Production and reproduction records that spanned a 24-yr period were used to study the effects of low coefficients of inbreeding in two experiment station Guernsey herds. Inbreeding ranged from 0 to 25.3% and averaged 4.1%. Milk and fat yields were all first lactations standardized to a mature equivalent, twice daily milking, and 305 d. Intrasire regressions of production traits on percent inbreeding were -23.8 kg of milk, -1.25 kg of fat, and .002% for fat percentage. Estimates of the effect of inbreeding on reproductive traits had large standard errors but suggested that inbreeding depressed reproductive performance. Age at first calving decreased 3.7 d per 1% increase in inbreeding.

Data were first lactation production and reproduction records initiated from 1958 to 1981 in two experiment station Guernsey herds. Heritability estimates using paternal half sib groups were .24 +/- .12 for milk yield, .27 +/- .12 for fat yield, and .77 +/- .15 for fat percentage. Heritability estimates for reproductive traits ranged from .01 to .04 for number of services, service period, conception rate, and days open, but were higher for days in milk at first breeding (.12) and age at first calving (.13). Except for age at first calving, coefficients of additive genetic variation were larger for reproductive traits than for productive traits. Genetic correlations between measures of production and reproduction were moderate to large and antagonistic, except that the relationship between production and age at first calving was favorable. Breeding value estimates for milk yield and reproduction were negatively correlated for sires with above average breeding values for milk yield. Huge phenotypic variances for reproductive traits masked substantial additive genetic variation for these traits. When all things are considered it seems unwise to ignore reproductive performance in selection programs for dairy cattle.