Spring is finally here and with the breaking of dormancy, the reports are coming in of extensive winterkill of alfalfa across Michigan and surrounding states. The combination of a late, wet fall, little insulating snow cover in many counties, frigid temperatures and ice sheeting was the last straw for many alfalfa stands that were already stressed. Some survived unscathed, but Michigan State University Extension is hearing about 80% loss of stand in some places.
The first step in remediation is assessing the damage. Alfalfa may exhibit frost heaving where crowns are lifted out of the soil. Crowns that are lifted more than about an inch are likely to die, if they have not already. Plants may also simply die. Be cautious about calling crowns dead too early. Alfalfa set its crown buds for first spring growth last fall. If those buds are damaged during the winter, the crown has to start again with a new round of buds, and this may take 10-14 extra days before new growth is visible. This is why winter damaged stands are often very uneven in height early in the spring. Also, remember that alfalfa varieties with low fall dormancy (FD) ratings are later to break dormancy in spring than varieties with higher FD ratings and will therefore lag a few days behind in development.
Another indicator of stand health is root condition. Dig some crowns and split the taproots lengthwise. A healthy crown and root is white throughout and has many crown buds emerging. Diseased, stressed roots have brown or rotten areas extending down from the crown through the center of the taproot, and few buds. Crown and root disease is progressive. Mildly affected plants may limp along a little bit longer, but they cannot be saved and are likely to die during subsequent summer stresses like heat or drought. Therefore the full impact of winter damage may not be evident until later.
The main factors that contribute to crown disease in alfalfa are wet soils that encourage fungal infection, wheel traffic damage that allows the fungi entry into the root, and lack of adequate potassium fertility that prevents plants from being able to resist the fungi. Lack of potassium fertility also directly reduces alfalfa’s ability to survive cold. Cutting alfalfa during the fall no-harvest zone, the six weeks before first killing frost, depletes root energy stores and can increase winter damage. Because the ground was slow to freeze last fall and the soil was wet, late manure applications caused permanent wheel traffic damage on many fields. See the University of Wisconsin bulletin listed in the resources for an excellent photo series illustrating degrees of root damage.
What to do?
Damaged stands present producers with multiple decision points. First, should they keep a damaged field despite reduced production potential? Second, should they try to boost the production of forage from that field by supplemental seeding? Third, should they just terminate the alfalfa and rotate to a different crop to provide emergency forage?
In fields where stand loss is patchy, decisions whether to keep the stand should be based on the health of the remaining plants and the total area lost. In fields with more than 50% alfalfa loss, starting over may be the best bet. Fields with less than 50% alfalfa loss and more than 30% healthy plants in the surviving sections may be worth salvaging for a year or two by inter-seeding with another forage to fill in the gaps.
Can I inter-seed with alfalfa to thicken the stand?
No. Michigan State University Extension does not recommend this practice because alfalfa exhibits autotoxicity to its own seedlings. Interseeding alfalfa to thicken a uniformly thin stand or replanting alfalfa into the same field that was just terminated will often fail if surviving alfalfa plants are less than 18 inches apart or alfalfa has been grown in the field in the previous six months to two years.
Autotoxic compounds can also permanently damage the root systems of seedlings that manage to survive, reducing the total yield potential and lifespan of plants. Competition from surviving plants in the original stand further reduces the success of overseeding with alfalfa. Although there is conflicting data on autotoxicity and how it can be mitigated by management, most agronomists recommend waiting six months to one year before reseeding a field back to alfalfa.
Can I inter-seed to thicken the stand with other non-alfalfa forage species?
Yes. Fortunately, inter-seeding with other forage species is not affected by alfalfa autotoxicity. Inter-seeded forage grasses or clovers will fill in the gaps left by winterkilled alfalfa, preventing weed invasion while producing nutritious forage. Overseeding with red clover (6-10 lb/acre) or Italian ryegrass (5-10 lb/acre) can prolong the useful life of a damaged alfalfa stand by up to two years. These two options are good for producers that harvest their forage primarily as chopped haylage, but they are difficult to dry as hay. Small grains and annual cool-season grasses (oats, wheat, rye, or triticale at 50-75 lb/acre, or annual ryegrass at 5-10 lb/acre) can provide high quality forage quickly and effectively prolong stand life for one year. If using annual or Italian ryegrasses, use care to prevent seed set because these grasses may readily develop glyphosate resistance and can potentially become weed problems when a field is subsequently rotated to corn or wheat. Interseeded perennial grasses like orchardgrass (5-10 lb/acre), timothy (3-5 lb/acre), or tall fescue (4 lb/acre) take longer to establish than annual grasses, but can enhance stands for two or more years.
For best success, inter-seeding should be done with a no-till drill. Large dead patches can be disked first and then seeded. Broadcast overseeding is unlikely to give satisfactory results unless it can be done in March as a frost-seeding. Grass or clover can be drilled early in the spring if alfalfa is still small. Once surviving alfalfa has reached 6-8 inches in height, it is extremely competitive with grass seedlings, and it will be more effective to drill the new seeds immediately after first cut is harvested.
Replacing alfalfa forage
When a damaged alfalfa stand must be terminated, a rotation to an annual forage grass will make good use of the nitrogen left behind by the alfalfa while maintaining a supply of forage. Depending on the crop to be planted, it may be practical to harvest first cutting of the damaged alfalfa stand before terminating the alfalfa. Corn silage can be planted as late as July 1 and has high potential total season dry matter and energy yield. Small grains such as oats or spring triticale grow quickly and are a good option for hay or silage when harvested at boot stage, oats with peas make excellent silage, and annual and Italian ryegrass make good quality silage or baleage. Warm-season annual forage crops like teff, pearl or foxtail millet, sorghum, sudangrass, or sorghum-sudangrass establish readily in summer and can provide good quality hay, silage, baleage, or pasture. If a pure legume replacement is preferred, proven options are forage soybeans, red clover or peas.
A newer option is berseem clover, which is an annual. Recent MSU research showed that berseem clover yielded more forage than seedling red clover, with potential for one to two cuttings. In northern Michigan, crimson clover yielded more as an emergency hay crop than berseem or seeding year red clover.
- University of Wisconsin, “Alfalfa stand assessment: Is this stand good enough to keep?” YouTube version.
- MSU Extension,Managing Alfalfa Autotoxicity, Kim Cassida.
— Kim Cassida, Michigan State University Extension, Department of Crop, Soil, and Microbial Sciences
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