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

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Abstract
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 1930-1964 ranged from 0 to 47% and averaged 10.2%. Selections were made primarily on an index based on the milk fat production and type of individual and its close relatives. Intra-sire partial regressions and standard errors of production in inbreeding, holding the dam''s production constant, were -23 [plus or minus]11 and -.78 [plus or minus] .36 kg for milk and milk fat respectively. Inbreeding had little effect on the percentage of milk fat. Type score and percentage of white coloration were essentially unaffected by increased inbreeding. Inbreeding reduced weight and body measurements, but the reduction decreased with increasing age. This may be due, in part, to the indirect selection of large 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.