Building soil organic matter and protecting soil from erosion are two well-known benefits of cover crops. Cover crops are also a common weed control tool.
According to Extension specialist Mark VanGessel of the University of Delaware, what is often overlooked, however, is the impact of herbicide carryover on the establishment and growth of cover crops. This in turn impacts the success of a cover crops’ weed control. VanGessel discussed this topic at a Northeast Cover Crop Council meeting.
Herbicide carryover can range from a minimal amount of injury to a cover crop (such as discoloration of the leaves) to plant death. The potential damage from carryover varies due to a number of factors.
One factor is that while herbicides break down through a number of different processes, the main process is microbial decomposition. This occurs when herbicide molecules in the soil are used as a feed source by microbes which break herbicides down into compounds with no more herbicidal properties.
Climate, such as adequate soil temperatures and moisture, can therefore impact this process. For example, dry soil will inhibit microbial activity and therefore increase the amount of time it takes for microbes to break down herbicides. And in colder climates, there is simply less time for the microbes to do this work.
Another factor affecting carryover is that the decomposition of herbicides varies from chemical to chemical. There’s no one set method of herbicide loss that’s uniformly effective across all chemicals. Not all herbicides break down at the same speed or the same rate, and some herbicides remain viable in the soil for only a few weeks while others may take months to fully break down. Application rates also play a role in how long it takes for an herbicide to break down.
Soil variables also have an impact on carryover. Medium- and fine-textured soils with organic matter over 3% have a higher risk of carryover due to more adsorption in the soil. Adsorption is the adhesion in an extremely thin layer of molecules (in this case the herbicide) to the surfaces of solid bodies (the soil) with which they are in contact.
“There’s more herbicide that’s bound to that soil that then can desorb and move off the soil particles later in the year. So, the more that’s adsorbed, the more likely that it can come off the soil matrix later and potentially injure the cover crop,” VanGessel said.
Herbicide carryover damage can be minimal – such as leaf discoloration – or it can result in cover crop plant death. Photo courtesy of Mark VanGessel
pH also has a role in herbicide carryover. Triazines and sulfonylurea have a slower breakdown at a pH of greater than 7.0. Imidazolinone breaks down more slowly if the pH is less than 6.0.
Given the impact that herbicide carryover can have on cover crops, VanGessel said it’s critical to understand how specific herbicides might impact a cover crop planting. This is increasingly important as growers shift from planting cover crops after a cash crop is harvested to more sophisticated strategies such as interseeding a corn field five or six weeks after the corn is planted. In this example, the herbicide used during the corn planting has had very little time to break down before the cover crop seed gets planted.
Some cover crops are highly sensitive to herbicides and even a small amount in the soil can damage them. For instance, clovers and radishes are sensitive to atrazine whereas winter grains tend to be the most tolerant of atrazine.
Similarly, clovers and radishes are sensitive to mesotrione (commonly used as a post-emergent herbicide) whereas grasses are very tolerant of mesotrione. A University of Missouri fact sheet provides further details about the impacts of specific herbicides on different cover crops at CoverCropCarryover.pdf.
According to VanGessel, research shows that the more biomass a cover crop has, the more successful it will be in controlling weeds. This is true during establishment – a denser and more robust stand is better able to out-compete the weeds. It is also true during termination because more dead biomass on the ground provides better weed suppression.
“Herbicides have the potential to reduce the growth of the cover crop or at least part of a cover crop mixture. This means less biomass, less ecological services that might be provided particularly as it relates to weed control and weed suppression,” said VanGessel. “Knowing which cover crops you want to plant, which herbicides you’re using and matching it up [means] you don’t set yourself up for failure in terms of getting a good establishment of that cover crop.”
VanGessel also provided a word of caution for growers who may be using cover crops for something other than strictly ecological services, such as grazing or harvesting them as cash crops. He urged growers to understand and follow rotational restrictions that vary widely in commercial herbicides.
by Sonja Heyck-Merlin
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