Evaluate alfalfa stands for winter injury

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As alfalfa stands break dormancy and begin growth, growers should make plans to take some time to evaluate the health of those stands and determine if there was winter injury. Some early bud growth was observed the last full week of March in the southern half of Ohio. This evaluation is especially important in those areas of the state where we had periods of near zero to below zero temperatures this winter combined with little to no snow cover during some of those cold temperatures.

After doing a quick literature review, it appears that there is general agreement that temperatures in the 5 to 15 degree F range as measured at the alfalfa crown can begin to damage the plant and prolonged exposure to these and lower temperatures can kill the plant. Generally, the soil temperature at a 2 inch depth is associated with the temperature of the alfalfa crown. Snow cover is an important component of protecting an alfalfa plant from sub-zero temperatures since even a cover of 4 inches of snow can provide 10 to 15 degrees of protection. Once again, the concern is for those areas that experienced periods of zero and subzero temperatures without a 4 inch or greater snow cover. For many areas of the state, however, soil temperatures at the 2-inch depth remained at or above 26F even through the coldest days this winter.

An alfalfa stand health evaluation and winter injury assessment needs to be done by getting out into the field and doing a combination of stand counts and digging up some plant roots. Generally that evaluation should be done when there is 3 to 4 inches of growth from the plant. Evaluation involves selecting random sites throughout the field and counting the plants in a one foot square area. Check at least one site for every 5-10 acres, and like soil sampling, more random sampling is better. In addition to counting the plants per square foot, a count of the total stems per square foot is also useful because healthy plants can often produce more stems per plant thereby compensating for potential yield loss from fewer plants per square foot. After counting the plants, dig up all the plants in a one foot square area for every 5 to 10 acres and examine the crown and roots of the plants.

The winter survival rating determined by the plants per square foot is based upon the age of the stand. The following table is from a 2012 article on the Iowa State University (ISU) Integrated Crop Management web site by Stephen K. Barnhart, ISU Extension forage specialist.


Plants per square foot

Stand Age

Good

Marginal

Consider Reseeding

Year after seeding

+12

8-12

Less than 8

2 years

+8

5-6

Less than 5

3 years

+6

4-5

Less than 4

4 years and older

+4

3-4

Less than 3

As mentioned previously, counting the total stem number in a square foot is another method of evaluating winter survival and yield potential of a stand and has been promoted by Dan Undersander, Extension forage specialist at the University of Wisconsin. Here is a summary of that system:

Stem number/square foot

Expected result or action

Over 55

Stem density not limiting yield

40 - 55

Some yield reduction expected

Less than 39

Consider stand replacement

Source: Undersander et al 2011, University of WI Extension publication A3620

While plant and stem counts are useful, to get a true determination of stand health, plants must be dug up so that crown and root tissue can be evaluated. To do this you must split the crowns/roots. The inside should be a creamy white color. If it is yellowish brown to chocolate brown color the tissue is damaged or dying. If more than 50% of the roots show these symptoms, reduce your stand counts and yield potential.

One other weather condition that can have a detrimental impact on alfalfa stands is freeze/thaw cycles. These cycles that typically occur in February through March often present the greatest danger of winter injury in Ohio. There is the potential during these cycles to physically lift or heave alfalfa plants out of the soil. This heaving exposes the crown of the alfalfa plant making it more susceptible to temperature and physical injury. In some cases, heaving breaks the root system, effectively killing the plant. Heaving tends to be more of a problem in wet, saturated soils and clay soils.

Although winter temperatures and snow cover amount are primary driving factors affecting alfalfa winter survival, there are also management factors that growers can control to decrease the chance of winter injury. Those factors include:

  • Selecting varieties with good winter hardiness and disease resistance.
  • Maintaining soil fertility levels. Potassium in particular is associated with enhancing alfalfa tolerance to winter injury.
  • Improving soil drainage.
  • Harvest management: more cuts is generally associated with a higher risk of winter injury, particularly if the last fall cut falls in that mid-September to mid-October time frame.


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