The first criterion the farmer should use in selecting a variety for his farm is whether or not the variety is adapted to his region’s climate and elevation.

For example, if a Wisconsin farmer’s variety choice didn’t survive the first winter, then it was a poor choice for his farm, despite any other desirable traits it might have had. Taking this into consideration, growers often ask the question “How do I narrow the national alfalfa variety list down to those best suited for my farm?”

There are two basic alfalfa variety rating systems all growers should take advantage of to initially narrow the national alfalfa list of 202 varieties to a workable number. These systems rate varieties on their adaptation and ability to grow in various climates, ranging from those with very cold winter conditions to very mild winters with little or no frost. The first is the fall dormancy rating (FDR) and second is the winter survival rating (WSR).

Fall dormancy rating (FDR)
After alfalfa was first introduced into North America, farmers growing it in northern areas of the U.S. noticed that some alfalfas were prone to winterkill and some were not.

It was also noted that if seed of varieties that evolved in cold climates was used, the resulting plants responded to the fall’s shortening daylength and cooling temperatures by reducing their fall growth.

In contrast, the varieties that evolved in warmer climates continued to grow in the fall since they were not genetically programmed to respond to the short days or cold temperatures that occurred in more northern latitudes.

As a result, they experienced repeated frost damage and were often winterkilled as a result of the added plant stress.

Eventually this “fall dormancy” observation resulted in the development of the fall dormancy rating system (FDR) to estimate winter survival using the amount of fall growth as an indicator.

Using the FDR system, alfalfa varieties are currently subdivided into ten dormancy groups based on their fall growth response to the shortening fall daylength or cooling temperatures.

Varietal differences are based on average plant heights in comparison to 10 standard check varieties. Varieties displaying the most fall dormancy (shortest height) are given a rating of 1, and those with the least fall dormancy (tallest) a rating of 10 (see Figure 1).

For many years, this fall dormancy rating was a useful tool for researchers and farmers in estimating the survival or adaptation of varieties to cold or hot climates.

However, this system did have its limitations and drawbacks. Winter survival was estimated solely on varieties’ survival mechanism of conserving food reserves by reducing fall growth and not on actual winter survival performance data.

Winter survival rating (WSR)
The 2010 alfalfa variety list currently has 79 varieties classified as FDR=4, and although all their fall growth habits are statistically the same, some of the varieties demonstrate actual winter survival as good as or better than varieties rated with an FDR of 2. This discrepancy prompted the development of the more accurate rating now called the winter survival rating (WSR).

In contrast to the FDR system, the WSR system directly rates a variety’s level of survival following harsh winter conditions in regions of the U.S. that are known to have severe winters. Varieties are compared to standard check varieties and rated on a scale of 1 to 6 depending on their degree of winter damage. Varieties with a score of 1 display little or no winter damage and those with severe damage are given a WSR= 6, and are considered very susceptible to winterkill.

Selecting a variety
Alfalfa researchers have known for years that factors associated with higher yields (vigor, fast regrowth, extended fall and spring growth) increased as you moved up the fall dormancy rating scale from 1 to 10.

Farmers and researchers often saw the extra yield potential of an FDR=5 or even an FDR=6 over an FDR=4, but were at a loss over how to capture that yield without risking winterkill. As a result, growers were often conservative in their selection of a variety based on FDR.

Now armed with a direct measurement of winter survival, growers can feel more confident in selecting a variety with a higher FDR number for increased yield. We now know that if that variety has a low WSR score, he can still take advantage of increased yield potential of a high FDR but feel confident that the low WSR number will ensure he will not sacrifice winter survival.

Summary
For years the FDR system was used to predict winter survival of alfalfa varieties, but it had some limitations. The FDR utilized the amount of fall growth of a variety as an indirect means of predicting winter survival. In contrast, the relatively new WSR directly measures a variety’s survival following severe winter conditions and is a more accurate winter survival estimate.

Despite its limitations of predicting winter survival, the FDR’s fall growth estimate can be a very good indicator of yield potential, and when used in conjunction with the WSR, the combination of the two is a powerful tool for the grower. The tandem use of the two ratings in variety selection allows the grower to pick a variety that maximizes yield without sacrificing winter survival.

For optimal variety selection, growers should initially select all the varieties with the desired winter survival rating for their location using the WSR.

The grower should then narrow that list by selecting the varieties with the highest available fall dormancy using the FDR (i.e. WSR=2 and an FDR=5). The remaining varieties can then be evaluated for other desirable agronomic traits such as pest resistance, quality and yield before final selection is made.  FG