Inbreeding and pedigree structure in Standardbred horses

Pedigree structure and inbreeding levels were examined for 5207 Standardbred horses from six breeding farms in the northeastern United States and Canada. Inbreeding coefficients were calculated from pedigrees traced back as far as 30 ancestral generations. A detailed analysis of inbreeding and pedigree structure was done for the first 14 ancestral generations. Although Standardbred breeders tend to avoid very close inbreeding, nearly all horses with complete pedigrees are inbred within the first five ancestral generations. Standardbreds are shown to be interrelated in complex ways, through multiple remote ancestors. Inbreeding coefficients increase markedly with increasing pedigree depth, leveling off only after 10–12 generations. When completeness of pedigrees is taken into account, there is little evidence for an increase in inbreeding through time. A preliminary analysis of inbreeding as a function of gait suggests that trotters are more highly inbred than are pacers.