Check Herbicide Restrictions before Planting and Using Cover Crops

Unplanted acres with corn residue
Figure 1. Where flood-damaged fields prevented timely planting, growers are considering planting cover crops to help stabilize the soil and provide grazing or forage. (Photo by Jenny Rees)

Check Herbicide Restrictions before Planting and Using Cover Crops June 28, 2019

With our challenging early spring weather across Nebraska and late spring flooding, many acres across the state will be claimed for prevented planting. With prevented planting, using cover crops is a good option. Cover crops can help anchor the soil, and provide weed suppression, grazing, and haying opportunities. Cover crops may also be considered in the event of hail, flash floods, or tornado damage.

Corn Approved as a Cover Crop

When previously applied herbicides limit your cover crop options, corn may be just what you need. See NRCS recommendations.

Change in Prevented Planting Grazing/Haying Date

In regards to the grazing and haying opportunities, changing the allowable use date from November 1 to September 1 opens up more forage options. If a pre-emergence herbicide was not applied, there are more planting options for warm-season annual forages for grazing, haying, and silage. A story, Annual Forage Options for July or August Planting, in the July BeefWatch offers further information on various options.

Check Herbicide Restrictions

Cover crops offer many benefits; however, including them in a rotation adds another layer of complexity that may be unfamiliar. Specifically, look at labels for herbicides applied in the previous growing season or any pre-emergent/post-emergent application earlier this season. The biggest things to look for on the label are:

  1. plant back interval for the specific cover crop and
  2. the use rate related to plant-back intervals.

Rotation restrictions or plant-back intervals for herbicides will be key when selecting a cover crop to plant. These intervals indicate the length of time after herbicide application that another crop can be planted. It is important to note that this is different from the grazing or forage restrictions posted on a herbicide label for crops to which a herbicide is directly applied. If the forage cover crop species you intend to plant is not listed on the label, you must follow the rotation restriction listed for “other crops.” In addition, the rotation restriction for a forage cover crop mixture is dictated by the species in the mixture with the longest restriction.

For labeled crops, studies have been conducted to identify safe replant intervals that prevent bioaccumulation of herbicides. In the case of non-labeled crops such as many cover crops or “other crops,” these tests were not conducted. In this case, labels list the longest plant-back restriction where herbicide companies can be confident potential herbicide carry-over effects are negligible. Without crop-specific tests, the risk for herbicide toxicity to animals consuming cover crop forage or herbicide residues in human food is unknown.  

As a point of clarification, if cover crops will NOT be harvested or grazed, they can be planted before the label replant restriction allows. Growers would assume the risk for cover crop failure. However, if cover crops are to be used as a forage, planting before the label replant restriction date violates federal law. An Environmental Protection Agency (EPA) registered herbicide label is a legal document and must be followed.

For example, many pre-emergence herbicides for corn and milo will injure teff, pearl millet, and foxtail millet. Sudangrass, forage sorghum, and sorghum-sudan hybrids will tolerate moderate levels of atrazine. If several other herbicides have been applied, safened seed may be advised. These sorghums also tolerate most herbicides labeled for use with grain sorghum.

Soybean herbicides that have residual soil activity can cause even bigger problems for replanting to forages. All summer grasses are sensitive to most soybean herbicides. Planting sunflowers for silage or replanting soybeans for hay or silage are among the few alternatives compatible with soybean herbicide carryover. For example, Fierce® herbicide would have no planting restrictions for soybean, up to 7 days for corn, and up to 4 months for sunflower with essentially no other options for July/August seeded forage.

Another possible emergency forage crop is short-season corn as silage or as late season pasture, especially if corn and/or soybean herbicides eliminate other possibilities. If you are considering this, the first step is to contact your crop insurance agent. Then go to your Farm Service Agency (FSA) office and fill out their form for failed crop and/or prevent plant. There may be additional considerations you need to follow from NRCS regarding the use of corn for silage as a forage crop in prevented planting acres.

Several websites post herbicide labels, including CDMS and Greenbook.net. In addition to herbicide labels, you can view restrictions for crop rotation restrictions in the 2019 Guide for Weed, Disease, and Insect Management in Nebraska (pg. 178-191) or in the excerpted table, Replant Options and Rotation Restrictions.

Examples

Following are specific examples to help walk a grower through reading herbicide labels for restrictions.

Corn
  • If corn was planted this year and Acuron® was applied pre-emergence before the field was flooded out, options for planting forage crops would be minimal because the replant interval for most cover crops is four months or more. There would be no restrictions on corn used for silage after Sept. 1, 2019.
  • If corn was planted, sprayed post-emergence with Harmony® SG, and then the field was flooded out, the herbicide label indicates there would be no plant-back restrictions for soybean, corn, grain sorghum, wheat, barley, oat, and triticale. The grower would need to wait 45 days to plant any “other crops” not listed, including cereal rye, turnip, and radishes.
Soybean
  • If soybean was planted this year and Authority® First or Sonic® pre-emergence was applied before the field was flooded out, there are no options for planting summer forage crops including corn because planting interval is more than four months. Wheat and rye could be options after four months.
  • The rate of the product used can also determine the plant-back interval or restriction. For example, Dual II Magnum® applied pre-emergence in soybean that were then drowned out has a plantback interval of 4 ½ months for oat, rye, and wheat. Radishes and turnips can be planted 60 days after Dual II Magnum® was applied. However, both applications are dependent on the rate of Dual II Magnum® used. If the rate of 1.33 pints/acre of Dual II Magnum® was applied, then all of the above options would hold true. However, if the application was made at a higher rate, the radishes and turnips cannot be planted until the following spring based on the label.
  • If glyphosate (such as Roundup PowerMAX®) is the only herbicide applied in glyphosate-resistant corn or soybean, any cover crop species can be planted without any planting interval.

No-Crop Situation

If no crop has been planted this year, the grower may need to check which herbicide was applied the previous year and determine the planting interval of cover crop to be planted this year. This information can be found here. Few herbicide labels suggest to conduct a bioassay test to determine if it is safe to plant a cover crop; however, it is only applicable if the intent of cover crop is NOT for forage or grain harvest. A bioassay is a technique for determining if herbicide residues are present and bioavailable in soil at high enough concentrations to adversely affect plant growth. A small area in the prevented planting can be planted with the cover crop species of interest and based on its emergence and initial growth, remaining field can be planted.

Conclusion

If you find yourself faced with limited replant options after recent weather events, a forage crop may offer benefits. Just be sure it's compatible with your herbicides and livestock. Considering previous herbicides first can save a headache, money, and cover crop failure. Remember to always use the label as the final determinant for plant-back recommendations. Also check with your crop insurance agent before taking any steps in fields covered by crop insurance.

The herbicide examples used here are to illustrate differences between plant back intervals and to guide the user through the process of reading these on the label. Always read and follow the label directions directly. For clarification on label verbiage, please contact your local extension office, agronomist, or chemical representative.

Additional Resource

Herbicide Options for Planting Forage Cover Crops after Corn and Soybean. Amit Jhala, Daren Redfearn, Bruce Anderson, Mary Drewnoski, and Chris Proctor. 2016.