Cold and moist weather ended last week, as temperatures across South Dakota plummeted below freezing. The mornings on May 13-15 were near to below freezing across much of South Dakota. The map below shows the temperatures (minimum in blue, maximum in green) for May 14, 2016 at the SD Mesonet sites (Figure 2). Many sites in the west and north were below freezing for more than a few hours.
As we enter this growing season, alfalfa fields are showing great potential for good tonnage production. Alfalfa can withstand temperatures as low as 20° F. However, several hours of 25 to 27° F or lower temperatures could damage leaf tissue and the alfalfa growing points.
Newly Seeded Alfalfa
Seedlings between the 2 and 5 trifoliate leaf stage are susceptible to being killed if exposed to 26° F for four or more hours. Using companion crops (oats and orchardgrass) can help seedlings survive cooler temperatures for longer periods of time. Seedlings from emergence to the second trifoliate leaf stage tend to be more cold tolerant (Undersander et al., 2011). Re-seeding with a drill into the damaged areas may be warranted.
Established Alfalfa (2+ years)
A light freeze where temperatures do not go below 27° F is likely to freeze several sets of trifoliate leaves on alfalfa tops and terminal buds, but plants will grow out of it with warmer temperatures. A moderate freeze where temperatures go between 26 to 27° F is likely to damage the upper part of the stem and can kill terminal buds. Usually, growth and first cutting will be delayed. However, if producers are approaching their first cutting and there is damage, they should consider harvesting as soon as possible.
A hard freeze where temperatures go below 26° F is likely to damage and kill leaves, buds, and stem tissue. Plants will regrow from crown buds. Compared to a moderate freeze, first cutting will be delayed and significant yield losses might be expected.
What to Expect After this Event
Forage producers should start monitoring their fields and look for any possible damage since temperatures are warming up. If there is no significant harm, go ahead and scout for possible insects and proceed to harvest on your normal schedule. If there is 20-30% damage in the stems and leaves consider harvesting as soon as possible to avoid sacrificing yield and quality.