GENETIC AND ENVIRONMENTAL-FACTORS THAT AFFECT THE BEST TIME OF THOROUGHBRED HORSES IN BRAZIL

On the basis of information provided by the Brazilian Association of R ace Horse Breeders, we analysed the racing performance of 947 Thorough bred horses in races held from 1985 to 1992. The performance was evalu ated using the best, time of the animals. The variance component was o btained by the derivative-free restricted maximum likelihood method, a nd the model used contained fixed effects of the racing month and year , sex, race track, track condition, animal age, number of competitors in race, and distance, and the random animal effect. The low heritabil ity estimate obtained (0.12) indicates that selection based on animal phenotypic value must induce small genetic changes in this trail.