For much of the Midwest, a mild winter followed by an early spring gave this year’s alfalfa crop a nice boost. However, dipping temperatures in the past couple of weeks left some fields susceptible to frost damage.
    We cannot draw a direct line between temperatures reported by the local news outlets and the damage done to our alfalfa fields. Temperature readings are captured by controlled weather stations generally located in areas with different conditions than most alfalfa fields.
    Temperatures immediately surrounding an alfalfa plant can vary as well. Temperatures in the plant canopy level are usually layered, meaning the temperature at the top of the canopy is colder than the temperature at the soil surface, which is colder than the soil in the taproot and crown area. This canopy temperature will also be higher than reported air temperatures. Soil temperatures, slope, wind and the microclimate within a field can all have an effect.
    The extent of frost damage to alfalfa depends on the severity of the freezing and the results of the frost are not immediately apparent. Frozen alfalfa usually needs some time to recover before damage can be estimated. To complicate things a bit more, Dr. Stephan Barnhart, professor emeritus at Iowa State University, indicates tolerance of leaves to frost varies among varieties and individual plants and is not related to winter hardiness of the variety.
    In mild frost cases, leaves at the tops of the plants become wilted and discolored, but plants should completely recover. If the tip of the plant straightens, normal growth resumes. Frosts that occur at 26 degrees Fahrenheit and warmer can freeze leaf margins (resulting in white spots on leaves) but not damage stems or growing points.
    To assess the extent of the damage, do not only look for frozen or wilting leaves. You need to determine if the growing point was killed. This growing point (apical meristem) is where all new leaves, stems and branches initially develop on alfalfa. It is located inside the dense cluster of unfolded leaves at the top of the main stem.
    Because the growing point is inside a cluster of leaves, it 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 the growing points in your alfalfa survived the freeze, wait for growth to begin again.
    If the growing point was killed, however, growth will cease on that stem. Any new growth must come from new crown shoots or from unfrosted axillary stems in the lower branches. Plants use up some of their root reserves for their initial spring growth. After growing points are frozen, these plants will then have to initiate regrowth from new crown or axillary buds. This will delay the growth and developments of the crop as well as use up more of the remaining root reserves. Healthy stands will recover more quickly. Depending on weather conditions, some first-cut yield reduction and a delay in maturity can be anticipated. Where damage is uneven across a field, there can be some unevenness in maturity. If possible, delay first or second cuttings of severely affected fields to allow rebuilding of root reserves and full recovery.
    There is usually no benefit to cutting frosted plants, especially if growth is less than 20 inches. Cutting will not enhance recovery and forage quality drops rapidly. Severe frost kills the growing points, the same as cutting does. Regrowth would have to come from new crown buds and may further weaken the stand. Yield may be extremely low with poor quality.
    Damage to new seedings of alfalfa is usually minimal. Immediately after emergence, alfalfa seedlings have fair to good tolerance to cold injury. Once they reach the second trifoliate leaf stage, tolerance to cold injury declines markedly. Four or more hours of exposure to 26 degrees can kill seedlings at this growth stage. Companion crops somewhat protect new alfalfa seedlings from frost damage.
    Before reseeding, assess the damage in new stands three to five days after a frost. Plants will initially wilt back. If the entire plant dies back to the ground, it is dead. To survive and recover, one set of leaves must survive. Reseeding may be required if less than 15-20 viable plants per square foot survive.
    Unfortunately, if your crop has been weakened by a frost, the effect may not be immediate. It may instead come later in the season when this stress to plant tissue makes it more susceptible to damage from foliar diseases, and sometimes insects, on regrowth. Work with your agronomist and continue scouting fields to determine if management strategies such as fungicides or insecticides can reduce risk.
    Barry Visser is a nutritionist for Vita Plus.