The objective of this study was to determine the effects of recombinant bovine somatotropin (rbST) on performance, reproductive precocity and hyperthrophy in muscle fibers in Nelore heifers. In the present study, 58 heifers seven months of age weighing 193 kg were used. The experimental design was completely randomized and divided in two phases. In the first phase of 150 days, the levels of rbSt were 0 mg and 500 mg at 28 day intervals. In the phase II, each group of phase I was divided in two new groups and the levels of rbST were 0 mg and 500 mg for the control group, and 500 mg and 1,000 mg for 500 mg group of phase I, at 28 day intervals. Diet was exclusively pasture by 180 days. Biopsy of semitendinosus muscle was done for analysis of fibers at the beginning and at the end of phase I. The animals were weighed monthly (28 days). Results showed no significative differences (p>0.05) among treatments for gain of weight and for sexual precocity. For SO (slow oxidative), FOG (fast oxidative glycolitic) and FG (fast glycolitic) muscle fiber diameter a significative difference (p<0.01) with higher values for rbST treated heifers was observed.
animal growth hormones; sexual reproduction; precocity; weight gain; beef cattle; animal production
