Treating heifers with GnRH from 4 to 8 weeks of age advanced growth and the age at puberty

In heifer calves, an early transient increase in circulating concentrations of LH is associated with early follicular development and is thought to regulate the timing of puberty. In an attempt to hasten the onset of sexual maturity, the early rise in LH concentration was advanced by injecting heifer calves with 120 ng/kg of GnRH (n = 6) twice daily from 4 to 8 weeks of age; control calves received saline (n = 6). Blood samples were collected every 15 min for 10 h at 4, 8, 14, 20, 26, 32, 38, 44 and 50 weeks of age. Treatment with GnRH increased mean circulating concentrations of LH at 8 weeks of age (P < 0.05), LH pulse frequency at 4 and 8 weeks of age (P < 0.05), and reduced the mean age at puberty by 6 weeks (56.8 ± 1.7 versus 62.8 ± 2.4 weeks of age, for GnRH treated and control calves, respectively; P = 0.04). Body weight gain was greater in GnRH-treated calves than control calves (P < 0.05), and the rate of weight gain was shown to be a significant covariate within age at puberty. In conclusion, we suggest that the timing of the early rise in LH concentrations is a critical signal involved in the timing of puberty in heifers.