Altrenogest (Regu-Mate, Matrix)

Oral Progestin

Highlights Of Prescribing Information

Progestational drug used in horses to suppress estrus or maintain pregnancy when progestin deficient; used in swine to synchronize estrus

May be used in dogs for luteal deficiency or as a treatment to prevent premature delivery

Many “handling” warnings for humans (see below)

Very sensitive to light

What Is Altrenogest Used For?

Altrenogest (Regu-Mate) is indicated (labeled) to suppress estrus in mares to allow a more predictable occurrence of estrus following withdrawal of the drug. It is used clinically to assist mares to establish normal cycles during the transitional period from anestrus to the normal breeding season often in conjunction with an artificial photoperiod. It is more effective in assisting in pregnancy attainment later in the transition period. Some authors () suggest selecting mares with considerable follicular activity (mares with one or more follicles 20 mm or greater in size) for treatment during the transitional phase. Mares that have been in estrus for 10 days or more and have active ovaries are also considered excellent candidates for progestin treatment.

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. Although the drug is labeled as contraindicated during pregnancy, it has been demonstrated to maintain pregnancy in oophorectomized mares and may be of benefit in mares that abort due to sub-therapeutic progestin levels.

The product Matrix is labeled for synchronization of estrus in sexually mature gilts that have had at least one estrous cycle. Treatment with altrenogest results in estrus (standing heat) 4-9 days after completion of the 14-day treatment period.

Altrenogest has been used in dogs for luteal insufficiency and as a treatment to prevent premature delivery.


Progestins are primarily produced endogenously by the corpus luteum. They transform proliferative endometrium to secretory endometrium, enhance myometrium hypertrophy and inhibit spontaneous uterine contraction. Progestins have a dose-dependent inhibitory effect on the secretion of pituitary gonadotropins and have some degree of estrogenic, anabolic and androgenic activity.


In horses, the pharmacokinetics of altrenogest have been studied (). After oral dosing of 44 mg/kg PO, peak levels usually occur within 15-30 minutes post-dose; 24 hours post-dose, levels were below the level of quantification. Elimination half-lives are approximately 2.5-4 hours. Altrenogest appears to be primarily eliminated in the urine. Peak urine levels occur 3-6 hours after oral dosing. Urine levels were detectable up to 12 days post-administration.

Before you take Altrenogest

Contraindications / Precautions / Warnings

The manufacturer (Regu-Mate — Intervet) lists pregnancy as a contraindication to the use of altrenogest, however it has been used clinically to maintain pregnancy in certain mares (see Dosages below). Altrenogest should also not be used in horses intended for food purposes.

Adverse Effects

Adverse effects of altrenogest appear to be minimal when used at labeled dosages. One study () found negligible changes in hematologic and most “standard” laboratory tests after administering altrenogest to 4 groups of horses (3 dosages, 1 control) over 86 days. Occasionally, slight changes in Ca++, K+, alkaline phosphatase and AST were noted in the treatment group, but values were only slightly elevated and only noted sporadically. No pattern or definite changes could be attributed to altrenogest. No outward adverse effects were noted in the treatment group during the trial.

Use of progestational agents in mares with chronic uterine infections should be avoided as the infection process maybe enhanced.

Overdosage / Acute Toxicity

The LD50 of altrenogest is 175-177 mg/kg in rats. No information was located regarding the effects of an accidental acute overdose in horses or other species.

How to use Altrenogest

Altrenogest dosage for dogs:

For luteal insufficiency:

a) Document luteal insufficiency and rule out infectious causes of pregnancy loss. Best to avoid during first trimester. Give equine product (Regumate) at 2 mL per 100 lbs of body weight PO once daily. Monitor pregnancy with ultrasound. Remember that exogenous progesterone is the experimental model for pyometra in the bitch, so monitor carefully. ()

b) For luteal insufficiency, pre-term labor: 0.1 mL per 10 lb body weight PO once daily. ()

c) To maintain pregnancy if tocolytics (e.g., terbutaline) do not control myometrial contractility: 0.088 mg/kg once daily (q24h). Must be withdrawn 2-3 days prior to predicted whelp date. ()

Altrenogest dosage for horses:

