Chairperson, Graduate Committee: R. Clyde Greer.Carroll, Llane Glenn2013-06-252013-06-251990https://scholarworks.montana.edu/handle/1/1046An important decision a rancher makes is the age at which a bull will first be used for breeding. While yearling bulls are used by many ranchers in the Great Plains States, there is concern that lack of maturity among yearling bulls leads to lower breeding performance. To compare the breeding performance of yearling and two-year-old bulls Line One Hereford bulls in single sire breeding herds at Fort Keogh Livestock and Range Research Laboratory (LARRL) were analyzed. Tests for differences in pregnancy rates, calving dates, calf birth weight, and calf average daily gain were conducted. The physical attribute distributions were converted to a returns distribution for each age of bull. The distributions were then compared in a stochastic dominance framework. From the results it was concluded that the alternative "use the bull first as a two-year-old" dominated the alternative "use the bull first as a yearling" in the first order stochastic sense. While ranchers are using yearling bulls, the expected income is higher and the dispersion of observed income smaller from herds bred to two-year-old bulls. Ranchers may be deriving other benefits from the use of yearling bulls such as decreased intervals for introducing special genetic traits. Ranchers may be using yearlings in multiple sire settings which may diminish the downside risk of using yearling bulls as compared to using yearling bulls in single sire settings.enCattleBullsAgeLivestock--Performance recordsAnimal breedingEconometric modelsThesisCopyright 1990 by Llane Glenn Carroll