Plasma Luteinizing Hormone (LH) and Testosterone Levels during Sexual Maturation in Beef Bull Calves1

Abstract

Blood samples were taken by jugular venipuncture from bull calves every other week from 1 to 40 weeks of age. Samples were also drawn by jugular cannula, every 15 min for 2 h, from a different group of 4–5 calves at 10 week intervals, from 1 to 40 weeks of age. Following the period of intensive blood collection each group of calves was castrated. Based upon histological examination of the testis, spermatogenesis was initiated after 10 weeks of age, with mature spermatozoa being present in the testis by 40 weeks of age. Body weight and epididymal weight increased linearly with age, while testicular weight increased in a curvilinear pattern, with the most dramatic increase occurring from 30 to 40 weeks of age. Plasma concentrations of LH were low prior to 8 weeks of age, between 10 and 20 weeks of age episodic fluctuations were observed, with a return to low and stable concentrations being seen before a final gradual increase to adult levels at 40 weeks of age. Plasma concentrations of testosterone were variable, but appeared to rise markedly after 28 weeks of age, reaching adult levels by 40 weeks of age. The mode of variation of plasma concentrations of testosterone appeared to resemble that of the adult in 3 of 5 bull calves at 20 weeks of age and all 5 bull calves at 40 weeks of age; more transient peaks were observed at 10 weeks of age.