April 6, 2012

With the potential for frosts across western and northern areas of the state this weekend, alfalfa growers will want to be alert to potential damage.

A "light" frost/freeze where temperatures don't go below 28°F for very long is likely to singe alfalfa tops a bit and set back growth rates, but plants will grow out of it. There's no need to cut this alfalfa, although some growers seeking high quality alfalfa may if standing yield is high enough to justify harvest. In this case, the plants will be weakened by early cutting and should be allowed extra time to recover before the next cutting.

An extreme freeze around 20°F or less, as occurred in 2007, likely will freeze plants all the way to the ground and they will collapse soon afterward. Harvest is warranted if yield is sufficient but must be done immediately.  Once plants collapse, much of the biomass will be unattainable and leaves will shatter quickly from those stems that can still be cut. Experience from 2007 showed little or no benefit to regrowth by cutting or shredding damaged tissue since the freeze was so thorough that plants reacted to the killed tops just as they would if tops were killed by cutting instead of freezing.

A freeze that penetrates about halfway down into the alfalfa canopy makes decisions more difficult. Cutting will weaken plants that weren’t ready to be cut. Uncut plants will be confused, some continuing to grow, others creating new shoots from aboveground stems, and others with new shoots coming from the crown. Much of that regrowth will be slow to initiate. If yield is high enough to justify harvest, this alfalfa probably should be cut, knowing that extra time will be needed for recovery before the next cutting. If yield of standing crop is low, it's probably best to just wait out the delay in regrowth. It will be hard to justify the time and expense of cutting/shredding with no immediate harvestable crop.

After a freeze that causes visible damage to alfalfa tissue, be extra observant of how the plants respond. In 2007, some regions experienced significantly higher than usual damage from foliar diseases and sometimes, insects, on regrowth following the freeze. At that time it was speculated that weakened plants may have been less able to resist these pests or that the abundant amount of dead alfalfa plant tissue on the ground or surrounding the regrowth provides a more desirable environment for pests to develop.

Bruce Anderson
Extension Forage Specialist