Emergency Hormones for Embryo Transfer Recipient Mares  

When donor/recipient mare cycles don’t match perfectly, veterinarians can synchronize them using exogenous hormones.
pregnant recipient mare used for embryo transfer
Hormone therapy for non-cycling recipient mares can achieve the same pregnancy and pregnancy loss rates as embryo transfers with cycling mares. | Getty Images

Recipient mares are hot commodities these days, even though embryo transfer (ET) is performed routinely. In “emergency” situations, when an ET mare is desperately needed last minute, hormone therapy is required for mares in winter anestrus, in transition, or in nonsynchronized diestrus. 

“Managing recipient mares is challenging and time-consuming and yet the most important factor affecting the successful outcome of ET,” said Brittany Middlebrooks, DVM, MS, third-year resident in Theriogenology at Colorado State University, at the 2023 American Association of Equine Practitioners Convnetion held Nov. 29 to Dec. 3 in San Diego.  

Factors to consider when choosing a recipient mare include her age, overall health, reproductive health, size compared with the donor, and behavior (handleability).  

“When the donor comes into heat, the dominant hormone is estrogen,” Middlebrooks explained. “We can admininster either human chorionic gonadotropin (HCG) or gonadotropin-releasing hormone (GnRH) to induce ovulation in one to two days. Following administration of an ovulatory induction agent, the mare is inseminated. After ovulation, the corpus luteum is present, and the dominant hormone is progesterone. That donor mare can then be flushed eight days post-ovulation.” 

A cycling recipient—an optimal situation—would ideally be in heat around the same time as the donor. The recipient would then receive an ovulation induction agent on the day the donor mare ovulates, putting the recipient mare two days “behind” the donor. After a successful embryo flush from the donor mare, the recipient mare would receive an embryo six days post-ovulation.  

“There is a range though, and the recipient mare can ovulate one day ahead and up to four days after the donor mare,” noted Middlebrooks.  

When the donor/recipient cycles don’t match perfectly, veterinarians can synchronize them using exogenous hormones. This is not a new concept but one that deserves to be highlighted due to the dearth of recipients.  

Deep Anestrus 

Imagine it’s March, and the donor mare was under lights and is ready to be bred. But the emergency recipient has not been under lights and has a small, flaccid uterus with minimal follicles on her ovaries.  

“Treat the donor mare as previously described,” said Middlebrooks. “But on the day of donor ovulation, the recipient mare will receive injectable estradiol for two days then a short-acting progesterone to mimic ovulation. Administer the recipient mare progesterone daily until day of transfer, and continue until the time of pregnancy diagnosis.”  


If we’re desperately searching for a recipient mare midseason, then we can use a mare in mid-diestrus (e.g., the planned recipient has endometritis and can no longer be used but another open mare is available).  

“This mare in mid-diestrus has a progesterone-producing corpus luteum that we need to eliminate,” said Middlebrooks.   

To do this, administer prostaglandin F2 alpha (PGF2α) the same day the donor mare ovulates plus a dose of estradiol.  

“When you check the recipient mare the following day, if she has a good amount of uterine edema, you can administer only a dose of estradiol,” said Middlebrooks. “If there is not sufficient uterine edema, a second dose of PGF2α should be utilized. Then, after two days of estradiol treatment, one or two doses of PGF2α, and visualization of appropriate uterine edema via transrectal ultrasonography, the recipient mare can receive a dose of short-acting progesterone. Short-acting progesterone is administered through day of transfer and until pregnancy diagnosis.”   

She then relayed five years of data (2018-2022) for recipient mares in the various abovementioned stages of their estrous cycles: traditional cycling, deep anestrus, and short-cycled diestrus mares. All recipient mares were on exogenous progesterone until pregnancy confirmation at Day 120. 

In Middlebrooks’ study, the initial pregnancy checks were performed on Days 12 to 16 then on Days 25, 35, and 45. 

“No difference in pregnancy rates occurred between the three groups of mares in different stages of their estrous cycles,” she relayed.   

Follow-up data using these same protocols in a large number of anestrous and transitional mares in 2023 were analyzed and, again, the researchers noted no difference in pregnancy rates at the initial check on Day 12 to 16 and Day 25. 

“Even deep anestrus and transitional mares can be manipulated using specific hormone protocols to receive embryos in an emergency, and those pregnancy rates and pregnancy loss rates are no different than normal, cycling, recipient mares,” concluded Middlebrooks.  

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