Factors affecting mating activity and pregnancy rates with beef bulls mated to estrus synchronized females

Abstract

Ninety-two beef bulls and 2,316 estrus synchronized beef females were used in single-sire mating trials to evaluate effects of age and breed of bull, bull-to-female ratios, number of females exhibiting estrus, and number of times a heifer or cow was serviced on mating activity and pregnancy rates. Bulls ranged in age from 1 to 7 yr and were either Angus or Hereford. Each bull received a breeding soundness examination and was exposed twice to a libido/serving capacity test. Females received either the Syncro-Mate B (SMB) treatment or the double injection treatment of Prostaglandin F 2α (PGF). Bull-to-female ratios ranged from 1:7 to 1:51. Bulls were joined with their groups of females at 27 h (SMB) or 48 h (PGF) posttreatment, and sexual activity was observed continuously for 30 and 48 h, respectively. Each bull remained with his group of females until the end of a 120-h posttreatment period. There were no differences between Angus and Hereford bulls in measurements of mating activity or pregnancy rate. It appeared that bulls became more efficient in mating activity as they increased in age up to 3 yr and older. Pregnancy rates based on number of females exhibiting estrus were lowest in yearlings (30.2 ± 5.1%), intermediate in 2-yr-olds (40.3 ± 6.0%) and highest in 3-yr and older bulls (50.7 ± 4.5%). Pregnancy rates were not affected by bull-to-female ratio or number of females exhibiting estrus per bull. However, females serviced two or more times had higher (P < 0.05) pregnancy rates than those serviced one time.