April 6, 2007

With this week's frigid temperatures, the appearance of alfalfa stands may be causing growers concerns. Regardless of what your alfalfa looks like, wait until you have had a couple days of non-freezing weather to evaluate your crop. Until then, alfalfa plants are not going to begin to recover or regrow.

Alfalfa plant sensitivity will be strongly related to the amount of growth it achieved before the freeze, i.e. 12-inch plants are much more likely to experience significant damage than 3-inch plants.

If the low temperature was above 28°F, I expect little significant impact on alfalfa except for some singed leaf edges. Remember, though, that 28°F at the farmstead may not mean the same temperature in the field; low spots easily could have been 3 to 5 degrees colder for a longer time. To make management decisions, though, ignore the thermometer and watch the plants. 

1. New seedlings. Generally these plants are tolerant of cold temperatures, partly due to heat arising from the soil and partly due to natural plant tolerance. Seedlings no older than first trifoliolate growth stage probably handle temperatures in the low 20s. As they advance in growth, cold tolerance lessens; seedlings at the fourth or fifth trifoliolate stage may be injured similarly to alfalfa seeded late last summer; however, I doubt that there were many new fields with already emerging seedlings and likely none with seedlings at the fourth or fifth trifoliolate stage. 
2. Well-established stands. Focus attention on the "growing point." This growing point, also called the apical meristem, is the initial development source of all new leaves, stems, and branches on the aboveground alfalfa. It is located inside the dense cluster of unfolded leaves at the top of the main stem. Because it is inside a cluster of leaves, the growing point is somewhat protected from cold injury. Exposed leaves and stems all around it can be frozen, wilted, and dying while the growing point cluster survives, waiting for warm weather before continuing to grow.

   If external parts of the canopy are the only parts showing freeze damage (wilting) so that the growing point survived, little impact is expected from the freeze. This assumes that the plants straighten back up if they laid over due to the freeze. If they don't stand back up, you probably should remove the existing growth by grazing, haying, or shredding to enable new regrowth to get started rapidly. If the growing point cluster appears healthy, stands straight, and remains green with little or no wilting, plant may be stunned but should start growing again with warm temperatures. Take no action except keep observing; if nothing happens following 10 days of favorable temperatures, harvest or shred the crop.

   If the growing point froze and plants are wilting below that cluster, a significant delay in recovery is likely. During the next few warm weather days, watch for: 

   • new growth emerging from the tip. This means the plant is recovering nicely. Take no action.
   • new growth emerging as branches below the tip. This means the growing point was killed, but the plant is recovering. Plant development will be significantly slower. Take no action.
   • new shoots emerging from crown buds. This means the growing point was killed and 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. Caution: Cutting or damaging new regrowth shoots will cause severe, sometimes even fatal, damage. Otherwise, just let the new shoots develop and expect to take first cutting much later than normal.
   • nothing happens. Based on experience during recent springs, this is the most likely reaction. If the growing point cluster froze and has wilted severely, additional growth from the existing plant is highly unlikely. Harvest or shred plants to encourage new shoots from the crown as quickly as possible. Or wait — new shoots will come eventually, but much slower and less dense than if existing plants are removed. In all situations, if plants that laid over by the freeze do not straighten back up, removing injured growth will hasten regrowth significantly. 
3. Last year's planting. Same as well-established stands except recovery, especially from crown buds, is likely to be much slower due to small root and crown containing low level of nutrient reserves. Might be wise to give plants an extra week to start recovering before taking any cutting, shredding, or grazing action that removes green leaves. 
4. Plants (any age) frozen to ground level. Remove frozen plants by grazing, shredding, or harvest to hasten initiation of new shoots and remove smothering mulch. 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 dead — reseed ASAP or plant to another crop.

Any plants damaged beyond singed leaf margins can be expected to become ready for first harvest later than if the freeze had not occurred. Be aware, though, that these plants may not bloom as they normally would. Don't rely strictly on bloom for harvest decisions. Instead, use your usual calendar date or appearance of new crown shoots as a harvest guide. Plan accordingly.

Haying

Any hay made from this early growth will be extremely rich and susceptible to microbial activity and heating. It needs to be extra dry when baled and/or should have plenty of preservative applied at baling, otherwise mold, heat damage, or even fire may occur. If silage is an option, this would be the best harvest method.

Grazing

Bloat risk likely remains high until plants have actually dried considerably. Wilting is not enough. In all cases, use animal husbandry practices to minimize bloat risk. For more information see NebGuide G1393, "Grazing Alfalfa" available on-line.

Bruce Anderson
Extension Forage Specialist