Mark Renz Extension Weed Scientist, University of Wisconsin-Madison
While removal of old stands is recommended with fall applications, many fields are now slated for removal due to winter-kill and changes in cropping plans for the field. This can be challenging, but options exist. Below I discuss management options for common scenarios this spring. If using herbicides, remember to read the label of the products used, as plant-back restrictions can vary between products.
SPRING REMOVAL AS ALFALFA GREENS-UP
Alfalfa can be removed with herbicides and/or aggressive tillage. While tillage can result in > 80% mortality, tillage implement, operation of equipment as well and environmental conditions can dramatically affect control. Spraying an herbicide prior to tillage is usually conducted as this combination greatly improves chances of alfalfa mortality. For no-till fields, spring herbicide application can provide good to great removal depending on the year. In years with little regrowth and stressed plants alfalfa plants often resprout and require control in the planted crop. Most labels recommend at least four inches of regrowth to maximize control. While many herbicides are available to remove alfalfa, the most popular active ingredients include glyphosate, 2,4-D, and/or dicamba. While glyphosate has no plant-back restrictions for other crops, 2,4-D and dicamba do. The restriction varies depending on the crop, rate, and product so read the label carefully. In a typical year the restriction for 2,4-D (7-14 days for corn) and dicamba (0-30 days for corn) can be met easily prior to planting corn.
SPRING REMOVAL DURING/AFTER ALFALFA HARVEST
Producers are sometimes interested in harvesting the first crop of alfalfa before terminating the stand. The best option for this scenario is to apply glyphosate 36 hours or longer before harvesting. Up to 1.5 lbs ae/A is registered for this type of application. This will allow for hay harvest, and improve effectiveness of removal due to the delayed application timing and larger sized alfalfa. I am not aware of any other herbicides that can be used in this fashion. Another option would be to harvest the hay, let the alfalfa regrow then apply an herbicide to eliminate the stand. This option, while effective would result in at least 2 weeks of additional time before planting to allow harvest and regrowth. Plant-back restrictions would then also need to be followed for the products selected.
CONTROL OF VOLUNTEER ALFALFA IN OTHER CROPS
While the goal is to kill plants with the removal treatments, some alfalfa plants can survive and will need to be managed in the following crop, especially in spring removal. For Roundup-Ready crops, glyphosate is the logical choice and is effective, unless removing Roundup-Ready alfalfa. Other products that are effective in corn include products that contain dicamba or clopyralid. I am not aware of effective non-glyphosate options in soybeans. Make applications in a timely fashion, as I expect significant yield loss could be seen from volunteer alfalfa in fields.