Gonadotropin releasing hormone-induced change in serum luteinizing hormone, testosterone and androstenedione in bulls, steers and steers given testosterone.

Two experiments were conducted to determine the effect of gonadal hormones on LH release by successive injections of Gonadotropin Releasing Hormone (GnRH). In the first experiment 1 month-old Holstein bulls were used. Bulls were assigned to be: 1) left intact (n=5), 2) castrated (n=6), or 3) castrated and given 10 mg testosterone thrice daily for 15 days (n=4). All animals received three intramuscular injections of 20 /lg GnRH administered at 12-hr intervals beginning 14 days post-castration. Average magnitude of LH release by GnRH, measured as area under the response curve was greatest (P<.01) in steers and least (P<.01) in steers given testosterone. Magnitude of LH release in response to the first GnRH challenge was greater (P<.01) than that of the second or third. Serum androstenedione was increased (P<.05) after GnRH treatment. These data indicate that testicular hormones affect the ability of the pituitary of bulls to respond to GnRH as early as 1 month of age. A second experiment utilized pubertal bulls and was conducted in two parts. In the first part, four bulls were left intact, four were castrated and four were castrated then given testosterone (20 rag) thrice daily from the day of castration (day 0) through day twenty-one.