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

SIMPLE SUMMARY: Natural service is currently the predominant method of breeding in extensively managed beef herds. In these herds, the importance of the bull has been largely overlooked, focusing instead on female fertility. However, the bull might be the most important factor in determining the reproductive performance of the herd, because one subfertile or infertile bull could mean a decrease of 25 to 40 calves per year. Thus, bull management is critical to increase herd fertility. Infectious bovine rhinotracheitis (IBR) and bovine viral diarrhea (BVD) are infections associated with reduced conception rates. In this study, we analyzed the effect of IBR, BVD, and bull: cow ratio on seminal parameters of the bull and herd fertility, finding a significant negative effect of BVD on sperm concentration and motility, and a negative correlation between the number of cows per bull and herd fertility. Our data suggest that serological control of BVD and sperm parameters, as well as including less than 40 cows per bull, could improve the reproductive efficiency of the herd in extensively managed herds. ABSTRACT: 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.