Producers looking to plant cover crops in their corn and soybean fields often ask what to do about their herbicide program. There’s not a one-size-fits-all answer to this question and, unfortunately, there are many unknowns.
However, you can help increase your odds of successfully incorporating cover crops into a corn and soybean system by paying careful attention to:
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Herbicide labels.
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Cover crop selection.
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Research results.
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The timing of herbicide application and seeding.
One challenge when adding cover crops into a corn and soybean production system is herbicides with residual activity may interfere with the establishment and growth of cover crops. Residual herbicides, however, are a key weed management tool, especially for managing herbicide-resistant weeds and combating weeds with extended emergence patterns like waterhemp.
Grazing or harvesting the cover crop for feed or forage
If you plan to graze or harvest the cover crop for feed and forage, then you must follow any rotational or plant-back restrictions listed on the label. The herbicide label is a legal document and instructions must be followed to avoid violating federal law.
There’s more flexibility if the cover crop will not be grazed or harvested. The producer assumes all risk of injury to the cover crop if label restrictions aren’t followed, but there would be no legal issue of trying to sell an adulterated crop because the cover crop isn’t entering the food or feed chain.
Many herbicides have rotational restrictions, which specify the length of time until a crop can be planted in the same field after application.
Rotational restrictions are placed on herbicide labels to protect the food chain from pesticide residues and/or the succeeding crop from injury. If a rotational restriction isn’t listed for a particular cover crop, you must follow the rotational restriction listed for “other crops”.
Here are some resources that list rotational restrictions of many corn and soybean herbicides:
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Herbicide rotation restrictions in forage and cover cropping systems
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Herbicide options for planting forage cover crops after corn and soybean
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.
For cover crops that won’t be grazed or harvested
If the cover crop won’t be grazed or harvested and the cover crop isn’t listed on the herbicide label, consider the following six criteria to help reduce the risk of crop injury while increasing the potential for successful cover crop establishment.
Key criteria
Several universities have been conducting research to understand cover crop sensitivity to herbicide carryover.
Most and least sensitive crops
University of Missouri research conducted in Columbia, Missouri from 2013 to 2015 looked at the general sensitivity of fall-seeded cover crops to carryover from common preemergence and postemergence corn and soybean herbicides.
Of the cover crops evaluated, they found tillage radish was the most sensitive to herbicide carryover, while cereal rye and hairy vetch were the least sensitive. From most to least sensitive, here’s the general order of sensitivity of cover crops to herbicide carryover:
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Tillage radish
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Austrian winter pea
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Crimson clover and annual ryegrass
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Winter wheat and winter oats
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Hairy vetch and cereal rye.
Other findings include:
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Herbicide carryover injury on cover crop species will vary from year to year, largely due to rainfall and time of application.
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Soybean herbicide treatments most injurious to cover crops are fomesafen (Flexstar/Prefix), pyroxasulfone (Zidua), imazethapyr (Pursuit), acetochlor (Warrant) and sulfentrazone (Authority products).
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Corn herbicide treatments most injurious to cover crops are topramezone (Impact), mesotrione (Callisto, Halex GT, etc.) clopyralid (Stinger, SureStart), isoxaflutole (Balance Flexx), pyroxasulfone (Zidua, etc.) and nicosulfuron (Accent Q, etc.).
Similarly, in greenhouse trials conducted by Iowa State University Extension to simulate herbicide carryover in the field, researchers found radish was the most sensitive of the cover crops evaluated while cereal rye was the most tolerant (Table 1).
Table 1: Relative tolerance of several cover crop species to herbicides commonly used in corn and soybean production.
| Herbicide | Group no. | 1X rate | Injury potential: Cereal rye | Injury potential: Oat | Injury potential: Hairy vetch | Injury potential: Lentil | Injury potential: Radish |
|---|---|---|---|---|---|---|---|
| Corn products | -- | -- | -- | -- | -- | -- | -- |
| Atrazine 90DF | 5 | 1.1 pounds | 2 | 2 | 2 | 2 | 2 |
| Dual II Magnum | 15 | 1.5 pints | 2 | 1 | 1 | 1 | 1 |
| Balance Flexx | 27 | 5 fluid ounces | 1 | 1 | 2 | 2 | 3 |
| Callisto | 27 | 3 fluid ounces | 1 | 1 | 1 | 2 | 2 |
| Laudis | 27 | 3 fluid ounces | 1 | 1 | 2 | 2 | 2 |
| Corvus | 2, 27 | 5.6 fluid ounces | 2 | 2 | 2 | 2 | 3 |
| Hornet WDG | 2, 4 | 5 ounces | 1 | 1 | 3 | 3 | 3 |
| Soybean products | -- | -- | -- | -- | -- | -- | -- |
| Classic | 2 | 1 ounces | 1 | 1 | 1 | 1 | 2 |
| Pursuit | 2 | 4 fluid ounces | 1 | 1 | 1 | 1 | 2 |
| Prowl H2O | 3 | 3 pints | 2 | 2 | 1 | 1 | 1 |
| Reflex | 14 | 1.25 pints | 1 | 1 | 1 | 1 | 2 |
Impact on establishment
Researchers at Purdue University have also been studying the impact of herbicide carryover on cover crops. In general, residual herbicides with grass activity can interfere with the establishment of some grass cover crop species, particularly, smaller-seeded ryegrass species.
