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The earliest sign of winter injury to forage crops is at first green-up in the spring. In most years, this occurs in late March through mid-April, though this of course can vary year to year.
To evaluate fields for winterkill, count plants per square foot at several locations in each two-acre portion of the field. Dig representative plants at these sites to assess the health and vigor of the crown and taproot. Slowly recovering areas may have buds just breaking dormancy and are still vigorous, potentially productive stands. Others that have had sufficient late-winter cold injury may show declining vigor, or the taproot and crown may be starting to die.
Inspect split taproots for indications of root health. A healthy taproot is creamy-white and has a firm texture. Cold-injured plants have tan or yellow taproots and a soft, watery texture, and may degrade into a stringy yellow or brown color. Pay close attention to the top 1 to 11/2 inches of the taproot below the crown. A second check 7 to 10 days later in the field will show you whether development is advancing or declining.
Old stands (three or more years) often have an increasing degree of crown rota dark brown disease that develops in the taproot and crown and causes normal thinning of stands over time. The number of plants per square foot and the general health of their taproots are two major factors in assessing stand adequacy.
Steve Barnhart, Dept. of Agronomy, Iowa State University
Alfalfa Winterkill and Injury
Can the low, dead spots be reseeded in an otherwise good stand?
Yes, but the success will depend upon the amount of winterkill in the area to be reseeded. Areas with 100 percent kill will reseed best. These areas can be no-tilled or lightly cultivated and seeded conventionally. It's sometimes a good idea to seed a little red clover with the alfalfa to insure forage establishment for the remaining life of the stand. If possible, avoid cutting these areas when the first crop harvest is made.
Can I interseed alfalfa into a thin stand of alfalfa?
No. It rarely is successful. To thicken a stand some producers have had good luck using red clover or a fast growing, cool season grass like perennial ryegrass. Sorghum-sudan grass has also been tried after a first crop harvest but the success of this practice depends largely upon adequate moisture conditions in June for germination and early growth.
How soon can I reseed this old, winterkilled stand back into alfalfa?
The research is pretty conclusive that waiting one year offers the least risk and greatest yield potential for the new stand. However, producers can seed a short season small grain or small grain/pea mixture this spring, harvest it for forage in late-June or early July, and reseed alfalfa around the first of August. This will result in the near equivalent of an established stand the following year. Quackgrass will need to controlled before reseeding in August.
How soon can I reseed a new seeding field?
Usually new seeding fields can be reseeded immediately. If doing so, till the field before significant vegetative growth occurs in the spring from surviving plants. Also, assess quackgrass encroachment and control as needed.
Will reseeding alfalfa on corn fields be a problem?
Usually not, but be sure to consider your corn herbicide use last year. Corn herbicides with limiting alfalfa rotational restrictions are as follows:
Atrazine related products 2 years
Exceed 15 months
Harness 2 years
Permit 2 years
Surpass 2 years
Mike Rankin, Crops and Soils Agent, UW Extension - Fond du Lac County
Current recommendations regarding cutting height of alfalfa are designed to maximize yield while maintaining high quality forages and stand longevity. Forage growers frequently cut forages at a height of 3-inches or more. However, recent reports indicate that there may be an advantage to cutting alfalfa closer, leaving an inch or less of stubble height (@griculture Online, 1999). Research indicates that dry matter yields and nutrient yields are higher for shorter cutting heights as compared to leaving taller stubble (Sheaffer et al., 1988). Obtaining higher yields requires that the plants are healthy and that carbohydrate root reserves are adequate for plant regrowth following harvest. Wisconsin studies conducted in the 1960s using Vernal alfalfa showed that forages harvested three or four times per season produced more total forage when cut at a 1-inch height versus cutting at 3 inches or more (Kust and Smith, 1961, Smith and Nelson, 1967). Recent North Dakota research evaluating the effects of cutting height shows similar results where shorter cutting height results in higher yields.
Current recommendations for alfalfa cutting height in Wisconsin include the following:
Daniel W. Wiersma - Marshfield Agricultural Research Station, Ron Wiederholt - UWEX - Clark County Extension, University of Wisconsin
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