By Meaghan Anderson

Many farmers have added fall- or interseeded cover crops to their corn and soybean fields in recent years. Increased adoption of cover crops has resulted in a significant increase in questions and concerns regarding herbicide product choice and use patterns in row crops that will be safe for planting summer-seeded or fall-seeded cover crops.

Now is the time to consider how herbicide selection can impact your cover crop options. Let’s look at a few tips to choose herbicides that will minimize potential harm to the cover crop investment:

Check the product label first
Most herbicide labels were not developed with cover crops in mind. This makes choosing herbicides for compatibility with cover crops difficult. Each herbicide label has a rotational crop section that describes the time interval required between herbicide application and planting of specific crops. The interval is based primarily on the sensitivity of a crop to the herbicide, but other factors are often considered.

With significant dollars invested in development of herbicide labels, companies must carefully choose which species they include in the rotational crop section of a label. Those crops are likely to be the crops commonly grown in rotation with the labeled crop. This means many popular cover crop species are excluded from the rotational crop section of herbicide labels, because until recently, they were not commonly used in rotation with corn and soybeans.

Checking the herbicide label rotational crop section can provide some insight as to how sensitive a listed crop might be to the herbicide. This is especially useful if you are planting a cover crop like cereal rye or wheat, as those species are often listed on herbicide labels. If the rotational interval to planting these species is short (less than nine months), it’s likely they could be planted in the fall with little concern for safety.

Researching beyond label
Since the rotational crop section of a label will not provide you all the information you might need, you should use resources that describe how persistent herbicides are and how sensitive cover crops will be to those products. Herbicide persistence is affected by many factors including rainfall following application, soil texture, soil pH and organic matter content. 

Herbicide half-lives are often used as an indicator of the persistence of a given herbicide in the soil. A half-life describes the approximate time required for the herbicide to degrade to one-half of the original amount applied to the soil; each subsequent half-life degrades the amount by one-half again. The half-life varies with soil type and weather, but it can give you an idea of the persistence of a product and is useful in comparing herbicides.

Half-life is the best indicator of persistence, but it does not give you any information about how sensitive a given cover crop species will be to the herbicide. Cover crops will vary widely in their sensitivity.

Several universities have tested common cover crop species for sensitivity to herbicides and found that radish is the most sensitive, while cereal rye is quite tolerant to most herbicides.

Generally, brassicas and small-seeded legumes and grasses are much more sensitive to herbicides than their larger-seeded counterparts. Penn State University has the most comprehensive list of herbicides and their half-lives available for cover croppers, in its 2018 Mid-Atlantic Field Crop Weed Management Guide.

Another resource is representatives from the company manufacturing the herbicide you plan to use. Those companies likely have agronomists who can provide insight about potential injury to other species from their products.

Testing soil yourself

Even after taking precautions to use herbicides that will be relatively safe with cover crops, concerns can arise. This generally happens after unusual weather patterns, like drought conditions, where more herbicide may persist in the soil than in a normal year. Whether you plan to seed cover crops in the summer or in the fall, a final test can help provide peace of mind prior to seeding.

Farmers can do a simple evaluation called a herbicide bioassay on their own farm prior to seeding cover crops in the summer or fall to help determine if they will be injured by a spring-applied herbicide. The steps to do this are quite simple, but require a little bit of planning and work.

1. Collect soil from multiple, representative areas of a field. Individual samples should be taken from the upper 2 to 3 inches of soil and mixed thoroughly.

2. Find a similar soil that had no herbicide applied to use as a control and collect it in the same way as the soil of concern.

3. Mix each soil sample thoroughly and place in pots or flats, making sure to keep the control soil separated from the soil of concern.

4. Seed several cover crop seeds of your species of interest into both the control soil and soil of concern. If a cover crop mix will be seeded in the field, make sure to collect enough soil to check every species.

5. Place the soils in a sunny location, preferably outside; water them regularly to keep them moist but not soaked.

6. Monitor the seedling growth and development for at least two weeks but preferably four weeks. Cover crop seed will almost always germinate but may die or have stunted growth after emergence.

By planning herbicide programs with cover crops in mind, we can reduce potential injury to those cover crops, allow for better growth of the species and improve benefits associated with the cover crop. If necessary, there are herbicide bioassays that can be a final test for potential problems from herbicides prior to seeding across tens or hundreds of acres. Contact your local ISU Extension field agronomist with additional questions or concerns.

Anderson is the ISU Extension field agronomist for east central Iowa. Contact her at [email protected].