Effects of Inbreeding and Selection in a Closed Holstein-Friesian Herd

The effects of mild inbreeding and selection were measured in the Iowa State University Holstein herd. Inbreeding of all animals born in the herd from 1930 through 1964 ranged from zero to 47% and averaged 10.2. Selections were made primarily on an index based on the milk fat production and type of the individual and its close relatives. Intra-sire partial regressions and standard errors of production on inbreeding, holding the dam's production constant, were −23 ± 11 and −.78 ± .36 kg for milk and milk fat, respectively. Inbreeding appeared to have little effect on the percentage of milk fat. Type score and percentage of white coloration were essentially unaffected by increased inbreeding. Analyses of weight and body measurements both showed a detrimental effect of inbreeding, but the reduction of weight and measurements due to inbreeding decreased with increasing age. This may be due, in part, to the indirect selection of larger cows that tend to have lower coefficients of inbreeding. Genetic gain through the selection of parents was estimated to be 0.52 and 0.54% of the mean yield for milk and milk fat, respectively. About 80% of this gain came from the selection of dams of sires.