Herbicide residuals are a double-edged sword. While residuals are incredibly important for weed control and reducing herbicide resistance in weeds, addressing potential herbicide carryover can ensure proper winter cover crops establishment.
For soybean and corn growers who want to use cover crops in their rotations, several factors should be considered when choosing the right herbicides, cover crops and rotation patterns.
Retailers can help growers balance the fine line between effective weed control and successful cover crop establishment.
What is herbicide carryover?
Herbicide residuals help farmers reduce their spray costs by extending the window of control. Often, herbicides with effective residual activity will persist in the soil and remain active to control any unwanted plants that may emerge during that window of coverage. This is especially valuable in corn and soybean systems where growers are managing tough, competitive weeds and trying to reduce selection pressure for herbicide resistance.
Herbicide carryover occurs when the herbicide used in one season remains active in the soil long enough to injure a later crop. The risk of carryover can be hard to predict because many herbicide labels do not provide rotational intervals for every cover crop species a grower may want to plant.¹
Retailers can help growers close that planning gap by reviewing the herbicide products used, application rates, application timing, label restrictions, field conditions and intended cover crop species before seed is purchased or planted.
Variables that affect herbicide carryover impacts
Herbicides degrade at different rates depending on soil conditions and weather. Dry soils after application can slow herbicide breakdown, while soils with either extremely high or low pH can increase the persistence of some chemistries.²
Other factors, such as low organic matter or coarse-textured soils, can increase the risk of herbicide carryover injury to planted crops. Cool conditions can also slow microbial degradation, which may allow some herbicides to remain active in the soil longer than expected.³
Help growers assess these factors in their situation because the same herbicide program may behave differently from field to field and from year to year.
Match cover crop species to minimize herbicide carryover issues
Cover crop sensitivity varies by species, herbicide chemistry and field conditions, so a species that works well after one herbicide program may be a poor fit after another.
In general, cereal grains such as cereal rye, wheat and oats are often more tolerant following many residual herbicide programs. Broadleaf cover crops, legumes and some brassicas may be more sensitive, depending on the active ingredients used earlier in the season. Cover crop mixes can add another layer of complexity because one sensitive species in the mix may be injured even if other species establish normally.⁴
It can be difficult to account for all interactions between cover crops and herbicides. Guide your growers to extension resources and tables for comparing common corn and soybean herbicides, identifying cover crops that are safer to plant in rotation and flagging species at higher risk of injury.
Recommendation checklist before planting
Before farmers seed winter cover crops potentially at risk for herbicide carryover injury, you can help them work through a simple field-by-field checklist:
- Which residual herbicides/active ingredients were applied?
- What were the application rates and dates?
- How much rainfall occurred after application?
- What are the field’s soil texture, soil pH and organic matter levels?
- Which cover crop species or mix does the grower plan to plant?
- Do the herbicide label or university resources identify rotational concerns?
If the risk of herbicide carryover injury is high, growers may need to adjust the cover crop, avoid sensitive species in a mix, delay planting where appropriate or test a small area before seeding the whole field. Residual herbicides remain an important tool for managing weeds in corn and soybean rotations, especially where herbicide resistance is a concern. With the right planning, retailers can help growers protect the value of residual weed control while improving the odds of successful fall cover crop establishment.
Experts are available to help you with your cover crop recommendations. Reach out to a nearby extension office agent or a seed company professional like your regional BASF representative.
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Endnotes
- Zimmer, Marcelo, and Bill Johnson. “Soil Residual Herbicides and Establishment of Cover Crops in the Fall.” Pest&Crop Newsletter, Purdue University Extension, 14 Aug. 2020, extension.entm.purdue.edu/newsletters/pestandcrop/article/soil-residual-herbicides-and-establishment-of-cover-crops-in-the-fall/
- Zimmer and Johnson. “Soil Residual Herbicides and Establishment of Cover Crops in the Fall.”
- Owen, Micheal D. K. “Herbicide Carryover This Spring.” Integrated Crop Management, Iowa State University Extension and Outreach, 16 Apr. 2024, crops.extension.iastate.edu/post/herbicide-carryover-spring/
- Zimmer and Johnson. “Soil Residual Herbicides and Establishment of Cover Crops in the Fall.”
