Considerations for Fall Seeding of Cover Crops Related to Herbicide Use and Weed Control

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

It’s time to be thinking about planting fall cover crops and here are a few things to consider related to weed control and herbicide use (there are other considerations as well).

 

What is the reason for planting the cover crop or what do you want to achieve by planting a cover crop?

There are a lot of reasons for planting cover crops (i.e., erosion control, nutrient management, improving soil structure, adding nitrogen, improve weed management) and cover crops vary in their effectiveness in achieving these objectives. Select the cover crop species (or mixture) based on your intended objectives. The Northeastern Cover Crop Council has developed excellent tools to help select the best species for your goal and your field conditions (https://covercrop.tools/species-selector).

 

What herbicides were sprayed this summer and when?

Typically, microorganisms break down herbicides and they need warm soils, moisture, and time to do so. Consequently, herbicides applied at planting in early May are at less risk of impacting your cover crop than herbicides applied postemergence in early July. Also, the timing of cover crop planting influences how much herbicide residue will be present (for instance early September versus mid October). In general, broadleaf cover crops species are more sensitive to herbicide carryover than grass species. For soybean herbicides, fomesafen (Reflex, FlexStar), pyroxasulfone (Zidua and others), acetochlor (Warrant), chlorimuron (Classic), and sulfentrazone (Authority products) are of highest concern. Corn herbicides at the highest risk for injury are topramezone (Impact), mesotrione (Callisto), isoxaflutole (Balance), and nicosulfuron (Accent). Cover crops like cereal rye, hairy vetch and wheat tend to be the least sensitive to herbicide carryover. The more sensitive species are tillage/forage radish, Austrian winter pea, crimson clover and annual ryegrass.

 

Will the cover crop be used for grazing?

If you intend to use the cover crop for anything other than leaving it on the field and killing before next cropping season, you need to check the herbicide labels for any potential issues. For instance, if you intend to graze the cover crop, the herbicides may have restrictions on how much time there needs to be between application and grazing.

 

When will the cover crop be terminated?

Also consider termination timing in the spring. If you are killing your cover in March, chances are the weather is not going to be as favorable for control than if you spray in late April. Likewise, annual ryegrass is always difficult to control. If you intend to kill your cover crop late in the spring, it is likely to have produced seed, so will that cover crop become a problem in rotation? For instance, hairy vetch can be an issue to control in small grains.

Check out the Northeastern Cover Crop Council website for more information on cover crop management (https://northeastcovercrops.com/)

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