Dealing with winter injury in alfalfa fields

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.

Significant areas of alfalfa winterkill are now evident in Michigan. The worst areas are in southern Michigan from Lapeer County on the east side of the state to Ottawa County on the west side. Most of the damage is from saturated fine textured soils that heaved resulting in alfalfa crowns that were exposed to cold temperatures after heaving. In addition, lower areas in fields were killed from ice sheeting and excessive moisture.

The decision producers must make is whether to keep a less productive field, whether to try to boost the production of forage from that field by supplemental seeding, or to plan on planting a new alfalfa field as well as planting an “emergency” short-term forage crop for immediate future forage needs. Some of the most common questions arising include:

What fields are still worth keeping?

The answer to this varies greatly. Research would say that a “keeper” field with no appreciable yield loss would be a first production year field with 12 or more healthy plants per square foot; second and third production year fields, six or more crowns per square foot; and older fields, four or more healthy plants per square foot. Associated forage grasses may compensate some toward higher seasonal yields as alfalfa stands decline.

Can I interseed to thicken the stand – with more alfalfa?

Interseeding alfalfa to thicken a uniformly thin alfalfa stand will generally not work. If the stand is one year or less old, plants will generally come up and then be eliminated from competition from older plants from last year. Large dead spots should be disked first and then seeded. If the stand is two or more years old, interseeded alfalfa will be adversely affected by autotoxicity. Interseeding with red clover will not be affected by autotoxicity.

For two or more year old alfalfa stands, autotoxic compounds will likely reduce the stand or future yield of the alfalfa, and you should wait one year before reseeding. Although there is some conflicting data on autotoxicity, most agronomists recommend waiting one year before reseeding back to alfalfa.

You can interseed grasses (Italian ryegrass (not annual ryegrass) for one year or orchardgrass or tall fescue for two or more years) or clovers to thicken a stand.

Some options

  • If the winter-injured alfalfa field was seeded less than one year ago (spring or late summer 2008), alfalfa can be re-seeded safely without autotoxicity concerns. Thin stands or bad spots can be inter-seeded 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 inter-seeding 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 is safe to re-seed to alfalfa this spring.
  • If the field was seeded more than one year ago (2007 or earlier), plant a different crop for a season before planting alfalfa again to avoid autotoxicity problems. If damage is spotty, inter-seed Italian ryegrass, a small grain, or red clover. Attempting to inter-seed 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 best bet, but small grains and forage grasses can make good use of the N left behind by the alfalfa.
  • Thin or dead spots may be a good place to try Italian ryegrass. Seeded soon, Italian ryegrass would be on schedule to contribute significantly to the 2nd and later alfalfa cuttings with quality similar to alfalfa. Italian ryegrass can be challenging to cure to hay moisture, so plan to chop, make balage, or graze. If seeding after complete alfalfa winterkill or stand termination, use 25-30 lb/ac 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 (<1.5 bu/ac) or Italian ryegrass (<5 lb/ac) nurse crop will provide some quicker tonnage in these fields. The nurse crop stubble may also help with snow catch and/or alfalfa crown insulation from cold temperatures.
  • Plant a short-season forage crop to get some near-term forage with a goal of re-seeding 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 re-seed alfalfa; but on lighter soils, if moisture is adequate, August re-seeding should be safe.
  • For dry cows and young stock, sudangrass, sorghum-sudan, or pearl millet are good, multi-cut/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. Teff is an annual warm season forage crop which can be planted as it will yield about 4 tons of dry matter per acre in three cuttings. 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 from your local NRCS office to help increase your use and management of this low-cost feed source on your farm.
  • Add 40-50 lb N/ac/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.

Where winterkill occurred and the field will be rotated to a non-legume crop such as corn, it is essential to assign appropriate nitrogen (N) credits for the previous alfalfa crop. This will help avoid unnecessary fertilizer costs and minimize the risk of losing excess N to the environment.

The key information needed to determine alfalfa N credits is the alfalfa stand density before going into winter and before winter kill occurred. The amount of regrowth after the last harvest in 2008 also influences the N credit value; therefore the status of regrowth at end of October 2008 is important information for arriving at the right N credit value. Dr. Larry Bundy, from University of Wisconsin has developed recommended N credits for alfalfa. Where winter kill is not an issue, these suggestions are also appropriate for planned rotations from alfalfa to corn.

Dr. Leep’s work is funded in part by MSU‘s AgBioResearch.

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