Trend of age at first calving.

Mean ages at first calving were computed from over 6 million cow records from 1960 through 1982 for six breeds. Average Jerseys first calved at 26.18 mo compared with 28.75 mo for Ayrshires. Other breed means were intermediate: Holsteins, 27.52 mo; Guernseys, 27.72 mo; Brown Swiss, 28.04 mo; and Milking Shorthorns, 28.11 mo. Mean age at first calving in 1982 was .16 to .32 mo less than for all data. Standard deviations of annual mean ages were similar and ranged from 3.23 to 3.67 mo for the 138 breed-year means. Frequency distributions for calving ages were skewed to the left. Cows born from January through March were about .5 mo older at first calving than those born in August and September for most breeds. Annual means did not show a significant trend although all breeds had highest means around 1976 and regressions of age on year of -.075 to -.111 mo per year for 1976 through 1982. Heifer fertility as indicated by age at first calving did not appear to change over 23 yr.