How do you choose among the many options? First, distinguish between winterhardiness and fall dormancy.
Fall dormancy is a measure of how tall the alfalfa grows in the fall, and is ranked from 1 to 11 with 1 being Canadian types and 11 being non-dormant types grown in southern California and Florida.
In the northern states, you want as little fall dormancy as possible (higher number). Less fall dormant types tend to start growing faster in the spring and after each cutting, yielding more than less fall dormant types. Less fall dormant types also grow more in the fall, making a late fall cutting more worthwhile.
The old Vernal types, having a fall dormancy of 2, would never grow more than about six inches tall in the fall, no matter how warm the weather was. They were marginally worth harvesting. New types with fall dormancy of 4 will grow much taller and produce more tonnage in the fall if weather cooperates.
Historically, we used fall dormancy to indicate winter survivability potential. This relationship is still true. Varieties with fall dormancy 1, from Canada, will survive best; 10s or 11s will always winterkill in the Northeast.
Link between dormancy and survivability is broken
Plant breeders have broken the relationship between winter survival and fall dormancy over the range of fall dormancies that we grow -- generally fall dormancies of 2 to 4. Thatâ€™s good news; It means we can grow the less fall dormant, higher-yielding types without the fear of winterkill if we select for good winter survival.
So, how much winter hardiness you need? Locations with colder average temperatures over winter tend to need more winterhardiness in alfalfa for good winter survival. But snow cover is a major factor. As little as 4 inches of snow can result in a 10-degree difference in soil temperatures.
Regions with good snow cover can grow less winterhardy alfalfa varieties than those with little or no snow cover for significant portions of the winter.
Your management also has a significant effect on alfalfa stand survival. Older stands are more likely to winterkill than younger ones. Alfalfa grown on well drained soils are less prone to winter injury. Lastly, both harvest frequency and timing of fall cutting affect alfalfa winterhardiness. Stands with shorter intervals between cuttings have greater risk of injury.
Winter injury probably causes more economic loss to more farmers than actual alfalfa winterkill because injury occurs more commonly and results in yield loss.
Your fields hold the answer
To determine if youâ€™re growing alfalfa varieties with enough winter hardiness for your situation, examine the stands as they green up this spring. Do your stands green up uniformly and thick or do they often green up with few shoots which fill in a week or two later?
In the later case, you often see two heights of alfalfa stems from green up in the spring until the stand is 6 to 10 inches tall, which indicates the alfalfa was injured. If you generally see good thick, uniform stands in the spring, you are using alfalfa varieties with adequate winterhardiness for your situation.
Dan Undersander is a University of Wisconsin Extension and Research forage agronomist.