Fall Dormancy and Snow Depth Effects on Winterkill of Alfalfa

The lack of a definitive method to assess winter hardiness in alfalfa (Medicago sativa L.) remains a challenge in the north‐central region of the USA where winterkill of alfalfa can be severe. The reliability of fall dormancy ratings for describing alfalfa cultivar susceptibility to winter injury and the role of snow depth in moderating temperatures near the plant were investigated at Chatham, MI on a Chatham Stony loam (Typic Haplorthod). Four cultivars were selected with a range of fall dormancy ratings: ‘Nitro’, ‘Magnum IV’, ‘Saranac’, and ‘Vernal’. The cultivars were planted in 1993–1994, 1994–1995, and 1995–1996 seasons in plots over which 0‐, 10‐, and 20‐cm winter snow depths were maintained. Temperatures were monitored for each plot, and stand counts were made each fall and spring to assess winter injury. Nitro suffered the most winterkill across snow cover treatments. The total yield range was 0 to 9 Mg ha−1 in the absence of a snow cover and 0.4 to 12 Mg ha−1 for a snow depth of at least 10 cm, except in 1996. Extreme minimum canopy‐level (6 cm) temperatures for 10‐cm snow depth averaged over three winter seasons were 12.1°C higher than the 0‐cm snow cover treatment, which translated into higher yields. The results suggest that snow cover of 10 cm adequately protects alfalfa from winter injury. Cultivars within the same fall dormancy rating did not necessarily perform similarly, suggesting the need to develop other methods for assessing winter survival.