29 Oct Fall Dormancy Classification of Alfalfa Explained
Fall Dormancy Class is often the first characteristic that alfalfa growers consider when deciding which variety to grow. What is fall dormancy, what does the fall dormancy rating really mean? And how should you use the fall dormancy rating in your alfalfa variety selection process?
What is fall dormancy? Fall Dormancy is a measure of fall growth. As autumn days bring shorter day length, alfalfa plants begin to slow their vegetative growth, pushing more carbohydrate into storage in the taproot in preparation for winter. But there is a wide spectrum in this response with some varieties having almost no slowing of fall growth (non-dormant). Others slow considerably as autumn day length shortens (fall dormant), and some are in between (semi-dormant).
Fall Dormancy Class is assigned to an alfalfa variety by comparing new varieties of interest to a set of standard check varieties that were agreed upon by alfalfa researchers decades ago. The standard check varieties provide stable comparison data so that fall dormancy ratings can be compared across varieties with relative confidence in the ratings. Fall Dormancy classes can be assigned from 1 through 11, but ratings above 9 are not common and more variable (less significant) in trials.
Here’s how the ratings are made: Alfalfa Plots are clipped on a normal schedule during the summer with the next to last cut no later than August 1. A final cut is then made in early September and plant height is measured 25 days later, by early to mid-October. That October plant height is compared to the height of the standard check varieties grown in the same trial(s), and a fall dormancy rating is assigned based on fall height. In general, fall dormancy classes are separated by a fall height difference of about 2 inches. For example the height difference between a fall dormancy rating of 4 versus a fall dormancy rating of 5 should be about 2 inches. Fall dormancy can be reported in increments of 0.1, for example 4.4 or 5.2, but that level of so-called ‘accuracy’ has more marketing interest than agronomic usefulness.
The association between fall dormancy and winter hardiness is well established, but not entirely a one-to one relationship. Fall Dormancy class can be a loose proxy for winter hardiness, but winter survival is related to crown depth and highly dependent on late summer and fall cutting management. In general, the more dormant varieties (lower fall dormancy number rating) will be a bit slower to initiate re-growth after cutting and will have less fall yield. Just remember that a fall harvest, if taken, is probably one of the lightest cuts of the year.
A sound variety selection process should focus on the fall dormancy (FD) classification best suited to your area, but should also consider high yielding adapted varieties with a fall dormancy rating adjacent to your target. For example if your target fall dormancy class is FD4, you could also consider FD3 and FD5 varieties provided they can keep up in yield and provide adequate winter-hardiness. Yield potential of an established alfalfa stand can be more affected by genetics and agronomic management when considering varieties within a narrow range of fall dormancies adapted for your geography.
Consider varieties in a target range of fall dormancies that can be best suited to your growing environment. Fall dormancy classes 3-5 (fall dormant) are highly suited to the northern half of the US and areas that experience soil freezing during winter. Fall Dormancy classes 6-7 (semi-dormant) have more fall growth and less tolerance to freezing. Semi-dormant varieties are often preferred in areas with little or no soil freezing, yet with sustained wet winter soil conditions making harvest difficult, and no desire for sustained winter growth. Fall Dormancy classes 8-10 (non-dormant) are well-suited to areas where winter growth can be sustained with warm daytime temperatures, with a desire for continuing harvest during winter months.
Photo 1. Fall dormancy trial October 22. Note the fall height differences among Fall dormancy 2 (upper left), fall dormancy 4 (upper middle), fall dormancy 5 (left foreground), and fall dormancy 8 (right foreground).
Photo 2. This fall dormancy trial at the S&W research farm in Nampa Idaho contains eleven standard check varieties plus more than fifty experimental varieties replicated four times across the trial, for characterization of fall dormancy.