Effects of rate of gain on scrotal circumference and histopathologic features of the testes of half-sibling yearling beef bulls

OBJECTIVE: To determine the effects of high and low gain on production variables and reproductive characteristics of bulls, including histopathologic lesions in the testes, as determined by quantitative measurements. ANIMALS: 23 half-sibling, 11.6-month-old beef bulls, averaging 326 +/- 8.3 kg in weight and having a mean scrotal circumference of 28.7 +/- 0.4 cm. DESIGN: Bulls were randomly assigned within pairs to high- or low-gain rations and fed ad libitum for 112 days. PROCEDURES: Scrotal circumference and body weight were measured on days 0, 28, 56, 84, and 112. Bulls were castrated on day 112. Morphologic characteristics of spermatozoa were determined from samples taken on days 50 and 111. Testis sections from each bull were examined, using light microscopy, for changes in the seminiferous epithelium and interstitium. A grading scheme was used to quantify testicular lesions by histologic techniques. Pathologic changes in 600 cross sections from each bull were classified into 9 grades. The degree of germinal epithelial loss per 100 tubules was calculated by assigning a value to each grade according to the severity of loss of germinal epithelium. RESULTS: Scrotal circumference (mean +/- SEM cm), 34.0 +/- 0.8 vs 31.7 +/- 0.5 (P < 0.001), body weight (kg), 512.3 +/- 19.6 vs 421.9 +/- 9.4 (P < 0.001), and paired testicular and epididymal weight (g), 532 +/- 31 vs 425 +/- 24 (P < 0.0001) and 73.8 +/- 1.9 vs 56.3 +/- 3.7 (P < 0.001) were greater in the high-gain than in the low-gain group. Morphologic characteristics of spermatozoa were similar for both groups. Means for graded seminiferous epithelial effects were similar in each group. Hypoplastic-type tubules were found in bulls with small scrotal circumference in both the high- and low-gain groups. CONCLUSION: High gain resulted in greater testicular growth but testicular structure and function were similar for both groups.