Sessions

Sex ratio is an important characteristic in both wild and domestic animals. In populations of wild animals, a skew towards either side can alter social structure and reproductive behaviour. In the equine industry, different sexes are preferred for different disciplines. Colts are favoured over fillies in thoroughbred racing, as stallions generally outperform mares and can sire multiple foals per breeding season. Breeding methods in the Thoroughbred industry are strictly limited to natural conception, making studies of potential predictors of offspring sex highly valuable.

In this study, we analysed factors that could affect the secondary sex ratio (SSR, the sex ratio of offspring at birth) in equines and assessed whether these effects are replicated across domestic populations in different regions.

Our dataset consisted of 279,779 Thoroughbred foals with sire and dam breeding records, of which 262,362 were obtained from the Weatherbys Stud Book, 10,703 from the Indian Stud Book, and 6,714 from the Hungarian Stud Book.

We used generalized linear models to study the relationship between SSR and sire and dam age at conception, foal parturition order, sex of previous parturition, birth year of foal, birth date of foal (day of year) and inter-foaling interval.

Out of these, the day of year and inter-foaling interval showed the most significant results across all three datasets. Later days of the year were associated with higher (male-biased) SSR throughout the main birth season. In the Weatherbys population, SSR increased from 49% to 55%, from January to July, followed by a decline from August to December. Similarly, inter-foaling interval showed a significant non-linear effect, where SSR increased from 35% to 54% between days 320 and 450, with a decrease afterwards. The highest SSR was estimated to be around 57% to 61% in foals born during June and July after a relatively long inter-foaling interval of 400 to 450 days, while the lowest SSR of around 35% to 40% was estimated in foals born in January and February after a shorter inter-foaling period of 320 to 350 days. The analyses of the Hungarian and Indian thoroughbred populations revealed similar relationships.

Sire age at conception had a weak negative effect, while dam age did not. Birth year showed a strong positive effect; however, this was restricted to foals from the Hungarian stud book. Foaling parturition order and sex of previous parturition showed no significant effect.

Our findings indicate that day of conception relative to the breeding season and time elapsed since the dam’s previous foaling have the strongest relationship with SSR, suggesting that duration of daylight exposure, intensity of lactation, and change in body condition are the main drivers of SSR, as they affect gonadotropin hormones. We propose that day of year of birth and inter-foaling interval may be used synergistically to influence the outcome of the desired sex of the offspring.