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Winter Injury and Spring Assessment of Alfalfa

Winter Injury and Spring Assessment of Alfalfa
By Dwane Miller
 
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Early springtime, after alfalfa has broken dormancy, is an excellent time to evaluate the effect of winter injury on your alfalfa stand. According to this University of Wisconsin factsheet, there are a number of factors that can impact winter injury. These include:
  • Stand age - Older stands are more likely to winterkill than younger ones.
  • Variety - Varieties with superior winter hardiness ratings and a high disease resistance index are less likely to experience winter injury.
  • Soil pH - Stands growing on soils with a pH above 6.6 are less likely to experience winter injury.
  • Soil fertility - Stands with high fertility, particularly potassium, are less likely to experience winter injury than those with low fertility.
  • Soil moisture - Alfalfa grown on well-drained soils is less prone to winter injury.
  • Fall soil moisture status - As dehydration is the primary means of tolerating freezing temperatures, stands that go into winter with low soil moisture are better able to lose moisture and are less likely to winter kill.
  • Cutting management - Both harvest frequency and timing of fall cutting affect alfalfa winterhardiness. The shorter the interval between cuttings, the greater is the risk of winter injury. Stands in which the last cutting is taken between September 1 and October 15 are at greater risk, as plants are unable to replenish root carbohydrate reserves before winter.
  • Snow cover - Snow is an excellent insulator. Temperature fluctuations are much less under snow cover. As little as 4 inches of snow can result in a 10oF difference in soil temperatures. Stands which have not been cut after September 1 or which have at least 6 inches of stubble left will be able to retain more snow cover and be less susceptible to winter injury.
Signs of winter injury include slow green-up, uneven or asymmetrical growth of plants, or root damage. Root assessment is the best way to assess winter injury. According to Dr. Dan Undersander, retired forage specialist at the University of Wisconsin, springtime stand assessment can begin as soon as the frost is out of the ground and continue until spring green-up occurs. The process is to dig a few plants 4 to 6 inches deep and look at the condition of the taproot. If the taproot is turgid (like a potato, leftmost plant), it is alive and healthy. If the root is browned, dehydrated, and ropey (like two plants on the right), it is dead or dying. If 50% or more of the root is blackened from root rot, the plant will most likely die during spring green-up or later in the year.
 
Alfalfa taproot evaluation. Credit: University of Wisconsin
Alfalfa taproot evaluation. Credit: University of Wisconsin
 
Another way to assess your stand in the spring would be to look at the buds. Alfalfa forms buds in the fall for spring growth. If these buds are killed the plant must form new buds in the spring, delaying growth and reducing yield. The three taller stems in the picture (above line) are from buds formed in the fall and the shorter stems are from buds formed in the spring. The delayed, shorter growth will reduce yield of first cutting and then plants will recover. If you see this, consider management to reduce this in the future, such as adequate soil pH, fall application of potassium, more winter-hardy varieties.
 
Evaluation of winter injury by comparing fall versus springtime buds
Evaluation of winter injury by comparing fall versus springtime buds. The line in the photo separates fall buds (top growth) to spring buds (lower growth). Credit: University of Wisconsin.
 
Finally, you can take a look at the number of stems in the field. A healthy stand should have 55 stems/ft2. Early assessments, before stems are visible, may need to assess based on plant count. A high yielding alfalfa stand seeded last year should have 20 plants/ft2. Counts as low as 12 will produce good yields but result in shortened stand life. Stands seeded last spring or fall with less than 12 plants/ft2 should be disked and reseeded.
 
Thin stand (left) versus adequate stand (right). Credit: University of Wisconsin
Thin stand (left) versus adequate stand (right). Credit: University of Wisconsin
 
A high yielding alfalfa stand over 1 year old should have at least 6 plants/ft2. If plant density is less than 6, oats (2 bu/a) or Italian ryegrass (10 lbs/a) can be overseeded to increase yield this year. The stand should be turned over either immediately or at end of the year.
 
If you have determined that you have winter injury to your stand, here are some management factors from the University of Wisconsin that can increase your chances of keeping that field in production:
  • Allow for longer maturity - Allowing plants to mature too early, mid or even full bloom will help the plants restore needed carbohydrates for subsequent production. How long and during which cutting depends on the extent of winter injury. For severely injured stands, allow plants to go to nearly full bloom in first cut and to early flower in subsequent cuttings. This will give these stands the best chance at survival. Stands with less injury could be harvested somewhat earlier depending on the extent of the injury.
  • Increase cutting height - This is particularly important when allowing plants to flower before cutting. At this time, new shoots may be developing at the base of the plants. It is important to not remove these shoots as it will further weaken the plant.
  • Fertilize - It is particularly important that winter injured stands have adequate fertility. Soil test and apply needed fertilizer prior to first cutting if possible.
  • Control weeds - Herbicide applications to control weed competition will help the stand by eliminating weeds that compete for moisture, light, and nutrients.
  • Fall cutting - Do not cut winter injured stands after September 1 to allow for the buildup of food reserves prior to winter unless the intent is to terminate the stand.
Source : psu.edu

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