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Altering Soluble and Potentially Rumen Degradable Protein for Prepubertal Holstein Heifers¹

Department of Dairy and Animal Science The Pennsylvania State University, University Park 16802

Corresponding author:
A. J. Heinrichs; e-mail ajh{at}psu.edu.

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 (B₂ B₃) (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% B₂B₃) at 2.0% BW affected rumen fermentation but did not affect DM digestibility or N utilization.

Key Words: heifer metabolism • protein to energy ratio • soluble protein • potentially rumen degradable protein

Abbreviation key: AB₁ = soluble protein, B₂B₃ = potentially rumen degradable protein, CP:ME = crude protein to metabolizable energy ratio, HH = high AB₁-high B₂B₃, HL = high AB₁-low B₂B₃, LH = low AB₁-high B₂B₃, LL = low AB₁-low B₂B₃, ME = metabolizable energy, NSC:RDP = non-structural carbohydrate to rumen degradable protein ratio, PD = purine derivatives, PUN = plasma urea nitrogen

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