By Nathan Winter — U of M Extension Educator

Will April’s chill spell cause “winter” kill? State Extension Forage Specialist Paul Peterson posted some discussion recently at http://www.extension.umn.edu/cropenews [1], discussing conditions around Minnesota and what to watch for when spring comes around again. Here are some of the points Paul makes (with some editing).

The potential for winter injury is always difficult to predict. Perhaps the most useful point of this discussion is to get out into alfalfa fields to see how they’re doing over the next two to three weeks.

In the northern two-thirds of Minnesota, recent weather has been favorable for alfalfa survival. Early reports indicate that little if any alfalfa broke dormancy in March. And most of the northern two-thirds of the state received an insulating blanket of snow prior to the cold snap. There may be more concern in low areas where water may have pooled and frozen for extended periods during the winter, on wind-swept hill tops, or south slopes where snow melted ahead of cold winter weather.

In the southern third of Minnesota, however, alfalfa did begin to break dormancy in late March, and did not have insulating snow cover during some of the cold weather in early April. Healthy, well-hardened, dormant alfalfa is very cold tolerant; crowns and crown buds are thought to be able to withstand soil temperatures as low as 5 to 15 degrees.
Plant stubble and residue helps to buffer cold temperatures along with holding snow. From April 5 through April 8, soil temperatures at depths of 1 and 5 centimeters in bare soil on the St. Paul Campus dropped to lows of 20 and 27 degrees respectively; and reached highs of 54 and 40 degrees. During that same four days, the temperatures at depths of 1 and 5 centimeters under grass sod gradually dropped from 35 to 33 degrees.

Plant cover can protect the crown and root from freezing damage, and also protects the crown from the deception of warm early-spring temperatures that would stimulate early dormancy breakage. An alfalfa stand is neither bare soil nor a grass sod, so soil temperatures around alfalfa crowns is somewhere between, and likely closer to the sod situation where stands are thicker and more residue is present.

The critical low soil temperature that actively growing (fully out of dormancy) crowns can tolerate is unknown and influenced by many factors; our best guess for healthy plants is somewhere in the range of 20 to 25 degrees. There are lower “guesses”. Alfalfa plants don’t go from fully dormant to fully out of dormancy overnight, at least from a chemical composition perspective. Freezing tolerance is probably lost gradually as the plant comes out of dormancy and crown buds develop into new stems. Plants with growth, say 1 to 3 inches, likely have more freezing-tolerant crowns than those that are further along, say 6 inches or more. But as growth progresses and ground cover increases, crown insulation is also improved. Yet the new growth is fully exposed. Air temperatures in the low 20s can kill the growing point of shoots, so at a minimum, it is likely that there was considerable damage to any fully emerged shoots in early April. Healthy plants (with unfrozen crowns) will recover with a second round of crown buds once temperatures warm up again.

Some recommendations:
• Areas to watch closely this spring include south-facing slopes and coarse-textured soils where alfalfa likely got the earliest start, as well as wind swept hill tops and low areas with extended water ponding.

• With shovel in hand, take a walk through all of your alfalfa fields soon. Watch for growth and how symmetrical that growth appears. Asymmetric spring growth is a sign of winter injury. Evaluate whether the shoot tips appear killed.

• Dig up some plants and look closely at the crown, crown buds, and taproots. Any evidence of rot now would be from previous stress. Crown and root tissue that has been frozen recently will be soggy initially when warmed, then appear more dehydrated in a week or two.

• Where initial shoots get frozen, healthy crowns have adequate crown buds to replace those shoots, using more stored energy. These stands would benefit from a delayed cutting at some point during 2007 to replenish root reserves.

• Monitor stands on a weekly basis since injury may not be readily apparent.

• After there is about 6 inches of viable shoots, take stem counts in several places in each field. Fewer than 40 stems per square foot usually mean the stand isn’t worth keeping; an average of 40 to 55 stems per square foot is borderline. More than 55 stems per square foot means the stand is in good shape.

• If you’re seeing enough damage to cause reasonable concern, begin planning for other fields you could sow to new thick stands of alfalfa. In southern Minnesota, its best to try to have spring alfalfa seedings in by mid-May.

• If high quality hay/haylage inventories are low, stands that appear somewhat questionable may be worth keeping at least for a first cutting.

• If you decide to terminate the stand either now or after one cutting, plan to follow with a grass crop that can benefit from the free-fixed nitrogen left behind by the alfalfa. Corn silage is generally the highest tonnage option.

(Source: Paul Peterson - State Extension Forage Specialist and Dan Martens – Benton, Stearns, and Morrison County Extension Educator)

Nathan Winter is an agriculture educator at the University of Minnesota extension — McLeod & Meeker Counties, 840 Century Ave. S.W., Hutchinson.