Hard freezes in early April damaged alfalfa in Kansas, says Jim Shroyer, Kansas State University extension agronomist. That means producers will have stand management decisions to make.
“Where the alfalfa is damaged by freeze, the leaves will probably turn dark, then start falling off a few days later. The plants may also collapse or fall over if the stems are injured,” Shroyer says. “Where this occurs, producers may want to mow or shred the plants and let them start over with fresh regrowth.” But that isn’t always necessary, Shroyer says. “This should only be done if the growing point clusters are frozen, the new regrowth is occurring only from the base of the plants, and the plants can be cut without damaging regrowth.” If producers do plan to shred or cut damaged stands, they should leave 2-3” of stubble, Shroyer recommends. This will help encourage regrowth after plants have been cut.
If damaged stands are cut, watch regrowth for alfalfa weevil and pea aphid infestations, and treat immediately if needed, says Jeff Whitworth, K-State extension entomologist. “Weevil larvae that survive in the leaf litter on the soil surface will start feeding on new growth once temperatures warm up,” he says. “The hard freezes will probably kill a certain percentage of weevil larvae, but it’s not certain how many. Some weevils have already pupated.”
There have been few reports of pea aphids on Kansas alfalfa yet this spring, but the pest could become a problem on regrowth after the stand is cut, Whitworth adds. If an insecticide had been applied to alfalfa for weevil control before the freeze, growers should be aware of residual insecticide that may affect how the crop can be utilized.
Freeze-damaged alfalfa only 6-8” tall or less will be slower to regrow after mowing or shredding than taller alfalfa, Shroyer says. “That’s because alfalfa plants are depleting carbohydrate reserves from the roots during the first 6-8” of growth, and will not have as many carbohydrate reserves for regrowth as taller alfalfa,” he explains. “With slower regrowth, producers will have to watch especially closely for insect infestations and treat if necessary. Alfalfa taller than 8” will have manufactured a new supply of carbohydrate reserves for the root and crown, and will be able to regrow more quickly after mowing or shredding.”
Alfalfa plant sensitivity to freeze is strongly related to the amount of growth it had before the freeze, says Bruce Anderson, University of Nebraska extension forage specialist. Plants that are 12” tall are much more likely to experience significant damage than 3”-tall plants. For more from Anderson, visit cropwatch.unl.edu/archives/2007/crop6/alfalfa_freeze.htm.