Herbicide influence on cover crop establishment

by Sara Berges, Allamakee SWCD Project Coordinator

Cover crops have received increased interest in the last few years due to their many soil health benefits. There has also been increased utilization of herbicides with residual activity to allow them to have long-lasting impact on weed management. However, this may also interfere with the establishment and growth of cover crops. When planning cover crops for the fall, it is important to consider the weed management program that will be used.

The first question to consider is whether or not you plan to graze the cover crop or harvest it for feed. If you answer “yes” to this question, then you, by law, must follow the rotational restrictions listed on the chemical label.  However, many cover crops are not listed in the rotational restriction information. If that is the case, the rotational restriction for “other crops” must be followed.

There are several publications available that can be used as resources. The University of Wisconsin Extension has a publication titled “Herbicide Rotation Restrictions in Forage and Cover Cropping Systems.”  ISU Extension has one titled “Herbicide Use May Restrict Grazing Options for Cover Crops.” Cover Crop Solutions utilized research from Penn State University to develop the guide titled “Herbicide Persistence and Rotation to Cover Crops.”  All of these publications can be found by searching online.

If you do not plan to graze or harvest the cover crop, you are less restricted regarding what product you use. However, you assume the risk of cover crop failure even if the crop is not listed on the herbicide label. Many factors influence the persistence of the herbicide in the soil including soil type, soil pH, rainfall, amount of organic matter and temperature. Dry and/or cool conditions the summer after herbicide application can increase injury risk to a fall-seeded cover crop. The likelihood of injury increases if interseeding the cover crop into standing corn or soybeans because the time between herbicide application and cover crop seeding is less.

Herbicide labels often do not list commonly used cover crops. Therefore, many universities and organizations have been evaluating the impact of commonly used herbicides on the establishment of many cover crop species.  In general, they found that cereal rye is fairly tolerant of most herbicides, depending on rate used. However, ryegrass has the potential to be affected by herbicides that have activity on grass weeds.

Many brassicas (radishes, turnips, etc.) and legumes are sensitive to herbicide damage. If you plan to use a brassica or legume, it is suggested that you plant a mix containing a grass. Pay attention to herbicide labels and cover crop restrictions to increase your odds of successful cover crops establishment.