To suppress estrus for synchronization:

a) Administer 1 mL per 110 pounds body weight (0.044 mg/ kg) PO once daily for 15 consecutive days. May administer directly on tongue using a dose syringe or on the usual grain ration. (Package insert; Regu-Mate — Intervet)

b) 0.044 mg/kg PO for 8-12 days ()

To maintain pregnancy in mares with deficient progesterone levels:

a) 22-44 mg daily PO ()

b) 0.044 mg/kg PO once daily. Three options for treatment: 1) treatment until day 60 of pregnancy or greater AND measurement of endogenous progesterone level of >4 ng/mL; 2) treatment until day 120 of pregnancy; or 3) treatment until end of pregnancy. ()

To maintain pregnancy in mares with placentitis: a) 10-20 mL (22-44 mg) daily PO () To suppress estrus (long-term): a) 0.044 mg/kg PO daily ()

Altrenogest dosage for swine:

For synchronization of estrous in sexually mature gilts that have had at least one estrous cycle:

a) Follow label directions for safe use. Administer 6.8 mL (15 mg) per gilt for 14 consecutive days. Apply as a top-dressing on a portion of gilt’s daily feed allowance. Estrous should occur 4-9 days after completing treatment. (Package insert; Matrix — Intervet)

Client Information

■ The manufacturer (Regu-Mate, Matrix — Intervet) lists the following people as those who should not handle the product:

1. Women who are or suspect that they are pregnant

2. Anyone with thrombophlebitis or thromboembolic disorders or with a history of these events

3. Anyone having cerebrovascular or coronary artery disease

4. Women with known or suspected carcinoma of the breast

5. People with known or suspected estrogen-dependent neoplasias

6. Women with undiagnosed vaginal bleeding

7. People with benign or malignant tumor that developed during the use of oral contraceptives or other estrogen containing products

■ Altrenogest can be absorbed after skin contact and absorption can be enhanced if the drug is covered by occlusive materials (e.g., under latex gloves, etc.). If exposed to the skin, wash off immediately with soap and water. If the eyes are exposed, flush with water for 15 minutes and get medical attention. If the product is swallowed, do not induce vomiting and contact a physician or poison control center.

■ This medication is prohibited from use in an extra-label manner to enhance food and/or fiber production in animals

Chemistry / Synonyms

An orally administered synthetic progestational agent, altrenogest has a chemical name of 17 alpha-Allyl- 17beta-hydroxyestra-4,9,11-trien-3-one.

Altrenogest may also be known as: allyl trenbolone, A-35957, A-41300, RH-2267, or RU-2267, Regu-Mate, or Matrix.

Storage / Stability

Altrenogest oral solution should be stored at room temperature. Altrenogest is extremely sensitive to light; dispense in light-resistant containers.

Dosage Forms / Regulatory Status

Veterinary-Labeled Products:

Altrenogest 0.22% (2.2 mg/mL) in oil solution in 150 mL and 1000 mL bottles; Regu-Mate (Intervet); (Rx). Approved for use in horses not intended for food. This medication is banned in racing animals in some countries.

Altrenogest 0.22% (2.2 mg/mL) in 1000 mL bottles; Matrix (Intervet); (OTC, but extra-label use prohibited). Approved for use in sexually mature gilts that have had at least one estrous cycle. Gilts must not be slaughtered for human consumption for 21 days after the last treatment. The FDA prohibits the extra-label use of this medication to enhance food and/or fiber production in animals.

Human-Labeled Products: None


Aglepristone (Alizin, Alizine)

Injectable Progesterone Blocker

Highlights Of Prescribing Information

Injectable progesterone blocker indicated for pregnancy termination in bitches; may also be of benefit in inducing parturition or in treating pyometra complex in dogs & progesterone-dependent mammary hyperplasia in cats

Not currently available in USA; marketed for use in dogs in Europe, South America, etc.

Localized injection site reactions are most commonly noted adverse effect; other adverse effects reported in >5% of patients include: anorexia (25%), excitation (23%), depression (21%), & diarrhea (13%)

What Is Aglepristone Used For?

Aglepristone is labeled (in the U.K. and elsewhere) for pregnancy termination in bitches up to 45 days after mating.

In dogs, aglepristone may prove useful in inducing parturition or treating pyometra complex (often in combination with a prostaglandin F analog such as cloprostenol).

In cats, it may be of benefit for pregnancy termination (one study documented 87% efficacy when administered at the recommended dog dose at day 25) or in treating mammary hyperplasias or pyometras.


