Effect of feed restriction during calfhood on serum concentrations of metabolic hormones, gonadotropins, testosterone, and on sexual development in bulls.

The objective of the present study was to evaluate the effects offeed restriction during calfhood on serum concentrations of metabolic hormones, gonadotropins, and testosterone, and on sexual development in bulls. Eight beef bull calves received a control diet from 10 to 70 weeks of age. An additional 16 calves had restricted feed (75% of control) from 10 to 26 weeks of age (calfhood), followed by either control or high nutrition (n=8/group) during the peripubertal period until 70 weeks of age. Restricted feed during calfhood inhibited the hypothalamic GnRH pulse generator, reduced the pituitary response to GnRH, impaired testicular steroidogenesis, delayed puberty, and reduced testicular weight at 70 weeks of age, regardless of the nutrition during the peripubertal period. Restricted feed reduced serum IGF-I concentrations, but concentrations of leptin, insulin, and GH were not affected. In conclusion, restricted feed during calfhood impaired sexual development in bulls due to adverse effects on every level of the hypothalamus-pituitary-gonad axis and these effects were not overcome by supplemental feeding during the peripubertal period. Furthermore, based on temporal associations, the effects of restricted feed on the hypothalamus-pituitary-gonad axis might be mediated by serum IGF-I concentrations. These results supported the hypotheses that the pattern of LH secretion during the early gonadotropin rise during calfhood is the main determinant of age of puberty in bulls and that gonadotropin-independent mechanisms involved in testicular growth during the peripubertal period are affected by previous LH exposure.