ALFALFA MATURITY AND CULTIVAR EFFECTS ON CHEMICAL AND IN-SITU ESTIMATES OF PROTEIN DEGRADABILITY

The relationships among plant maturity or genotype and forage quality and ruminal in situ degradability of alfalfa (Medicago sativa L.) are inadequately understood. A 2-yr experiment was conducted to study the effects of cutting (spring or summer), plant maturity (six sampling dates), and cultivar (Vernal, Arrow, WL-320, and Target II or Legend) on chemical composition and in situ degradability of alfalfa. Plant maturity was quantified by the mean stage weight (MSW) system. Herbage cell wall, cell-wall bound N, and in situ escape protein (EP) increased significantly with MSW; crude protein (CP) and in situ dry matter degradability (ISDMD) decreased. Forage nutritive quality declined less with increasing MSW in summer than in spring cuttings. In 1991, the cultivar Target II had lower ISDMD than Vernal, Arrow, or WL-320, while summer growth of Arrow contained less EP than other cultivars. In 1992, Vernal had higher EP than Arrow, WL-320, or Legend. High EP related to increasing maturity or cultivar differences was negatively correlated with other measures of nutritive quality. Differences in chemical composition and in situ degradability among alfalfa cultivars exist, but are dependent on growing conditions. Usefulness of the MSW system to predict alfalfa nutritive quality is limited to within cuttings, and cell-wall bound N measures were better predictors of alfalfa EP than was MSW.