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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Mesotrione injury on radish seedlings (Photo credit: Dr. Mark Bernards, Western Illinois University).

Year-round planning and careful herbicide selection required for fall cover crops

It seems as if cover crops have gotten a lot of press attention in the past several years, reaching a crescendo each year right before the time that they are established – late-summer to early fall. Unfortunately, decisions that were made at the beginning of the season can affect how well certain cover crops establish months later.

Application rates listed on herbicide labels were developed in part to maximize efficacy and minimize crop injury. Specimen labels are considered legal documents and need to be followed to avoid violating the law. Those herbicides that have longer half-lives, including soil-applied residual herbicides, include guidelines that are intended to reduce the occurrence of crop injury. In addition to application rates, these guidelines may list rotational crop restrictions, specific replanting or rotation crop planting intervals, grazing restrictions, and quantity of rainfall or duration of freezing temperatures required before attempting to establish a sensitive crop. The label may also specify whether safe and/or legal use of the product may be restricted to a specific growing region, soil pH or soil organic matter content, or a specific rate given the intended use.

Very little published data is available regarding the effect(s) of the herbicides labeled for weed management in corn or soybean on fall cover crop establishment. Drs. Mandy Bish and Kevin Bradley, Weed Scientists with the University of Missouri-Division of Plant Sciences, recently summarized results of a 2013 research trial in which they investigated the effects of 14 different commonly used herbicide treatments labeled for use in corn or soybean production on fall cover crop establishment. They estimated percent stand reduction (when compared to untreated control plots) at 28 days after emergence of wheat, tillage radish, cereal rye, crimson clover, oats, Austrian pea, annual ryegrass and hairy vetch seedlings. Percent reduction in stand emergence was grouped into three groupings, milder (less than 15), moderate (between 15 and 30) and severe (greater than 30). Five of the different soybean herbicides and four of the corn herbicides caused severe stand reductions in fall-seeded cover crops. Each of the different herbicides caused at least moderate stand reduction in one or more of the fall cover crops.

With an aim towards testing cover crop species under the "worst-case scenarios" regarding potential herbicide carryover injury, Western Illinois University Weed Scientist Dr. Mark Bernards led a team that in 2014 conducted a series of dose-response studies with 11 different corn and/or soybean herbicide active ingredients (and one two-a.i. combination) on 10 different cover crop species. To generate the curves, and simulate herbicide degradation over time, they applied herbicides at the label rate and 50, 25 and 12.5 percent of the label rate to plots that had been planted to cover crops the previous day. In these studies, the brassicas (turnip, radish and rapeseed) were very sensitive to both ALS- and PPO-inhibitor herbicides and red clover sustained the most injury from HPPD-inhibitor herbicides.

For those that are already investing the additional time and money to establish a cover crop, it would likely be worthwhile to develop a plan to strike an effective between the issues that are most important for both the primary corn or soybean cash crop and the cover crop: season-long, effective weed management; using multiple and diverse herbicide modes of action to best delay the development of herbicide resistant weeds; and the timely establishment of a cover crop without significant carryover injury.

Additional Resources:

Bish, M. and Bradley, K. 2014. Influence of Corn and Soybean Herbicide Treatments on Cover Crop Stands. Integrated Pest & Crop Management. University of Missouri Division of Plant Sciences Integrated Pest Management Program.

Bosak, E. and Davis, V. 2014. Herbicide Rotation Restrictions in Forage and Cover Cropping Systems. Nutrient and Pest Management Program, University of Wisconsin Extension.

Crop Data Management Systems, Inc. 2015. Pesticide Label Database.

Curran, W. and Lingenfelter, D. 2013. Herbicides Persistence and Rotation to Cash and Cover Crops. Online. Penn State Extension.

Curran, W. and Lingenfelter, D. 2013. Herbicides Persistence and Rotation to Cover Crops II – Soybean. Online. Penn State Extension.

Parker B., Heaton B.S., Bernards M.L. 2014. Cover crop species response to simulated half-life doses of soybean and corn herbicides. North Central Weed Science Society Proceedings 69:24.

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