The effects of inbreeding on the lifetime performance of dairy cattle.

The effects of inbreeding on the lifetime performance of dairy cattle were examined using data for production, somatic cell score, and linear type for all Holstein cows that were scored between 1983 and 1993. The results of fixed and mixed animal models differed. Relative net income adjusted for opportunity cost for the 2,610,123 cows with an 84-mo opportunity for herdlife was depressed by $14.79 for fluid market pricing and by $12.40 for manufacturing pricing per 1% increase in inbreeding. Mixed model estimates of depression per 1% of increase in inbreeding were +0.55 d for age at first calving, -6 d for days of productive life, and -4.8 for days in milk. Inbreeding decreased the mature equivalent production of milk, fat, and protein during first lactation by 27, 0.9, and 0.8 kg and the lifetime production of milk, fat, and protein by 177, 6.0, and 5.5 kg, respectively, per 1% increase in inbreeding. Inbreeding had little effect on conformation traits. The effects of inbreeding were cumulative, and effects on lifetime profit functions were relatively larger than the effects on lactation traits. Registered cows had higher levels of inbreeding and larger standard deviations than did grade cows. Inbreeding in registered cows depressed relative net income adjusted for opportunity cost for fluid and manufacturing prices by $24.43 and $21.78, respectively; income was depressed $9.43 and $9.02, respectively, for grade cows. The difference between registered and grade cattle is likely due to the incomplete pedigree information in grade animals. Inbreeding among cows in this study was not high on average, but economic losses represented a significant cost to the producer.