VARIATIONS IN AND RELATIONSHIPS BETWEEN REPRODUCTIVE PERFORMANCE AND GROWTH PARAMETERS IN YEARLING BEEF BULLS IN SINGLE-SIRE MATING AT PASTURE

Abstract

Yearling bull fertility, measured as pregnancy rate, percentage of calves born during the first 4 and 6 wk of calving and mean and median of calving date distribution, were studied in 109 single-sire breeding herds of two breed groups over a 10-yr period. The breed groups were Herefords bulls mated to purebred and crossbred Hereford cows, and Beef Synthetic bulls mated to Beef Synthetic cows. The bulls averaged 14 mo old at the start of breeding. The average bull-to-cow ratio was approximately 1:22, and the breeding season started on 1 July each year and extended for 60 d. Two out of 109 bulls (1.8%) were subfertile (less than 50% pregnancy of the herd). The Beef Synthetic, which had crossbred foundation, had significantly higher fertility, calved earlier, had smaller phenotypic variance, and lower year-to-year fluctuation in the reproductive traits than the Hereford. Variations in reproductive performances of bulls of the same breed group used in the same year were much larger than those among different years, the former comprised over 75% of phenotypic variance of the traits in the two breed groups. Relationships between fertility of bulls and their weight and growth parameters were negligible, but bull fertility tended (P < 0.10) to improve with age. Bulls that were heavy at weaning, end of feedlot test and breeding settled their mates earlier during the breeding season. Key words: Beef cattle, yearling bull, fertility