Do maiden mares produce smaller foals?

Is bigger better? That's what society thinks, and the same is true for horse breeders. They tend to think that maiden mares, those which have not had a foal, have smaller foals.

Researchers at Kansas State University used embryo transfer to produce full-siblings from different mares.

They had a group of nulliparous (maiden) mares and a group of multiparous (had several foals) mares.

All of the mares were mature. Nulliparouos mares averaged 6.5 years old, while multiparous mares averaged nearly 11 years of age.

Fertilized embryos were taken from the multiparous mares and transferred to the nulliparous mares. Then, the multiparous mares were re-mated to produce their own foals.

From this procedure, six pairs of full-sibs were produced. Each group (nulliparous and multiparous mares) had four fillies and two colts. Since two sires were used, each sire had two fillies and a colt in each group.

Multiparous mares significantly produced more milk in both early and mid-lactation. They produced 26.6 pounds of milk daily in early lactation, compared to 23.6 pounds from the nulliparous mares.

At mid-lactation, both groups produced less milk, which is natural. Multiparous mares produced 25.7 pounds, compared to 22.9 pounds from the nulliparous mares. In late lactation, milk production was similar.

Did decreased milk production from the nulliparous mares result in less growth of their foals?

It did during early lactation. Foals from multiparous mares weighed 155 pounds, compared to 139 pounds for those from nulliparous mares. By late lactation at 120 days of age, there was a 6-pound difference in foals, which was not significant.

Foals from multiparous mares were significantly taller (109.9 cm) than foals from the nulliparous mares (106.9 cm) at mid-lactation. This difference disappeared by late lactation.

Hip height and length of front cannon bone were not different between foals from nulliparous and multiparous mares.

At the end of the 120-day experiment, full-sibs from nulliparous and multiparous mares were not different in body weight, length of cannon bone and wither or hip height.

So, breeders should not discriminate against maiden mares if they are mature. but, it is not recommended to mate two-year-old fillies. If properly managed, three-year-old fillies should be mature. So, there should not be any difference in the size of foals from mature maiden mares and those from mares that have had several foals.

Any difference observed early in these foals should disappear by 120 days of age.