Fall dormancy of alfalfa is an important trait to consider when selecting a variety, but how relevant is it to farm-scale production?

Fall dormancy is described as the physiological response of the alfalfa plant causing a slowdown and cessation of regrowth. The primary response factor is shorter day lengths, though temperature and light quality also come into play. This response is consistent and, having experience with dormancy trials on the Mexican border in El Centro, Calif., can be quite surprising. Even as the weather would seem perfect for plant growth, with temperatures in the low 80s, you will see the most dormant varieties with nearly no regrowth in the winter months. This demonstrates how strong and consistent the dormancy response displays itself.

Fall dormancy rating (FDR) is the number assigned through a trial and is a quantitative measure. Fall dormancy class (FDC) is a discrete integer classification of 1 through 11 for the fall dormancy of a variety. Essentially, FDR will be calculated in a trial and rounded to the nearest FDC. These terms are often used interchangeably, and collectively are colloquially referred to as the fall dormancy of a variety, with the most dormant (least fall regrowth) being a FDC 1, and the least dormant (most fall regrowth) being FDC 11.

A standard test is used

Fall dormancy is a directly measured trait governed by a standard testing procedure established by alfalfa scientists (public and private) through the North American Alfalfa Improvement Conference (NAAIC). The standard test procedure outlines a test that has been proven across the U.S. to provide consistent dormancy rankings.

This standard test is performed in space-planted plots where plants can be established by direct seeding and thinning, but is usually done by transplanting, at a 1-foot spacing on 30- to 36-inch centers. There are 11 “check” varieties that span across the fall dormancy spectrum from FDC 1 to 11. These varieties have been shown through much previous study to be consistent across locations in their relative dormancy response and are used to establish a regression for estimating FDC of new varieties. The standard test dictates that four replications of 25 plants each be established at each location.

In the fall, when day length is shortening, a final cut is taken from the trial. Twenty-five to 30 days later, individual plant heights are measured. These heights can range from 2 to 25 inches, depending on the variety’s degree of dormancy. The heights measured for the check varieties, along with their known dormancy class, are used to establish a relative regrowth response equation in that environment. That equation is then used to predict the dormancy class for new varieties under test at the location.

Testing the standard test

Keen-eyed readers will realize this system of testing spaced plants is not exactly an accurate reflection of how alfalfa is grown on farms and may question its relevance. If you look at any cornfield, you can see a good representation of the edge effect and the difference that population density can make on plant height.

A group of alfalfa and forage scientists at the University of Wisconsin; University of California, Davis; and Cornell University chose five field locations across the U.S. to represent the breadth of latitude and longitude of most alfalfa growing regions. Locations were in El Centro, Calif.; Davis, Calif., in the Central Valley; Tulelake, Calif., near Oregon; Arlington, Wis.; and Ithaca, N.Y.

At each location, paired trials were established. One trial followed the fall dormancy standard test, transplanted at 1-foot spacings, while the other was direct seeded in 4- by 8-foot plots at typical seeding rates, four replications within each treatment. Twenty varieties were seeded in these trials, comprising 11 standard checks and nine experimental or commercial varieties.

These fields were managed similarly, following the same cutting schedules through the growing season. At each location, a suitable final cutting date was chosen based on previous experience, being sure to cut with enough time before a killing frost that could interfere with characterization in Northern locations. In the space-planted plots, individual plants were measured in accordance with the standard test. In sward (solid-seeded) plots, a technique was devised to randomly measure plants in 12 places.

Little difference in dormancy rating was attained between locations and planting methods (see Figure 1). Depending on location, the correlation between the two methods for all 20 varieties ranged from 0.89 to 0.99. When all locations were considered together, the correlation between the two methods was 0.99 (see Table 1). It should be noted that higher correlations were seen when considering 11 standard check varieties alone, showcasing the nature of these check varieties to perform true to type.

Opens up options

This knowledge has advantages for both the seed buyer and seller. For the farmer, you can be assured that the seed you are buying will perform as expected in field conditions, despite the nature of the standard fall dormancy test being so different from field practices. As a result of this study, standard test procedures were updated to include the use of sward plots as an alternative testing method to classify fall dormancy. This opens the possibility of including dormancy testing with alfalfa yield trials, as an example. For the alfalfa breeding company, this reduces the cost of determining alfalfa fall dormancy and may allow for multiple-location testing.

At present, one location and year is sufficient to certify the fall dormancy of an alfalfa variety. This is likely a compromise due to the onerous nature of the standard test. However, yield testing must be done over multiple locations and years. Combining fall dormancy with yield testing could prove feasible and lead to higher standards of testing, resulting in better characterization of varieties.

A recent trend has been to report fall dormancy with an accuracy to 0.1 (for example, FDR=4.3). The difference in fall regrowth between each FDC is, on average, about 2 to 2.5 inches, depending on location. Therefore, a tenth of a dormancy class is less than a quarter inch in the fall, which equates to about 0.03 tons of dry matter per acre of yield. In truth, management practices and careful machinery adjustments will make a bigger difference to yield in the fall than will a tenth of a dormancy class rating. Perhaps some intermediate values could prove useful, but more stringent testing procedures should be agreed upon.

Fall dormancy continues to be an important trait to consider when selecting a variety for use, along with many other factors, including disease and pest resistance and winter survival rating. It is critical to know that what you are buying will generally perform as advertised.

This article appeared in the March 2024 issue of Hay & Forage Grower on pages 16-17.

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