Those balmy temperatures in late March sure felt good, but they may have gotten our alfalfa into trouble. Fortunately, however, while the weather progression from a warm, dormancy-breaking late March to a bone-chilling early April has provided reasonable cause for concern; early field observations are providing some comfort, at least for the moment.
The potential for winter injury is always difficult to predict, but we continue to try to do it anyway. Whether our predictions turn out right or wrong, perhaps there is benefit in getting us to think about it and into alfalfa fields to see how they’re progressing.
In the northern two-thirds of Minnesota, recent weather has been pretty favorable for alfalfa survival. Early reports indicated 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. So concerns in central and northern Minnesota are largely limited to low areas where water may have pooled and frozen for extended periods during the winter, enhancing the potential for either suffocation or heaving.
In the southern third of Minnesota, however, alfalfa did begin to break dormancy in late March and did not have insulating snow cover in early April. Healthy, well-hardened, “fully” dormant alfalfa is very cold tolerant; crowns and crown buds are thought to be able to withstand soil temperatures as low as 5-15 degrees F. Snow cover is one of the best forms of insulation, but plant stubble/residue also helps – not just indirectly by helping to catch snow, but directly, too.
For example, from April 5 through April 8, soil temperatures at 1- and 5-cm depths in bare soil here on the St. Paul Campus dropped to lows of 20 and 27 degrees, respectively; and reached highs of 54 and 40 degrees, respectively. During that same four-day period, soil temperatures at the 1- and 5-cm depths under grass sod gradually and steadily dropped from 35 to 33 degrees. So plant cover not only provides protection against freezing damage to below-ground tissues, but it 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 temperatures to which alfalfa crowns are exposed are 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 certainly influenced by many factors; our best guess for healthy plants is somewhere in the range of 20-25 degrees, but I’ve seen lower estimates. And alfalfa plants don’t go from fully dormant to fully out of dormancy overnight, at least from a chemical composition perspective. It’s likely that freezing tolerance is lost gradually as the plant comes out of dormancy and crown buds elongate to form legitimate amounts of herbage. Thus, plants that have little herbage development, say 1-3”, likely have more freezing-tolerant crowns than those that are farther along, say 6” or more. But as herbage development progresses and thus ground cover increases, crown insulation is also improved. Yet the herbage 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 shoot damage to fully emerged shoots in early April. Healthy plants (with unfrozen crowns) will recover via a second round of crown buds once temperatures warm up again.
I dug up alfalfa plants from several fields of varying ages at UMore Park near Rosemount on Monday, April 9. The ground was somewhat frozen only in a few of the north-facing areas I sampled. In general, I was pleasantly surprised at the apparent condition of the crowns and crown buds. There was less injury than I expected to see. But these stands went into the winter in good health and, likely, well-hardened. I’ll be keeping a close eye to see if injury symptoms are delayed.
• Areas to watch most closely this spring include south-facing slopes and coarse-textured soils where alfalfa likely got the earliest start.
• With shovel in hand, take a walk through all of your alfalfa fields soon. Look at how much growth has progressed and how symmetrical that growth appears. Asymmetric spring growth is a sign of winter injury. Ascertain 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/root tissue that’s 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, but probably at some energy cost. These stands would benefit from a delayed cutting at some point during 2007 to ensure root reserves get replenished.
• Monitor stands on a weekly basis since injury may not be readily apparent.
• After there is about 6” of viable shoots, take stem counts in several places in each field. Fewer than 40 stems/square foot means the stand isn’t worth keeping; an average of 40 to 55 stems/square foot is borderline. An average of more than 55 stems/square foot means the stand is in good shape.
• If you’re seeing enough damage to give you reasonable cause for concern, begin planning for other fields you could sow to new, thick stands of alfalfa. In southern Minnesota, it’s 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. Italian ryegrass is a high-quality “annual” grass option that can provide multiple cuttings and high yields in the seeding year. Small grain-pea mixtures (if peas are affordable and planting can be done early) can provide good-quality forage within two months after planting.