Unusually cold weather late last week and through the weekend damaged alfalfa fields in much of the Upper Midwest and Northern Plains. It's too early to accurately assess the damage, but experts suspect that it was mostly limited to alfalfa topgrowth. Frost-damaged plants will likely be stunted but eventually will recover. However, alfalfa crowns may have been damaged below the soil surface in some fields in Minnesota, permanently injuring or even killing the crop, fears Paul Peterson, University of Minnesota extension forage agronomist.
Alfalfa crowns become less cold tolerant as they come out of dormancy, he says. But the temperature at which they're permanently damaged is unknown and probably quite variable since it's influenced by many factors. Air temperatures dropped to the mid-teens for several nights throughout Minnesota, and on Saturday morning, the soil temperature on the university's St Paul campus hit 27 degrees 2" deep and 20 degrees 1/2" deep. That was below bare soil; Peterson emphasizes that even a small amount of crop residue may have a significant insulating effect.
On Monday, Peterson dug up plants from several fields of varying ages at Umore Park near at Rosemount, just south of the Twin Cities, and was relieved and somewhat surprised to find limited evidence of crown damage. Those plants had recently broken dormancy and had up to 2" of new growth. He fears that the crop may not have fared as well in areas where it started growing earlier and had more growth, such as on sandy soils and south-facing slopes. Fields that entered the winter with a stress factor such as low fertility or a fall cutting also are more at risk, says Peterson.
He suggests that growers who think their alfalfa might have been permanently damaged dig up a few crowns, thaw them if they're frozen and examine them. If they're mushy inside, they've been injured and the field may need to be replaced.
In Nebraska, permanent damage is not a concern of Bruce Anderson, University of Nebraska extension forage agronomist. Despite several nights of well-below-freezing temperatures, he doubts that many fields in his state were injured that seriously. "I expect that we're going to have some pretty slow regrowth in many cases, but I think survival of the plants is not really a big issue," says Anderson.
Alfalfa came out of dormancy earlier than usual, he says, and some fields had 10-12" of growth before the freeze. Those fields are likely to have more frost damage than fields with less growth, he says. He expects to see a lot of variability in the amount of damage. Plants in all frozen fields have likely drooped, but some will straighten up and continue growing normally. In other cases, the topgrowth is probably dead and plants will send new shoots from crown buds.
When assessing the damage, Anderson says to focus on a plant's growing point, which is inside the dense cluster of unfolded leaves at the top of the main stem. It's somewhat protected and may be still alive, even if the leaves and stems around it are dead. If it appears healthy and remains green, the plant may be stunted but should start growing again.
If the growing point is dead, additional growth from the existing plant is unlikely. Anderson recommends shredding or cutting the killed topgrowth to speed development of new shoots.
"We had somewhat similar conditions three years ago, and we had much faster recovery in fields that were cut off than in those that we just left," he says. "The alfalfa came back when we just left it, but it was much slower and the number of shoots per crown seemed quite a bit lower. So the above-ground growth inhibited the release of new shoots."
For more details, read Anderson's article, "Assessing Alfalfa Post-Freeze," at cropwatch.unl.edu/archives/2007/crop6/alfalfa_freeze.htm.
For a report on freeze damage in Kansas, click on hayandforage.com.