A reduced animal model has been developed to genetically evaluated racing performance in the American Quarter Horse. The model includes repeated records on horses to improve prediction accuracy. Breeding values are determined for racing finish time and expressed in + seconds for each horse evaluated. Finish times are currently adjusted for horse sex and age, but not for other effects. An analysis was undertaken to re-examine whether finish times should also be adjusted for jockey weight, where jockey weight includes the rider, tack and any assigned handicap. The analysis included racing starts for three distances: 274.2, 365.5 and 402.2 m. Racing starts were classified according to type of race: allowance, stakes, handicap and others. Results found finish time to be significantly affected by jockey weight for all types of races, with the exception of races classified as handicap races. For the 274.2 m race, linear regressions on the jockey weight covariate were highly significant. For the 365.5 and 402.2 m races, both linear and quadratic regressions on the covariate were highly significant.

D. E Wilson

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XVI. Poultry, fish and horse genetics and breeding, growth and reproduction, immune response and disease resistance., , 198–201, 1990
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