Four Holstein heifers with initial body weight (BW) of 297.5 ± 27.7 kg fitted with ruminal cannula (i.d. 7.5 cm, Bar Diamond, Parma, ID) were used in a crossover design to study A. Rotger et al. / Animal Feed Science and Technology 125 (2006) 73–87 75 ruminal fermentation in animals fed two high concentrate diets with different forage to concentrate ratios, and to estimate ruminal degradability of plant protein supplements under those ruminal conditions.