Show Your Mare and Breed Her, Too!

Today’s assisted reproductive technologies mean you can show your mare and breed her in the same season.

By Barb Crabbe, DVM, Photos by Kaycie Timm

There’s nothing like a good mare—especially yours. She’s smart, strong, and spins on a dime. It took a while to convince her, but now that she’s decided you’re “the one,” she gives you all she has in every run. If last year’s earnings mean anything at all, you can hardly wait to see what’s coming this year. But, on the other hand, you’d love to breed her so you have a young prospect with all her outstanding qualities.

Enter assisted reproductive technologies—advanced breeding techniques that make it possible to have it all: another stellar year in the show pen and that foal you can point toward futurities! You’ve probably heard about these techniques that allow your mare to produce a foal (or maybe more than one) while she continues to train and compete. In this article, we’ll take a deep dive into how each of these techniques works.

The Basics

To understand how advanced reproductive technologies work, it helps to know a little bit about the early stages of conception and what it takes to create a foal.  

When your mare has a heat cycle, fluid-filled vesicles called follicles develop on her ovaries and grow during the period of estrus. Within each follicle is an egg (called the oocyte).  At the end of estrus, one dominant follicle will typically release an egg, which then passes into the oviduct (a tube connecting the ovary to the uterus). In some situations, there will be multiple dominant follicles, resulting in multiple ovulations. 

For a mare to become pregnant, semen must be deposited into the mare’s uterus very close to the time of ovulation. Individual sperm travel to the tip of the mare’s uterine horn and enter the oviduct where the egg is fertilized. That fertilized egg then remains in the oviduct for five or six days before passing down into the mare’s uterus. 

Once in the uterus, the tiny embryo travels around for approximately 15 to 16 days before implanting itself in the uterine lining. Once settled, the embryo develops and grows. The following requirements must be met for a successful pregnancy to result for a mare. 

• The mare must produce a viable egg and release it from her ovary. 
• Viable sperm must be available at the right time and in the right place to fertilize the egg. 
• The egg/fertilized embryo must successfully pass through the oviduct to reach the uterus. 
• The uterus must provide a healthy environment for the embryo to successfully implant and grow. 
• The uterus must provide a healthy environment for the embryo to successfully implant and grow. 

For assisted reproductive technologies such as embryo transfer  (ET) or intracytoplasmic sperm injection (ICSI) to be successful, they must accommodate all these steps. Here’s how they work. 

Embryo Transfer

What it is: Embryo transfer means an embryo is taken from one mare and put inside another for gestation and delivery of the foal.  

How it works: During embryo transfer, your mare (the donor mare) is bred just as she would be if she were to carry a pregnancy to term. The breeding will be closely monitored with ultrasound examinations so your vet can determine the exact time of ovulation.  

At the same time your mare is being monitored and bred, the mare that’ll carry her foal (the recipient mare) is also being monitored. For best success, the recipient mare will ovulate one to three days after the donor mare. Because this timing is so important for success, embryo transfer facilities often have large herds of potential recipients, which increases the odds that there will be a suitable mare available to receive an embryo at just the right time. 

On approximately day seven or eight after ovulation, your veterinarian will flush the embryo out of your mare’s uterus by infusing fluid, then recovering that fluid through a special filter that catches the embryo. Your vet will identify the embryo under a microscope and then prepare it to be placed into the uterus of the recipient mare. If you choose to work with an embryo transfer facility with a large recipient herd, the embryo may be shipped to a distant location before being placed in the mare. 

Why choose ET: Embryo transfer provides a way to breed your mare while she continues to train and compete. It’s also possible for your mare to produce multiple foals each year. ET has become quite common, and many veterinarians are comfortable with the procedures of breeding a mare and flushing embryos—making it easily accessible for most horse owners. Success rates for embryo transfer are high. Estimates say you can expect a pregnant recipient mare in approximately 50% of breedings, and with an experienced veterinarian and a well-managed recipient herd that percentage may be much higher.   

In addition, if you’re breeding an older mare or one with poor fertility due to problems with her uterus or cervix that would prevent her from successfully carrying a foal to term, embryo transfer is a good option. 

The downsides: Convenience and scheduling constraints are downsides of ET, particularly if your mare has a heavy training and competition schedule. Time of ovulation can’t be completely controlled—nor can ideal scheduling for the embryo flush. You’ll be at the mercy of your mare’s cycle during those periods. Additionally, she may require sedation or other medications throughout the procedure that can create conflicts with medication rules.   

Finally, embryo transfer will only be successful if you mare’s reproductive function is normal regarding ovulation and movement through the oviduct following fertilization.

This is typically less of a problem for a young, healthy performance mare, but can create challenges for an older mare or a mare with fertility issues that involve more than just the uterus. And if you are breeding to a subfertile stallion or one with limited semen available, successful embryo transfer does require a full dose of good quality semen. 

Intracytoplasmic Sperm Injection 

What it is: During ICSI, an egg is harvested from your mare, inseminated in a laboratory, and implanted in another mare for gestation and delivery of a foal. 

How it works: The first step for ICSI is aspiration of the oocytes from your mare’s ovaries, which is performed with an ultrasound-guided needle inserted through her vaginal wall and into individual follicles.  Oocytes can be collected from either dominant, mature follicles near the time of ovulation or from immature follicles. 

In the early stages of development of these techniques, collection of dominant follicles was more popular—primarily because mature oocytes are easier to aspirate and have a higher rate of developing into a viable embryo. Once harvested, mature oocytes are much more sensitive than immature oocytes, making successful transport much more complicated.  

Collection of immature oocytes is much less time-sensitive, and multiple eggs are commonly collected at the same time. On average, six to eight eggs can be collected as often as every 14 days. Although the success rate with each individual oocyte may not be as high for immature versus mature oocytes, the larger numbers collected mean overall success in establishing a pregnancy is just as good, if not better.  This, combined with the scheduling convenience and improved technical skills of practitioners performing the aspirations, has resulted in a gradual shift toward ICSI using immature oocytes as a more common procedure. 

If collection doesn’t occur at a facility with an ICSI laboratory, the oocytes will be transported to an ICSI facility where they’ll be fertilized using  microinjection of a single sperm cell, then held for six to eight days. If the fertilized egg successfully develops into an embryo, it’ll then be transferred into a recipient mare’s uterus for gestation. 

Why choose ICSI: Like embryo transfer, ICSI provides a way that you can breed your mare while she continues to train and compete and also allows her to produce multiple foals each year. Success rates are good,  but depend on a number of different variables, including the number of oocytes that can be collected with each attempt and the skill of the practitioner. If your mare has a heavy training and competition schedule, pursuing a pregnancy via ICSI will disrupt this schedule less than breeding via embryo transfer.  

Finally, if your mare has fertility issues that include abnormal cycles/failure to ovulate, a blocked oviduct, or an unhealthy uterine environment, she can still produce a foal as long as she has healthy oocytes available for collection. If you hope to breed to a subfertile stallion or one with limited semen availability, ICSI has a much lower requirement for sperm than
embryo transfer. 

The downsides: Although ICSI is becoming more prevalent, fewer practitioners are comfortable with oocyte aspiration techniques compared with embryo transfer, and availability of ICSI laboratories capable of performing the sperm injections is still limited. As expertise becomes more widespread, however, the breeding industry is headed in this direction. If you plan to breed your performance mare, ICSI is clearly something to consider.