Effects of scrotal circumference of sire on age of calving in Ontario beef heifers.

Calving records for daughters of 667 Record of Performance tested sires were examined to study factors related to age at first calving. For the 3025 heifers analyzed, mean age (+/- SD) at first calving was 791.5 +/- 161.6 days (26.4 +/- 5.4 months). There were significant effects of the breed of sire on age of heifer at first calving. Daughters of Angus sires calved earlier than daughters of Charolais, Hereford, Limousin, and Simmental sires. Mean ages at first calving for daughters of Hereford, Limousin, and Simmental bulls were significantly (p < 0.05) correlated with the sire's end-of-test age-adjusted scrotal circumference, but these correlations were not significant when farm effects were controlled. It appears that unknown herd level factors are more important in determining age at first calving in Ontario herds than using early maturing sires as determined by scrotal circumference measurements.