Aglepristone is a synthetic steroid that binds to the progesterone (P4) receptors thereby preventing biological effects from progesterone. It has an affinity for uterine progesterone receptors approximately three times that of progesterone. As progesterone is necessary for maintaining pregnancy, pregnancy can be terminated or parturition induced. Abortion occurs within 7 days of administration.

Benign feline mammary hyperplasias (fibroadenomatous hyperplasia; FAHs) are usually under the influence of progesterone and aglepristone can be used to medically treat this condition.

When used for treating pyometra in dogs, aglepristone can cause opening of the cervix and resumption of miometral contractility.

Within 24 hours of administration, aglepristone does not appreciably affect circulating plasma levels of progesterone, cortisol, prostaglandins or oxytocin. Plasma levels of prolactin are increased within 12 hours when used in dogs during mid-pregnancy which is probably the cause of mammary gland congestion often seen in these dogs.

Aglepristone also binds to glucocorticoid receptors but has no glucocorticoid activity; it can prevent endogenous or exogenously administered glucocorticoids from binding and acting at these sites.


In dogs, after injecting two doses of 10 mg/kg 24 hours apart, peak serum levels occur about 2.5 days later and mean residence time is about 6 days. The majority (90%) of the drug is excreted via the feces.

Before you take Aglepristone

Contraindications / Precautions / Warnings

Aglepristone is contraindicated in patients who have documented hypersensitivity to it and during pregnancy, unless used for pregnancy termination or inducing parturition.

Because of its antagonistic effects on glucocorticoid receptors, the drug should not be used in patients with hypoadrenocorticism or in dogs with a genetic predisposition to hypoadrenocorticism.

The manufacturer does not recommend using the product in patients in poor health, with diabetes, or with impaired hepatic or renal function as there is no data documenting its safety with these conditions.

Adverse Effects

As the product is in an oil-alcohol base, localized pain and inflammatory reactions (edema, skin thickening, ulceration, and localized lymph node enlargement) can be noted at the injection site. Resolution of pain generally occurs shortly after injection; other injection site reactions usually resolve within 2-4 weeks. The manufacturer recommends light massage of the injection site after administration. Larger dogs should not receive more than 5 mL at any one subcutaneous injection site. One source states that severe injection reactions can be avoided if the drug is administered into the scruff of the neck.

Systemic adverse effects reported from field trials include: anorexia (25%), excitation (23%), depression (21%), vomiting (2%), diarrhea (13%) and uterine infections (3.4%). Transient changes in hematologic (RBC, WBC indices) or biochemical (BUN, creatinine, chloride, potassium, sodium, liver enzymes) laboratory parameters were seen in <5% of dogs treated.

When used for pregnancy termination, a brown mucoid vaginal discharge can be seen approximately 24 hours before fetal expulsion. This discharge can persist for an additional 3-5 days. If used in bitches after the 20th day of gestation, abortion maybe accompanied with other signs associated with parturition (e.g., inappetance, restlessness, mammary congestion).

Bitches may return to estrus in as little as 45 days after pregnancy termination.

Overdosage / Acute Toxicity

When administered at 3X (30mg/kg) recommended doses, bitches demonstrated no untoward systemic effects. Localized reactions were noted at the injection site, presumably due to the larger volumes injected.

How to use Aglepristone

WARNING: As accidental injection of this product can induce abortion; it should not be administered or handled by pregnant women. Accidental injection can also cause severe pain, intense swelling and ischemic necrosis that can lead to serious sequelae, including loss of a digit. In cases of accidental injection, prompt medical attention must be sought.

Aglepristone dosage for dogs:

To terminate pregnancy (up to day 45):

a) 10 mg/kg (0.33 mL/kg) subcutaneous injection only. Repeat one time, 24 hours after the first injection. A maximum of 5 mL should be injected at any one site. Light massage of the injection site is recommended after administration. (Label information; Alizin — Virbac U.K.)

