Get a Head Start and Dormant Spray For Weeds in Alfalfa
Timing is crucial when controlling winter annual weeds in alfalfa. Be prepared to take advantage of nice weather when you can get it in the next few weeks to control weeds before alfalfa greens up.
Weeds like pennycress, downy brome, mustards, cheatgrass, dandelion, and shepherd's purse are common in first-cut alfalfa. They lower alfalfa yields, reduce quality, lessen palatability, and slow hay drydown. Walk over your fields during the next few weeks once snow is gone, especially in areas where these weeds grew last year. This spring you'll probably be able to see their small, green, over-wintering growth.
Before deciding to spray these weeds, consider whether they are causing economic damage. If you kill these weeds, you may harvest more alfalfa, but total tonnage at first cutting may be less without the weeds. If dairy quality hay is your objective, controlling these weeds might be very important due to their impact on forage quality, palatability, and hay drydown. In some other livestock operations, though, these weeds may not cause serious problems so use of herbicides may not be cost effective.
If your alfalfa is a Roundup Ready variety, you have some flexibility in timing the application of Roundup. Don't wait too long, though. If weeds get more than 3 inches tall or alfalfa begins to form a canopy, weed control may not be as effective.
Once conventional alfalfa starts growing, you can't control these weeds very well without hurting your alfalfa. However, if you treat your alfalfa as soon as possible during this winter's next spring-like weather, you can have cleaner, healthier alfalfa at first cutting.
Herbicides for Winter Annuals
Several herbicides can help control winter annual grasses and weeds in alfalfa. Herbicides that are used before alfalfa breaks dormancy include metribuzin (Sencor), Velpar, Sinbar, and Karmex. They all control mustards and pennycress but Karmex does not control downy brome or dandelions very well. Metribuzin usually is the least expensive.
To be most successful, though, you must apply these herbicides before alfalfa shoots green-up this spring to avoid much injury to your alfalfa. During mild winter weather would be a great time. If you wait and alfalfa shoots are green when you spray, your alfalfa growth might be set back a couple weeks.
If it does get late and alfalfa turns green before you can spray, switch your herbicide choice to either Pursuit or Raptor. These herbicides tend to cause less injury to your alfalfa.
UNL Forage Specialist