Date of this Version
Although artificial insemination is widely used in dairy cattle, it has found only limited application in beef cattle. Use of bulls in natural mating programs accounts for over 90% of the pregnancies achieved each year in the U.S. beef cattle industry, and a large percentage of the beef bulls used for natural mating are purchased as yearlings. Many of these yearling bulls undergo a breeding soundness examination prior to sale or the breeding season. This involves visual and manual examination of the genital system as well as assessment of semen, which is usually collected by electroejaculation. However, sex drive and mating ability, which are essential for the delivery of viable spermatozoa and impregnation of females, are not commonly measured.
Among beef bulls used for single-sire mating, large ranges in pregnancy rates have been reported, and only low correlations were found between pregnancy rates and semen characteristics. These differences may be potentially explained by differences in the levels of serving capacity (SC) between bulls. Several procedures for testing SC have been used; however, studies investigating the relationship between bulls' SC and herd fertility are inconclusive, with some researchers finding no relationship and other researchers reporting SC test results to be a good predictor of bull fertility. These studies differed in testing procedures used for measuring SC. Thus, differences in the findings may lie in the procedures used for tests or may be due to differences in bull-to-female ratios used when measuring fertility.
At present, there is a lack of research relating the SC of yearling beef bulls in a standardized test with their behavior and fertility under pasture mating conditions. The first objective of this study was to evaluate the sexual and grazing behavior of low and high SC yearling bulls when placed with naturally cycling and estrus synchronized cows under pasture mating conditions during both