Animal Science Department


Date of this Version

December 1991


Published for Proceedings, The Range Beef Cow Symposium XII December 3, 4 & 5, 1991, Fort Collins, Colorado.


One of the biggest challenges facing cow/calf producers today is cutting their costs of production. One possible strategy for lowering costs is to increase the production efficiency of the cow herd. A key area to focus on is the reproductive function of the beef bull because natural mating accounts for over 95% of the pregnancies achieved each year in the 33.7 million beef cows in the U.S.

It is becoming apparent that perhaps because of increased selection pressure on scrotal circumference, the modern beef bull is more fertile than the bull of yesterday, yet the majority of beef bulls are still being mated at traditional bull to female ratios of 1:20 to 1:30. Utilizing bulls to their full breeding potential is one of the quickest and simplest ways a producer can cut costs. Unfortunately, however, determining a bulls breeding potential has been either elusive in the case of yearling bulls or labor intensive in the case of mature bulls.

As a predictor of natural service fertility, most beef bulls undergo a breeding soundness examination prior to either sale or breeding; however, sex drive and mating ability are not commonly measured though both are essential for impregnation of females. Tests have been developed to measure a bulls mating ability or serving capacity. However, past studies investigating the relationship between serving capacity and herd fertility have been inconclusive with some researchers finding no relationship and other researchers reporting serving capacity tests to be an accurate predictor of bull fertility. Many of these studies differed in testing procedures and utilized bulls of different ages with varying levels of sexual experience; thus, differences in the findings may rest with these factors. Because most cattle producers in the United States are purchasing bulls at 12 to 15 months of age (virgins), most serving capacity work in the United States has focused on the serving capacity testing of yearling bulls.