Killing an alfalfa stand during the fall is generally easier and yields better results; however, partial winterkill or a change in cropping plans sometimes necessitate the need for a spring farewell to established alfalfa fields.
“Spring kills can be challenging, but options do exist,” notes Mark Renz, University of Wisconsin extension weed scientist. “If using herbicides, remember to read the product label as plant-back restrictions vary between products.”
According to Renz, tillage alone can result in over 80 percent mortality to alfalfa plants following spring green up, but the type of tillage implement, its operation, and environmental conditions will dramatically affect the final results.
“Spraying an herbicide prior to tillage greatly improves your chances for a more complete kill,” Renz says in a recent Wisconsin Crop Manager newsletter. “For no-till fields, using an herbicide alone can provide good to great results, depending on the year.”
Most herbicide labels recommend at least 4 inches of regrowth to maximize control. Renz points out the most popular active ingredients are glyphosate, 2,4-D, and/or dicamba. Of course, glyphosate will not be an option for Roundup Ready alfalfa varieties.
Though glyphosate has no plant-back restrictions for a subsequent crop, 2,4-D and dicamba do. The crop to be planted, rate of application, and the product used (marketer) govern the length of the restriction. For corn, Renz notes that many 2,4-D labels require seven to 14 days between herbicide application and planting. Dicamba may range from zero to 30 days.
It’s common to want to take a first cutting before killing an alfalfa stand and planting the field to a different crop. Renz outlines several options for this scenario.
If the alfalfa isn’t a Roundup Ready variety, glyphosate can be applied 36 hours or longer before harvesting. Up to 1.5 pounds of active ingredient is registered for such a preharvest application. Renz says this is a very effective termination approach because of the larger-sized alfalfa being sprayed coupled with the cutting process and subsequent tillage.
A second approach is to harvest the first cutting and then wait for the alfalfa to regrow before making an application. The significant downside is that there is a two- to four-week delay before planting the subsequent crop.
A final option is to kill the stand with tillage following harvest, plant corn, and then remove any surviving alfalfa plants using a postemergence corn herbicide that is also effective on alfalfa. Glyphosate is the logical choice in Roundup Ready corn following a non-Roundup Ready alfalfa crop, but other active ingredients are also available if both crops are glyphosate tolerant. These include herbicides that contain dicamba or clopyralid.
“Make these postemergence applications for corn in a timely fashion,” Renz says. “Volunteer alfalfa can be extremely competitive and cause significant crop yield losses.”