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Time to dormant spray for alfalfa weeds
Stacy Campbell
Stacy Campbell

As our weather warms, alfalfa producers are reminded to eliminate weeds in their fields before plants break dormancy. Herbicides for controlling winter annual weeds such as pennycress, mustards and shepherd’s purse should be applied before alfalfa regrowth reaches two inches of height. If alfalfa shoots are green when sprayed, its growth may be set back a couple weeks.

Broadleaf weeds can lower yields, reduce hay quality and slow first-cutting alfalfa dry-down. Grassy weeds such as downy brome, cheatgrass and bromegrass can have even higher impact on lowering hay quality. 

If your alfalfa variety is Roundup Ready, then glyphosate can be applied to the alfalfa almost any time without harming the fields. Fortunately, if you treat your alfalfa soon, you can have cleaner, healthier alfalfa at first cutting.

Before spraying these weeds, be sure they are causing economic damage to your crop. Spraying will provide a purer alfalfa stand but may reduce your overall total tonnage. 

Our new 2023 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland guide, lists herbicides labelled to control weeds in alfalfa. To be most successful, apply herbicides before alfalfa shoots green up this spring to avoid alfalfa injury. If it does get late, use either Raptor or Pursuit, because they tend to cause less injury to your alfalfa.

Get ready to control unwanted weeds now before alfalfa plants break dormancy. Spray timing is critical.

Alfalfa weevils start scouting soon

Eggs will hatch after about 300 growing degree days, a physiological measurement of temperature. That puts the newly hatched weevils in farmer’s field roughly in late March. It soon will be time to start scouting your alfalfa fields. 

Alfalfa weevils leave behind tiny holes – about the size of a pin prick – in the leaves of alfalfa plants.

It is recommended that producers wait until there is a 30-50% infestation of alfalfa weevils in a field before deciding to spray. To test the infestation level, pick stems and then vigorously shake them into a white bucket to count the number of larvae that fall off. 

If a producer shakes 10 stems but only counts two larvae, then there is a 20% infestation level and should be checked within the next 4-5 days, depending on temperatures. Ideally it is recommended to do this sampling in at least two to three different areas or spots in the field to get a better estimation. 

“Don’t go out there and spray too early,” advises Jeff Whitworth, Extension Entomologist. “These insecticides only last 10-14 days.”  

He recommends checking the weather forecast before treating for weevils. “Anytime the temperature gets down into the mid-20s, the young larvae will be killed.”  

If rain is forecast in the coming three to four days, Whitworth suggests holding off on spraying. 

Stacy Campbell is an Agriculture and Natural Resources agent for Cottonwood Extension District. Email him at scampbel@ksu.eduor call the Hays office, 785-628-9430.