With the recent cold weather and always present danger of severe frosts in April, many of you have been concerned with frost damage to alfalfa. Note: the temperature that matters is in the plant canopy, not at the 4-6’ height where NOAA usually measures temperature.
Light Frost: temperatures in the range of 27 to 31° F. Leaves may be impacted and some twisting of stems ends when temperatures drop below 28° F. Buds and growing points are rarely damaged. Alfalfa will normally outgrow this damage and no changes in management practices are warranted.
Moderate frost: canopy temperatures in the range of 26 to 27°F. Freezing causes significant injury in the terminal growing point near the end of the stem, however the entire stem is not killed. Axillary and crown buds begin to develop into new stems. Lower stems, not impacted by frost, will continue to grow normally. This results in a mixture of mature and vegetative growth that makes harvest decisions difficult. It is important to avoid cutting moderately frosted alfalfa early, since the stress of early cutting decreases both yield and regrowth rate.
Severe frost: temperatures below 26° F in the canopy for extended periods. Stems that were growing at the time of frost will not regrow. Regrowth comes from crown buds at the base of the plant. Alfalfa stands with sufficient growth to justify harvest should be cut immediately after a severe “killing” frost. This helps to maintain forage quality and allows regrowth from crown buds. When stands are short, frost damaged plant material can be left in place, but leaving damaged or killed plant material may delay regrowth. Severe frost damaged plants can be grazed off. Waiting for frosted alfalfa to begin to dry will significantly reduce the chances of bloat or use a bloat preventative. Since severe frost has the same effect on alfalfa as an early harvest, regrowth will likely be slower. Delay second harvest a week or more to allow the plant to recover its stored carbohydrates.
New Alfalfa Seedings: Immediately after emergence, alfalfa seedlings have fair to good tolerance to cold injury. Once they reach the 2nd trifoliate leaf stage, tolerance to cold injury decreases markedly. Four or more hours of exposure to 26° F can kill seedlings at this growth stage. Before reseeding, the extent of damage in new stands should be assessed. The ideal stand density at the end of the first production year is 12-20 plants per square foot. If this number of healthy seedling per square foot is present following frost damage, then no reseeding is needed. If stands do not consistently have 12-20 healthy plants per square foot, then thickening stands with no-till seeding may be the best option. In most cases, seeding rates in the range of 10-12 lb/A should be sufficient for thickening frost damaged stands.
~Chris Teutsch, Jimmy Henning, Ray Smith, and Tom Keene. Full article on home page of the UK Forage Website. http://www.uky.edu/Ag/Forage/.