The American Shire horse breed is currently listed in “critical condition”, the most serious threat category assigned by the American Livestock Breeds Conservancy. Despite concern for the breed's future, however, a thorough genetic analysis of the breed has not been conducted to date. Pedigree data from 85 671 Shire horses born between 1806 and 2011 were used to determine current and historical genetic variability in the Shire horse since the first imported animals arrived from the United Kingdom to the United States. From this larger dataset, a reference population of 1 152 animals representing the most current generation was defined. Effective population size was 330.5, with 832 total founders and high pedigree completeness (98.2 percent) to five generations. Effective number of founders, ancestors and genomes were 104.5, 28.6 and 13.9, respectively. Average inbreeding coefficient was 2.4 percent from complete pedigree data and 1.3 percent when the most recent five generations were considered. Demographic analysis also revealed a population bottleneck in the 1950s. The ten most influential male and female ancestors contributed 48.5 percent and 11.6 percent, respectively, to the reference population, and only three foundation sires are responsible for 95.6 percent of all sire lines in the current generation. Although current assessment of genetic variability indicates a relatively healthy population, the small number of breeding animals, low birth rates and steady rise in inbreeding coefficients mark the American Shire as an at-risk breed for which conservation strategies should be considered.
