Myostatin is a member of the transforming growth factor-β family with a key role in inhibition of muscle growth by negative regulation of both myoblast proliferation and differentiation. Recently, a genomic region on ECA18, which includes the MSTN
gene, was identified as a candidate region influencing racing performance in Thoroughbreds. In this study, four SNPs on ECA18, g.65809482T>C, g.65868604G>T, g.66493737C>T, and g.66539967A>G, were genotyped in 91 Thoroughbred horses-in-training to evaluate the association between genotype and body composition traits, including body weight, withers height, chest circumference, cannon circumference, and body weight/withers height. Of these, statistically differences in body weight and body weight/withers height were associated with specific genotypes in males. Specifically, body weight/withers height showed statistically significant differences depending on genotype at g.658604G>T, g.66493737C>T, and g.66539967A>G (P
<0.01) in males during the training period. Animals with a genotype associated with suitability for short-distance racing, C/C at g.66493737C>T, had the highest value (3.17 ± 0.05 kg·cm-1
) for body weight/withers height in March, while those with a genotype associated with suitability for long-distance racing, T/T, had the lowest (2.99 ± 0.03 kg·cm-1
). In females, the trends in the association of body weight/withers height with genotypes were similar to those observed in males. As the SNPs are not believed to be linked to coding variants in MSTN
, these results suggest that regulation of MSTN
gene expression influences skeletal muscle mass and hence racing performance, particularly optimum race distance, in Thoroughbred horses.