Cover crops and weed control

By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and soybean check-off

At the 2022 Ohio No-Till Conference, Alyssa Essman, visiting assistant professor of weed science at The Ohio State University, discussed one of the most pressing topics in agriculture today — weed control. While there are a number of herbicide options to control various weed species, some of those herbicides have residual carryover that can negatively impact the establishment of cover crops. An alternative method of weed control may be the actual use of cover crops to suppress weeds.

Cover crops suppress weeds multiple ways. There is the physical suppression of weeds in which the cover crops act as a mulch layer that hinders weed germination and growth. This can be measured by the cover crop biomass and percent groundcover. 

Competition is another way cover crops suppress weeds. The cover crops compete with the weeds for light, nutrients, water, and physical space. 

Alleopathy also allows cover crops to suppress weeds. Alleopathy occurs when certain species of cover crops release chemical compounds into the soil that inhibit germination and growth of weed seeds. 

The process of raising cover crops also gives the option to control weeds at the time of cover crop termination. As the cover crops are either sprayed, mowed, crimped, or tilled, the weeds are as well. The practice of raising cover crops also alters the seed environment for weed species. Cover crops impact the seed environment as they alter the level of soil moisture, temperature, light availability, and provide an environment for pests.

As farmers battle the increasing number herbicide resistant weeds, cover crops can be a tool as well.

“Herbicide resistance has steadily increased in the last 50 years since the time that herbicide application became a tool for farmers in the battle against weeds,” Essman said. “In 2020 there were over 500 unique herbicide resistance cases globally.” 

As herbicide use increased, overuse of the same sites of action herbicides became an issue. Currently there are nearly 160 species of weeds resistant to the Group 2 ALS Inhibitors. There are almost 100 weed species resistant to the Group 5 PSII Inhibitors. The number of weed species resistant to Groups 1, 4, and 9 sites of action are also on the increase.

Herbicide resistance develops when farmers repeatedly use the same herbicide, which leads to selection pressure. A lack of crop rotation narrows herbicide options available to producers, which also increases the chance for resistance to develop. By using a limited number of sites of action, the likelihood of resistance development increases. Also using reduced rates of the products can increase resistance.

“In the battle of resistant weeds, residual herbicides are going to be more important than ever,” Essman said. “This is especially true for weeds like waterhemp.” 

In Ohio, species like common lambsquarters, wild carrot, tall waterhemp, Palmer amaranth, common ragweed, giant ragweed, common cocklebur, and shattercane are resistant to one or more sites of action. The challenge is combating the weeds that are currently resistance and preventing future instances of resistance.

Essman suggests an integrated pest management strategy to tackle this challenge. 

“Instead of one big hammer, it will take several small hammers,” Essman said. “The use of biological controls, cultural controls, mechanical controls, and chemical controls are all examples of small hammers. The use of cover crops is also one of the many small hammers we can use to fight herbicide resistant weeds.”

Factors to successful cover crop establishment include planting date, seeding rates, the planting method, as well as not having a herbicide carryover situation. Herbicide carryover in cover crops will depend largely on the cover crop species selection and the use of residual herbicides. The amount of rainfall received, and herbicide application timing also play a role in herbicide carryover. There can be large variations from year to year. Factors such as the water solubility and soil half-life affect carryover. Soil organic matter, clay content, and high CEC can increase carryover risk. Warm temperatures and rainfall increase herbicide degradation. Cool temperatures and drought increase carryover.

Factors that can affect herbicide carryover include the application, tillage, and species selection. Residual herbicides used at planting are less risky than those post-emerge applied. Incorporation also can increase herbicide persistence. Tillage increases microbial degradation, reduces volatilization and photodecomposition of herbicides. Cover crop species selection is also important. Herbicides are more effective on small-seeded weeds and many cover crops. Rye tends to be lease problematic.

The University of Missouri has conducted studies that identified the most vulnerable species and damaging herbicides. The general order of sensitivity of cover crops to herbicide carryover from greatest to least sensitive is tillage radish > Austrian winter pea > crimson clover and annual rye grass > winter wheat and winter oats > hairy vetch and cereal rye.

The soybean herbicides that were most injurious to cover crops include: Fomesafen (Flexstar/Prefix), pyroxasulfone (Zidua), imazethapyr (Pursuit), acetochlor (Warrant), sulfentrazone (Authority products).

The corn herbicides that were most injurious to cover crops include: Topramezone (Impact), mesotrione (Callisto, Halex GT, etc.), clopyralid (Stinger/SureStart), isoxaflutole (Balance Flexx), pyroxasylfone (Zidua), nicosulfuron (Accent Q).