Altering soluble and potentially rumen degradable protein for prepubertal Holstein heifers

Four prepubertal Holstein heifers, average age 184 +/- 2.1 d and 169.1 +/- 15.8 kg of body weight, fitted with rumen cannulae were used to evaluate effects of altering the soluble CP and potentially rumen degradable protein fractions of diets containing 16% CP and 71.5% TDN. Parameters studied included rumen ammonia, volatile fatty acid production, N balance, total tract apparent digestibility, and urinary excretion of purine derivatives in a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treatments. Heifers were fed treatment rations containing 62.1 +/- 0.8 g CP/Mcal ME at 2.0% BW with altered soluble CP fractions (AB1) (33.6 or 40.6% of CP) and potentially rumen degradable protein fractions (B2B3) (20.9 or 28.2% of CP). Increased intake of AB1 increased rumen ammonia and decreased total volatile fatty acid concentrations with molar proportions of isovalerate and isobutyrate decreased. Increased intake of B2B3 tended to increase volatile fatty acid concentrations, increased molar proportions of propionate, and decreased the acetate to propionate ratio. Nitrogen utilization was not affected by increased intake of AB1 or B2B3, although increased intake of AB1 tended to increase urine urea excretion. Increasing solubility (40.6 versus 33.6% AB1) of dietary CP when feeding a 16% CP and 71.5% TDN ration, decreased total volatile fatty acid concentrations and molar proportions of isovalerate and isobutyrate. Feeding 62.1 +/- 0.8 g CP/Mcal ME with increased potentially rumen degradable protein (28.2 versus 20.9% B2BS) at 2.0% BW affected rumen fermentation but did not affect DM digestibility or N utilization.