Herbicide application to corn from Hagie sprayer Hagie
TO LATE NOW? Selecting cover crop-compatible herbicides begins long before crop spraying — or a soil assay may be needed.

Choose herbicides now with cover crops in mind

Growing Healthy Soil: Herbicide half-life is biggest factor to consider when adding cover crops to cash crop rotations.

Adding cover crops to your cash crop rotation should cause you to rethink weed control options. With the steady rise of cover crop use, it’s somewhat surprising this hasn’t come up more often. Now’s the time to strategize it, not after spring planting.

Complicating this issue is that some herbicide manufacturers haven’t tested their products in the context of cover cropping. Reason: It’s a significant research investment. Fortunately, you can use basic knowledge of how herbicides work and the types of weeds they control to get a good idea.  

3 key factors 
Consider these factors when “shopping” for herbicides:

Herbicide half-life. How long a product persists in the soil is critical. For instance, Verdict, the corn and soybean herbicide combination of Outlook and Sharpen, has a very short half-life. It’s unlikely to negatively affect a fall-planted cover crop. On the other hand, Reflex, a soybean herbicide, has a very long half-life. It’s being resurrected from 20 years ago due to its ability to control weeds now resistant to newer chemistry. If applied to soybeans, it’ll probably hold back fall-seeded cover crops, especially brassicas.  

Rainfall. It’s another significant factor. More rain will dilute the chemical quicker; less rain may allow it to linger longer.  

Organic matter. A soil with high organic matter levels will degrade a herbicide faster. But regardless of rainfall or organic matter, half-life is the dominate factor — one you can have control over by selection.

I’m most familiar with Penn State research on interaction of herbicides and cover crops. Much of it relates to interseeding cover crops in corn. Check out extension.psu.edu/evaluation-of-residual-herbicides-for-interseeding-in-corn, or check with your local Extension Service to see if they have evaluated herbicide effects on cover crops.

Recipe for success 
The best way to really know if a herbicide is still active in the soil is to take a soil assay. Here’s a simple process to determine if a herbicide residue will stress a succeeding cover crop:

• Collect at least three representative soil samples about 2-inches deep. Place that soil in a small pot and add cover crop seeds that you plan to plant.

• Water as needed and observe seedling health in two weeks. If they sprout and then die, this field probably wouldn’t be worth the risk of planting a cover crop.

• If seeding a mix of cover crop species, observe any differences. Some species may be more herbicide-susceptible than others. If they look normal, you’re good to go. A bit of perspective is warranted, though, if young shoots seem slightly affected.

By the time that a cover crop seedling is 2 weeks old, its roots are probably more than 4 inches into the soil. In most cases, no residual herbicides will be at that depth, and the cover crop will take off and grow normally. To put it another way, signs of a little herbicide damage within a soil assay may not preclude you from planting that cover crop. If the cover crop survives your test, it’s at little risk.    

Some herbicide manufactures are starting to list general cover cropping information on their labels. Company representatives are also getting up to speed and can be a great regional source for understanding which herbicides will be compatible in rotations including cover crops. 

The Coach’s Closer
Understanding how herbicides work, strategic planning and a soil assay (if needed) will help you successfully manage weed control along with your cover crops.

Groff is a cover crop pioneer and innovator who farms on the Chesapeake Bay watershed. Check out his website, covercropcoaching.com. 

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