Ages at Calving and Calving Intervals for Dairy Cattle in the United States

Calving ages and intervals for cows first calving in 1966 to 1986 were examined for five dairy cattle breeds. For parities one through seven, mean age at each parity was lowest for Jerseys and highest for Ayrshires and Brown Swiss. Registered cows usually were older than grades for a specific parity. Notable exceptions were that registered Guernseys, Holsteins, and Jerseys were younger than grades at first parity. Trends in calving ages over time generally were positive for given parities; if parity was not considered, trends were negative except for Jerseys and significant except for Ayrshires and Holsteins. In later years, trend for Guernseys was no longer significant. Calving intervals were shortest for Jerseys and longest for Brown Swiss and Guernseys. Mean calving intervals decreased from first to second interval and then increased through sixth for all breeds. Calving intervals for Holsteins ranged from 393 d following second parity to 405 d following sixth. Registered cows had longer calving intervals than did grades. Trends for calving intervals generally were positive for given parities; trends for more recent data were less positive and significant only for Guernseys. No evidence of generally deteriorating reproductive performance across time was found.