Two digestion experiments and an in situ nylon bag experiment were conducted to determine the correlations between chemical composition, ruminal in situ degradation kinetics and OM I, and OM and CP digestibilities of diets in wethers fed forages of varying nutritive value without and with protein supplementation. Wethers were fed ground corn crop residues, oat-berseem clover or alfalfa hay in the first experiment, ground corn crop residues with or without alfalfa or oat-berseem clover hay supplemented with SBM and grain corn mixture to meet the DIP requirement, by using level 1 of the Nutrient Requirements of Beef Cattle computer program at ad libitum intake. To determine the kinetics of N and OM degradation in the rumen, a composite sample of each feed was prepared. Duplicate bags of each forage were attached to weighted rings, placed in the rumens of two fistulated steers grazing smooth bromegrass, and allowed to incubate for 0, 3, 6, 12, 24. 48 and 72 h. While the concentrations of NDF and ADF and the percentage of potentially digestible organic matter fraction and indigestible organic matter fraction were greater, the percentage of water soluble organic matter fraction was lower in corn crop residue compared with alfalfa hay or oat/berseem clover (P<0.05). Addition of alfalfa or oat-berseem clover hay supplemented with SBM and grain corn mixture to corn crop residues linearly decreased the concentrations of NDF and ADF, and the percentage of the indigestible organic matter fraction, but increased the percentage of potentially digestible and water soluble organic matter fractions. Organic matter intake, organic matter and crude protein digestibilities were significantly lower (P<0.05) in wethers fed corn crop residues alone compared with wethers fed alfalfa or oat/berseem clover hays. Addition of protein supplement sand addition of increasing levels of oat/berseem clover or alfalfa hays to corn crop residues linearly increased organic matter intake, organic matter and crude protein digestibilities of diets in wethers. The highest correlations were observed between water soluble organic matter fraction and organic matter intake (r(2) = .80), ADF and apparent total track OM digestibility (r(2)=.94), and indigestible CP fraction and true ruminal CP digestibility (r(2)=.82). It was concluded that chemical composition with ruminal degradation characteristics can provide a reliable prediction of the intake and digestibility of forage-based diets.