By: Liz Stahl, Extension Educator

With post-emergence herbicide supply issues creating challenges for 2022 and possibly into 2023, using a pre-emergence herbicide with residual activity as part of a weed management program is as important as ever. Pre-emergence herbicides can aid in the control of troublesome weeds such as waterhemp and giant ragweed. They can also result in lower weed densities and a more uniform weed height at the time of postemergence herbicide application, aid in the management of herbicide-resistant weeds, and ultimately help protect crop yield potential and profitability. What if you want to plant a cover crop later in the season? Will a residual herbicide that controls weeds also affect your cover crop?

Determine your cover crop goals

If you are planting a cover crop for feed or forage (e.g., to graze it), you must follow any restrictions listed on the herbicide label as the cover crop will be entering the food and feed chain. Most herbicide labels do not list the time you must wait from application until a cover crop could be grazed or harvested for forage. In these cases, you need to follow the rotational restriction listed on the label. This restriction is the time you must wait after herbicide application before you can seed the cover crop. If the cover crop you want to plant is not listed, you must follow the most restrictive rotational interval. Some products have extremely long rotational restrictions for certain cover crops (e.g., 30 months), so these products may not be suitable in fields where you want to plant a cover crop for feed or forage.

On the other hand, if you are planting a cover crop for soil health reasons and the cover crop will not enter the food or feed chain, you have more flexibility. In this case, you can plant the cover crop when you want, but you assume any risk of cover crop injury or failure.

Considerations when using a cover crop for soil health

Species of cover crop

Research shows that in general, cereal rye, wheat, and oats tend to tolerate many of the herbicides commonly used in a corn/soybean system. If you are seeding a cover crop mix, keep in mind that broadleaf herbicides tend to pose more risk to broadleaf cover crops, and likewise for grass herbicides and grass cover crops.

Length of time between herbicide application and cover crop seeding

In general, the longer the time between herbicide application and cover crop seeding, the less potential there is for injury to the cover crop. For example, a fall-seeded cover crop is less likely to be impacted by an herbicide applied in the spring than a cover crop interseeded into the cash crop soon after application.

Herbicide characteristics

Although an herbicide with a long residual may be appealing from a weed control standpoint, this characteristic may pose greater risk to a cover crop seeded after application. An herbicide with a long half-life - the length of time it takes for a pesticide to break down to 50% of the original amount - may pose a greater risk to a cover crop seeded after application, but only if the product has soil activity (i.e., the herbicide can be taken up by plants after application).

Soil conditions

Depending on the herbicide, extremes in soil pH or soil organic matter can influence the length of time an herbicide remains active in the soil, and thus the potential to affect a cover crop planted after application. For example, atrazine and chlorimuron (a.i. in Classic) are not readily broken down in soils with a pH greater than 6.8, while imazethapyr (a.i. in Pursuit) is more likely to cause injury to a sensitive crop in soils with a pH less than 6.5.

Weather conditions

Microbial activity is a driving factor in the breakdown of most herbicides. As a result, conditions that favor microbial activity, such as warm conditions and adequate moisture, tend to favor herbicide breakdown. On the other hand, extreme conditions (e.g., drought, cold/freezing temperatures) hinder microbial activity. These conditions can increase the longevity of a product in the soil and the potential to injure a sensitive cover crop planted after application.

Research results from MN

Research conducted in several states has demonstrated the potential for herbicides to impact cover crop establishment and growth. To help address this concern in MN, the University of MN conducted research looking at the tradeoffs in weed control when cover crops were integrated into a corn/soybean system at 3 locations (Waseca, Rochester, and Rosemount) from 2019-2021.

Since waterhemp is one of the main weed challenges for many farmers across the state, herbicides with residual activity on waterhemp were selected. Resicore (3 qts/ac), Verdict (18 oz/ac), Outlook (21 oz/ac), and Outlook (16 oz/ac) fb Outlook (8 oz/ac) 30 days later, were applied in the spring, and compared to a control which had no residual herbicide. The cover crops evaluated (red clover @ 12 lb/ac, camelina @ 10 lb/ac, and cereal rye @ 60 lb/ac), were seeded by a drill in the fall after corn silage harvest. Establishment of red clover was poor across all treatments both years, so results are only shown for camelina and cereal rye.

In these trials, the herbicides evaluated did not have an impact on cereal rye and camelina biomass in the spring (Figures 1 & 2). This is encouraging as it shows the potential to use residual herbicides that are effective on waterhemp, even when you plan to seed a cover crop later in the season. It is important to note, however, that these results are based on only 6 site years of data. We could not evaluate all the possible weather & soil scenarios and there is risk of poor/no establishment if warnings or restrictions on the herbicide label are not followed. Individual results are expected to vary based on the factors listed earlier.

Figure 1. Average spring biomass of a fall-seeded camelina cover crop at three locations in southern MN across two years (2020-2021) as impacted by the pre-emergence program used in the spring prior to seeding. No difference was detected among treatments (p=0.05).
Figure 2. Average spring biomass of a fall-seeded winter rye cover crop at three locations in southern MN across two years (2020-2021) as impacted by the pre-emergence program used in the spring prior to seeding. No difference was detected among treatments (p=0.05).


Weighing risks and trade-offs

Planting a more tolerant cover crop such as cereal rye in the fall poses less risk to establishment when a residual herbicide is used in the spring, compared to interseeding a cover crop shortly after application in the spring or planting a mix that contains potentially sensitive species.

Keep in mind that using a reduced rate of an herbicide is NOT recommended. Reduced rates can result in reduced weed control while increasing the selection for herbicide-resistant weeds, and you could still have injury to a sensitive cover crop.

Cover crops can provide many soil health benefits and be a food/feed source for livestock. Using a robust weed management program that tackles your biggest weed challenges, even when planting a cover crop, will help lead to long-term success in your cropping system.