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The estimate of inbreeding for registered Holstein females born during 1990 was 5.1 +/- 0.4% for a random sample of 600 two-line pedigrees. Previously published estimates were 4.7% for 1970, 3.8% for 1976, and 4.3% for 1982 from similar samples of pedigrees. These estimates were calculated using a base of approximately 1884, or the initiation of herd-book registration in the US. The estimate of relationship between randomly paired females born during 1990 was 10.2 +/- 0.8%; relationships were 5.2% for 1970, 7.3% for 1976, and 9.8% for 1982. Average relationship within the breed increased significantly without a corresponding increase in inbreeding. Inbreeding was also estimated for alternative base years with 10-yr increments to determine the effect of base year on estimates of inbreeding. Using 1950 as the base year, inbreeding was estimated at 0.5% for 1970, 0.3% for 1976, 1.6% for 1982, and 2.1% for 1990. Estimates using 1920 as the base year were 2.0% for 1970, 1.9% for 1976, 2.3% for 1982, and 3.2% for 1990. More recent base years gave lower estimates of inbreeding, as expected, but showed an exaggerated percentage of increase in inbreeding from 1970 to 1990. However, the annual amount of increase of inbreeding from 1976 to 1990 was not significantly affected by time of base year and ranged from 0.08 to 0.12/yr. The five most influential bulls of the breed were Pawnee Farm Arlinda Chief 1427381, Round Oak Rag Apple Elevation 1491007, S-W-D Valiant 1650414, Osborndale Ivanhoe 1189870, and Sir Pietertje Ormsby Mercedes 44931; estimated direct relationships to the breed were 0.123, 0.122, 0.096, 0.083, and 0.074, respectively.
Young et al. (Fri,) studied this question.