Alfalfa winterkill: Now what?

Paul Peterson, Dan Martens, Dave Nicolai and Krishona Martinson; University of Minnesota

If you haven’t done so already, get out and look closely at alfalfa fields. Even if plants are not dead, they may be injured and in need of attention. Look for slow or no green-up and uneven, asymmetrical growth. Dig up some plants and inspect the crown and root. Split the crown and root open lengthwise.

Healthy roots are firm and white inside with little evidence of root rot. Winter-injured roots are gray and water-soaked or brown due to root rots. If the crown or root is soft and water-soaked, it is most likely dead. If over half of the root is damaged, the plant will likely die this year. If less than half of the root is injured, the plant will likely survive for another year.

A minimum of three to four alfalfa plants per square foot must be present for a stand to still have good production potential. However, stem density is a better determinant of yield potential since plants that are injured can still survive but will have significantly reduced stem number and thus lower yield potential. Use the following guidelines to aid in making a decision about keeping a winter-injured stand:

Stem density per square foot Action
Greater than 55: Stem density not limiting yield
40 to 55: Stem density limiting yield potential
Less than 40: Stem density severely limiting yield potential; consider replacing the stand

Stands can be slow to recover when injured, so don’t be in too much of a hurry to tear it up until you’ve looked at some roots to determine health. If surviving plants are injured but have adequate stem density to maintain the stand, it’s probably best to plan to push the stand with annual forages (e.g. Italian ryegrass or oats) to get the most out of it this year, then plan to terminate after this growing season. If most surviving plants look reasonably healthy (losses were localized or ice-sheeting related) you might consider interseeding perennials to stretch the stand beyond this year.

Some options

•If the winter-injured alfalfa field was seeded less than one year ago, alfalfa can be reseeded safely without autotoxicity concerns. Thin stands or bad spots can be interseeded with alfalfa. Italian or annual ryegrass, oats or red clover can be added to thin alfalfa stands to increase short-term yields. Orchardgrass is a good perennial grass option for interseeding if you intend to try to keep the stand beyond this growing season. If dead areas are essentially bare, a conventional drill can probably do the job while the ground is still somewhat soft. If the damage is widespread, the stand should probably be torn up and it is safe to reseed to alfalfa this spring.

•If the field was seeded more than one year ago, plant a different crop for a season before planting alfalfa again to avoid autotoxicity problems. If damage is spotty, interseed Italian ryegrass, a small grain or red clover. Attempting to interseed alfalfa into older alfalfa stands is risky and unlikely to be successful because of autotoxicity. If most of the stand is dead, corn for grain or silage may be the best bet, but small grains and forage grasses can make good use of the nitrogen left behind by the alfalfa.

•Thin or dead spots may be a good place to try Italian ryegrass. Seeded at 15 to 25 pounds per acre, it would be on schedule to contribute significantly to the second and later alfalfa cuttings with quality similar to alfalfa. Italian ryegrass can be challenging to cure to hay moisture, so plan to chop, make baleage or graze. If seeding after complete alfalfa winterkill or stand termination, use 25 to 30 pounds per acre of Italian ryegrass.

•Plant new alfalfa stands in different fields. Make sure herbicides used last year are not a problem for seeding alfalfa and that soil pH is okay. Planting a small grain (less than 1.5 bushels per acre) or Italian ryegrass (less than 5 pounds per acre) nurse crop will provide some quicker tonnage in these fields. The nurse crop stubble may also help with snow catch and alfalfa crown insulation from cold temperatures.

•Plant only alfalfa varieties with proven winter hardiness unless you’re planning for short-term stands (three years or less). Consider seeding a low-alkaloid reed canarygrass variety with new alfalfa seedings. A sod-forming grass in mixture with alfalfa may provide some insulation and reduced heaving potential.

•Plant a short-season forage crop to get some near-term forage with a goal of reseeding alfalfa in early August. This might include oats or another small grain harvested for forage at the boot stage, mixed with peas and harvested at heading, or Italian ryegrass for silage or grazing. If the pH needs to be improved for an early August planting target, work lime into the soil now. On heavier soils, it’s safest to wait a full year before attempting to reseed alfalfa, but on lighter soils, if moisture is adequate, August reseeding should be safe.

•For dry cows and young stock, sudangrass, sorghum-sudan or pearl millet are good multi-cut or graze options if planting gets delayed into early June. These crops can be seeded following a final spring harvest and termination of a winter-injured alfalfa stand. Foxtail millets can provide some moderate-quality forage in one cutting within 60 days after seeding, and can germinate in cooler soils than the sorghums or pearl millet. But if maximum total-season DM and energy yield are needed, corn silage is probably the best bet, even if planting as late as July 1.

•Make maximum use of pasture. Good fertilization and rotational grazing management will increase pasture productivity. Look into cost-sharing opportunities to help increase your use and management of this low-cost feed source on your farm.

•Add 40 to 50 pounds nitrogen per acre per cutting or manure to boost yields of grass and legume-grass hay fields where the legumes are thin.

•Do some shopping and see whether you can buy hay or other feeds to build a ration at a cost that still provides an acceptable return.

•Discuss ideas and experiences with others. Experiment with new ideas on a small scale where possible. HG

References omitted but are available upon request.

—Excerpts from University of Minnesota Crop e-News