Progesterone + Altrenogest

During the ovulatory season, healthy mares cycle normally and exhibit sexual receptivity to the stallion on a regular basis; they are also producing normal follicles that ovulate. The ovulatory season begins in middle of Spring (early April) and continues through the Fall unless the mare becomes pregnant. The mare undergoes a succession of cycles, with each being around 22 days in length. 

The equine estrous cycle is divided into two parts: estrus and diestrus. Estrus is usually referred to as the “heat” portion of the cycle, and is the time of follicular maturation and ovulation.1 During diestrus, the mare is not receptive to the stallion. Ovulation usually occurs between 24 to 48 hours prior to the end of estrus. The duration of estrus is around three to seven days. The mare will enter diestrus following ovulation and the end of estrus. The follicle that came about due to ovulation develops into the corpus luteum (CL).2 If the mare has not become pregnant, the corpus luteum will be absorbed and follicular development will proceed at the end of diestrus.

Timing Ovulation in the Mare

Considering the unpredictability of mares as regards the timing of their ovulation, horse owners and managers often have a need to cause a mare to enter into estrus and ovulate within a specific time period. Reasons for this can include the intent to use frozen semen to impregnate the mare; others often involve a stallion being delivered for a specific (limited) time, or in synchronizing the mare's ovulation with a shipment of semen.

Thus, the question then becomes one of what is the optimal method for achieving the objective of timing control.

Several drugs are useful to hasten ovulation to a more fixed time to coordinate a schedule that works with breeding farms for both the stallion and the mare.3 While there are a number of effective compounds that can help horse owners and managers induce ovulation, few are as widely applicable as progesterone, an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis.

While progestins are routinely used to manage ovulation, studies have shown that by themselves, they will not necessarily control follicular development and it is possible to have ovulations occurring during treatment, or large pre-ovulatory follicles present at the end of treatment, either of which will result in unpredictable timing of estrus and ovulation.4

Progesterone + Altrenogest for Breeding Management

Progesterone has been used for many years to prevent mares from coming into heat, to synchronize estrus cycles for better breeding efficiency, to organize or regulate heat cycles during the mare's seasonal transition, and to help maintain pregnancy. Progesterone is also used to modify estrus-related behaviors that interfere with performance and pleasure riding in non-breeding mares, and sometimes in stallions.5

Because an increase in estrogen accompanies the development of follicles, there is a marked impact on the mare’s behavior, which is calculated (by nature, of course) to attract a stallion. Unfortunately, these behaviors (frequent urination, squealing, tail swishing, and threatening other horses) are disagreeable in horses that have to work with humans. Consequently, progesterone has been widely used to manage “moody mares” as well as for breeding management.

Altrenogest is a progestin of the 19-nortestosterone group commonly used to suppress or synchronize estrus in horses. It is used extensively in the reproductive management of mares to control the reproductive cycle, help maintain pregnancy, and help with behavior modification.

Altrenogest is effective in normally cycling mares for minimizing the necessity for estrus detection, for the synchronization of estrus, and permitting scheduled breeding. Estrus will ensue 2-5 days after treatment is completed and most mares ovulate between 8-15 days after withdrawal. Altrenogest is also effective in suppressing estrus expression in show mares or mares to be raced. Altrenogest has been demonstrated to maintain pregnancy in oophorectomized mares and may be of benefit in mares that abort due to subtherapeutic progestin levels.6

Contraindications and Warnings

Altrenogest is contraindicated in mares having a previous or current history of uterine inflammation (ie, acute, subacute, or chronic endometritis). “Natural or synthetic gestagen therapy may exacerbate existing low-grade or ‘smoldering’ uterine inflammation into a fulminating uterine infection in some instances (package insert; Regu-Mate®). When used to maintain pregnancy, it should be discontinued within 24 hours of the due date.”7 Pregnant mares with gastrointestinal diseases may not absorb altrenogest adequately, thus injectable progesterone is preferred.8 Altrenogest should not be used in horses intended for food purposes and should not enter water courses, as this may be dangerous for fish and other aquatic organisms.6

Skin contact must be avoided as altrenogest is readily absorbed through unbroken skin. Protective gloves must be worn by all persons handling this product. Pregnant women or women who suspect they are pregnant should not handle altrenogest. Women of child bearing age should exercise extreme caution when handling this product, as accidental absorption could lead to a disruption of the menstrual cycle or prolongation of pregnancy. Direct contact with the skin should therefore be avoided. Accidental spillage on the skin should be washed off immediately with soap and water.5 Extra-label use of drugs in treating animals is allowable only by licensed veterinarians.

Where to buy Progesterone + Altrenogest 

Progesterone and altrenogest formulations are available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies.In combination, Progesterone 150 mg/ml / Altrenogest 18 mg/ml by NexGen is a superior solution for synchronizing estrus cycles for better breeding efficiency.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1Hemberg E, Lundeheim N, Einarsson S. Successful timing of ovulation using deslorelin (Ovuplant) is labour-saving in mares aimed for single AI with frozen semen. Reprod Domest Anim. 2006 Dec;41(6):535-7. doi: 10.1111/j.1439-0531.2006.00709.x. PMID: 17107513.

2Merck Veterinary Manual.

3equimanagement.com.

4equine-reproduction.com.

5equimed.com.

6Plumb’s Veterinary Drugs.

7Baker TW, Davidson AF. Obstetrical Emergencies I. Western Veterinary Conference 2012 2012. 2012. 

8Volkmann D. Rational Use of Progestagen Therapy During Pregnancy. Western Veterinary Conference 2010 2010. 2010.