Sex ratio of equine offspring is affected by the ages of the mare and stallion

The aim of this study was to determine the influence of parental age on the sex ratio of offspring in horses. Two trials were performed. In the first trial, the data from a randomly obtained population with a 1:1 sex ratio of 59,950 Mangalarga Marchador horses born in Brazil from 1990 to 2011 were analyzed. The sex ratios of the offspring were compared among groups according to the mare and the stallion ages (from 3 to 25 years). In the first step of the analysis, the mares and stallions were grouped according to age in 5-year intervals. In the second step, the groups were based on the parental age gap at conception. In the third step, the group of the mares and stallions with similar ages from the second step was subdivided, and the different parental age subgroups that were divided into 5-year intervals were compared. In the fourth step, the sex ratio of the offspring was determined according to the ages of the mares and the stallions at conception. The second trial was based on the data from 253 horses of several breeds that were born after natural gestation into a herd from 1989 to 2010, and the offspring of groups that were younger or older than 15 years were compared. The data from both trials were analyzed using a chi-square test (P ≤ 0.01 for the first trial; and P ≤ 0.05 for the second trial) for the comparisons of the sex ratios. In the first trial, the Spearman test (P ≤ 0.01) was used to verify the correlations between the parental age and the offspring sex ratio. In the first trial, the offspring sex ratio decreased as the mare or stallion age increased, and the decrease was more marked for the mares than for the stallions. In the second trial, the mares older than 15 years had more fillies than the younger mares, but the stallion age had no effect on the sex of the offspring. The first trial, with a large number of horses, revealed the pattern of the distribution of the sex ratios of offspring according to the parental age in horses, whereas the second trial, with a more restricted number of horses, confirmed the influence of the age of the mare on the offspring sex ratio. We concluded that the parental age affected the offspring sex ratio in horses and that this effect was stronger for the mares than for the stallions.