By far the best use of the actual scrotal circumference measurement on a bull is to ensure he has adequate size testes to make him a satisfactory potential breeder. The scrotal circumference EPD is the best indicator of a bull's genetic potential to sire sons with larger or smaller testicles than the breed average. The following paragraphs explain the concepts needed to devise a breeding strategy that includes selection of sires based on scrotal circumference or scrotal circumference EPDs. To be a successful breeder a bull must detect and successfully breed females in heat which includes delivering an adequate number of motile, morphologically normal sperm to achieve acceptable pregnancy rates. The breeding soundness exam devised by the Society for Theriogenology is designed to identify bulls which are most likely to be satisfactory breeders by measuring their scrotal circumference, estimating the motility of ejaculated sperm and evaluating the percentage sperm in the ejaculate that appear normal in shape.
Scrotal circumference is the best estimate of a bull's testes volume and daily sperm production.
Finally, the scrotal circumference measurements of yearling bulls are positively correlated with sperm motility and percent normal sperm. Hence, scrotal circumference of a young bull directly and indirectly affects his likelihood of passing all three components of the breeding soundness exam. Despite the positive relationships between scrotal circumference and semen traits, a consistent relationship between scrotal size and reproductive efficiency of bulls has been difficult to demonstrate. Several other studies failed to demonstrate higher pregnancy rates for groups of cows bred by bulls with scrotal circumferences that differed, but were all above the minimum threshold recognized by the exam. The common sense position on predicting a bull's breeding potential is to consider the importance of a bull's scrotal circumference as a threshold point.