Fetus Sexing in Horses
Determining the Sex of the Mare’s Foal
By: Melissa Sykes
- The mare, heavy in foal, enters the sales ring. The auctioneer begins giving the details highlighted on the catalog page. The crowd waits, as she is pronounced in foal. Then the auctioneer elaborates a little more – “…believed to be carrying a colt by…”
Immediately, this mare’s selling price has gone up or down, depending on what is most valuable from that particular stallion.
Dr. Sandy Curran of the University of Wisconsin wrote the first published paper on sexing the equine fetus. She was studying embryo development in cattle when the idea of crossing over to horses occurred to her.
It took her roughly 3,000 ultrasounds to perfect the procedure in horses.
The ideal time to ultrasound a mare is between 57 and 62 days gestation. “You get a very clear view” at this juncture.
The ‘view’ is of the area between the umbilicus and hind end. Until about day 55 of gestation, both sexes look the same. As the ultrasound scans the umbilicus (appearing as a black dot on the screen), a bright white equals sign (the genital tubercle) is evident behind the umbilicus and in front of the hind legs. After 55 days, this tubercle begins to migrate toward the umbilicus in a colt and toward the anus in a filly.
“Since we have to wait for the genital tubercle migration, we cannot determine the sex before approximately 60 days of gestation,” explained Dr. Richard D. Holder of Hagyard-Davidson-McGee Associates in Lexington.
Holder has been perfecting his fetus sexing technique for over ten years. He now can determine the sex of the fetus up to 140 days gestation.
As the fetus grows, it begins to extend back into the pelvic cavity (at about 90 – 95 days) and once again can be viewed via sonogram.
When the mare is past 90 days a different system is used. Instead of looking for the migration of the sex tubercle, the practitioner is viewing the external genital tubercle.
“These are frequently difficult to see because they are not well developed until around 110 days,” Holder explained. “External genitalia is usually more difficult to differentiate from other surrounding tissues.”
During the 90 – 140 day window, the fetus is developing muscle tissue and bone. The bone can cast shadows over the areas that need to be observed and a mistake can easily be made.
After 140 days, sex determination is very difficult because the fetus has grown so much that the sonographic view cannot encompass the entire area. Only parts can be viewed at a time and these individual pictures make it difficult to determine exactly what the practitioner is looking at.
The technology is now becoming readily available and many horsemen are taking advantage of this information to make informed decisions about their equine operations.
“Knowledge of the fetal sex enables the horse owner or breeder to better manage several different aspects of his horse business,” explained Holder. “Most clients are having fetal sex determinations made to decide how to manage their mare and future offspring,” he said.
In many cases, owners will keep one sex to show/race or breed and sell the other. Or they may be trying for a specific sex from a certain sire – if the mare is not carrying the desired sex, they may choose to sell her, or book her back for the next breeding season in the hopes of getting what they want.
“We can be more accurate in assessing the value of a pregnant mare by knowing the sex of her fetus,” said Holder. Many things can be influenced by this knowledge – appraisals, selling decisions, auction reserves, foaling location and mating decisions.
Holder has been contacted by various sales companies to develop a certificate to accompany a mare when the sex of the fetus is announced at auction.
“People wanted to announce at the sales” and buyers wanted some kind of guarantee as to the accuracy of this information.
Holder’s personal accuracy rate has been 99.9% out of over 2,000 determinations. His signature on the sex determination certificate carries a lot of weight.
But, offering any type of guarantee opens the seller up to the possibility of liability. That’s why, when Holder shows other veterinarians how to sex the fetus, he stresses that they must be positive in their diagnosis before they commit themselves to identifying the fetus as a specific gender.
“If you cannot make a determination, do not guess,” he said. “Guessing leads to errors and errors lead to loss of confidence in the procedure. 90% correct is not acceptable information for a business decision.”
Many critics of the path science is taking cite on-demand abortions if the animal is not carrying the gender of choice.
But actually choosing the sex of the fetus is something Holder feels is unlikely to happen.
“If you abort the fetus and try to bring the mare back into heat, it would take 3-4 months” just to get the mare cycling again. “I don’t know of any case where it has been done, but I have heard people talk about it (the possibility).”
“If they did try it, I think it would be very unsuccessful,” he said.
Holder has gone from performing 24 sex determinations in 1992 to an expected 800+ in 1999. The technology is becoming more and more accepted.
“I think it will become quite commonplace someday.”
My vets are currently trying to perfect their fetus sexing technique. I had them out to check my mare, Sally at about 60 days gestation. Sally Port is a 15 year old bay Thoroughbred mare. She was bred last season to Diligence (a gorgeous gray Thoroughbred). They tell me she’s carrying a filly. But, doc – is it a gray?
