When adding cover crops into a corn and soybean production system, one challenge is herbicides with residual activity may interfere with the establishment and growth of cover crops.
However, residual herbicides are a key weed management tool, especially for managing herbicide-resistant weeds and combating waterhemp and other weeds with extended emergence patterns.
By paying careful attention to herbicide labels, cover crop selection, research results and the timing of herbicide application and seeding, you can help increase your odds of successfully incorporating cover crops into corn and soybean systems.
Grazing or harvesting cover crops for feed or forage
Any rotational or plant-back restrictions listed on the label must be followed if you graze or harvest cover crops for feed or forage. The herbicide label is a legal document and instructions must be followed to avoid violating federal law.
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, the rotational restriction listed for “other crops” must be followed.
More about rotational restrictions of corn and soybean herbicides:
As always, refer to the product label for the most up-to-date information.
There’s more flexibility if the cover crop won’t be grazed or harvested. The farmer 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.
Rotational restrictions listed on the label must be followed if the cover crop will be grazed or harvested. If the cover crop will not be grazed or harvested, a farmer has more flexibility. In this case, however, the farmer assumes all risk of injury to the cover crop if label restrictions are not followed.
If you won’t graze or harvest the cover crop
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.
Several universities have been conducting research to determine how sensitive cover crops are to herbicide carryover.
Research: University of Missouri
Research conducted by the University of Missouri evaluated the sensitivity of fall-seeded cover crops to postemergence corn and soybean herbicides in Columbia, Missouri from 2013 to 2015.
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.
Following is a summary of research:
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Herbicide carryover injury on cover crop species varies from year to year, largely due to rainfall and time of application.
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The evaluated cover crops’ sensitivity to herbicide carryover, from most to least sensitive are: Tillage radish, Austrian winter pea, crimson clover and annual ryegrass (tie), winter wheat and winter oats (tie) and hairy vetch and cereal rye (tie).
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Soybean herbicide treatments most injurious to cover crops, of those evaluated, 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, of those evaluated, are: Topramezone (Impact), mesotrione (Callisto, Halex GT, etc.), clopyralid (Stinger, SureStart), isoxaflutole (Balance Flexx), pyroxasulfone (Zidua, etc.) and nicosulfuron (Accent Q, etc.).
Research: Iowa State
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 shows herbicides commonly used in corn and soybean production. Injury potential ratings are based on greenhouse trial: 1 = little or no risk; 2 = some risk depending on herbicide rate and environmental factors; 3 = high potential for injury affecting cover crop establishment.
Table 1: Relative tolerance of cover crop species to herbicides
| Herbicide | Injury potential: Cereal rye | Injury potential: Oat | Injury potential: Hairy vetch | Injury potential: Lentil | Injury potential: Radish |
|---|---|---|---|---|---|
| Corn products | -- | -- | -- | -- | -- |
| Atrazine 90DF | 2 | 2 | 2 | 2 | 2 |
| Dual II Magnum | 2 | 1 | 1 | 1 | 1 |
| Balance Flexx | 1 | 1 | 2 | 2 | 3 |
| Callisto | 1 | 1 | 1 | 2 | 2 |
| Laudis | 1 | 1 | 2 | 2 | 2 |
| Corvus | 2 | 2 | 2 | 2 | 3 |
| Hornet WDG | 1 | 1 | 3 | 3 | 3 |
| Soybean | -- | -- | -- | -- | -- |
| Classic | 1 | 1 | 1 | 1 | 2 |
| Pursuit | 1 | 1 | 1 | 1 | 2 |
| Prowl H2O | 2 | 2 | 1 | 1 | 1 |
| Reflex | 1 | 1 | 1 | 1 | 2 |
Research: Purdue University
Researchers at Purdue University also have been studying the impact of herbicide carryover on cover crops.
Researchers report that, 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 following sites of action can interfere with the establishment of some broadleaf cover crop species:
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Group 2: Acetolactate Synthase (ALS) inhibitors
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Group 5: Triazine family
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Group 14: PPO inhibitors
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Group 27: Pigment inhibitors
Following are the results to-date.
Corn herbicides
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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 per acre will be problematic for legumes and mustards unless rainfall is high. Less than 0.75 pounds per acre may allow for establishment of most legume cover crops, mustards and annual ryegrass.
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Atrazine at less than 1 pound per acre 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 herbicides
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Chlorimuron (e.g., Classic), Imazethapyr (Pursuit) and fomesafen (Reflex, Flexstar) could be a problem for fall-seeded legume or mustard covers, including radish. However, establishment of cereal grains should be okay.
General guidelines
If little is known about a species’ sensitivity to the herbicides used or you can’t find information on a particular cover crop species, 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 (length of time they remain active in the soil). If a product has soil activity, consider the half-life (time it takes for 50 percent of the active ingredient to dissipate) of all active ingredients when evaluating risk to later-planted cover crops.
According to Penn State Extension, 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 that influence 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 has a significant impact on herbicide persistence, and is the dominant mechanism of herbicide breakdown or degradation in the soil. Conditions favorable 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 as temperature and moisture increase, 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.
To-date, much of the University research conducted on cover crops’ sensitivity 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 the herbicide’s persistence and the cover crop’s 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.
When interseeding a cover crop, non-residual herbicides may be the best chance for successful establishment. However, you should 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 this isn’t a recommended practice.
Sub-lethal doses select for tougher-to-control weeds. This can result in poor weed control that impacts cash crop yields and future weed control if weed escapes go to seed, replenishing the weed seedbank.
Using an herbicide’s full-labeled rate also is a key tactic for preventing and remediating herbicide-resistant weeds. Many factors impact herbicide longevity in the soil (as previously discussed), 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 can be a risky proposition to potentially sacrifice long-term weed control by reducing herbicide application rates of residual herbicides.
Crop producers often ask about the possibility of increasing cover crop seeding rates if herbicide carryover is a concern.
Increasing the cover crop seeding rate is a potential option to consider if the cover crop is marginally sensitive to the herbicide used. However, there are no guarantees that a higher seeding rate would result in a higher cover crop stand if the cover crop is sensitive to the herbicide used.
Higher seeding rates also increase cover crop seeding costs. Factors that influence the longevity and activity of the herbicide in the soil (as described previously) 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 then 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, you can best manage risk by only selecting the herbicide and cover crop combinations allowed on the herbicide label and following any application restrictions listed on the label.
Reviewed in 2018