EVALUATION OF BULL AND COW FERTILITY AT PASTURE IN SINGLE-SIRE MATING

Abstract

Pregnancy rates and calving dates of beef cows in 27 single-sire breeding herds mated to yearling or 2-yr old bulls were used in this study. The breed groups of bulls were Hereford and Beef Synthetic and cow herds were Hereford, Hereford cross and Beef Synthetic, respectively. A week before the start of the breeding season the bulls were scored for libido, their scrotal circumferences were measured and semen samples were collected. Semen volume, sperm motility and percent normal cells were evaluated. The average bull to cow ratio was 1:20. Pregnancy rate increased as age and body weight of cows increased, but it was independent of the previous calving date. Calving date was not related to body weight or age of cow, but it was influenced by the previous calving date. Each day delay in calving after the first 5 wk of calving season resulted in a 0.7-day delay in the date of calving during the next calving season. The Beef Synthetic-sired herds had higher pregnancy rates (P < 0.05) than the Hereford-sired herds, but the average calving dates of the two groups were similar. Cows mated to 2-yr-old bulls had 5% higher (P = 0.12) pregnancy rate than those mated to yearling bulls. Five of the herds showed low fertility and late calving, probably due to low fertility of the bulls. Yearling weight and feedlot average daily gain were positively (P < 0.05) related to bull fertility. The number of services performed by a bull during a 10 min libido test and total libido score were also found to be useful in assessing bull fertility, but scrotal circumference or semen traits were not related to fertility of bulls when used in single-sire mating at pasture. Key words: Beef cattle, bull fertility, single-sire mating