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Serum LH and Testosterone and Sperm Numbers in Pubertal Bulls Chronically Treated with Gonadotropin Releasing Hormone^{1, 2,}

Michigan State University, East Lansing 48824⁴

Abstract

This experiment with 18 bulls, six each at 2, 4 and 6 months of age, was designed to test whether chronic treatment with GnRH may hasten puberty. Each of three bulls within each age was given 40 µg (im) GnRH twice daily for 28 days and three were given saline. Blood was sampled at 0, .5 and 3 hr after each injection on days 1, 7, 14, 24 and 28. Serum LH was increased (P<.01) 30 min after GnRH, and the increase in LH was greater (P<.05) on day 1 than on days 7, 14, 24 or 28; averages at 30 min were 20.0, 5.5, 6.5, 5.0 and 6.2 ng/ml, respectively. Serum testosterone in 6-month-old bulls treated with GnRH was greater (P<.05) than that in controls of the same age, but this difference was not observed in 2- or 4-month-old bulls. Overall averages for serum testosterone in 2-, 4- or 6-month-old bulls treated with GnRH were .7, 2.0 and 5.8 ng/ml, and the value at 6 months was higher (P<.05) than those for 2- or 4-month-old bulls. Testosterone for control bulls averaged .5, 2.4 and 3.8 ng/ml for 2-, 4- and 6-month-old bulls, respectively. Despite increased serum LH after GnRH at all ages and increased testosterone after GnRH in 6-month-old bulls, testicular size, testicular sperm numbers and epididymidal sperm numbers were unaltered at 60 days after GnRH treatment.

¹ Published with approval of director of the Agriculture Experiment Station as Paper No. 6895.

² Gonadotropin releasing hormone (GnRH) was generously supplied by Dr. R. Rippel, Abbott Laboratories, North Chicago, Ill.

³ Present address: Kasetsart University, Bangkok 9, Thailand.

⁴ Animal Reproduction Laboratory, Department of Dairy Science.