Herbicides Persistence and Rotation to Cover Crops
If you look at the rotation crop restrictions for corn and soybean herbicides in the Penn State Agronomy Guide (Tables 2.2-17 and 2.4-15), you will see that many products limit rotation to alfalfa and/or the clovers as well as some of the small grains. This is a good place to start when thinking about rotation to fall cover crops. However, these tables are inadequate because these cash crop rotation restrictions may be due to the concern for herbicide residues accumulating in forage or feed rather than carryover injury. 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. 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 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 table below provides some persistence and carryover information for some commonly used corn and soybean herbicides. Some of this information is our best guess and only pertains to the eastern US, not heavy Midwest soils or the western US where soils have high soil pH and rainfall is lower.
Prepared by Bill Curran and Dwight Lingenfelter, Extension Weed Science, Penn State
TitleHerbicides Persistence and Rotation to Cover Crops
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