AUTHORS: MARTA ARMANGAUa; THIBAUT LURIERbc; ANNE JOSSONa
PHOTOGRAPHY: OWNERS’ ARCHIVES
Complete title: Paternal determinants of Offspring Sex Ratio in horses: A retrospective analysis of stallion breed, age, and breeding method.
Offspring Sex Ratio (OSR) at birth in mammals is generally assumed to follow a 1:1 male-to-female distribution, yet deviations in OSR have been observed and may be influenced by maternal and paternal factors. In this study we have focused on the effect of paternal factors (stallion age and breed) and breeding methods on the OSR. Using a retrospective dataset of 251,783 foal births recorded in France between 2015 and 2024, we applied generalized linear mixed models to evaluate the effects of these variables on the likelihood of producing a female foal. Stallion identity was included as a random effect.
Our analysis revealed that stallion breed and breeding methods significantly influenced the sex ratio, while stallion age did not. The Anglo-Arab, Comtois, and French Sport Horse stallions were more likely to sire female foals compared to the French Trotter. Pasture breeding was associated with a higher proportion of female offspring, whereas artificial insemination using frozen or refrigerated semen reduced this likelihood. Overall, deviations in OSR were toward female offspring and were associated with stallion breed and breeding method.
These patterns suggest possible contributions from paternal factors, including breed-related differences in semen characteristics and post-meiotic or peri-fertilization processes – such as semen handling, preservation, and insemination timing- that may differentially affect X- and Y-chromosome–bearing spermatozoa. Maternal factors previously reported to influence Offspring Sex Ratio cannot be excluded as contributors to the observed patterns.
1. Introduction
Selecting the sex of the offspring prior to conception is an emerging area of interest in the equine breeding industry in recent years. Unlike the cattle industry, where dairy farmers have a higher economic interest in producing female calves and beef producers favour males, the economic impact of producing a male or a female foal in the horse industry is related to the need to produce competitive horses in different areas of equestrian sports and racing. The preferred sex of the offspring is often subjective, based on the belief that a horse’s personality or trainability may be influenced by its sex. In the near future it might become possible to inseminate mares routinely with sexed stallion semen. This possibility would be welcomed by the equine industry as some breed societies prefer a specific sex for their competition horses.
In the Thoroughbred racing industry for instance, where the use of artificial insemination, embryo transfer, and cloning is prohibited, the male sex is preferred because stallions can sire multiple foals per breeding season, while the female can only carry one foal per year (Hall et al., 2022). Moreover, females rarely outperform males in mixed races.
In the polo horse industry on the other hand, the female sex is preferred as players believe mares have a more tolerant and balanced temperament to maintain focus under pressure in the game. At the same time these mares can be used in embryo transfer programs without interrupting their athletic career (Panarace et al., 2014).
In Western riding, the preferred sex varies depending on the discipline, with both females and males being desirable.
In the Warmblood sport horse, the main breeding objective is to produce competitive show jumper or dressage horses. Stallions seem to be more successful in dressage competition than mares and dressage riders prefer stallions because they are more even-tempered than mares. There does not appear to be a significant effect of sex on performance in showjumping and cross-country competitions.
Understanding what determines the Offspring Sex Ratio (OSR) has been the subject of many studies. One study of maiden and aged mares revealed that OSR markedly deviated from the expected 1:1 ratio with a higher percentage of female offspring in these two categories (Kuhl et al., 2015). It is also possible that the age of the mare could have an effect on nutrition and uterine environment, with body condition score, food availability, and semi-feral conditions at conception potentially having pronounced effects on OSR (Monard et al., 1997, Cameron et al., 1999).
