Effects of inbreeding on milk and fat production, stayability, and calving interval of registered Ayrshire cattle in the northeastern United States.

Inbreeding coefficients of 30,794 registered Ayrshire cows were calculated from relationships between sire and maternal male ancestors. Average inbreeding coefficient of all cows was less than 1% and of the 5,202 inbred cows was 5.4%. Fewer than 2% of inbred cows had coefficients greater than 15%. Percent of inbred cows increased from 23% of cows born in 1972 to 43% in 1980. Over the same period average inbreeding coefficient of all cows increased from 1.2 to 2.0%, but average coefficient of inbred cows decreased from 5.6 to 4.7%. Effects of inbreeding on first lactation, 305-day, 2X, mature equivalent milk and fat production (kg), 48-mo stayability (proportion of cows surviving to 48 mo of age), and first calving interval (days) were estimated by a model that included fixed effects for herd-year-seasons, sire-maternal grandsire groups, inbreeding, and random effects for sires and maternal grandsires within groups. Inbreeding was included in the model as a classification (six classes according to inbreeding coefficient: 0, 0+ to 5-%, 5 to 10-%, 10 to 15-%, 15 to 25-%, and 25 to 35-% and one class for cows with indeterminate inbreeding). Estimates of differences between inbreeding classes 0+ to 5-% through 25 to 35-% and the zero inbreeding class indicated that milk and fat production decreased with increased inbreeding. Effects of inbreeding on stayability and calving interval were small. Inbreeding was fitted also as a linear covariate. Regressions of milk, fat, stayability, and calving interval on inbreeding coefficients were -23, -1, -.008, and -.095 per 1% increase of inbreeding coefficient.