Seeding Cover Crops Following Residual Herbicides in a Dry Year

Even with some recent rainfalls across the state, many areas across Indiana are still experiencing a drier than normal planting season (Figure 1). While the primary concern is receiving enough rain this month to have a good finish for our cash crops, many cover croppers have reason for concern about establishing cover crops. We are only a few weeks away from the primary establishment period for cover crops across the state, so we wanted to post a reminder about some herbicide/cover crop combinations that can be problematic. Here is a summary of research conducted at Purdue and other Midwest Universities about cover establishment following corn and soybean residual herbicides:

  • Corn herbicides
    • Pyroxasulfone (Zidua) and metolachlor (Dual, etc) can hinder annual ryegrass establishment.
    • Atrazine or simazine at > 1 lb/A will be problematic for legumes and mustards unless lots of rainfall occurs after application.
      • < 0.75 lb/A may allow for good establishment of most legume cover crops, mustards, and annual ryegrass.
      • Atrazine < 1 lb/A can allow cereal grain establishment. We have observed cereal rye survival with atrazine rates as high as 1.5 lb ai/A if we have near normal precipitation patterns.
    • Mesotrione (Callisto, Lumax, Lexar etc.), flumetsulam (Python) and clopyralid (Stinger, Hornet, SureStart) can be problematic for legumes and mustards like canola and forage radish.
  • Soybean herbicides
  • Chlorimuron (Classic, Canopy, Cloak, etc.), imazethapyr (Pursuit), and fomesafen (Reflex, etc.) could be a problem for fall seeded legume or mustard covers including radish. However, establishment of cereal grains should be OK.

As a general rule of thumb, the later an application is made, and the higher rate of herbicide applied will lead to more establishment issues. These problems become worse when we do not receive much rainfall after application. If any of the herbicides listed above were used postemergence, we would expect more herbicide to still be available in the soil and hinder cover crop establishment.

If you have questions about specific situations, one way to address the residual herbicide left in a field is to do a bioassay.  Simply collect soil from the area you would like to seed the cover crop into and an area with a similar soil type, but no herbicide residue, and plant seed from the cover crop you would like to use. Observe growth for 3 weeks and if the plants look the same in the untreated and treated soil, you should be safe to plant to desired crop. Another consideration if you do not have time to do a bioassay is to plant a cover crop mixture. Cover crop establishment may be more reliable when mixtures of grass and broadleaf species are purchased and planted. Residual herbicides may interfere with establishment of some species in the mix, but have no effect on other species. The use of mixtures may allow one more protection from complete failure due to excessive residues in the soil. It would be important however to be sure that at least one or two of the species in the mixture is tolerant to the herbicides used in a specific field.

For a more detailed explanation of residual herbicides and cover crops, please see our earlier article that covered this issue (https://extension.entm.purdue.edu/newsletters/pestandcrop/article/residual-herbicides-and-fall-cover-crop-establishment/).

Figure 1. Rainfall across Indiana from May 1 until August 1 2018. In general, the yellow areas have received less precipitation than normal, while the green areas have received more precipitation than normal.

Figure 1. Rainfall across Indiana from May 1 until August 1 2018. In general, the yellow areas have received less precipitation than normal, while the green areas have received more precipitation than normal.

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