by Jim Morrison, Crops System Educator
Winterkill in alfalfa has been reported in several fields in northern Illinois. The extent of this injury varies from field to field, and within fields. Let's address the causes and symptoms of winterkill, alfalfa stand evaluation, and forage options following winterkill.
Factors contributing to winterkill are deficient snow cover, insufficient crop residue, lack of winterhardiness, low soil fertility, and low soil pH. When alfalfa crowns are exposed to temperatures below 15 °F, plants may be killed.
Two other contributing factors are ice sheeting and stress on the plant by cutting during the "no harvest period" of September 1 to mid-October. Very low temperatures following ice sheeting can result in rapid killing of plants as cold temperatures are easily transferred through ice to plants. Ice sheeting causes suffocation and plants can start to die in about a week. One way to reduce damage from ice sheeting is the recommended practice of leaving 6 to 8 inches of stubble in the fall. This will also increase the chances "catching" snow, which acts as an insulator.
Plants suffering from winterkill have soft and fibrous crowns and taproots and many times a distinct brown line is visible across the taproot 2 to 3 inches below the soil surface. In assessing alfalfa stands, it is important to dig some plants (include the top 6 inches of the root) and examine the crown for size, symmetry, and number of shoots. Split the crown and taproot lengthwise and check for the degree of discoloration, which is indicative of disease and/or injury. In addition to assessing crown and root health, evaluating stands are commonly done by counting plants or stems per square foot.
Stem counts would be the preferred method (they relate better to potential yield), but alfalfa top growth needs to average at least 6 inches. 55 stems per square foot is optimum and replacing the stand should be considered if there are 39 stems or less per square foot. Plant counts per square foot can be used prior to alfalfa being 6 inches tall. Density less than 8, 5, 4, 3 plants per square foot for 1 year, 2 year, 3 year and 4-year-old stands, respectively, will not yield well.
What are some forage options to seed into winterkilled alfalfa? Choices depend on the desired forage quality, yield, length of stand needed, and how the crop will be harvested (bale, silage, or grazing). Some possibilities include:
- If the stand is one year old or less, alfalfa may be reseeded. However, overseeding with additional alfalfa if the stand is over one year of age is not recommended due to the likelihood of autotoxicity. Red clover is not affected by the autotoxicity of alfalfa, so it can be used to "thicken" an alfalfa stand.
- Small grain (oat, spring triticale, barley) seeded solo or in a field pea mixture, harvest should be based on the maturity stage of the small grain.
- Annual (or Italian) ryegrass (select rust resistant varieties) with or without clover. Depending upon conditions next winter, ryegrass may not survive and grow in the spring 2006.
- Sorghum-sudangrass hybrids (consider the brown midrib type), pearl millet, or sudangrass hybrids. All produce prussic acid except pearl millet.
- Corn silage.
- Orchardgrass (select rust resistant varieties) and red clover.
Each of the above mentioned options have advantages and disadvantages. Additional information is available at Extension offices.
Jim Morrison, (815) 397-7714, firstname.lastname@example.org
Date: April 17, 2005