Influence of broodmare aging on its offspring's racing performance

Abstract

Maternal aging has negative influences on the development and racing performance of their offspring in racehorses. However, the mechanism by which pregnancy at old age reduces the race performance of the offspring is unknown. Here, two hypotheses were posited: 1) Foals born to older mares are more likely to have muscular, skeletal, and cognitive disadvantages (direct effects). 2) Foals born to older mares are more likely to be affected by non-physiological factors correlating with the mare's age, such as the quality of sires (e.g. low-quality sires are likely to be chosen as partners of older broodmares). To test these hypotheses, the effect of the broodmare's age on the offspring's racing performance was examined, while controlling for the effects of the stallion's quality, age, and ID, offspring's sex, trainer, and the location of the training center. Information of racehorses registered to the Japan Racing Association were collected from the Japan Bloodhorse Breeders' Association website. Overall, results showed that the racing performance of horses born from older mares was lower than that of horses born from younger mares. However, generalized linear mixed models (GLMM) indicated that the quality of sires was significantly associated with the offspring's racing performance, rather than the broodmare's age itself. Furthermore, the age of broodmares was negatively correlated with the quality of sires, although the variance inflation factor was low. Therefore, the effect of maternal aging was negligible or only limited, and rather, the sire's quality had an important influence on the offspring's racing performance. Low quality sires, or cheap stallions in other words, are likely to be chosen as partners of older blood-mares, which may have reproductive risks such as lower fertility and higher rate of miscarriages. The present study suggests that the conventional belief that racehorses born from older mares show lower performance may not always be accurate.

Fig 1. Relationship between the number of horses and the number of winning races (N = 15,461).

Fig 2. Relationship between the racing performance of offspring and parents’ age.

The upper row shows the proportion of offspring which did not win any races. In the lower row the proportion of offspring which won more than two races are drawn. The dashed line indicates 16 years old, which was determined as a threshold of age category. This figure contains results for parents over the age of 3 and under the age of 24. A figure including all data is shown in SI 2 in S1 File.

Fig 3. Estimated posterior densities of each parameter in the continuous model.

95% of posterior distributions are drawn here. Edges of distributions indicate the lower bound of 2.5% and upper bound of 97.5%. The dark blue vertical lines indicate the median. The dark blue area in each distribution shows the high density interval (HDI) which includes 89% of samples. The red dashed line indicates zero.

Fig 4. Posterior densities of each parameter in the transition model.

95% of posterior distributions are drawn here. The edges of distributions indicate the lower bound of 2.5% and upper bound of 97.5%. The dark blue vertical lines indicate the median. The dark blue area in each distribution shows the high density interval (HDI) which includes 89% of samples. The red dashed line indicates zero.