Source: Alyssa Essman, Mark Loux, OSU
Herbicides with residual that are used in corn and soybeans can affect the establishment of fall-planted cover crops, and should be taken into account when planning cover crop practices and selecting species. Soil characteristics and weather also play a role in the persistence of residual herbicides, which can vary by field and year. More information is needed on rotational intervals for many cover crop species, and this information is often not included on herbicide labels. University weed scientists have studied the effect of residual herbicides on some of the most popular cover crop species in order to provide this information to growers. In general, residual herbicides that control grass weeds can hinder establishment of grass cover crop species. Broadleaf cover crop species are most impacted by group 2 (ALS inhibitors), 5 (PSII inhibitors), 14 (PPO inhibitors), and 27 (HPPD inhibitors) herbicides (Purdue University).
A multi-state study found that the general order of sensitivity of cover crops to herbicide carryover, from greatest to least sensitive, is:
- Tillage radish > Austrian winter pea > crimson clover = annual ryegrass > winter wheat = winter oats > hairy vetch = cereal rye.
Soybean herbicides that tended to be most injurious were:
- Fomesafen, pyroxasulfone, imazethapyr, acetochlor, and sulfentrazone.
Corn herbicide treatments that were most injurious to cover crops were:
- Topramezone, mesotrione, clopyralid, isoxaflutole, pyroxasulfone, and nicosulfuron
(University of Missouri).
Below is a table of commonly used corn and soybean herbicides, the fall cover crops that are safe to plant in rotation, and cover crop species that may be injured following these herbicides (Adapted from Lingenfelter D. and Curran W., Penn State University).
Cover crops provide a multitude of benefits and their use is becoming an increasingly popular practice in Ohio. Including cover crops in rotation with agronomic crops to realize these benefits costs time and money. It is important to evaluate the potential risk of herbicide residue on the establishment of cover crops in order to ensure success. Residual herbicides applied at the time of planting typically interfere with cover crop establishment less than those applied POST. Weather can affect the persistence of herbicides also, especially rainfall in summer. The risk of residual herbicides affecting cover establishment will be higher in areas that have been dry since herbicide application. Risk will be lower where the herbicide application was followed by some wet weather to get herbicide degradation started, compared with an application during prolonged dry weather. One of the least problematic cover crop species is cereal rye, which can be successfully established following a late corn or soybean harvest, and is tolerant to a most of the most commonly used corn and soybean herbicides. Weed control should continue to be the priority in selecting herbicides, and cover crop species selection should be based on potential injury and goals for the use of cover crops. The introductory section of the “Weed Control Guide for Ohio, Indiana, and Illinois” has some of the same information presented here, and OSU weed scientists also summarize this in a video: https://www.youtube.com/watch?v=ylr0zGnXMfs
The following resources contain information on residual herbicides and cove crops also: