GROWTH AND GROWTH-HORMONE KINETICS IN HOLSTEIN STEER CALVES

Average daily gain (ADG), basal plasma growth hormone (GH) concentration, and plasma GH kinetics following a bolus injection of GH were determined at birth and at 75, 150, 250, 350 and 450 kg body weight in 19 Holstein steer calves. From 100 kg body weight, calves were fed one of three diets: 85 % grass hay and 15 % concentrate; 85 % grass hay and 15 % concentrate with rumen undegradable protein (formaldehyde-treated canola meal); and, 85 % concentrate and 15 % grass hay. Addition of rumen undegradable protein to the hay-based diet did not (P > 0.05) increase ADG. However, ADG were higher (P < 0.001) for calves on the 85 % concentrate diet. Calf basal plasma GH concentrations, plasma GH clearance rates, and GH secretion rates were not affected by either diet or ADG adjusted for the effect of diet. In contrast, body weight had a large effect on basal plasma GH concentration and plasma GH kinetics. As body weight increased, basal plasma GH concentration and GH half-life decreased. Growth hormone steady state volume of distribution, metabolic clearance rate and secretion rate per kg of body weight all decreased as calves became heavier.