Scientific Papers
Associations between yearling body measurements and career racing performance in Thoroughbred racehorses

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Abstract

Yearling morphometric measurements and variables related to career racing performance were collected on 260 Thoroughbreds to determine correlations between and among these traits. Morphometric measurements were adjusted to 365 days based on interpolation from adjacent age points and included wither and hip height, body length, distal limb length, cannon circumference, heartgirth circumference, and chest width. Measures of career racing ability included total earnings, win percentage, and Standard Starts Index (SSI). Wither height and hip height were favorably correlated with lifetime earnings, SSI, and win percentage. Body length and heartgirth were positively correlated with SSI and win percentage. Leg traits were not correlated with any performance parameters. Horses that had placed in or won a stakes race tended to be taller at the wither and hip as yearlings. Horses categorized as preferring turf tended to be taller at the wither and hip at 365 days than those that preferred to run on dirt. No significant difference in yearling morphometric measurements between sprinters and routers was detected. In addition to phenotype and pedigree information, morphometric measurements taken in the yearling year may allow for more accurate estimation of future performance in the Thoroughbred racehorse.

Introduction

American Thoroughbreds are often sold at auction during their yearling year. However, because Thoroughbreds do not begin their racing careers until 2 years of age, no direct measures of racing performance are available at the time of purchase. Buyers are instead forced to assess yearlings based on subjective evaluation of conformation and gait and recorded performance data of relatives.

Wither height of mature horses is positively correlated with racing performance1 and stride length.2 In Thoroughbreds, yearling measurements can be used to predict mature dimensions with reasonable accuracy.3, 4 Thus it is logical to assume that morphometric measurements of yearling horses may have predictive value for future racing performance. The objective of this study was therefore to estimate correlations between yearling skeletal measurements and subsequent career racing performance, with the hypothesis that a significant association exists.

Section snippets

Materials and methods

Data were collected on 260 Thoroughbreds born on 13 farms in Virginia: 121 foals born in 1981 (67 fillies; 54 colts) and 139 foals born in 1982 (71 fillies; 68 colts). These data were a subset of a larger dataset5 limited to only those foals that went on to successfully start in at least one race in their lifetime. Morphometric measurements of horses born in 1981 were obtained at 6-week intervals from February of their yearling year through the 3rd week in July of their yearling year.

Results

No significant effects due to birth year were detected for morphological measurements, so data were collapsed and analyzed as one dataset. No significant correlations between measurements and ER were found, although a trend (P < .10) for WH (r = 0.19) and HH (r = 0.20) with ER was noted in females. WH, HH, BL, and HG were moderately and favorably correlated with SSI for females, whereas for males only HH was moderately and favorably correlated with SSI (Table 2), with a trend toward association

Discussion and conclusion

The favorable correlation of wither height to lifetime earnings, SSI, and win percentage is not entirely surprising, as wither height in mature horses is favorably correlated with stride length,2 and in turn a longer stride length is favorably correlated with a higher winning percentage in Thoroughbreds.9 Because yearling morphometric measurements can be used to predict adult dimensions,3 the current study confirms the suggested correlation between yearling wither height and lifetime earnings,

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