Drought and herbicide carryover

By James Hoorman, Hoorman Soil Health Services

The 2020 summer was hotter and drier than normal for most farms, so herbicide carryover will be a major issue for planting cover crops.  Herbicides degrade based on soil temperature, rainfall, time of application, organic matter, soil type, soil pH, and sunlight.  Generally, microbially active soils break down herbicides quickly.  Moisture is critical for microbe activity, so drought or dry summers means slower herbicide breakdown.  High soil temperatures can also reduce microbial activity and herbicide breakdown.  High soil microbial activity occurs between 75-850F but once soil temperatures get above 900F, generally microbial activity declines.  On bare soils, the soil temperatures in the top inch may reach 110-1400F on a hot sunny day, greatly reducing microbial activity and herbicide breakdown.

Mark Loux OSU Extension Weed Scientist
Dr. Mark Loux, OSU Extension Weed Scientist

Herbicide application timing also determines herbicide degradation.  Herbicides applied in the spring or early summer have a longer time to break down.  Applying a postemergence herbicide late in the summer, even with a short half-life may cause problems.  Sandy soils with low organic matter have more problems than clay soils with higher soil organic matter.  Soil pH may also affect herbicide breakdown depending on the herbicide chemistry.

Jeff Stachler, OSU Extension, Auglaize County

Sunlight and ultra-violet light generally also cause herbicides to break down.

Several local resources can help farmers make decisions on herbicide carryover issues. First, contact your local Ohio State University Extension Office.  Dr. Jeff Stachler, OSU Extension in Auglaize County or Dr. Mark Loux at Ohio State in Columbus, are two local expert on herbicides.  Second, many online resources exist. Always check the date on these articles for the most updated information.

1) Dr. Bill Curran and Dwight Lingenfelter, Penn State: Common corn and soybean herbicides, estimated half-lives, cash crop restrictions and their potential to injure fall cover crops.

2) Dr. Kevin Bradley from Missouri: Influence of Corn and Soybean Herbicide Treatments on Cover Crop Stands. 

3) Dr. Laura Barrera, Purdue University: Evaluating Herbicide Carryover for Cover Crops.

4) University of Wisconsin: Herbicide Rotation Restrictions in Forage and Cover Cropping Systems.

Generally, grass herbicides interfere with grass cover crop species. Broadleaf residual herbicides like group 2 (ALS), group 5 (triazine), group 14 (PPO), or group 27 (bleacher) interfere with the establishment of broadleaf cover crop species like legumes (peas, vetches, or clovers) or mustards/brassicas (radish, kale, rape). Each cover crop species may be more or less susceptible to certain herbicides.

In grasses, cereal rye generally has fewer problems than annual ryegrass.  For cereal rye, use caution if sulentriazone (Spartan) or imazethapyr (Pursuit) were used.  For annual ryegrass, herbicides like Pyroxasulfone (Zidua),  metolachlor (Dual, etc), flumioxazin (Valor), and imazethapryr (Pursuit) hinder annual ryegrass establishment.

For legumes, clovers, mustards or brassicas and even some grasses; atrazine or simazine at > 1 lb/A will be problematic unless lots of rainfall occurred after application.   Lower rates of atrazine or simazine may allow establishment of most legumes, clovers, mustards, and annual ryegrass but drought and dry weather may limit herbicide breakdown this year.  Mesotrione (Callisto, Lumax, Lexar etc.), flumetsulam (Python) and clopyralid (Stinger, Hornet, SureStart) can be problematic for legumes and mustards like canola, rape seed, and forage radish. Herbicides like chlorimuron (Classic, Canopy, Cloak, etc.), imazethapyr (Pursuit), fomesafen (Flexistar, Reflex, etc.), flumioxazin (Valor) and pyroxasulfone (Zidua) could be a problem for fall seeded legumes, clovers, or mustard covers including radish.  On crimson clover and radish, use caution if using metolachlor (Dual, etc) or Pyroxasulfone (Zidua).

Bioassays may evaluate how much residual herbicide may be left in a field.  Collect soil from two areas, one with the herbicide treatment  and one untreated area with a similar soil type without the herbicide being evaluated. Plant seed from the cover crop you would like to use in both areas. Observe growth for 2-3 weeks and if the plants look the same in the treated and untreated soil, you should be safe to plant the desired cover crop. If you do not have time to do a bioassay, plant a cover crop mixture. Cover crop establishment may be more reliable when mixtures of grass, broadleaf, mustards or brassica species are planted together. Residual herbicides may interfere with establishment of some species in the mix, but have little or no effect on other species. Cover crop mixtures may offer protection from complete failure due to herbicide carryover. Make sure that at least one or two of the species in the mixture is tolerant to the herbicides used in a specific field.

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