Effect of Sire-Associated Factors on Secondary Sex Ratio of Offspring in Equine

The objective of the present study was to evaluate the effect of breed of stallion and individual stallion on secondary sex ratio (SSR; the proportion of male foals at birth). Data associated with the sex of foals and the sire, as well as the breed and age of sire were retrieved from the database of the Equestrian Federation of the Islamic Republic of Iran. In total, data consisted of 4,491 birth records from 92 stallions. Stallions were from three breeds of Arabian, Thoroughbred, and Akhal-Teke. Data were analyzed using univariable and multivariable logistic regression. Proportion of colts was 63.0% (427/678), 46.1% (1,545/3,355), and 53.9% (247/458) in Arabian, Thoroughbred, and Akhal-Teke stallions, respectively. In Arabian stallions, SSR was skewed toward males (P < .0001; odds ratio, 1.701), whereas in Thoroughbred stallions, it was skewed toward females (P = .001; odds ratio, 0.853). Secondary sex ratio was not skewed in Akhal-Teke stallions (P > .05). Secondary sex ratio in Thoroughbred stallions was lower than that in Arabian (P < .0001; adjusted odds ratio, 1.983) and Akhal-Teke (P = .010; adjusted odds ratio, 1.527) stallions, but SSR did not differ between Arabian and Akhal-Teke stallions (P > .05). There was the effect of individual stallion on SSR in Arabian and Thoroughbred breeds (P < .0001) but not in Akhal-Teke breed (P > .05). In conclusion, the present study showed the effect of breed of stallion and stallion itself on sex ratio of foals.