When concentrates are rapidly digested in the rumen of the dairy cows, turnover rate will increase and protein degradability consequently decrease. This may reduce the need for low degradability protein supplements but the rapid post-prandial release of energy for microbial digestion could cause a deficit of rumen - degradable nitrogen and low rumen pH. In two experiments two rapidly-digested energy sources were examined, molasses and extruded barley, and three nitrogen supplements, urea, soyabean and fishmeal, as well as the use of sodium bicarbonate to stabilize rumen pH.