With silage harvest well underway and corn grain and soybean not far behind, a number of farmers are considering cover crop establishment this fall. Remember that some herbicides can persist and potentially influence successful cover crop establishment.
Both the 2015/16 Agronomy Guide (Table 1.10-6) and the 2016 Mid-Atlantic Weed Management Guide (Table 1-5), contain a table titled “Corn and soybean half-lives, cash crop restrictions, and their potential to injure fall cover crops” that includes the major corn and soybean herbicides restrictions and guidelines.
Remember that cash crop rotation restrictions may be due to the concern for herbicide residues accumulating in forage or feed rather than carryover injury. Cover crops that are not harvested can be planted after any herbicide program, but the grower assumes the risk of crop failure.
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.