Inbreeding Within an Artificially Bred Dairy Cattle Population

Abstract

Purebred artificially bred daughters of 200 Holstein bulls were investigated for the occurrence of inbreeding by utilizing the relationship between their sire (S) and maternal grandsire (MGS). The existence of such a relationship would denote an inbred individual. Among the DHIA lactation records available, this group of 200 bulls sired 87% of the artificially bred purebred cows. Seven artificial breeding studs were represented among the 200 bulls studied. The proportion of related bull pairs was 4.4% and among the related pairs the mean coefficient of relationship was 16%. Within the five studs having at least ten bulls represented in the group, there were 2, 7, 7, 8, and 15% related pairs. The 17,490 cows, whose S and MGS were among the selected sample of 200 bulls, comprised 21% of the total number of purebred cows. Some 1,300, or 7.4% of the cows, were estimated to be inbred. The mean inbreeding per cent among the inbred cows was 4.9%. The mean inbreeding per cent for the entire sample was .4%. The inbreeding depression of mean milk and fat deviations from herd-mates as measured by the intrasire linear regression on inbreeding per cent was −33.7 and −.9 lb, respectively. © 1965, American Dairy Science Association. All rights reserved.