Date of this Version
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.