Effect of somatotropin on hyperthrophy of skeletal muscle fibers and reproductive precocity of nelore heifers

The objective of this study was to determine the effects of recombinant bov ine somatotropin (rbST) on performance, reproductive precocity and hyperthr ophy in muscle fibers in Nelore heifers. In the present study, 58 heifers s even months of age weighing 193 kg were used. The experimental design was c ompletely randomized and divided in two phases. In the first phase of 150 d ays; the levels of rbSt were 0 mg and 500 mg at 28 day intervals. In the ph ase 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 pas ture 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) am ong treatments for gain of weight and far sexual precocity. For SO (slow ox idative), FOG (fast oxidative glycolitic) and FG (fast glycolitic) muscle f iber diameter a significative difference (p<0.01) with higher values for rb ST treated heifers was observed.