EFFECT OF DIET ENERGY CONCENTRATION AND OF AGE OF HOLSTEIN-FRIESIAN BULL CALVES ON GROWTH-RATE, UREA SPACE AND FAT DEPOSITION, AND RUMINAL VOLUME

Holstein-Friesian bull calves were fed diets of three metabolizable en ergy concentrations: 11.7, 10.9, and 10.0 MJ/kg of DM, and the same co ntent of CP, 145 g/kg. Two trials were carried out. Initial weights we re 195 and 180 kg and final weights were 490 and 600 kg for Trials I a nd 2, respectively. The live weight gains (kg/d) were .93, 1.06, and 1 .16 in Trial I and .98, 1.11, and 1.16 in Trial 2 on the low-, medium- , and high-energy diet, respectively. In Trial 2, measurements were ma de of ruminal fluid volume and urea space, which expresses the protein mass in the body. Ruminal fluid volume of young calves was high, exce eding 250 mL/kg live weight. Proportional to live weight, it tends to decrease from the age of 213 d to slaughter. It was higher on the lowe r-energy diet (P < .01). Rate of fat deposition between 250 and 600 kg body weight did not increase even on the diet with 11.7 MJ/kg, althou gh these calves were fatter (P < .05) at slaughter than those on the l ow-energy diets. The ratios of depot fats (g of fat/kg of hot carcass) in both trials were close for each energy level, in spite of a 100-kg difference in live weight at slaughter. It was 24.0, 29.8, and 40.7 f or calves fed low-, medium-, and high-energy diets, respectively, in T rial 1 and 21.7, 37.1, and 40.4, respectively, in Trial 2. The common belief that rate of protein deposition decreases, and that of fat depo sition increases, with age and live weight could be supported in this study by calculations of energy deposited in body tissues only up to a weight of approximately 250 kg. Above this weight, energy concentrati on in the body tissue gain did not increase with age on either of the tested diets. It is suggested that differences in fat content between treatments resulted from fat accumulation during the growth period rat her than to an increased rate of deposition in the final stage.