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Extension > Agriculture > Crops > Soil management and health > Cover crops > Diversifying herbicides doesn't have to hurt cover crops

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Diversifying herbicides doesn't have to hurt cover crops

Neith Little, Jeff Gunsolus and M. Scott Wells

This question comes up often when talking about cover crops: Should I worry about carryover injury from herbicides used on the previous crop?

For farmers who rely on glyphosate (SOA 9) for weed management, carryover injury to the following crop is not a concern, because glyphosate does not have residual activity – it does not kill plants that emerge after it has been applied. However, the spread of herbicide resistant weeds is prompting many Extension weed scientists to encourage growers to diversify the herbicides they use.

Diversifying corn and soybean herbicides

Jeff Gunsolus, University of Minnesota Extension weed scientist, and Rich Zollinger, from North Dakota State University, have published a set of tables (z.umn.edu/herbicidediversification) of preemergence (PRE) and postemergence (POST) herbicide options for diversifying the herbicide sites of action (SOA) used to control weeds in corn and soybean. These tables include rotation intervals for common crops, which can also suggest which related cover crop species are likely to be sensitive to the listed herbicides.

PRE herbicides work by using active ingredients with residual activity in the soil. This residual activity makes it important to consider how PRE herbicides might impact the following crop. Pre-mixed products that contain multiple active ingredients and tank mixes of multiple products require special attention, because the label restrictions for each product and the residual activity of each active ingredient must be considered.

Carryover concerns for the following crop

The first question to consider is whether the following crop is intended for use as forage or feed, or whether it is being planted solely as a cover crop.

Forage or feed as the following crop

Grass cover crop growing in Iowa

Photo: Lynn Betts, USDA-NRCS

Photo 1. A grass cover crop growing in Iowa.

If the following crop is intended for use as forage or feed, strict rules apply to prevent herbicide residue from contaminating the feed. If there is any chance the crop might be used as emergency forage, label rotation restrictions must be followed. University of Wisconsin Extension has a helpful publication on this issue, Herbicide rotation restrictions in forage and cover crop systems. See also Herbicide options for planting cover crops after corn and soybean from University of Nebraska-Lincoln.

In contrast, most herbicide labels are silent on rotational planting of cover crops not intended for forage (Hornet and Python WDG are two exceptions). When planting a cover crop in the fall, the main concern is whether any residual herbicide activity will damage the crop enough to hurt establishment. Unless specifically prohibited by the product label, a farmer can choose to plant a cover crop after any herbicide program, if the cover crop will not be used for forage and if the farmer is willing to assume the risk that the cover crop may fail.

Assessing herbicide carryover risk

To assess the risk of carryover damage, consider the following questions:

  1. What herbicides were applied? When? At what rate? Well-kept records are valuable!
  2. How long is the herbicide active in the soil after application?
  3. How sensitive to the applied herbicides are the cover crops you are considering planting?
  4. Since herbicide application, have the weather conditions favored herbicide degradation (high rainfall and warm temperatures) or has the weather favored herbicide persistence (drought and cool temperatures)?
  5. Do the soil characteristics contribute to longer herbicide persistence (high pH or fine-textured, "heavy" soil)?

Two publications are particularly useful when considering the above questions:

Cover crop and herbicide examples

As an example, let's walk through how one might use the information in those articles to assess the risk of cover crop damage from two common PRE herbicide premixes. Products mentioned are used only as an example, and are not an endorsement of specific products.

Example 1

Lumax EZ is applied as a PRE in corn, and is a premix of the active ingredients mesotrione (SOA 27), s-metolachlor (SOA 15), and atrazine (SOA 5). Looking at Penn State' table, atrazine poses the most risk out of the three of causing carryover damage, because it has a half-life of 60 days under normal conditions (longer in high pH soils), and because both grass and broadleaf cover crops are sensitive to its effects (Table 1). Similarly, in Iowa State's experiment, atrazine was observed to cause some damage to all five cover crops studied. Based on this information, concern is warranted that a cover crop might sustain enough damage to hurt establishment if planted after a corn crop treated with Lumax EZ.

Table 1. Estimated half-lives, cash crop restrictions and potential to injure fall cover crops from active ingredients contained in Lumax EZ1.


Trade names

Active ingredient
Half-life
Cash crop restrictions
Fall cover crops
SOA# (days) OK to plant Concern for Other
Atrazine 4L, AAtrex atrazine 5 60 Can plant corn, sorghum and soybean following year (some products allow others) Sorghum species Cereals, ryegrass, legumes & mustards More persistent in high pH soils (>7.0); Low rates (<1 lb a.i./A) may allow more flexibility
Callisto mesotrione 27 5 – 32 10 to 18 months for legumes & vegetables All grasses Small-seeded legumes, mustards Sequential applications (PRE followed by POST) increase injury potential
Dual II Mag 7.62E, Cinch s-metolachlor 15 15 – 50 Labeled for use on many crops Almost anything Annual ryegrass or other small-seeded grasses Higher rates and later applications more of a potential problem
1Adapted from Curran and Lingenfelter (2012).
Always follow the product's current label restrictions and instructions.
Example 2

Optill is applied as a PRE in soybeans and is a premix of saflufenacil (SOA 14) and imazethapyr (SOA 2). Saflufenacil has a relatively short half-life under normal weather and soil conditions. Imazethapyr has a long half-life, but it is ranked in Penn State's table as having low risk for damage to several common cover crops. This prediction was supported by Iowa State's experiment. There is always some risk, especially under dry, cool conditions, but based on this information, a rye cover crop planted after an Optill application in soybeans would have a relatively low risk of sustaining enough damage to prevent successful establishment.

Table 2. Estimated half-lives, cash crop restrictions and potential to injure fall cover crops from active ingredients contained in Optill1


Trade names

Active ingredient
Half-life
Cash crop restrictions
Fall cover crops
SOA# (days) OK to plant Concern for Other
Sharpen 2.85SC saflufenacil 14 7 – 35 Any crop can be planted 4 mo. after application All None This product reported to be more persistent in western Canada.
Pursuit 2S imazethapyr 2 60 – 90 Recrop restrictions range from 4 to 18 mo. Wheat, triticale, rye, alfalfa, clover Oats, sorghum, mustards Any crop can be planted 40 mo. after application
1Adapted from Curran and Lingenfelter (2012).
Always follow the product's current label restrictions and instructions.

Summary

Minimizing selection for herbicide resistance and protecting soil from erosion are both important management goals. The keys to making sure those goals do not conflict are to keep good records of your weed management program and to use the best available information to assess the risk of carryover damage. If you know you want to plant a cover crop in the fall, keep that in mind as you make weed management decisions. If you have already applied an herbicide earlier in the season, consider the susceptibility of different crops when you choose which cover crop to plant.

DISCLAIMER: The label on the product you are using always supersedes information in this article. Always refer to the product label for specific instructions and restrictions.

For more information, see Managing risk when using herbicides and cover crops in corn and soybean.

2016

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