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This study was carried out to investigate the fixed effects influencing racing traits of Thoroughbred horses and to estimate the genetic parameters for these traits, which are needed to design breeding programmes. The racing records used in the study were obtained from the Turkish Jockey Club. The traits used in the study were racing time, best racing time, rank, annual earnings, earnings per start, log annual earnings and log earnings per start. Genetic parameters were estimated by REML procedure using DFREML programme. The effects of age and sex were significant for each trait. The effect of year was significant on earning traits and time traits, except for best racing time on turf track. The effects of month on time traits were also significant. Heritability estimates of entire dataset were 0. 317, 0.467, 0.132, 0.194, 0.291, 0.188 and 0.341 for racing time, best racing time, rank, annual earnings, earnings per start, log annual earnings and log earnings per start, respectively. Estimates of repeatability varied from 0.293 to 0.465 for racing time, from 0.373 to 0.531 for best racing time and from 0.215 to 0.232 for rank depending on the dataset used in the analyses.

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The racing records for Arabian horses used in the study were obtained from the Turkish Jockey Club. The traits used in the study were racing time, best racing time, rank, annual earnings, earnings per start, log annual earnings and log earnings per start. Genetic parameters were estimated by the restricted maximum likelihood (REML) procedure using the DFREML program. The effects of age, sex and origin of horse were significant for each trait. The effect of year was significant on time and earning traits, but not rank. The effect of month on time traits was also significant. Heritability estimates of the entire data set were 0.280, 0.281, 0.069, 0.139, 0.174, 0.152 and 0.171 for racing time, best racing time, rank, annual earnings, earnings per start, log annual earnings and log earnings per start respectively. Estimates of repeatability varied from 0.349 to 0.500 for racing time, from 0.430 to 0.524 for best racing time and from 0.129 to 0.171 for rank depending on the data set used in the analyses. Best racing time was the most appropriate trait for selection in this study, as this might lead to genetic improvement than other traits.