Use of bulls in natural mating programs accounts for over 90% of the pregnancies achieved each year in the U.S. beef cattle industry (Huffine, 1987), and a large percentage of the beef bulls used for natural mating are marketed as yearlings. Many of these yearling bulls undergo a breeding soundness examination prior to sale or the breeding season. However, drive and mating ability are not commonly measured, though they are essential for the delivery of viable spermatozoa and impregnation of females.

Among beef bulls used for single-sire mating, a large range in pregnancy rates has been reported (Smith et al., 1981; Lunstra and Laster,1982), and correlations between pregnancy rates and semen characteristics were low. These differences may be explained by differences in the levels of serving capacity (SC) among bulls. Several procedures for testing SC have been described (Chenoweth and Osborne, 1975; Lunstra,1980; Blockey, 1981). However, studies investigating the relationship between SC and herd fertility are inconclusive, with some researchers finding no relationship (Farin etal.,1978;Christensen et al., 1982; Makarechianand Farid, 1985) and other researchers reporting SC test results to be an accurate predictor of bull fertility (Blockey, 1978a,b; Lunstra, 1980).Studies have differed in testing procedures used for measuring SC. Thus, differences in the findings may lie in the procedures used for tests or 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. Therefore, the first objective of this study was to evaluate the sexual behavior of low and high SC yearling bulls when placed with naturally cycling and estrus synchronized cows under pasture mating conditions during both daylight and dark hours. The second objective was to determine the effect bull SC has on herd fertility under these conditions and to determine the relationship between mating activity and body weight, testicular characteristics
and semen quality.  The movie below shows how Serving Capacity may be measured.