Ratios of serum concentrations of testosterone and progesterone from yearling bulls with small testes

Thirty crossbred bulls, 12 to 13 mo of age, were used to examine the relationship of testosterone and progesterone concentrations and testosterone:progesterone ratio to measurements of testicular function. Bulls were allotted to 1 of 2 groups based on scrotal circumferences (SC) as follows: the Small SC (n = 20) group had scrotal circumference less than 28 cm while the Large SC (n = 10) group had scrotal circumference greater than 28 cm. All bulls were administered GnRH (100 ug, im), and blood was obtained immediately prior to injection (t = 0), 30 min after injection (t = 30) and 2 to 3 h after injection (t = 150). Serum was assayed for concentrations of testosterone and progesterone. Semen was evaluated for the percentage of morphologically normal spermatozoa. Testicular parenchyma was sectioned and stained, and 300 cross sections per testis of seminiferous tubules were examined under a light microscope and classified as either active (spermatocytes and spermatids present) or inactive (no spermatocytes or spermatids present). Although progesterone concentrations varied widely (range: 21 pg/ml to 1070 pg/ml), repeated measurements from individual bulls were highly correlated (r(2) = 0.74) and did not change significantly (P > 0.1) in response to GnRH treatment. Small SC bulls had a higher percentage of inactive seminiferous tubules (P < 0.001) and a lower percentage morphologically normal spermatozoa (P < 0.001) than Large SC bulls, but no differences in testosterone or progesterone concentrations or in the ratio of testosterone:progesterone were detected. Mean serum testosterone concentration increased (P < 0.0001) by 30 min after GnRH treatment and continued to increase (P < 0.0001) through t = 150 but did not differ (p > 0.1) between groups. Normal testosterone secretion in response to GnRH injection suggested that no biochemical lesions in the testosterone production pathway were present in bulls with very small scrotal circumference.