Managing Freeze-Damaged Alfalfa

Manhattan, Kansas (April 10, 2007)--Hard freezes in early April damaged alfalfa in Kansas which means that producers will have to decide how to manage their stands in the aftermath of the freeze, said Jim Shroyer, agronomy state leader for K-State Research and Extension.

“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 said. “Where this occurs, producers may want to mow or shred the plants and let them start over with fresh regrowth.”

But mowing or shredding plants damaged by freeze isn’t always necessary, Shroyer said. “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 the new regrowth.”

If producers do plan to shred or cut their damaged stands, they should leave at least 2 to 3 inches of stubble, Shroyer said. This will help encourage regrowth after the plants have been cut.

If damaged stands are cut, producers should watch the regrowth for alfalfa weevil and pea aphids, and treat immediately if needed, said Jeff Whitworth, K-State Research and Extension entomologist.

“Weevil larvae that survive in the leaf litter on the soil surface will start feeding on the new growth once the temperatures warm up. The hard freezes will probably kill a certain percentage of the weevil larvae, but it’s not certain how many. Some of the weevils have already pupated,” he said.

There haven’t been many reports on pea aphids on alfalfa yet this spring in Kansas, but they could become a problem on new regrowth after the stand is cut, Whitworth added.

If an insecticide had been applied to alfalfa for weevil control before the freeze, growers should be aware of any residual insecticide in the alfalfa that may affect how it can be utilized.

Freeze-damaged alfalfa that is only 6 to 8 inches tall or less will be slower to regrow after mowing or shredding than taller alfalfa, Shroyer said.

“That’s because alfalfa plants are depleting carbohydrate reserves from the roots during the first 6-8 inches of growth, and will not have as many carbohydrate reserves for regrowth as taller alfalfa,” he explained. “With slower regrowth, producers will have to watch especially closely for insect infestations and treat if necessary. Alfalfa taller than 8 inches 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, said Bruce Anderson, University of Nebraska-Lincoln Research and Extension forage specialist. Plants that are 12 inches tall are much more likely to experience significant damage than 3-inch tall plants. Anderson’s comments:

• If the low temperature was above 28 degrees in the field, there should be little significant impact on any alfalfa, except for some singed leaf edges.

• New seedlings are generally tolerant of cold weather, partly due to heat rising from the soil and to natural plant tolerance. Seedlings no older than first trifoliate growth stage can probably handle temperatures in the low 20s. As they grow, cold tolerance lessens so seedlings at the 4th or 5th trifoliate stage may be injured similarly to alfalfa seeded late last summer.

• On well-established stands, during the next few warm weather days, watch for:

(a) New growth emerging from the tip. This means plants are recovering nicely and no action is necessary.

(b) New growth emerging as branches below the tip. This means the growing point was killed, slowing plant development significantly, but recovery is occurring. No action is necessary.

(c) New shoots emerging from crown buds. This means the growing point was killed and very little new growth can be expected from existing shoots. Cut or graze if sufficient growth is available for economical harvest before new shoots get tall enough to be damaged by the harvest. A word of caution: Cutting or damaging new regrowth shoots will cause severe, sometimes even fatal, damage. If the new regrowth has grown too tall to safely cut the plants, just let the new shoots develop and expect to take the first cutting much later than normal.

If the growing point cluster has been frozen and plants are severely wilted, the plants should be harvested or shredded as quickly as possible to encourage new shoots from the crown. If the plants are not harvested or shredded, new shoots will come eventually, but much slower and less densely than if existing plants are removed.

In all situations, if plants are laid over by the freeze and do not straighten back up, removal of this injured growth will hasten regrowth significantly.

• Well-established, healthy plants should start regrowing from new shoots emerging from the crown within seven days of favorable temperatures. Old diseased plants and last year’s planting will take longer to start regrowing and some may not survive at all. New seedings frozen to ground level are very likely dead, and the stand should be reseeded as soon as possible, or planted to another crop.

Contact:
Jim Shroyer
785-532-0397