One of the more common questions that I have been asked recently is whether the corn or soybean herbicide program will pose a problem for seeding fall cover crops. Many products limit rotation to alfalfa and/or clover as well as some of the small grains.

This is a good place to start when thinking about rotation to fall cover crops, but these tables are inadequate and these cash crop rotation restrictions may be due to the concern for herbicide residues accumulating in forage or feed rather than carryover injury, the focus of this article.

If the crop is not going to be harvested and consumed by livestock or humans, then the primary concern is carryover injury and achieving an acceptable stand that provides the benefits of a fall or winter cover. 

Covers such as tillage or daikon radish can be difficult to determine if herbicide residues will cause a problem. If herbicide labels list canola or other annual mustards as rotation crops, these might substitute well for radish. In general, products with a 4 month or less rotation restriction for the species of interest, close relative, or sensitive species (i.e. clovers) should pose little problem.

Some general guidelines: Avoid atrazine or simazine use if at all possible in corn if sensitive crops species and particularly legumes or annual ryegrass will follow. If 1 lb./acre (1 qt) or less of atrazine or simazine has been spring applied, cereal rye and other cereals will likely be OK to establish in the fall for cover.

For legumes and mustards like canola and daikon radish, 0.75 lb/acre (1.5 pt) or less may be OK. Atrazine and simazine will persist longer on higher pH soils (pH 7 or greater) or soils recently limed, so watch surface pH in particular.

Table1 (below) provides some persistence information for some commonly used corn and soybean herbicides. Not only is herbicide half-life important (the time it takes for 50% of the active ingredient to dissipate), but also soil availability. Herbicides with shorter half-lives are always less of a concern.

In addition, species sensitivity can play a role if only a small amount of residue is necessary to cause injury. Until more research has been conducted on herbicide carryover and cover crop establishment, rely on cash crop rotation restrictions along with what we know about herbicide persistence and availability.