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

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

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