Considering the cold nighttime temperatures across the state this past week, many forage growers are concerned about their alfalfa yields. Forage research indicates that although alfalfa is considered to have good cold temperature tolerance, minor frost damage may occur when plants are exposed to air temperatures slightly below freezing for several hours, and more severe damage will be seen when temperatures drop below 25°F for four or more hours.
Newly seeded alfalfa tends to be more frost-tolerant than well-established stands, but once plants surpass the second trifoliate leaf stage, seedlings become more susceptible to cold injury. Newly seeded alfalfa planted with a nurse crop may survive cold temperatures better than those seeded alone. As alfalfa plants age, winter hardiness is lost. When temperatures stay near or below 25°F for several hours, alfalfa leaf tissue, buds, and growing points are susceptible to frost damage. If temperatures remain in the upper 20s, damage to exposed trifoliate leaves near the top of the canopy could occur but will not likely affect yield.
Understanding Frost Damage
There are several factors that contribute to frost damage, including: soil temperature, soil moisture, age of stand, surface residue, field location, and more. Due to the wide range of factors that can affect frost damage, the best way to know if an alfalfa stand has experienced injury is to wait 2-4 days to determine if leaves have wilted. Look for the following effects on plants:
- Blackened leaf edges: This means damage may be minimal and yield loss is not likely to occur. Harvest should be able to occur at the expected time.
- Some leaves are entirely damaged, but buds are not affected: As long as some leaves are healthy and buds look unaffected, yield loss should be minimal and no further action should need to be taken. Harvest should be able to occur at the expected time.
- The entire stem wilts and turns brown (including some leaves and bud): This may mean that the growing point has been killed by frost injury and stems will cease to grow any taller; however, growth can still occur on plants that look entirely wilted.
- Axillary shoots may develop at leaf junctures on the stem along with other new growth from developing crown buds. Should this be the case, yield will likely be reduced but a delayed harvest will aid in allowing more time for the new shoots to develop.
- If frozen stems are 20 inches or taller, it may be economical to harvest, and the crop will still provide a quality forage product. There is little concern for toxins in frozen alfalfa top-growth; nitrate levels can increase, but not typically to hazardous levels. Following harvest, be sure adequate soil fertility is maintained, and allow the alfalfa to begin flowering before cutting a second time for improved stand. Regrowth could be very slow and total seasonal yield loss will occur over the course of the season.
In many cases, durations of below freezing temperatures were not likely to have been cold enough nor long enough to significantly affect alfalfa yields this spring. However, should you have further questions or continued concerns about your alfalfa, please visit any of the resources below or contact Sara Bauder.