Gonadotrophin secretion in prepubertal bull calves born in spring and autumn

The reproductive development of bull calves born in spring and autumn was c ompared. Mean serum LH concentrations in calves born in spring increased fr om week 4 to week 18 after birth and decreased by week 24. In bull calves b orn in autumn, mean LH concentrations increased from week 4 to week 8 after birth and remained steady until week 44. LH pulse amplitude was lower in b ull calves born in autumn than in calves born in spring until week 24 of ag e (P < 0.05). There was a negative correlation between LH pulse frequency a t week 12 after birth and age at puberty in bull calves, irrespective of se ason of birth, and LH pulse frequency at week 18 also tended to correlate n egatively with age at puberty. Mean serum FSH concentrations, age at pubert y, bodyweight, scrotal circumference, testes, prostate and vesicular gland dimensions, and ultrasonographic grey scale (pixel units) were not signific antly different between bull calves born in autumn and spring. However, age and bodyweight at puberty were more variable for bull calves born in autum n (P < 0.05). In a second study, bull calves born in spring received either a melatonin or sham implant immediately after birth and at weeks 6 and 11 after birth. Implants were removed at week 20. Mean LH concentrations, LH p ulse frequency and amplitude, mean FSH concentrations and age at puberty di d not differ between the two groups. No significant differences between gro ups in the growth and pixel units of the reproductive tract were observed b y ultrasonography. In conclusion, although there were differences in the pa ttern of LH. secretion in the prepubertal period between bull calves born i n autumn and spring, the postnatal changes in gonadotrophin secretion were not disrupted by melatonin treatment in bull calves born in spring. Reprodu ctive tract development did not differ between calves born in spring and au tumn but age at puberty was more variable in bull calves born in autumn. LH pulse frequency during the early prepubertal period may be a vital factor in determining the age of bull calves at puberty.