cover crops. In addition to suppressing weeds, cover crops reduce soil erosion and help protect the Chesapeake Bay and other waterways.

In addition to his work on behalf of Virginia, Flessner is also a leader in the mid-Atlantic region of GROW, a scientist-led network coordinating research and outreach on herbicide resistance. His participation advances collaborative research and helps standardize protocols for diversified, integrated weed management strategies.