Impact of Overuse and Sexually Transmitted Infections on Seminal Parameters of Extensively Managed Bulls.

Natural service remains the main breeding method in extensively managed beef herds. Although the bull might be the most important factor in determining herd fertility, its importance has been largely overlooked, focusing instead on female fertility. Management of the bull is critical to maximize the opportunities for cow conception. Infectious bovine rhinotracheitis (IBR) and bovine viral diarrhea (BVD) are infections associated with reduced conception rates. This study aimed to determine the effect of both IBR and BVD infection, and bull: cow ratio on seminal parameters in the bull and herd fertility. The presence of antibodies to IBR and BVD, seminal parameters (volume, concentration, mass, and progressive motility), and herd fertility were analyzed in 158 extensively managed bulls. Sperm concentration and mass motility, as well as herd fertility, were significantly lower in BVD-positive bulls. No significant differences were found between IBR-positive and -negative bulls in any reproductive parameter. Sperm concentration was negatively affected by BVD infection in both Charolais and Limousin bulls, whereas mass motility and herd fertility were reduced in Limousin bulls only. No differences were observed in the cow: bull ratio between BVD+ and BVD- bulls. A significant negative correlation was detected between the number of cows per bull and herd fertility, which was negatively affected when herds had more than 40 cows per bull. In conclusion, BVD and bull overuse negatively affect the reproductive performance of the herd.