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Genetics of Racing Performance in the American Quarter Horse: III. Estimation of Variance Components^{1, 2,}

Iowa State University, Ames 50011

Abstract

An ANOVA method (the prior method), the iterative version of Henderson's simple method (IHSM) and the Tilde-hat method (THM) were used to estimate heritability and repeatability for racing time in American Quarter Horses. Racing times were adjusted for age and sex, and individual races were defined as contemporary groups. A model including contemporary groups, sires and horses within sires was used because horses had repeated records. Both heritability and repeatability estimates were slightly higher when THM was used than when IHSM was used. Estimates of heritability ranged from .00 to .36 and from .02 to .38 with IHSM and THM, respectively. Repeatability estimates were from .20 to .42 with IHSM and .34 to .51 with THM. In some instances solutions for these methods did not converge because of negative estimates. The prior method generally yielded higher heritability estimates and lower estimates of repeatability than either IHSM or THM. Heritability and repeatability estimates were data-dependent, and weighted means of estimates from the prior method were used to compare estimates from different sources of data. Records of 2-yr-old horses yielded higher estimates of heritability (.28 vs .22) than records from older horses. The entire data set for each distance gave higher estimates of heritability (.24 vs .05) than a subset of records from a single track. Weighted means for estimates of heritability and repeatability of finish time using the prior method were .24 and .32.

¹ Journal paper no. J-12805 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.