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Serum Hormone Patterns Associated with Growth and Sexual Development in Bulls¹

Michigan State University², East Lansing 48824

Abstract

Two experiments were conducted to characterize changes in serum hormones prior to and during puberty in bulls and to study effects of castration or shortening the scrotum on these hormonal changes. In the first experiment, four of eight prepubertal bulls (135 days old) and four of eight pubertal bulls (278 days old) were left intact and four of each group were castrated. Two weeks later, serum LH concentrations did not differ between prepubertal and pubertal animals, but steers had higher mean LH and more frequent episodic peaks than bulls at both ages. Serum prolactin concentrations did not differ significantly between bulls and steers, but older steers and bulls tended to have greater prolactin than younger animals (17.3 ng/ml vs 8.1 ng/ml). Prepubertal bulls had higher mean growth hormone concentrations than pubertal bulls or steers at both ages. Testosterone concentrations were fourfold greater in pubertal than in prepubertal bulls.

In a second experiment, 12 2-week-old bulls were assigned to be bulls, steers or bulls with shortened scrotums (SS bulls). Blood was sampled at .5-hr intervals for 24 hr each month for 10 months. Serum LH concentrations increased significantly from 1 to 3 months in steers, from 1 to 2 months in SS bulls and from 1 to 4 months in bulls, while frequency of LH episodic peaks increased fivefold from 1 to 4 months in each treatment. Steers had greater serum LH concentrations and more frequent LH peaks than bulls from 3 to 10 months, while SS bulls had higher LH concentrations at 5 and 6 months and more LH peaks than bulls at 3 and 4 months. FSH concentrations did not differ between or change with age in bulls or SS bulls, but FSH increased between 1 and 2 months in steers and thereafter remained higher than in bulls or SS bulls. Serum testosterone increased from 1 to 10 months in bulls and SS bulls, with the increase being most pronounced between 4 and 6 months. Androstenedione increased transiently at 4 months and did not differ between bulls and SS bulls.

We speculate that increased testosterone secretion at 4 to 6 months may have been caused by increased frequency of episodic LH peaks at 4 months. Increased secretion of FSH and LH in steers at 1 month and increased frequency of LH peaks in SS bulls at 3 and 4 months indicate that gonadal factors normally inhibit gonadotropin secretion well in advance of adult levels of testosterone secretion.

¹ Michigan Agr. Exp. Sta. Journal Article No. 8758. This research was supported in part by USPHS Research Grants HD06948 and HD10123.

² Animal Reproduction Laboratory, Department of Dairy Science.

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