Factors Affecting a Laboratory Evaluation of Alfalfa Cold Tolerance

Currently, only test winter data are used to evaluate the winter hardiness of alfalfa ( Medicago sativa L.) cultivars. A test winter is defined as field conditions severe enough to kill nonhardy cultivars and to cause varying degrees of winter injury to cultivars of intermediate hardiness. Due to the unpredictability of test winter occurrence, alternative methods are needed for predicting alfalfa winter survival. This research tested the effects of temperature stress, storage time, and plant size, measured as the width of the root at the crown‐root juncture, on a laboratory cold tolerance evaluation of field grown and hardened alfalfa plants. Plants were dug from the field in mid‐November and stored at −2°C until subjected to cold stresses from 0 to −24°C. Differences among entries were greatest at the −12 and −16°C cold treatments. Crown injury score was positively correlated with field fall growth ( r = 0.91) and winter injury scores ( r = 0.95; P ≤ 0.01). Root injury score was correlated positively with field fall growth ( r = 0.87) and winter injury scores ( r = 0.95; P < 0.01). Large plants (>10‐mm root diameter) had lower cold injury scores than small plants (1‐ to S‐mm root diameter). Correlations between plant size and winter injury were greatest ( r > −0.55, P ≤ 0.05) at low freezing temperatures. We propose a laboratory test in which field‐hardened seeding‐year alfalfa entries of a similar size are packed in dry vermiculite, exposed to temperatures of −12 to −16°C for 1 to 2 h, thawed, and scored after 3 wk of regrowth.