Determining the sex of a foal before birth is becoming commercially available for most breeding barns in the United States (US). It is becoming more common that horse owners prefer knowing if their mare is going to have a colt (male) or a filly (female). Many business decisions can be made ahead of time based on this information.
There are several factors to consider in determining the sex of a foal before birth. Ultrasound has a 98.9% accuracy rate. This is a simple procedure where your veterinarian can look at the health, position and gender of your foal prior to birth. Most veterinarians now use portable ultrasound machines so it is no longer necessary to subject your mare to traveling away from the barn during this time.
It is common knowledge that sperm determines the sex of a foal. Since the 1990’s gender selection has become more available and more prevalent in the horse industry. Gender selection is accomplished by sorting semen according to gender before insemination. By using a flow cytometer, X- or Y- chromosomes can be distinguished in sperm cells. Then the semen is sorted according to the gender it would produce. The first filly was produced using this technique of predetermining the sex of a foal in 1998. Her name was Call Me Madam.
Studies of wild horses have reported that fat and glucose levels determine whether the in vitro environment is more favorable for male or female offspring. Overweight mares have a tendency to produce males and underweight mares, females.
In the feral horses studied, of the mares putting on fat during the time of conception, 80% were reported to have male offspring. During a time when the mares were losing weight, only 3% were male. For the female offspring, the premise was fillies survived in vitro better in a poor glucose medium, but would die off in an enriched glucose medium.
This premise explains that Nature creates this structure to maximize the number of descendants a sire and dam can produce under any circumstances. If the season is plentiful, the sex of a foal would be male so he could grow strong and have a better opportunity to compete for breeding rights and produce more descendants. Therefore it would be advantageous to produce males during this time.
In times of scarcity, it would be more questionable if a mare could raise a strong, competing colt. If he could not win breeding rights then the line of descendants would cease. Therefore it would be more advantageous to produce female offspring as a mare would at least produce one offspring each year and consequently offer more grand-offspring than a colt.
Except for breeding barns, most people are not as concerned about the sex of a foal until birth. For domesticated horses, there are advantages and disadvantages in either gender. There is also the joy of anticipation and surprise by waiting to see the sex of a foal. Then the focus remains on the health of the dam and her foal.