There are three major reasons to attempt to have a mare "superovulate" or ovulate more than one follicle at a time. First, in embryo transfer programs there is a greater chance of recovering an embryo with superovulation, thus reducing the expense of getting a viable embryo for transplanting. Second, it has been found that superovulation of a mare in transition (early in the year before she is cycling regularly) will often hasten the onset of normal estrous cycles. Third, in breeding mares to stallions with low fertility, superovulation can increase the chances of conception.

Ed Squires, PhD, an honorary Diplomate in the American College of Theriogeniology (reproduction), has long studied mare and stallion reproduction at Colorado State University. One of his recent studies involves a new commercially available product called equine follicle-stimulating hormone (eFSH, from Bioniche Animal Health). 

Squires reported that only 50% of mares flushed on any given cycle will produce an embryo for collection. He recommended that mares which will undergo a superovulatory attempt should have their ovaries scanned with ultrasound to select mares with follicles 20 mm or smaller, or perhaps suppress their follicular development with progestins and estradiol prior to eFSH treatment.

Studies have shown it takes about seven days for the twice-a-day treatment regimen to produce several follicles 35 mm or larger.

Protocol for eFSH is to begin treatment five to seven days after ovulation, with eFSH injected twice per day. On the second day give prostaglandin. Continue treatment until several 35 mm follicles develop, stop treatment, give HCG, and collect.

Squires reported on two controlled studies done at Colorado State University and in Brazil. The studies looked at two different doses of eFSH, either 25 mg or 12 mg twice a day. The mares which received 12 mg got either HCG or deslorelin (Ovuplant) once most of the follicles were at least 35 mm. Squires found he got a better ovulation and pregnancy rate using HCG than GNRH (gonadotropin-releasing hormone). The number of days treated didn’t seem to matter as long as the mares were treated a minimum of seven days. Mares which got the 25 mg of eFSH plus GnRH had an average of 6.7 follicles develop, but only 50% of the follicles ovulated.

The partner study in Brazil showed similar results. It was found that 80% of those mares given eFSH had multiple ovulations.

Squires said one way to enhance the response to eFSH is to select mares with appropriate follicles, and perhaps use progesterone and estrogen to suppress the follicle population prior to treatment. He recommended stopping treatment sooner than what has been the industry norm.

Potential problems include over-stimulation with greater than five ovulations.

Squires recommended using eFSH when breeding mares in the presence of low-quality semen.

Transitional Mares
These mares tend to have long, erratic cycles from February to April, depending if they have been placed under lights, said Squires. When using eFSH therapy on anestrus mares, he only had two ovulate. "If you wait until the mares are at least transitional, we got eight out of nine to ovulate within two weeks," he reported.

Squires designed a study to evaluate the efficacy of eFSH in stimulating ovulation in transitional mares. When study mares showed follicles in the 20-25 mm range and were in mid- to late transition, they were given eFSH. On average, in 7.6 days they ovulated. If mares in that same situation were given nothing, it took about 40 days for them to ovulate.

Squires says uses of eFSH include increasing ovulation rate, increasing embryo recovery and pregnancy rates, enhancing fertility of mares, stimulating multiple pre-ovulatory follicles for oocyte collection, and improving the time of first ovulation of the year.

He said if you do get a mare to ovulate too early in the season, she will ovulate once and return to anestrus/transition, but will cycle later in the season.

Q & A
When questioned about the use of eFSH in older mares, he replied that success would depend on whether they have some small follicles there and whether they have any FSH receptors. "It will work on some, so it’s worth a try," said Squires. "Some of those won’t respond."

Another practitioner asked if there should be any concern with superovulating young mares. Is there any potential problems of prematurely exhausting a finite supply of eggs? Squires said that is a commonly asked question. "There are hundreds of thousands of follicles in the ovary, and I think we are rescuing some of them (most are lost and never ovulate)," he said.

Another question was whether there were any adverse effects of repeated treatments. Squires said one study in France gave eFSH to a mare six cycles in a row and saw no adverse effects. "I want to get it done (collect embryos) in one cycle for guys who want multiple foals so there is less flushing and breeding," said Squires.

When questioned about the cost, Squires said it is about $400-500 per cycle for drug cost, depending on the mare.

Squires said the mare usually can maintain a pregnancy after being superovulated and collected. However, he cautioned, in some cases mares will spontaneously double ovulate during the next cycle. "My gut feeling is that there's very little adverse effect in next cycle," said Squires.

Squires said, ”eFSH is a tool that can be used by the practitioner, but it might take several years for researchers and practitioners to fine-tune the protocol for eFSH.”


About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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