Relationships of Scrotal Circumference to Puberty and Subsequent Reproductive Performance in Male and Female Offspring

Reproductive efficiency, obtained through cost-effective measures, is the most important factor affecting profitability of the cow-calf enterprise. The reproductive efficiency of bulls and females both contribute to expressed reproductive performance of the cow herd. It is quite possible that although females are culled for low expressed reproduction, sires with only average reproductive potential are subsequently used resulting in female offspring that again need to be culled based on low reproductive performance. This culling process, although repeated every generation, would improve reproductive potential of the herd very little as long as sires with average reproductive potential are used. Since nearly 90% of the genetic change over time is due to sires, heritable reproductive measures in bulls need to be identified and used to genetically improve reproductive potential in male and female offspring. This discussion reviews some of the literature relating to scrotal circumference (SC) and its possible use in improving overall reproductive efficiency of the cow herd. Breeding Soundness Examination The breeding soundness exam (BSE) recommended by the Society of Theriogenology is widely used for both yearling and older bulls and especially for yearling bulls at the conclusion of performance tests. I believe there are two primary uses for the BSE: 1) to cull bulls classified as unsatisfactory and possibly retest bulls (especially yearlings) which are classified questionable and 2) to select yearling bulls to genetically improve both bull and their heifer offspring reproductive efficiency. Both uses should improve immediate bull performance while the second use should also aid in improving long-term herd reproductive potential. This discussion will concentrate on the second use and on yearling bull information since this is when selection would be most effective. A portion of the present BSE information for yearling bulls (12 to 14 mo) is shown in table 1. The BSE includes information on SC, semen morphology and motility, and an internal and external physical examination. Bulls receiving 60 or more points are usually classified as satisfactory breeders if they pass the physical exam and have at least 30 cm SC. Scrotal circumference is an important component of the BSE accounting for up to 40 of the possible 100 points. SC Adjustment Factors If SC in yearling bulls is to be used as a selection criterion, some attention should be given to adjustments to account for differences in age and age of dam among bulls. Results from several major studies relating to adjustment factors for SC are listed in table 2. The formula for mature dam equivalent 365 d scrotal circumference