Associations among age, scrotal circumference, and proportion of morphologically normal spermatozoa in young beef bulls during an initial breeding soundness examination

Objective-To evaluate age and scrotal circumference as predictors of semen quality in young beef bulls. Design-Cohort study. Animals-1,173 beef bulls < 15 months old. Procedure-During initial breeding soundness examination, variables for bull s producing greater than or equal to 70% morphologically normal spermatozoa were compared with those for bulls producing < 70% morphologically normal spermatozoa. Mean and 95% confidence interval for age and scrotal circumfer ence were constructed to detect differences between groups of bulls over ai l observations and within the 5 most common breeds. For these 5 breeds, chi (2) analysis was used to evaluate differences in the proportion of bulls th at had values less than the population mean for scrotal circumference, age, and percentage of morphologically normal spermatozoa. Multivariate regress ion was used to quantify variation in the proportion of morphologically nor mal spermatozoa that could be explained by age and scrotal circumference. Results-Mean (+/- SD) age and scrotal circumference differed significantly for bulls that produced greater than or equal to 70% morphologically normal spermatozoa (12.7 +/- 1.1 months and 35.6 +/- 2.7 cm) and bulls that produ ced < 70% morphologically normal spermatozoa (12.1 +/- 1.2 months and 34.8 +/- 3.3 cm), The proportion of bulls younger than mean age at first examina tion and the proportion producing greater than or equal to 70% morphologica lly nor mal spermatozoa differed among breeds. Age and scrotal circumferenc e explained only 11% of the variation in semen quality. Clinical Implications-Among young beef bulls, those that were older and had larger testes were more likely to produce greater than or equal to 70% mor phologically normal spermatozoa. Age and scrotal circumference were not suf ficient predictors of semen quality.