PHYSIOLOGICAL TRAITS ASSOCIATED WITH GRAZING-TOLERANT ALFALFA

Many alfalfa (Medicago sativa L.) cultivars do not persist under long-term, continuous grazing. This study was conducted to examine the variation of physiological traits that may contribute to grazing tolerance. Plants of the grazing-tolerant cultivars Alfagraze and Travois and the grazing-intolerant cultivar Florida 77 were grown from seed (Study 1) and from cuttings (Study 2) in the greenhouse. Populations from each cultivar that survived 2 yr of continuous grazing in the field were also tested in Study 2. Plants were clipped either frequently (weekly) or infrequently (every 4 wk) for 20 wk. Measurements of root total nonstructural carbohydrates (TNC), root weight, top and stubble herbage yield, and stubble leaf area were taken at Weeks 0, 2, 4, 10, 12, 18, and 20. No differences were noted between populations in Study 2. Under infrequent clipping TNC levels for all cultivars were higher at 4 than 2 wk after clipping. Florida 77 produced high TNC and root and top weights only under infrequent clipping but had low TNC and root weights under both clipping regimes, produced low top and stubble herbage dry matter, but maintained high stubble leaf areas under frequent clipping. Alfagraze produced and maintained high TNC and root weights under both clipping treatments, but produced high stubble dry matter and leaf area under frequent clipping. The genetic potential of a cultivar to store large amounts of carbohydrates may enhance its grazing tolerance, although stubble leaf area is probably important in allowing maintenance of total nonstructural carbohydrates under frequent defoliation.