They also report that residual herbicides in the site of action group 2 (ALS inhibitors), group 5 (triazine family), group 14 (PPO inhibitors) or group 27 (pigment inhibitors) can interfere with the establishment of some broadleaf cover crop species.
Corn herbicide findings
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Pyroxasulfone (Zidua) and metolachlor (Dual, etc.) can hinder ryegrass establishment.
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Atrazine or simazine at more than 1 pound will be problematic for legumes and mustards unless there’s lots of rain. More than 0.75 pounds per acre may allow for most legume cover crops, mustards and annual ryegrass.
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Less than 1 pound per acre of atrazine can allow cereal grain establishment.
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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 herbicide findings
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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 okay.
Estimating sensitivity
If little is known about a species’ sensitivity to the herbicides used or if you can’t find information about a particular cover crop species, looking at information on a close relative can give you a guesstimate of sensitivity.
Planting a mixture of grass and broadleaf cover crops may also help increase the odds of successfully establishing at least one species in the mix.
Herbicides can vary greatly in their persistence, meaning length of time they remain active in the soil. If a product has soil activity, consider the half-life – the time it takes for 50 percent of the active ingredient to dissipate – of all active ingredients when evaluating risk to later-planted cover crops.
Long-lasting herbicides
According to Penn State Extension, examples of herbicides with soil activity and a relatively long half-life include:
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Atrazine: 60 days
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Authority: 32 to 302 days
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Capreno: 50 to 120 days
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Classic: 40 days
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Corvus: 50 to 120 days
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Flexstar: 100 days
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Stinger (in Hornet and SureStart): 40 days
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Pursuit: 60 to 90 days
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Python (in Hornet and SureStart): 14 to 120 days
Factors affecting herbicide persistence
Many factors, however, influence herbicide persistence. Soil pH, organic matter, cation exchange capacity and clay content can increase herbicide persistence.
Microbial activity also significantly impacts herbicide persistence, and is the dominant mechanism of herbicide breakdown or degradation in the soil.
Favorable conditions for microbial activity such as warm temperatures, adequate soil moisture, aerobic conditions, good fertility and medium pH will favor herbicide breakdown.
Herbicide degradation rates will generally increase with increases in temperature and moisture, while cool, dry conditions tend to slow degradation. As a result, dry and/or cool conditions the summer after herbicide application can increase injury risk to a fall-seeded cover crop.
Much of the University research conducted to-date that looks at the sensitivity of cover crops to residual herbicides has been done with fall-seeded cover crops. One can expect that the longer the time period between herbicide application and cover crop seeding, the lower the risk of injury to the cover crop.
As more farmers look at interseeding cover crops into standing corn and soybean, even more attention should be paid to persistence of the herbicide and cover crop sensitivity to the herbicides applied.
Early interseeding of cover crops shortens the time window between herbicide application and cover crop seeding, which may significantly limit herbicide and cover crop choices. Non-residual herbicides may be the best chance for successful establishment when interseeding a cover crop. However, consider the potential short- and long-term risks of reduced weed control.
Reducing herbicide application rates to help reduce the risk of injuring a cover crop is a potential option, but not a recommended practice.
Sub-lethal doses select for tougher-to-control weeds and can result in poor weed control. This can impact cash crop yields and future weed control if weed escapes go to seed and replenish the weed seedbank.
Using the full-labeled rate of an herbicide is also a key tactic to prevent and remedy herbicide-resistant weeds. Many factors impact herbicide longevity in the soil, so using a reduced herbicide rate could result in poor weed control but enough residual activity to hinder a sensitive cover crop’s establishment and growth anyway.
Considering the extent of herbicide-resistant weed populations in many fields across the state, it’s risky to potentially sacrifice long-term weed control by reducing residual herbicide application rates.
Producers often ask about the possibility of increasing cover crop seeding rates if they’re concerned about herbicide carryover.
If the cover crop is marginally sensitive to the herbicide, you may want to consider increasing the cover crop seeding rate. However, there’s no guarantee that a higher seeding rate will result in a higher cover crop stand if the cover crop is sensitive to the herbicide that’s being used.
Higher seeding rates also increase cover crop seeding costs. Factors that influence the longevity and activity of the herbicide in the soil will influence how successful this tactic might be.
No one wants to spend the time and money to interseed a cover crop early in the season to end up unintentionally killing it with an herbicide used for weed control in corn or soybean.
If you wish to apply an herbicide after interseeding a cover crop early in the season, manage the risk by:
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Only selecting an herbicide and cover crop combination that’s allowed on the herbicide label.
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Following the application restrictions listed on the label.
Bradley, K. (2016). The effects of herbicide carryover on cover crops.
Curran, W. & Lingenfelter, D. (2012). Herbicides persistence and rotation to cover crops.
Hartzler, B. & Anderson, M. (2015). Effect of residual herbicides on cover crop establishment.
Lingengelter, D. & W. Curran. (2013). Residual herbicides and fall cover crop establishment.
Reviewed in 2018