One of the challenges of cover crops is that if they don't winter kill they have to be chemically or mechanically controlled in the spring before planting corn or soybean. In some cases they are controlled well enough with a typical burndown herbicide program that would be used to control a winter annual weeds. Occasionally due to cool wet environmental conditions, late application timing, or contamination of seed the burndown does not completely control the cover crop in the spring.

One question that has been brought up in recent cover crop situations is replant timing when cover crops are planted after the use of residual herbicides used in corn or soybean. There are several plant species being promoted for the use of cover crops. Some of the plants include annual ryegrass, wheat, buckwheat, clovers, radish, cowpea and vetch. Information regarding effects or corn and soybean herbicides on these species is somewhat lacking. Herbicide label rotation restrictions often require substantial waiting periods before these species can be planted. These waiting periods are to assure that residual effects from the herbicide do not impact the following crop, the buyer (finished product) or livestock negatively. In some cases plants used as cover crops fall under the 'other' or 'not listed' category requiring the maximum duration before planting.

However, in recent conversations with the Office of the Indiana State Chemist we have learned the following. If the cover crop is not harvested, used as feed for livestock or sold in anyway and that the cover crop is terminated in the appropriate manner or left in the field, that the rotation restriction does not apply [personal communication Office of the Indiana State Chemist]. It comes down to the definition of a 'crop' and that a 'cover crop' is not really used as a 'crop.' Label rotation restrictions are in place for crops that are going to be harvested, cover crops are to be destroyed in the spring, left in the field or winter killed. The purpose is in having it there, but there is no finished product at the end of the season. However, because the company selling the herbicide does not recommend or approve a rotation other than the ones listed on the label, the company selling the herbicide is not liable for any injury or germination problems seen in the cover crop.

Purdue is presently working on a study that will look at the response of some cover crops to some of the common corn and soybean herbicides. Look for a future article with that data.

The table below list several herbicides used in corn and soybean and their rotation restrictions to a few of the plants used as cover crops. Remember, failure to follow labels can lead to possible injury of desired plants.

Examples of rotation restrictions of several corn and soybean herbicides and a few of the plants used as cover crops in months. See footnote for cover crops.