Early morning temperatures from May 14-16th have been below 32° F for much of the state, with some portions of the state reaching as low as 23° F. Crop damage will be a concern in a few instances.

The crop, crop growth stage, air temperature, near surface air temperature, soil temperature, length of freezing conditions, and local topography among other factors all influence the extent of crop damage that may occur during an early morning freeze event. Corn emergence is behind average as well as growth of spring small grains and pulse crops, which will minimize the impact of these freezing temperatures. Freeze damage to emerged soybeans would be a large concern, but very few acres of soybeans have emerged and those acres that have emerged in the southern part of the state did not see as low of temperatures. Alfalfa, on the other hand, may be more susceptible than the aforementioned, annuals.

South Dakota and Montana have the largest amount of acres dedicated to alfalfa and alfalfa mixtures in the U.S and therefore the biggest concern is the nearly 1.8 million acres of alfalfa and alfalfa mixtures in the state that will be impacted by these early morning freezing temperatures.

For established alfalfa stands, the following guidelines serve as a general rule of thumb (S. Barnhart, 2005 and direct communication with Vance Owens, SDSU):

Light Freeze (27 - 31° F): Alfalfa plants will outgrow the damage. Loss of several sets of trifoliolate leaves down the stem and some slight curling of the stem may be observed. Buds or growing points will continue to grow as normal. A majority of the terminal buds should still look green and alive 2 to 3 days after the freeze event.

Moderate Freeze (26-27° F): Upper part of the stem and terminal buds can be killed. Regrowth must occur from lower axillary buds on the stem or from new crown buds. Growth and first cutting will be delayed.

Hard Freeze (26° F and colder): Leaf, buds, and stem tissue will be killed and plants will regrow from crown buds. First cutting will be delayed significantly.

In newly seeded alfalfa:

Under normal conditions, alfalfa seedling populations will naturally thin down by the end of the establishment year with a target range of 25 to 35 plants per square foot in the establishment year. In the first full production year the following spring, 12 to 20 plants per square foot is sufficient.

New seeded alfalfa or seedlings between the 2 and 5 trifoliolate leaf stage are very susceptible to being killed if exposed to 26° F at 4 or more hours. Companion crops can help seedlings survive under colder or longer periods of exposure. Seedlings from emergence to the 2 trifoliolate leaf stage are more cold tolerant (D. Undersander et al., 2011). When seedlings lose all trifoliolates and look discolored, they will not likely regrow.

Re-seeding with the drill into thin and damaged areas may be warranted on new seedings. The presence of a companion crop like oats could be too competitive for interseeded alfalfa seedlings. In this case, replanting the areas without the surviving stand may need to occur.

Source - http://www.tristateneighbor.com/