Indiana growers have shown increased interest in utilizing cover crops in our corn and soybean production systems over the last decade. Marcelo Zimmer and Bill Johnson, Purdue weed specialists, have also noted there has been increased use of soil residual herbicides to help manage herbicide-resistant weeds such as marestail (horseweed), waterhemp, and giant ragweed. Soil residual herbicides can remain active in the soil for a period of weeks to months after application.
The use of residual herbicides in our corn and soybean production systems may interfere with establishment of fall-seeded cover crops under certain conditions. Unfortunately, many of the species being used for cover crops were not evaluated for herbicide carryover when field research was conducted to support EPA’s approved herbicide labels.
About five years ago, we conducted experiments designed to evaluate the impact of commonly used residual herbicides on the establishment of many cover crop species. As a general rule, residual herbicides that have activity on grass weeds can interfere with the establishment of some grass cover crop species, especially the smaller-seeded ryegrass species. Residual herbicides from group 2 (ALS), group 5 (triazine), group 14 (PPO), or group 27 (bleacher) can interfere with the establishment of some of the broadleaf cover crop species.
More specifically we have learned:
Corn herbicides: Pyroxasulfone (Zidua) and metolachlor (Dual, etc) can hinder annual ryegrass establishment; Atrazine or simazine at > 1 lb/A will be problematic for legumes and mustards unless lots of rainfall occurs after application; Atrazine or simazine at < 0.75 lb/A may allow for good establishment of most legume cover crops, mustards, and annual ryegrass; Atrazine < 1 lb/A can allow cereal grain establishment; 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: Chlorimuron (Classic, Canopy, Cloak, etc.), imazethapyr (Pursuit), and fomesafen (Reflex, etc.) could be a problem for fall-seeded legume or mustard covers including radish. However, establishment of cereal grains should be OK.
It is important to remember that herbicide application timing greatly influences the risk of carryover interfering with cover crop establishment. In general, herbicides applied at planting have a lower risk of interfering with cover crop establishment than herbicides applied postemergence later in the year. We can use the knowledge we have about herbicide interactions with specific cover crops to assess risk of certain herbicide programs interfering with cover crop establishment. However, it is important to prioritize controlling weeds in your cash crop rather than dropping certain herbicides from your program to ensure successful cover crop establishment.
The final two things to mention are that if you have questions about specific situations, one way to address the residual herbicide left in a field is to do a bioassay. Simply collect soil from the area you would like to seed the cover crop into and an area with a similar soil type, but no herbicide residue, and plant seed from the cover crop you would like to use. Observe growth for three weeks, and if the plants look the same in the untreated and treated soil, you should be safe to plant to desired crop. Another consideration if you do not have time to do a bioassay is to plant a cover crop mixture. Cover crop establishment may be more reliable when mixtures of grass and broadleaf species are purchased and planted. Residual herbicides may interfere with establishment of some species in the mix but have no effect on other species. The use of mixtures may allow more protection from complete failure due to excessive herbicide residues in the soil. It would be important however to make sure that at least one or two of the species in the mixture is tolerant to the herbicides used in a specific field.