Freezing temperatures at this point in early May can injure the stems of alfalfa and create management decisions for producers, said Jim Shroyer, K-State Extension agronomist.
"When temperatures get into the 20s for several hours, producers can expect to see injury to stems. With the amount of growth currently on alfalfa, I would expect to see stems being crimped high in the plant canopy on stands injured by the freeze," he said.
Heat radiating upward from the soil can counteract the freezing temperatures to some extent and may help protect the plants, he added. However, windy conditions can increase the likelihood of potential injury.
If the tops of stems suffer freeze injury, the uppermost leaves in the canopy will begin dropping within a few days, Shroyer said.
Beyond that, alfalfa plant sensitivity will depend on the amount of growth at the time of the freeze. Plants that are 12 inches tall are much more likely to experience significant damage than 3-inch tall plants.
During the next few warm weather days, Shroyer advised producers to watch for:
1. New growth continuing from the tip. This means plants are recovering nicely. Take no action unless the stand is nearly ready to cut anyway. If the stand is nearly ready to cut, do so quickly before any leaf drop occurs.
2. New growth emerging as branches below the tip. This means the growing point was killed, slowing plant development significantly, but recovery is occurring. As mentioned above, take no action unless the stand is nearly ready to cut anyway.
3. Normal regrowth emerging from crown buds. This means the growing point was killed and very little new growth can be expected from existing shoots. Cut the stand as soon as possible if sufficient growth is available for economical harvest before new shoots get tall enough to be damaged by the harvest. Cutting or damaging new regrowth shoots will cause severe damage. If the stand is too short, just let the new shoots develop and expect to take the first cutting much later than normal.
When cutting freeze-damaged alfalfa, be sure to leave at least 2 to 3 inches of stubble. This will help encourage regrowth, Shroyer said.
If damaged stands are cut, producers should watch the regrowth carefully for further infestations of alfalfa weevil and possibly pea aphids, and treat immediately, he added.
"Weevil larvae that survive in the leaf litter on the soil surface will start feeding on the new growth once the weather warms up. A hard freeze will probably kill a certain percentage of the weevil larvae, but it´s not certain how many. Some of the weevils have already pupated by now," he said.
"If an insecticide had already been applied to the alfalfa for weevil control, producers should be aware of any residual insecticide in the alfalfa that may affect how it can be utilized," he said.