Paternal determinants of Offspring Sex Ratio in horses: A retrospective analysis of stallion breed, age, and breeding method.

Overview

• This study investigated how paternal factors—specifically stallion breed, age, and breeding method—influence the sex ratio of foals born in horses.
• Using a large dataset of over 250,000 births in France, the research found that breed and breeding method affect the likelihood of producing female offspring, while stallion age does not.

Introduction and Background

• In most mammals, offspring sex ratio (OSR) at birth is generally expected to be about 1:1 male to female.
• However, deviations from this ratio occur and can be influenced by multiple factors from both the mother (maternal) and father (paternal).
• While maternal influences on OSR are well studied, the effect of paternal factors such as stallion breed, age, and breeding methods in horses had not been thoroughly examined before this study.

Data and Methodology

• The study used a retrospective dataset comprising 251,783 foal births recorded across France from 2015 to 2024.
• The main paternal variables examined were stallion breed, stallion age, and breeding method.
• Breeding methods included natural pasture breeding and artificial insemination using either frozen or refrigerated semen.
• Generalized linear mixed models (GLMMs) were applied to evaluate how these variables affect the likelihood of a foal being female.
• Stallion identity was included as a random effect to account for repeated measures within the same stallion.

Key Findings

• Breed Effects: Certain stallion breeds—Anglo-Arab, Comtois, and French Sport Horse—had a higher probability of siring female foals compared to the French Trotter breed.
• Age Effects: Stallion age did not have a significant influence on the offspring sex ratio.
• Breeding Method Effects: Pasture (natural) breeding resulted in a higher proportion of female offspring.
• In contrast, artificial insemination—either with frozen or refrigerated semen—was associated with a reduced likelihood of producing female foals (i.e., relatively more male offspring).
• The overall deviations in OSR observed tend towards producing more female foals.

Interpretation of Results

• Breed-related differences in semen characteristics could influence the proportion of X- versus Y-chromosome-bearing sperm, thereby affecting the sex ratio.
• Processes related to artificial insemination—such as sperm handling, preservation, and timing of insemination—may differentially impact X and Y sperm, altering the expected sex ratio.
• These paternal factors highlight potential mechanisms beyond simple chance affecting OSR, possibly due to post-meiotic sperm alterations or differential sperm survival.
• The findings do not exclude the role of maternal factors, which have been previously reported to affect OSR and could interact with these paternal factors.

Implications and Conclusion

• This study is among the largest to quantify paternal influences on offspring sex ratio in horses, offering valuable insights for breeders and reproductive specialists.
• Understanding that breed and breeding method can skew sex ratios enables more informed breeding decisions, especially when targeting foal sex for economic or management reasons.
• Future research could investigate the biological mechanisms underlying these breed differences and the effects of semen processing on sperm sex chromosome viability.
• Investigating maternal-paternal interactions may further clarify how offspring sex ratios are influenced in horses and potentially other mammals.