Four multiparous Holstein cows were used in a 4 x 5 incomplete Latin square design to study the effects of different dietary sources of energy and protein on digestion, ruminal fermentation, and degradability in cows fed high moisture grass silage. The five treatments were an all silage diet (control); silage and concentrate containing soybean meal fed with corn, beet pulp, or a mixture (50:50 on a DM basis) of oats and barley; and fish meal fed with beet pulp. Concentrates were fed between .70 and .76% of BW to give similar CP and NEL intakes. Total DMI and milk production were lower for unsupplemented than for supplemented cows, but digestion and ruminal fermentation did not differ. Digestibility of fiber and concentration of total VFA were higher for cows fed corn than for those fed the mixture of oats and barley, but starch source had no effect on total DMI or milk production and composition. Energy source had no effect on total DMI or milk production and composition. Digestibility of DM and NDF was higher, and ruminal concentration of NH3 N and degradability of silage N tended to be lower, for cows fed beet pulp than for those fed starch, suggesting an improvement in the microbial protein synthesis in the rumen when beet pulp was fed instead of starch.