Two factors become important when trying to predict the potential for carryover injury to rotational crops. 1.) How long does the herbicide last or persist in the soil assuming that it has soil activity, and 2.) How sensitive is the rotational crop to potential herbicide residues? Herbicides with shorter half-lives (the time it takes for 50% of the active ingredient to dissipate) are always less of a concern. Of course several factors influence the rate of dissipation such as rainfall, soil texture and soil pH, etc., however, most guidelines generally are for “normal” conditions (e.g. not severe drought). 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. These products typically have half-lives of less than 30 days. Species sensitivity can play a role if only a small amount of herbicide residue is necessary to cause injury and the herbicide persists. Quite often, small seeded legumes and grasses like the clovers and ryegrass and mustard species like canola are very sensitive to some herbicides. The good news is that for most winter cereals like rye or wheat, most products are fairly safe. The following table includes some “highlights” for products that can have the potential to be more problematic. For a more complete listing, refer to the Penn State Agronomy Guide or the Mid-Atlantic Weed Management Guide.
Selected Corn and Soybean herbicide half-lives and their potential to injure fall cover crops. Herbicides included below are generally more restrictive.