While Mother Nature functions very well for breeding horses, technology can help Mother Nature when necessary, or do things such as superovulation that Nature never really intended.
The mare’s estrus cycle typically lasts about 21 days, and she is in heat for four to seven of those days. When needed, there are a variety of hormones available to help the mare along, but they must be used carefully. Here we will briefly discuss the main hormones and their uses.
Human Chorionic Gonadotropin (hCG)
It may be desirable to induce timed ovulation in mares when you are using frozen or cooled-transported semen or to synchronize ovulation. The hormone hCG acts like a luteinizing hormone, which matures and induces ovulation of the dominant ovarian follicle. Within 48 hours, 75 to 85 percent of the mares will ovulate, with the average time being 36 hours. However, the percentage of mares ovulating within 24 to 48 hours after receiving hCG decreases as the mare’s age increases.
There is some controversy as to whether repeated use of hCG during the breeding season results in a decreased efficacy of timed ovulation. Repeated administration of hCG may cause anti-hCG antibodies to form, which bind to the hormone and make it less effective. Patrick McCue, DVM, PhD, Diplomate ACT with the Equine Reproduction Laboratory at Colorado State University, believes there is a slight decline in effectiveness, so it may be best to only use it once or twice in a breeding season. Dr. Dave Stanford of Woodside Equine Clinic in Ashland, Virginia, hasn’t necessarily seen that happen, but he has found certain mares that have developed a resistance to it.
Gonadotropin Releasing Hormone (GnRH)
This hormone is also known as Luteinizing Hormone Releasing Hormone (LHRH) and is also used to induce ovulation. According to McCue, natural GnRH is not effective because of its short half-life and low potency. Therefore, it is necessary to use a synthetic form. Ovuplant is the trade name for deslorelin acetate, a subcutaneous implant that stimulates the pituitary gland to release luteinizing hormone. Supplies of this implant were short in 2004 and not available at all in the U.S. in 2005. However, it can still be obtained in Canada, Australia, and Europe. An alternative to Ovuplant is a compounded, injectable version of deslorelin acetate, but it is not approved for use in the horse.
Equine Follicle Stimulating Hormone (eFSH)
When utilizing superovulation to recover multiple embryos, eFSH is the hormone used. On average, you will get three to four ovulations with 1.5 to 2 embryos collected per cycle. However, McCue stresses that it should be tailored to the needs of the individual mare. “Unfortunately, some mares treated with eFSH develop multiple large follicles that do not ovulate in response to hCG. Superovulation treatment is more effective in younger mares than in older mares,” he says. Another reason for using eFSH is to help mares transition and ovulate for the first time that year.
Progesterone is a product of the ovarian corpus luteum and the placenta of a pregnant mare. It is used to suppress estrus, maintain pregnancy and synchronize ovulations of two or more mares, and it is one of the most common hormones Stanford uses in his practice. Natural progesterone is not normally utilized because of local swelling and pain that often occurs at the intramuscular injection site. The most common form of synthetic progestin used is Regumate.
Prostaglandins, such as Lutalyse (dinoprost tromethamine or PGF2) and Estrumate (cloprostenol or CLO) are used to manipulate a mare’s estrus cycle. Stanford uses these frequently to short cycle a mare for breeding or synchronize estrus for embryo transfer. Other uses are for termination of pregnancy and for treating a persistent corpus luteum. Side effects of prostaglandins include abdominal discomfort, diarrhea, increased heart rate and sweating. However, at the 2004 AAEP Convention (American Association of Equine Practitioners), Gary J. Nie, DVM, PhD, consultant for World Wide Veterinary Consultants, LLC, reported on a study he did that showed that microdoses of CLO are as effective, decrease drug costs and virtually eliminate side effects. He did not recommend the low dose of PGF2 because of a delayed progesterone drop seen in the treated mares. Stanford also used a lower dose this past year and saw much fewer side effects.
Oxytocin is commonly given to mares to help the uterus clear fluid, induce labor and manage retained placentas. If a mare is not able to expel fluid from her uterus due to advanced age or ill health, the oxytocin causes the uterus to contract and do the work for her. McCue feels that inducing labor for nonmedical purposes is not advisable, but can be done using oxytocin. Finally, a retained placenta is one of the most common postpartum problems in mares. If the placenta has not been passed within two to three hours after foaling, oxytocin can be given to help expel it.
Some experts feel that using hormones to help advance equine reproduction can be very beneficial for some horses, but it should be done under the careful guidance of your veterinarian. Others, however, are not in favor of interfering with Mother Nature, who, all agree, does it best. Ultimately, it’s a choice the breeder makes, depending on each individual situation.