What sex is the foal?
Horse Breeding is an exciting time! Your mare has had a positive scan and now you want to know, what sex is the foal? Will you be welcoming a filly or a colt and how can you tell?
Our co-founder and equine veterinary surgeon, Kevin Corley, is sharing his expertise on Fetal Sexing. Do you have questions or comments? Drop them in the comment form below, and Kevin will be more than happy to help! Subscribe to our newsletter for more articles on horse breeding and our latest news.
Introduction
A veterinarian performs foetal sexing to confirm a foal’s sex before birth. This guide walks through the steps a veterinarian takes to determine the foetal sex and update the horse breeding records accordingly.
Knowing the sex of the foal a mare is carrying can be useful for many reasons. Firstly, it can help with future horse breeding plans. Secondly, it can help with planning sales. Some owners might be more likely to send a mare to the sales if they knew she was carrying a filly. The sex of the foetus may also influence the reserve price set.
Some owners want a certificate from the vet stating that the mare is believed to be carrying a colt to try and enhance the sales value. If you want a filly from the mare and you know she is carrying a filly, you might be more likely to sell a female weanling.
Lastly, it may affect whether you choose to insure the pregnancy during horse breeding.
Equipment Needed
- Stocks with an adjustable bar side to protect the ultrasound machine and the veterinarian.
- Ultrasound machine with colour Doppler.
- Probe phased array or sector probe between 1-5 MHz typical distance required 15-20cm depth.
- Clipping: clipping is not necessary in thoroughbreds; alcohol can be used for contact. In heavier coated animals, clip between the xiphoid and the udder midline or 10cm off midline, depending on the position of the foal.
Technique
The foetus can be sexed in two ways – either by rectal examination and trans-rectal ultrasound or by ultrasound through the abdominal wall. The time window where foetal scanning can be achieved is smaller and it can be harder to get the necessary images rectally than transabdominally. There are risks to the mare of a protracted rectal examination to be considered too.
These are as follows:
- Trans-rectal scanning: 55 to 70 days, 110 to 140 days.
- Trans-abdominal scanning: 120 days to 260 days.
Smaller foals can be sexed slightly later than 260 days, and bigger foals can be sexed slightly earlier than 120 days.
For trans-abdominal sexing, the sex of the foetus is determined by the appearance of the gonads.
Male Foetal Sexing
The gonad in colts is heterogenous. There is no clear differentiation between the cortex and the medulla. Colts have a central vein in the gonad which is relatively straight. See illustration one.
Female Foetal Sexing
Fillies have a distinct cortex and medulla in the gonads, divided by a thin concentrical line. There are blood vessels within the junction between the cortex and medulla, which can be highlighted by colour Doppler ultrasonography.
It can be hard to see a complete ring of blood vessels on one section but it should be able to be traced through different planes. See illustration two.
Useful Resources
Explore the reproduction exams that feature in our horse breeding solution: Breeding
Scientific Publications can be found here: https://www.sciencedirect.com/science/article/abs/pii/S0093691X05001731#:~:text=Early%20fetal%20gender%20determination%20in,the%20clitoris%20in%20the%20female.