Short communication: Effect of dietary protein on growth and nitrogen balance of Holstein heifers.

A growth study and a companion N balance study were conducted to evaluate the effect of dietary protein on growth and N utilization of postpubertal Holstein heifers. Forty heifers (398 +/- 9.4 kg) were fed one of four diets containing 8, 11, 13, or 15% crude protein (CP) for 121 d. Body measurements were taken at the beginning and end of the experimental period. Blood was collected via jugular vein every 28 d and evaluated for serum protein, albumin, and urea nitrogen. Curvilinear relationships were observed between dietary CP and gains in wither height and hip width, with maximal gains occurring at 13% CP. Feeding heifers higher amounts of dietary CP resulted in linear increases in heart girth gain, serum protein, albumin, and blood urea nitrogen. In the companion N balance study, feeding increasing amounts of dietary CP to heifers resulted in linear increases in N intake, fecal-N, urinary-N, and absorbed-N. There was a numerical trend towards maximal N retention in heifers fed diets containing 13% CP. Curvilinear relationships also were observed between dietary CP and dry matter, organic matter, and CP digestibility with maximal nutrient digestibilities occurring when heifers were fed diets containing 13% CP. Data suggest 13% dietary CP was optimal for postpubertal (400 kg) Holstein heifers.