After the long frigid winter followed by cold wet spring, there is much concern about potential winterkill in alfalfa stands. After a slow start to the growing season, we are now seeing the extent of alfalfa winterkill. Damage is spotty across the southern part of the state, with many stands that looked questionable a few weeks ago now growing out of the problem because they have had time to form new buds to replace those damaged by cold. The Thumb region of Michigan is most affected, with many fields showing 30-80 percent loss and little recovery. Damage in northern Michigan and the U.P. is less widespread than in Central and Southern Michigan. Remember that alfalfa varieties with low fall dormancy (FD) ratings are later to break dormancy in spring than varieties with higher FD ratings and will therefore lag behind in development.
Where damage has occurred, it is concentrated in areas of fields where ice sheets formed, water ponded or channeled, where soil was continuously saturated for a long period of time during snow melt, or in wind-scoured areas where there was not enough snow cover to insulate alfalfa against extreme temperatures. Intensively and late-harvested stands that are three or more years old are showing more damage than younger stands under moderate management, indicating that accumulated stress is a factor in losses. Because plants damaged by winter cold or spring waterlogging are more likely to die during subsequent stresses like heat and drought, the full effect of the cold winter may not be evident until several cuttings are harvested.
What to do?
Damaged stands present producers with multiple decisions. Should they keep a damaged field despite reduced production potential, try to boost the production of forage from that field by supplemental seeding, rotate to a different crop or plant a new alfalfa field? The latter choice also requires attention to securing an emergency short-term forage crop to cover forage needs during the year it takes for the new alfalfa stand to reach full production potential.
All alfalfa stands naturally thin over time, but yield can still be maintained because surviving plants become bigger. Therefore, stands can tolerate some loss if enough healthy crowns remain. The number of crowns is very difficult to estimate accurately without digging plants, but fortunately the number of alfalfa stems per square foot is an even better indicator of stand vigor than the number of plants. Stem counts are easy to measure. Simply wait until new growth is 6 or more inches tall and count all stems longer than 2 inches within a one square foot area. Healthy stands contain more than 55 stems per square foot, regardless of stand age. Once stem count drops below 40 stems per square foot, it is time to seriously consider intervention.