Priming Anestrous Mares for Embryo Transfer

Assisted reproduction techniques such as intracytoplasmic sperm injection (ICSI) and embryo transfer continue to gain popularity among horse breeders. One barrier to success is finding appropriate recipient mares in which to transfer embryos. According to Spanish researchers, anestrous (noncycling) mares can be used as recipients for embryos produced by ICSI if appropriately “primed.”^*

In the study, four mares in anestrus were primed by using a combination of estrogen and progesterone. Specifically, mares were administered estradiol benzoate intramuscularly for five consecutive days (0.5 mg the first day, 1 mg the second day, and 1.5 mg on days three, four, and five). Three days later, all mares began receiving low-dose altrenogest (0.088 mg/kg) daily for the duration of the study.

Embryos were transferred into these primed mares 3.5 days after starting the altrenogest. Then, all mares were subsequently flushed 48 hours afterwards to collect the embryos to microscopically evaluate embryonic development.

Fifteen embryos were transferred into the four mares, and 12 were successfully recovered 48 hours later. Eight of the 12 embryos (67%) of the recovered embryos showed signs of intrauterine development, affirming that anestrus mares can be primed to act as recipient mares in embryo transfer programs.

“This is the first study to evaluate the feasibility of using estradiol benzoate and a short-acting progesterone to prime anestrus mares for receiving in vitro produced embryos,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

This is important research as the general unavailability of recipient mares is currently limiting the widespread use of ICSI and embryo transfer.

“This assisted reproductive technique plays an important role in equine practice today as it allows us to produce foals from valuable sires with limited semen available, deceased sires, mares in competition, and mares that are too old or cannot carry their own foals to term,” Crandell said.

Maintaining broodmares or prospective recipient mares in optimal body condition with the best nutrition can increase the chances of successful reproduction.

A carefully balanced diet with adequate nutrients and a judicious number of calories can go a long way to supporting this success,” advised Crandell. “In addition, supplementing recipient mares with DHA and EPA has reproductive benefits in the mare. These include improved fertility by supporting the uterine environment.”

EO-3 contains both DHA and EPA in a palatable liquid top-dressed on your horse’s ration. Mares should be supplemented year-round to maximize EO-3’s benefits. If year-round supplementation is not possible, begin feeding 60 days before the breeding season for optimal effect.

^*Muñoz-García, C.C., M. Luis-Calero, P. Fernández-Hernández, A. Pizarro-Hernández, R. Fernández-González, A. Gutiérrez-Adán, M.J. Sánchez-Calabuig, J. Cuervo-Arango, L. González-Fernández, B. Macías-García. 2024. A new hormonal protocol supports early development of in vitro-produced embryos after transfer to anoestrus mares. Reproduction in Domestic Animals 59 Suppl. 3:e14622.