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How can I Determine the Sex of a Foal Before Birth?

KD Morgan
KD Morgan

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.

Woman holding a book
Woman holding a book

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.

Frequently Asked Questions

Can you determine the sex of a foal through ultrasound?

Yes, the sex of a foal can often be determined through ultrasound, typically performed by a veterinarian. The optimal time for sex determination via ultrasound is between 60 and 70 days of gestation, when the genital tubercle, which will develop into the penis or clitoris, is visible and its position can indicate the sex.

Is there a genetic test to determine a foal's sex before birth?

Genetic testing can be used to determine the sex of a foal before birth. This involves collecting DNA samples from the pregnant mare's blood, which contains fetal DNA, and analyzing it for the presence of Y chromosomes, indicative of a male foal. This method is non-invasive and can provide reliable results.

How accurate are these methods for determining a foal's sex?

Ultrasound sex determination has a high accuracy rate when performed during the optimal time frame, with experienced veterinarians achieving over 90% accuracy. Genetic testing is also highly accurate, as it directly analyzes the foal's DNA for sex chromosomes, making it nearly 100% accurate when conducted properly.

Are there any risks involved in determining the sex of a foal before birth?

Ultrasound examination carries minimal risk when performed by a skilled professional. It is non-invasive and does not require sedation. Genetic testing is also low-risk, as it only requires a blood sample from the mare. Both methods are considered safe for the mare and the developing foal when conducted correctly.

At what stage of pregnancy is it best to determine the sex of a foal?

The best stage for determining the sex of a foal via ultrasound is between 60 and 70 days of gestation. This is when the genital tubercle's position is most indicative of the foal's sex. For genetic testing, the sex can be determined as early as 9 weeks into the pregnancy, as fetal DNA is present in the mare's bloodstream by then.

Can the position of the foal affect the ability to determine its sex?

The position of the foal can indeed affect the ability to determine its sex through ultrasound. If the foal is not positioned favorably, the genital area may not be visible, making it challenging to identify the sex. In such cases, the veterinarian may suggest a follow-up ultrasound at a later date for clearer results.

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