Effects of treatment with GnRH from 4 to 8 weeks of age on the attainment of sexual maturity in bull calves

Abstract

In bull calves an early transient increase in circulating concentrations of LH occurs between 6 and 20 weeks of age. This has been shown to influence reproductive development and performance later in life. In an attempt to hasten the onset of sexual maturity, bull calves (Hereford × Charolais) were treated (im) with 120 ng/kg of GnRH ( n = 6) twice every day from 4 to 8 weeks of age; control calves received saline ( n = 6). Injection of GnRH resulted in an LH pulse in all animals. GnRH treated bulls displayed more rapid testicular growth rates between 22 and 44 weeks of age. Sexual maturity (SC ≥ 28 cm) was achieved earlier in GnRH treated bulls compared to saline treated bulls (41.7 ± 2.22 and 47.0 ± 0.45 weeks of age, respectively) and this was confirmed by age of sexual maturity based on ejaculate characteristics (>50 million spermatozoa, >10% motility; 45.0 ± 0.86 and 49.0 ± 1.13 weeks of age for GnRH and control treated bull calves, respectively; P < 0.05). We concluded that treatment with GnRH, twice daily, from 4 to 8 weeks of age, prior to the endogenous early increase in plasma LH concentrations, could increase in plasma LH concentrations, advance testicular development and reduce age at puberty in beef bull calves. This may provide the basis for a simple regimen to hasten sexual development in the bull calf.