Abstract
Experiments were conducted to determine the reproductive potential of herd sires by: 1 ) an evaluation of age and genotype effects on semen quality and response to electroejaculation, 2) a serving capacity test to compare the profile of copulatory activity between two genotypes and, 3) a combination of both semen evaluation and a social interaction test to categorize bulls as either dominant or subordinate. In Experiment 1, fewer electronic impulses were needed to ejaculate 1-year-old than 2-year-old bulls (P<.001) via electroejaculation. In 3-year-old bulls, a genotype difference was observed with the Fl Gelbvieh x Red Angus requiring fewer electronic impulses than either the Santa Gertrudis or Simmental bulls (P<.002). Scrotal circumference was influenced by age and genotype. One-year-old bulls had a smaller scrotal circumference than 2-year-old bulls (P<.0004), and F, Gelbvieh x Red Angus bulls had a larger scrotal circumference than the Composite (1/2 Santa Gertrudis, 1/4 Red Angus, 1/4 Gelbvieh) and Santa Gertrudis genotypes (P<.003). An age by genotype effect (P<.02) on sperm motility was observed for the 1-and 2-year-old bulls. The percentages of normal and motile sperm increased from 1 to 2 years of age (P<.0001). Santa Gertrudis bulls had a lower percentage of motile sperm than the Fl Gelbvieh x Red Angus and Composite genotypes (P<.0053). Results of Experiment 2 indicate that serving capacity may be assessed by a 20minute test duration in Composite bulls, while a 30-minute serving capacity test is suggested to evaluate copulatory proficiency of Santa Gertrudis bulls. Pregnancy rate, interval to conception and calving date in single-sire breeding groups were significantly influenced by bull social rank in Experiment 3. Subordinate bulls had a shorter interval to conception (P<.0001) and had a higher pregnancy rate than dominant bulls (P<.0001). We conclude that yearling bulls respond more rapidly to electroejaculation stimuli than older bulls, duration of a serving capacity test can be reduced for Composite genotypes and social rank can influence pregnancy rate in a single-sire breeding group.
Smith, Ronald Gregg (1994). Effects of age, genotype and social rank on mating proficiency of beef bulls. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1994 -THESIS -S6577.