Influence of period between breeding and ovulation on foal sex ratio

Abstract

Objective was to determine the effect of period between breeding and ovulation (PBBO) on the sex of the foal. Foaling records (n
= 288) from 2015 - 2018 breeding seasons and from 2 veterinary hospitals were used. There were 122 (42%) fillies and 166 (58%)
colts. Foal sex records were categorized into 3 groups based on PBBO < 24 hours, 24 - 48 hours, and > 48 hours. There was an
effect (p < 0.0001) of PBBO on the probability of the foal being a colt. If the PBBO was < 24 hours, the odds for a colt were 23.3
times higher compared to > 48 hours group (OR = 23.6, 95% CI: 3.0, 185, p = 0.003). Additionally, if the PBBO was < 24 hours,
the odds for a colt were 4.6 times higher compared to 24 - 48 hours group (OR = 4.6, 95% CI: 2.0, 10.5, p = 0.0003). There were
no differences (p = 0.14) between odds for a colt between 24 - 48 and > 48 hours groups. Confounding variables such as sire, age
of dam, size of ovulated primary follicle, and type of semen (fresh or chilled) were not significant. Results indicated that the closer
a mare is bred to ovulation with fresh semen, it is more likely that she will have a colt.