AAEP Veterinarians Discuss Estrus Suppression in Mares

Many owners seek to eliminate estrus behavior when there is a negative effect on the mare’s performance.
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Many mare owners seek to eliminate estrus behavior when they do not want to breed their mare or when there is a perceived negative effect of estrus behavior on the mare’s performance. The "Estrus Suppression in the Mare" table topic session at the 2013 American Association of Equine Practitioners convention, held Dec. 7-11 in Nashville, Tenn., began with a discussion of reasons for estrus suppression and approaches practitioners use to guide their clients through selecting a method of estrus suppression. The veterinarians in attendance listed available methods for estrus suppression and discussed preference, efficacy, and problems associated with the treatment.

Many practitioners administer exogenous progesterone/progestin with reported good results. Altrenogest is the most commonly used drug. Practitioners most frequently use the oral formulation, although some have used a compounded slow-release intramuscular formulation. Other progestins owners and veterinarians often discuss are not always efficacious. Progesterone/progestin treatment is usually started just after ovulation. This treatment stops estrus behavior signs but does not eliminate follicular development.

For several years veterinarians have used intrauterine marbles to suppress estrus, but this method is slowly falling out favor because of poor efficacy and increasing reported incidence of complications such pain, uterine infection, and uterine irritation if the marbles break. In addition, removal of the marbles is often difficult.

Another approach practitioners are using increasingly is injecting oxytocin (60 IU) daily from Day 7 to Day 14 after an ovulation to prolong corpus luteum (CL, the structure formed after the follicle releases the egg, or ovulates, and then produces progesterone) life. This treatment places the mare in a state of “pseudopregnancy” and maintains her out of heat for 60 to 90 days. The technique has a high efficacy in preventing estrus manifestation. Although not verified experimentally yet, the treatment can be used again on subsequent ovulation. Other methods for prolongation of corpus luteum function have been discussed but they are not offered in practice. These include establishing a pregnancy and then manually reducing the embryo after maternal recognition of pregnancy is complete at Day 16; inducing a late-diestrus (when the mare is not in heat) ovulation, so there is a newly formed CL that is too immature to respond to luteolysis (degradation of the CL); and intrauterine infusion of plant oils, which seems to reduce/eliminate the endometrium’s (uterine body and horns) ability to secrete prostaglandin F 2 alpha, the hormone that causes lysis of the corpus luteum and initiation of another estrus cycle

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