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 da tes), and cultivar (Vernal, Arrow, WL-320, and Target II or Legend) on chemical composition and in situ degradability of alfalfa. Plant matu rity was quantified by the mean stage weight (MSW) system. Herbage cel l wall, cell-wall bound N, and in situ escape protein (EP) increased s ignificantly with MSW; crude protein (CP) and in situ dry matter degra dability (ISDMD) decreased. Forage nutritive quality declined less wit h increasing MSW in summer than in spring cuttings. In 1991, the culti var Target II had lower ISDMD than Vernal, Arrow, or WL-320, while sum mer growth of Arrow contained less EP than other cultivars. In 1992, V ernal 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 comp osition and in situ degradability among alfalfa cultivars exist, but a re dependent on growing conditions. Usefulness of the MSW system to pr edict alfalfa nutritive quality is limited to within cuttings, and cel l-wall bound N measures were better predictors of alfalfa EP than was MSW.