Cover crop adoption is continuing to increase throughout Nebraska. Respondents to the 2015 cover crop survey conducted during Nebraska extension meetings indicated the top five desired benefits from cover crops are
- building soil organic matter,
- reducing soil erosion,
- increasing soil water-holding capacity,
- producing forage, and
- improving soil microbial biomass.
While all of these benefits will improve cropping system productivity and sustainability over time, the direct economic value of cover crops is difficult to determine. For this reason, many producers are grazing or haying cover crops to offset seasonal expenses while still capturing the longer term benefits. An important consideration of using these forage cover crops following corn or soybean is that herbicide label rotation restrictions from the agronomic crop MUST be followed to avoid potential carryover to animals consuming this forage. The recommendations in this article are for forage cover crops because the requirements to integrate them in an herbicide program are the more restrictive than for other uses.
Herbicide Label Restrictions for Forage Cover Crops
Most herbicide labels list crop rotation intervals or plant-back restrictions that limit how soon a subsequent crop can be planted after an herbicide application. It is important to note that this is different from the grazing or forage restrictions posted on an herbicide label for crops to which an herbicide is directly applied. When planning a fall-planted forage cover crop following corn or soybean, an herbicide crop rotation interval or plant-back restriction should be short enough to allow the forage cover crop to establish before winter. 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. Meeting these requirements can be challenging as many forage cover crop species are not listed on herbicide labels and as a result replant restrictions fall outside the ideal fall planting window. Hail or other crop damage, seed corn, and silage corn all provide opportunities for earlier forage cover crop planting; however, herbicide plant-back restrictions must still be observed.
Risk of Herbicide Persistence
Herbicide persistence in the soil is affected by several environmental factors and ultimately, herbicide residues can inhibit forage cover crop growth or cause crop failure. Furthermore, herbicide residues can be taken up into forage cover crops and either be directly toxic to livestock or accumulate in their fat, meat, milk, or other tissues and pass into the human food supply. For labeled crops, studies have been conducted to identify safe replant intervals that prevent bioaccumulation of herbicides. In the case of non-labeled 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 it WILL be used as a forage cover crop, planting before the label replant restriction permits violates federal law. An Environmental Protection Agency (EPA) registered herbicide label is a legal document and must be followed, particularly where human and animal safety is concerned.
Plant-back restrictions of forage cover crops will limit herbicides that can be used in corn and soybean cropping systems. Many pre-emergence herbicides that allow fall planting of a forage cover crop have a four-month restriction interval. To compensate for this, consider planting forage cover crops in the earliest planted, sprayed, and harvested fields. Additionally, to avoid selection for herbicide-resistant weeds due to repeated application of the same herbicide, fields in which forage cover crops are planted should be rotated so that a broader range of herbicides can be used during the non-forage cover crop year.
It can be challenging to decipher herbicide label information when deciding which herbicides to use with forage cover crops. Tables 1 and 2 summarize the replant restrictions for several cool-season and warm-season forage cover crop species that may be planted in the fall. Restrictions are listed for common pre-emergence and post-emergence corn and soybean herbicides. When tank-mixing herbicides, follow the most restrictive interval listed on all the labels.
Integrating forage cover crops into your cropping system can provide economic and agronomic benefits. To maximize these benefits, it is important to consider how your herbicide program might impact the use of forage cover crops in your cropping system.
Herbicide information compiled here is intended to be as accurate as possible at the time of publication; however, refer to current herbicide labels for the latest and most detailed information.
(Source – http://www.farms.com/news/herbicide-options-for-planting-forage-cover-crops-after-corn-and-soybean-105115.aspx)