Inbreeding of artificially bred dairy cattle in the northeastern United States.

Inbreeding coefficients of artificially bred Ayrshire, Guernsey, Holstein, Jersey, and Brown Swiss cows were calculated from relationships between sires and maternal male ancestors. Percentages of inbred cows and average inbreeding coefficients of inbred cows were: Ayrshire, 26 and 11%; Guernsey, 11 and 4%; Holstein, 31 and 1%; Jersey 23 and 2%; and Brown Swiss, 23 and 2%. Percentages of inbred cows and average inbreeding coefficients of all cows increased over time whereas average coefficients of inbred cows decreased over time. These temporal trends may be due in part to having more pedigree information available for cows born in later years. Effects of inbreeding on milk and fat yields of first lactation (2 times milked, 305-day mature equivalent), 48-mo stayability, and first calving interval were estimated with a model including effects for fixed herd-year-seasons, fixed sire and maternal grandsire groups, random sires and maternal grandsires within groups, and random residual. Regression coefficients of milk (kg), fat (kg), 48-mo stayability (proportion of cows surviving to 48 mo of age), and calving interval (days) on inbreeding coefficient (%) were: Ayrshire -27.1, -1.2, -.005, .23; Guernsey -19.3, -.97, -.007, .27; Holstein -21.1, -.78, -.003, .09; Jersey -14.8, -.80, -.002, .63; and Brown Swiss -39.5, -1.36; -.011, .03. There is no cause for concern over current inbreeding, but active inbreeding is not recommended.