By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and soybean discount.

At the 2022 Ohio No-Till Conference, Dr. Alyssa Essman, Visiting Assistant Professor of Weed Science at Ohio State University, discusses one of the most pressing topics in agriculture today – weed control. Although there are a number of herbicide options to control various weed species, some of those herbicides have residual carryover that can negatively affect cover crop establishment. An alternative method of weed control could be the actual use of cover crops to suppress weeds.

Cover crops suppress weeds in several ways. There is the physical suppression of weeds in which the cover crops act as a mulch that inhibits weed germination and growth. This can be measured by the cover crop biomass and percent ground cover. Competition is another way cover crops suppress weeds. The cover crops compete with the weeds for light, nutrients, water and physical space. Alleopathy 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 growing cover crops also gives the option of controlling weeds at the time of cover crop termination. As the cover crops are either sprayed, cut, crimped or tilled, so are the weeds. The practice of growing cover crops also changes the seed environment for weed species. Cover crops affect the seed environment as they change the level of soil moisture, temperature, light availability and provide an environment for pests.

As farmers battle the increasing number of herbicide-resistant weeds, cover crops can also be a tool. “Herbicide resistance has steadily increased over the past 50 years since the time when herbicide application became a tool for farmers in the fight against weeds,” Essman said. “In 2020, there were more than 500 unique herbicide resistance cases worldwide.” As herbicide use increased, overuse of the same workplace herbicides became a problem. Currently, there are nearly 160 species of weeds that are resistant to the Group 2 ALS inhibitors. There are nearly 100 weed species that are resistant to the Group 5 PSII inhibitors. The number of weed species resistant to Groups 1, 4 and 9 sites of action is also increasing.

Herbicide resistance develops when farmers repeatedly use the same herbicide, leading to selection pressure. A lack of crop rotation narrows herbicide options available to growers, which also increases the chance of resistance developing. Using a limited number of sites of action increases the likelihood of resistance development. Also the use of reduced amounts 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 such as common lambsquarters, wild carrot, tall water shirt, Powell amaranth, common ragweed, giant ragweed, common ragweed, common ragweed, and shattercane are resistant to one or more sites of action. The challenge is to combat the currently resistant weeds and prevent future instances of resistance.

Essman proposes 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 combat herbicide-resistant weeds.”

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

Factors that can affect herbicide carryover include the application, tillage and species selection. Residual herbicides applied at planting are less risky than those applied after emergence. Incorporation can also increase the persistence of herbicides. Tillage increases microbial degradation, reduces volatilization and photodegradation 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 rental problems.

The University of Missouri conducted studies that identified the most vulnerable species and harmful herbicides. The general order of sensitivity of cover crops to herbicide carryover from most to least sensitive is Tillage radish > Austrian winter pea > crimson clover and annual ryegrass > winter wheat and winter oats > hairy vetches and grain rye.

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

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

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