Inbreeding affects racing performance negatively in the Standardbred trotter

ABSTRACT We estimated the effect of inbreeding on racing performance in the Standardbred trotter. A random sample of 1,217 horses with organized pedigree was utilized to calculate individual proportions of French and American ancestry and their heterozygosity and inbreeding coefficients. Average earnings per start for the 807 raced horses summarized between three and five years of age were analysed, after first applying a fourth-root transformation followed by standard-normalization within their birth-year group (1988 and 2006/2007). The resulting variable was analysed using a linear model with fixed effects of sex, additive breed effect and a regression on the squared inbreeding coefficient. The model was expanded with a fixed effect of birth-year and a random effect of animal. In both models, the regression coefficient for the squared inbreeding coefficient was found negative (P < 0.04). Breed effect was not significant. Heritability was estimated as 0.227 ± 0.106.