Don’t Count on Breeding Date and Moon Phase to Determine Fetal Sex in Horses

Your favorite broodmare is getting on in years and most likely won’t produce many more foals. She’s given you some wonderful colts, but this year you’d love to have a filly to carry on the maternal line. Is there anything you can do to shift the odds toward a female fetus?

One thing that probably won’t work is following lunar guidelines that are said to increase the chance of producing a filly. Statistics examined in a study conducted in Argentina showed that the percentage of colts and fillies did not correlate in any way to the phase of the moon when mares were bred.

Researchers looked at 65,535 registered matings for Thoroughbred and Arabian horses recorded in the Argentinian studbooks between 2003 and 2011. Numbers indicated that 50.96% of successful matings produced fillies and 49.04% produced colts. No statistically significant difference correlation was found between sex ratio and lunar phase.

So, should you give up on trying to get a filly from your mare? Not necessarily. The technique of sperm sorting has been perfected for horses and other animals. In this procedure, a semen sample can be processed to separate sperm that will produce a fetus of a particular sex. However, only a limited number of laboratories are able to carry out the procedure. Check with a veterinary clinic or university’s animal science department to see if this possibility exists in your area.