Weed management in alfalfa—postemergence options

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Although the wet weather has many people concerned about getting corn and soybeans planted, the first alfalfa cutting is fast approaching. The warm, dry weather early this season provided a good window for many people to make herbicide applications to dormant alfalfa, if scouting was timely. Weed control often isn’t at the top of the list in April, so this may be the first look at weed species in many alfalfa fields, making postemergence herbicide applications necessary. In a pure alfalfa stands, winter and summer annual grass weeds can effectively be controlled with Poast or Select, however, be aware of the harvest restrictions with these herbicides. Once alfalfa breaks dormancy and has appreciable new growth, most broadleaf herbicides can cause injury to alfalfa. When trying to determine whether to apply herbicides or not, remember in many cases the easiest and most economical solution for weed control is the first cutting.

Summer annual weeds will be emerging and becoming established at the first cutting. Control of these small weeds in pure alfalfa stands can be accomplished with herbicides such as Gramoxone Inteon at 1 pt/A, Pursuit at 4 oz/A, Velpar at 1 qt/A, Raptor at 5 oz/A when applied between cuttings. Be sure to make these between cutting applications before one to two inches of alfalfa growth occurs, usually within four to five days. Also be aware of use restrictions. For example, do not make sequential applications of Pursuit and Raptor within a 60-day period to avoid potential herbicide injury, and be aware of rotation restrictions if the alfalfa stand is nearing the end of its life. Herbicide rotation restrictions can be found on pages 147-150 in 2010 Weed Control Guide for Field Crops. The majority of herbicides also have a harvest restriction (usually 20 to 30 days), however a harvest restriction for an application made within four to five days after cutting shouldn’t delay the next cutting substantially. Harvest restrictions for herbicides in alfalfa can be found on page 110 in 2010 Weed Control Guide for Field Crops. As always, if there is ever any doubt, consult the herbicide label.

For mixed alfalfa stands, the weed control options are greatly reduced, essentially limited to applications of Buctil or Moxy at 1 pt/A or 2,4-DB amine at 2 qt/A applied early April for control of broadleaf weeds. Grass weed control is very difficult in a mixed stand, and research studies are being conducted to determine if options exist.

A proactive weed management program in alfalfa, as in all crops, is often the most effective means to keep your crop weed-free and help ensure maximum yield potential.

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