Cover crop termination review for 2023

The warmer weather forecast has me thinking about new beginnings and the resurrection of our dormant overwintering cover crops. While some will let the covers grow for some time yet, those covers in fields going to corn this spring will likely be terminated in the next few weeks. Regardless of when you choose to terminate your cover crop, remember that the best management practice is to terminate 10-14 days ahead of corn planting. Timing is less critical for soybean, but killing the cover crop prior to soybean emergence reduces risk to crop yield.

Herbicidal termination is going to be the most effective method available to most farmers and despite substantial research into alternatives, glyphosate remains the most consistent product to kill grass species.

Choose an appropriate glyphosate rate for the size and condition of the cover crop. In the past, we’ve recommended using a 1 lb acid equivalent (a.e.) rate, but this is a generous rate to account for variable spring weather, variation in cover crop size, and the addition of tank mix partners. A rate of 0.75 lb a.e. should be sufficient to kill a small (<18”) cereal rye cover crop if applications are made during favorable growing conditions and best management practices are followed (appropriate additives, good coverage, etc.).

Consider these BMPs:

• Pre-treat water with ammonium sulfate (AMS) to reduce hard water issues
• Follow label directions for adjuvants and tank mix partners
• Spray in the middle of a sunny, warm (>60F) day with a 3-5 day forecast that doesn’t get below 40F if possible

If you’re using a new glyphosate product or adjusting the rate from prior years, use the “Glyphosate math” blog to determine the equivalent rate of the new glyphosate product.

If you’re applying in particularly challenging conditions, multi-state research shows that the addition of clethodim to glyphosate will improve control, but keep in mind the planting restriction if using that mixture prior to corn. If cereal rye or other grass species are seeded with a legume, inclusion of 2,4-D or dicamba with glyphosate will improve consistency of control of the broadleaf. This addition can also be helpful when broadleaf winter annuals are present.

What other options are there?

I’m often asked about alternative chemicals or different strategies to manage cereal rye in the spring. Researchers have looked at other herbicides such as paraquat, glufosinate, and herbicide group (HG) 1 products like clethodim and quizalofop. Research continues to show they provide less consistent control than glyphosate. These products are more likely to be effective when weather conditions are favorable. The addition of metribuzin or atrazine to paraquat will improve its consistency. Both paraquat and HG 1 products list volunteer cereal grasses on the labels for effective burndown. These applications are most effective on cereals less than 6 inches tall. Researchers have noted mixed success in controlling cereal rye with the HG 1 products, including some new research from University of Wisconsin’s Rodrigo Werle’s lab. Applications of HG1 products to large cereal grasses often result in a slower death than with glyphosate, and cereal rye remains more upright than when terminated with glyphosate.

Termination using a roller crimper is gaining interest in parts of Iowa. While roller crimping can be quite effective at the correct growth stage for cereal rye, it is often less consistent than a glyphosate application, usually requires delayed crop planting, and the equipment can be challenging to procure. One substantial benefit to roller crimping is laying the cover down in an even mat on the ground, which may reduce shading on the crop and could improve weed suppression. The Wisconsin research noted above showed that when combined with roller crimping, the HG 1 herbicide treatments resulted in improved kill, though still not as effective as glyphosate in combination with the roller crimper. Read up more on using a roller crimper to terminate cover crops here and here.

Always look at the herbicide labels for directions and any restrictions for the subsequent crop. Two quick and easy places to look up herbicide labels are www.cdms.net or www.greenbook.net. I also encourage anyone looking into alternative methods to terminate cover crops to reach out to local resources like your local Iowa State University Extension and Outreach field agronomist or groups like Practical Farmers of Iowa.