Eight postpubertal Holstein heifers (455 ± 4.0), fit with rumen cannulas, were used in 2 experiments to investigate the effects of altering dietary protein type on nutrient digestibility, rumen fermentation, and nitrogen utilization. Heifers were fed diets containing low or high levels of soluble (SP) and low or high levels of rumen-undegradable protein (RUP) in a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. The treatment rations in experiment 1 were formulated with corn silage composing the majority of the forage fraction, whereas in experiment 2, grass hay composed the highest proportion of ration DM. Blood and rumen samples were collected over 2 d and total fecal and urine collections were conducted for 4 d. Dry matter, organic matter, and neutral detergent fiber digestibility were not different in either experiment 1 or 2. Increasing the proportion of dietary crude protein that was SP increased mean daily rumen ammonia concentrations in each experiment, although no other rumen parameter differed. Excretion of urinary nitrogen in experiment 1 was highest for diets with low SP and low RUP and with high SP and high RUP, which resulted in these rations being the least efficient in retention of apparently digested nitrogen'. The proportion of consumed or absorbed nitrogen retained in experiment 2 was not significantly different between treatments. Responses to alterations in crude protein degradability are observable in postpubertal heifers; however, the level of response may depend on the diet in which protein degradability is altered.
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