Effect of Inbreeding on Production in Holsteins

Abstract

Data collected over a 25-yr. period from the Iowa State College Holstein herd were used to study the effect of inbreeding on production. The inbreeding ranged from zero to 34% and averaged 7.4%. Records used were standardized to a 305-day-2×-M.E. base. The number of cows used in each analysis varied but, at the most, 534 were available. Intra-sire regressions, using the average of each cow's records, were −1.74 ± 0.57 lb. of butterfat, −54 ± 17 lb. of milk, and +0.003 ± 0.003% of butterfat for each increase of 1% in inbreeding. There was no evidence of curvilinearity in the effect of inbreeding on fat production. Differences among the individual sires’ regressions were not significant. Inbreeding affects production mostly through channels other than reducing general size. This was shown by comparing the regression of production on inbreeding with and without holding heart girth constant. The partial regression was still about three-fourths as large as the simple regression. Partial regression of production on inbreeding, with the dam's production held constant, did not differ appreciably from the regression obtained when the dam's record was disregarded. © 1958, American Dairy Science Association. All rights reserved.