Filly or Colt?

Filly or Colt?

Last Updated on February 21, 2022 by Allison Price

Research has shown that the sex ratio between foals and their parents is affected by the age of the horses. The effect is greater for mares than for stallions.

Recent research has shown that the age of the parents of equine children can have an impact on the sex ratio between foals. This effect is more pronounced for mares than for stallions. (c) Amy K. Dragoo

Will your mare’s foal be a filly if she is bred this spring? Research from Brazil suggests that the answer could depend on the mare’s age and the stallion she chooses. The Pontifical Catholic University Minas, in Betim, examined breeding records and found similar patterns in both groups.

The one group included nearly 60,000 Mangalarga Marchador horses that were born in Brazil between 1990 and 2011. This includes all registered males as well as a random selection of females. Researchers sorted horses according to their age at conception. This ranged between 3 and 25 years. They then calculated the sex ratio for the offspring based on their parental age. They came up with:

Filly or Colt?

* The chances of having a female foal increased with the age of the mare/stallion. This trend was more evident for older mares than it was for stallions. The likelihood of having colts was lower for mares between the ages of 10 and 15, and even less for mares between 15- to 20 years.

* The mare and stallion’s ages are often very different. When bred to young mares, older stallions were more likely than younger ones to produce colts. When bred to younger mares, older mares were more likely to give birth.

The second group consisted of 253 horses from different breeds, born to a herd between 1989 and 2010. These horses were the offspring 16 stallions and 119 mares. This smaller group also included mares over 15 who had more fillies than younger mares. However, the age of the stallion had no effect on the sexual performance of the offspring.

Researchers concluded that the parental age has a significant impact on foals’ sex ratios. This effect is stronger in mares than it is for stallions. If you are determined to have a colt from your mare’s mare, it is best to do so while she is still young. Elaine Pascoe

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