Accurate assessment of perennial forage stands for winter injury is an important and economically sound management practice. The degree of injury will vary depending on a number of climatic and cultural factors.
• Direct exposure of plants to extreme low temperatures.
Continued exposure of legumes to temperatures of 0ºF to 15ºF can be lethal. Under these conditions, plant survival depends upon the genetic cold tolerance of the variety, the insulating properties of the soil, vegetative cover and snow cover.
• Heaving of plants from wet soils during alternate freeze and thaw.
The heaved root and crown tissue is exposed to lethal air temperatures. The more branched, rooted legumes are less susceptible to heaving. Sod-forming grasses are the least susceptible to heaving and help hold legumes in place. Selecting or providing soils with good surface drainage can reduce the occurrence of heaving.
• Smothering of plants when covered by ice sheets.
Smothering of alfalfa may cause injury within one to three weeks, and death within two to six weeks. Red clover and white clover have tolerance to smothering that is similar to alfalfa, while birdsfoot trefoil and clover are more susceptible to injury.
Grasses are more tolerant than legumes to smothering and can withstand injury for up to 10 to 14 weeks. Leaving six to eight inches of vegetative stubble in the fall can help reduce the occurrence of ice sheet formation.
• Mid-winter dehardening of plants from exposure to extended warm periods.
Plants that deharden over winter during warm periods use some of their carbohydrate and nitrogen reserves during this premature regrowth attempt, leaving the plants with a reduced level of cold hardiness and less reserves available for continued survival during the rest of the winter and for spring regrowth.
The grower has little control over climatic factors that influence winter injury, but a number of cultural practices can be employed to reduce the severity of winter injury.
Injury is more likely to occur on species and varieties with low inherent cold hardiness. Disease resistance is also very important in stand persistence and may partially contribute to winterhardiness. Plants weakened by disease are less resilient and more susceptible to winter injury.
Young stands are less susceptible to winter injury than old stands. Old plants are more likely to be infected with root and crown diseases, and stand loss is apt to be more serious because old stands generally have fewer plants per unit area than young stands.
Injury is less severe where a grass is present. Grass reduces heaving of legumes and helps catch snow and provide insulation to crowns. Injury occurs more frequently where fall cutting or overgrazing is practiced. Fall cutting or grazing may not allow for accumulation of adequate carbohydrate reserves for the winter or leave stubble to catch snow.
Injury is generally less severe where a good annual soil fertility program is followed. The only exception would be fall-applied nitrogen on grasses that would encourage vegetative growth when the plant should be going dormant.
Winter-injured plants often are slow to recover in spring, so a quick decision to destroy a winter-injured stand is not recommended. When evaluating winter injury, consider both the number of plants per square foot and the age of the stand. Crown and root diseases also have a major effect on stand reduction of legumes, so plants should be checked for dead, dying or diseased tissue.
Wait until the spring regrowth is about three to four inches high. Select random stand count sites. Check a one-square-foot site for every five to 10 acres. Dig up all of the plants in the one-square-foot area. Pick at the crown and buds with a knife to determine if the tissue is still alive.
Also, split the taproots to determine if they are firm and creamy-white, or yellowish-brown and decaying. Count only the healthy plants. More than 50 percent of the taproot should be disease-free.
Evaluate other legumes similarly to alfalfa. The ability of red clover, white clover, and birdsfoot trefoil to reseed may compensate for some stand loss. Sod-forming grasses may spread and fill in for thin stands; bunch-type grasses will not.
Management options for winter-injured stands
Management alternatives to consider depend on severity of winter injury, age of stand and grower needs.
If the stand is “marginal,” it may be best to leave it and obtain what forage will be produced. Delaying first harvest until one-half to full bloom will let the stand regain productive vigor, and then a normal cutting schedule could be resumed. If the legume in a legume-grass mix has decreased by more than 50 percent, but the grass stand is good, applying nitrogen fertilizer to stimulate grass production over that of the legume may be beneficial.
Options after first harvest include keeping the stand or destroying the damaged stand and planting an alternative crop. However, under dry soil conditions, any alternative crop following alfalfa is questionable because the alfalfa stand will have depleted the subsoil moisture.
For unacceptable or damaged alfalfa stands, reseeding alfalfa into an injured stand may be desired. Successful reseeding of alfalfa will depend largely on age of stand and cause of plant injury, such as disease or lethal winter climate.
• For stands two years old or older, autotoxicity of alfalfa may cause renovation problems. Younger stands (within 12 to 15 months of the original planting) probably have not developed a high enough level of autotoxicity in the soil to interfere with reseeding.
• Older stands with less than 25 percent of the plants remaining (one plant or less per square foot) may be interseeded with red clover if the stand will be maintained for hay harvest for only one to two more years.
• Older stands with 25 to 50 percent of the plants remaining in the stand are not recommended for interseeding with alfalfa because of autotoxicity. However, interseeding red clover or a grass into a uniformly thin stand of alfalfa may be beneficial. It usually is recommended to destroy these stands and rotate to a different crop.
• Alfalfa stands thinned mainly by disease should be rotated to an alternative crop for a year or more, or seeded to a grass-based forage mixture to decrease the level of disease organisms in the soil.
If forage needs are great or will be in the upcoming year, and present alfalfa stands are three years old or older with suspected winter injury, it may be wise to seed a new stand in another field as soon as conditions permit to ensure an adequate forage supply. The existing stands then could be managed according to severity of injury.
If the injured stand is to be grazed in spring, graze conservatively to let the stand recover. Check on rental opportunities of other pastures. If extensive pasture renovation is required, it may be economical to rent pasture until renovation is completed.
Fertilization and weed control of the existing injured stand may be sufficient in improving the pasture to meet grower needs. A more productive forage component may be added to a thinned pasture or injured area. For more severely damaged pastures, consider no-till renovation on erodible land or complete renovation of the stand where erosion potential is minimal.
Choosing the best economic alternative after winter injury can be very difficult. The decision to renovate, delay harvest of first cutting and/or add nitrogen fertilizer to older, thin stands is influenced by yield, cost, alternative hay prices, forage supplies and forage demand.
A decision on alternatives, particularly those requiring borrowing money, ought to be considered in light of the financial status of the farm. Growers must make choices on their own, but choices dealing with winter injury have long-term consequences. The best decisions are derived from accurately assessing stand and winter injury. FG
References omitted due to space but are available upon request.
—Excerpts from Iowa State University Extension Agronomy newsletter, November 2008