Jerry Little is County Extension Agent for Agriculture/Natural Resources.

First, it is important to understand that determining the temperature that alfalfa stands were exposed to during a frost event is less than exact science. Air temperature reported by local news stations likely use data logged at a weather station. These guidelines state that sensors should be installed on level terrain, away from paved or concrete surfaces and upright structures, four to six feet above the soil surface, and in a radiation shield. Most pastures and hayfields do not meet these criteria. In fact, most alfalfa plants never reach a height of four to six feet. The height of alfalfa exposed to spring frosts is likely to be in the 6- to 10-inch range. In addition, conditions at the soil surface in a field can be quite different compared to the air temperature at five feet above the soil. Couple all this with variations in topography and you can have many “microclimates” within a single field.

Light frost: temperatures that the alfalfa plant experienced were in the range of 27 to 31ºF.  Alfalfa leaves may be impacted when temperatures drop below 28ºF. This will present itself as the loss of some trifoliate leaves and twisting of stem ends near the top of the canopy. In this temperature range, alfalfa buds and growing points will not likely be damaged and will continue to grow as normal. Alfalfa will normally outgrow this damage and no changes in management practices are warranted.

Moderate frost: temperatures that the alfalfa plant experienced were in the range of 26 to 27ºF.  Freezing causes significant injury in the terminal growing point near the end of the stem, however the entire stem is not killed. Axillary buds at each leaf position lower on the stem and crown buds will begin to develop into new stems. Stems lower in the canopy that were not impacted by the frost will continue to grow normally.  The net result is a mixture of mature and vegetative growth that can make harvest decisions difficult. If high quality forage is the number one priority, then harvest based on the maturity of the undamaged stems. If a lower feed quality can be tolerated, then you may want to delay harvest for a week or so to allow maximum accumulation of carbohydrates in the taproot. It is important to avoid cutting moderately frosted alfalfa early. This decreases both yield and regrowth rate of the stand due to the stress of early cutting.

Severe frost: temperatures that the alfalfa plant experienced were below 26ºF. Stems that were growing at the time of the frost will not regrow.  Regrowth will come from crown buds at the base of the plant. Since severe frost has the same effect on the alfalfa plant as early harvest, regrowth will likely be slower. If alfalfa fields are fenced, frost damaged plants could be grazed off. However, grazing should be carefully managed to reduce the chance of bloat.  Waiting for frosted alfalfa to begin to dry will significantly reduce the chances of bloat. Animals should never be turned into alfalfa hungry and should have free access to a high quality dry hay at all times.  Flash grazing damaged stands with a high livestock density for a short period of time is highly recommended. If possible delay the second harvest a week or so to allow the plant to recover it’s stored carbohydrates.

For more information on alfalfa and forage management, contact the Boyle County Cooperative Extension Office or visit http://www.uky.edu/Ag/Forage/.