How much winter injury did alfalfa receive from recent low temperatures?
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
The warm temperatures in late March may have gotten the alfalfa into trouble. Fortunately, 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 Michigan, recent weather has been pretty favorable for alfalfa survival. Early reports showed 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 Michigan 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 Michigan, however, alfalfa did begin to break dormancy in late March, and while the western part of southern Michigan had insulating snow cover in early April, the eastern part did not. 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 to 15°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.
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 to 25°F, 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 inches, likely have more freezing tolerant crowns than those that are further along, say 6 inches 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 areas in my alfalfa trials of varying ages in East Lansing last week Thursday, April 12. 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.
- Areas to watch more 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, and then it will appear more dehydrated in a week or two.
- Where initial shoots become 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 inches of viable shoots, take stem counts in several places in each field. Fewer than 40 stems/ft2 means the stand isn’t worth keeping; an average of 40 to 55 stems/ft2 is borderline. An average of more than 55 stems/ft2 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, 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 N 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.
Dr. Leep's work is funded in part by MSU's AgBioResearch.