Left-Sided Ovulation Favors More Male Foals Than Right-Sided Ovulation in Thoroughbred Mares

• Asymmetric distribution of male and female foals in the left and right ovaries in mares.
• Effect of parity on the proportion of male and female foals.
• Parallel distribution of ovulations in the left and right ovaries.

The side of pregnancy has been observed to impact the sex of offspring in different species of mammals. Accordingly, the present study was conducted to investigate the distribution of male and female foals in the left and right ovaries in Thoroughbred mares. To do so, data associated with sex of foals, side of ovulation, stallion ID, and age and parity of mares at conception were collected from four horse centers. In total, data consisted of 238 birth records from 238 dams and 23 sires. Data were analyzed using univariable and multivariable logistic regression analyses. The incidence of left and right ovulations were 51.3% (122/238) and 48.7% (116/238), respectively, which were not different (P > .05). Secondary sex ratio (SSR; the proportion of male foals at birth) in nulliparous mares (29/83, 34.9%) was lower than that in parous mares (82/155, 52.9%; adjusted odds ratio, 0.388; P = .005). Moreover, conception of oocyte ovulated from the left ovary resulted in higher proportion of males (70/122, 57.4%) than conception of oocyte ovulated from the right ovary (41/116, 35.3%; adjusted odds ratio, 3.636; P < .0001). Secondary sex ratio of right-sided ovulation was lower than the expected 50.0% SSR (odds ratio, 0.547; P = .026), but SSR of left-sided ovulation and overall SSR (111/238, 46.6%) did not differ from the expected 50.0% SSR (P > .05). In conclusion, the present study showed dissimilar distribution of male and female foals for left and right ovulations in ovaries of Thoroughbred mares. Moreover, the results indicated the effect of parity on the proportion of male foals in equine.