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Genetics of Racing Performance in the American Quarter Horse: II. Adjustment Factors and Contemporary Groups^{1, 2,}

Iowa State University, Ames 50011

Abstract

Individual racing records were used to estimate the effects of sex, age and handicap weight on racing performance in American Quarter Horses. Adjustment factors for pre-adjusting racing times were calculated for each of five distances. At 320 m, 2-yr-old stallions were .060 s faster than mares and .032 s faster than geldings of the same age. Multiplicative sex adjustment factors calculated from 2-yr-olds ranged from .9956 to .9988 for mares and .9946 to .9995 for geldings, depending on distance. Sex adjustments generally increased as distance increased. Additive age adjustments were calculated separately for males and females because young mares seemed to require larger adjustments than young stallions and geldings. Two-year-old males and females were .097 and .161 s slower than 4-yr-olds of the same sex in 320-m races. At the same distance, 3-yr-olds were .035 and .062 s slower than 4-yr-old males and females, respectively. Regression coefficients for racing time (s) on handicap weight (kg) ranged from –.0051 to .0158. A hierarchical ANOVA was used to evaluate the relative importance of tracks, years, days and individual races as sources of variation in racing time. Although tracks alone accounted for 10.6% to 31.8% of the variation, depending on the distance of the race, individual races within tracks, years and days should be used as contemporary groups. Contemporary groups defined in this way accounted for 49.4% to 70.9% of the variation in racing time.

¹ Journal paper no. J-12803 of the Iowa Agric. and Home Econ. Exp. Sta., Ames. Project no. 0155.

² The cooperation and financial assistance of the American Quarter Horse Association is gratefully acknowledged.

³ Present address: DeKalb Swine Breeders, 3100 Sycamore Road, DeKalb, IL 60115.

⁴ Dept. of Anim. Sci., Iowa State Univ., Ames 50011.