Fall dormancy and snow depth effects on winterkill of alfalfa

The lack of a definitive method to assess winter hardiness in alfalfa (Medi cago sativa L.) remains a challenge in the north-central region of the USA where winterkill of alfalfa can he severe. The reliability of fall dormancy ratings for describing alfalfa cultivar susceptibility to winter injury an d the role of snow depth in moderating temperatures near the plant were inv estigated at Chatham, MI on a Chatham Stony loam (Typic Haplorthod). Four c ultivars were selected with a range of fall dormancy ratings: 'Nitro', 'Mag num 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 win ter snow depths were maintained. Temperatures were monitored for each plot, and stand counts were made each fall and spring to assess winter injury. N itro suffered the most winterkill across snow cover treatments. The total y ield range was 0 to 9 Mg ha (1) in the absence of a snow cover and 0.4 to 1 2 Mg ha(-1) for a snow depth of at least 10 cm, except in 1996. Extreme min imum canopy-level (6 cm) temperatures for 10-cm snow depth averaged over th ree winter seasons were 12.1 degreesC higher than the 0-cm snow cover treat ment, which translated into higher yields. The results suggest that snow co ver of 10 cm adequately protects alfalfa from winter injury. Cultivars with in the same fall dormancy rating did not necessarily perform similarly, sug gesting the need to develop other methods for assessing winter survival.