Evaluation of American Wagyu sires for scrotal circumference by age and body weight

Thirty-six percent of American Wagyu bulls do not meet the current minimum standards set by the Society of Theriogenology for the breeding soundness exam. In contrast, only 15% of bulls of domestic breeds do not meet the minimum standards. Scrotal circumference measurements of Wagyu are smaller than those of other breeds. The objective of this research was to describe scrotal circumference of Wagyu bulls as it relates to age and BW. The data set consisted of 190 Wagyu bulls housed at two locations. One hundred forty-one bulls constituted the first set of data (location 1); scrotal circumference was measured one to six times per bull aged between 13 and 70 mo. Ninety-four of the bulls underwent semen evaluation for motility and morphology. Forty-nine bulls constituted the data set for which scrotal circumference and BW was measured one to nine times per bull between 5 and 21 mo of age (location 2). Mean scrotal circumference of bulls within each age group was as follows: 12 to 14 mo, 29.8 +/- 0.2 cm (mean +/- SE); 15 to 17 mo, 31.8 +/- 0.2 cm; 18 to 20 mo, 32.9 +/- 0.3 cm; 21 to 24 mo, 31.8 +/- 0.5 cm; and > 24 mo, 35.5 +/- 0.2 cm. Both age and BW were highly correlated to scrotal circumference (r = 0.81 and 0.82, respectively). Within each age group, there were a percentage of bulls that did not meet the minimum standard for scrotal circumference set by the Society of Theriogenology. The percentages were as follows: 12 to 14 mo, 46%; 15 to 17 mo, 25%; 18 to 20 mo, 33%; 21 to 24 mo, 42%; and > 24 mo, 32%. Morphology and motility were greater than or equal to 50% each in 91% of the bulls between ages 12 and 20 mo at location 1. Based on these data, it is recommended that Wagyu bulls be evaluated with the breed-specific minimum standards for scrotal circumference of 26 cm from 12 to 14 mo, 29 cm from 15 to 17 mo, and 30 cm from 18 to 20 mo of age.