The influence of recombinant bovine somatotropin on dietary energy level-related growth of Holstein-Friesian bull calves

Our objective for this study was to assess the effect of recombinant bovine somatotropin (rbST) in overcoming the biological effects attributed to liv e weight increase and age on growth and fat deposition in male cattle. Hols tein-Friesian bull calves (n = 56; 182.2 +/- 14.7 d old) were allotted to f our subtreatments in a randomized complete block with a factorial arrangeme nt of two levels of rbST: 0 and 500 mg of Posilac, every 2 wk, and two diet ary metabolizable energy (ME) concentrations: low metabolizable energy (LME ) and high metabolizable energy (HME); 10 vs 11.3 MJ/kg DM, respectively. T he effect of rbST treatment on daily gain was expressed mostly on the HME d iets. The rbST treatment had no effect on the animals fed the LME diets bef ore the age of 240 d was reached. Dry matter intake and the effect of rbST treatment on DM intake were inversely related to the energy concentration o f the diet. The degree of fatness of the animals was significantly reduced by rbST treatment and significantly increased by energy concentration of th e diet. The major effect of rbST, under the experimental conditions, in reg ards to adipose tissue deposition, was on the fat depots and not on the int ramuscular fat. The concentration of bST, ICE-I, and insulin in the plasma was increased (P < .001) owing to rbST treatment. Lower metabolizable energ y supply led to a higher (P < .001) plasma bST concentration, nonsignifican tly lower plasma IGF-I and thyroid hormone concentrations, and lower (P < . 001) plasma insulin concentration. A trend (P = .065) toward an increase in PUFA was found in the muscle of the rbST-treated and the HME diet animals. At a young age, when the natural growth potential is high, rbST treatment will be efficient only when a diet allowing a high digestible energy intake is provided.