By Todd Whitney
Record winter high temperatures are causing earlier weeds emergence, so now is the time to control winter annual weeds in alfalfa. Thin field stands combined with open soil surfaces and adequate moisture can result in high weed pressure. So, prior to alfalfa shoot green-up or dormancy break, scout fields for early green winter annual weed patches of pennycress, downy brome, mustards, cheatgrass and shepherd’s purse. Left unchecked, these weeds can lower first cutting hay yield, quality and palatability. Also, excessive weeds can extend dry-down and lengthen harvest time.
However, before applying any herbicide, scout current and potential weed pressure. Verify that fields have enough weeds to justify herbicide spray applications and then determine the best management options.
- Winter annual weeds can grow better during cooler spring temperatures than alfalfa, so dormant alfalfa herbicides such as MCPA Amine, metribuzin and Gramoxone® can still be applied.
- Pre-emergent herbicides such as Warrant®, Chateau® and Prowl H2O® can be applied soon after green-up, but these chemicals will not control weeds which have already germinated.
- Aim® herbicide will control early growing broadleaf weeds but not control grassy weeds.
- Poast® herbicide will control most growing weedy grasses but not downy brome.
- Pursuit® and Raptor® herbicides control growing henbit well during early alfalfa green-up, but control weeds best when air temperatures are warmer.
- Glyphosate products like Roundup Weathermax®/Powermax® will control growing weeds with warmer temperatures too; however, they must be used with Roundup-Tolerant alfalfa varieties.
Source : unl.edu