To induce parturition:

a) After day 58 of pregnancy: 15 mg/kg subcutaneously one time. 24 hours after aglepristone injection, give oxytocin 0.15 Units/kg every 2 hours until the end of parturition. ()

b) On or after day 58 of pregnancy: 15 mg/kg subcutaneously; repeat in 9 hours. In treated group, expulsion of first pup occurred between 32 and 56 hours after treatment. Use standard protocols to assist with birth (including oxytocin to assist in pup expulsion if necessary) or to intervene if parturition does not proceed. ()

As an adjunct to treating pyometra/metritis:

a) For closed cervix: 6 mg/kg twice daily on the first day followed by the same dose once daily on days 2, 3, and 4. Some prefer using larger doses (10 mg/kg) once daily on days 1, 3,and 8, then follow up also on days 15 and 28 depending on the bitch’s condition. ()

b) For metritis: 10 mg/kg subcutaneously once daily on days 1,2 and 8.

For open or closed pyometra: aglepristone 10 mg/kg subcutaneously once daily on days 1,2 and 8 and cloprostenol 1 meg/ kg subcutaneously on days 3 to 7. Bitches with closed pyometra or with elevated temperature or dehydration should also receive intravenous fluids and antibiotics (e.g., amoxicillin/clavulanate at 24 mg/kg/day on days 1 – 5). If pyometra has not resolved, additional aglepristone doses should be given on days 14 and 28. ()

Aglepristone dosage for cats:

For treating mammary fibroadenomatous hyperplasia: a) 20 mg/kg aglepristone subcutaneously once weekly until resolution of signs. Cats who present with heart rates greater than 200 BPM should receive atenolol at 6.25 mg (total dose) until heart rate is less than 200 BPM with regression in size of the mammary glands. ()


■ Clinical efficacy

■ For pregnancy termination: ultrasound 10 days after treatment and at least 30 days after mating

■ Adverse effects (see above)

Client Information

■ Only veterinary professionals should handle and administer this product

■ When used for pregnancy termination in the bitch, clients should understand that aglepristone might only be 95% effective in terminating pregnancy when used between days 26-45

■ A brown mucoid vaginal discharge can be seen approximately 24 hours before fetal expulsion

■ Bitch may exhibit the following after treatment: lack of appetite, excitement, restlessness or depression, vomiting, or diarrhea

■ Clients should be instructed to contact veterinarian if bitch exhibits a purulent or hemorrhagic discharge after treatment or if vaginal discharge persists 3 weeks after treatment

Chemistry / Synonyms

Aglepristone is a synthetic steroid. The manufactured injectable dosage form is in a clear, yellow, oily, non-aqueous vehicle that contains arachis oil and ethanol. No additional antimicrobial agent is added to the injection.

Aglepristone may also be known as RU-534, Alizine, or Alizin.

Storage / Stability/Compatibility

Aglepristone injection should be stored below 25°C and protected from light. The manufacturer recommends using the product within 28 days of withdrawing the first dose.

Although no incompatibilities have been reported, due to the product’s oil/alcohol vehicle formulation it should not be mixed with any other medication.

Dosage Forms / Regulatory Status

Veterinary-Labeled Products:

Note: Not presently available or approved for use in the USA. In several countries:

Aglepristone 30 mg/mL in 5 mL and 10 mL vials; Alizine or Alizin (Virbac); (Rx)

The FDA may allow legal importation of this medication for compassionate use in animals; for more information, see the Instructions for Legally Importing Drugs for Compassionate Use in the USA found in the appendix.

Human-Labeled Products: None


Perineal and Cervical Lacerations

Three natural barriers protect the uterus from bacterial contamination. The cervix, vulvar lips, and vestibular sphincter are all susceptible to trauma during foaling, and especially during obstetrical manipulations. Serious life-threatening complications, however, seldom result from these injuries.

Perineal Lacerations

Cervical Lacerations

Although cervical lacerations are most commonly associated with dystocia, they can occur during unassisted deliveries. The most common cause is a large fetus. Overzealous foaling attendants can increase the prevalence of cervical tears on a farm by applying traction to the fetal limbs before the cervix has fully dilated. Any assisted delivery should heighten awareness of the possibility of a cervical injury. Diagnosis at the time of fetal extraction is difficult because the cervix is edematous and widely dilated. Dilation of the cervical musculature results in a thin, flaccid cervix that is not readily identifiable on speculum or digital examination during the immediate postpartum period. Inability to confirm the presence of cervical tears at the time of injury is not an impediment since surgical repair should not be attempted until complete uterine involution has occurred. The acutely damaged cervical tissue will be edematous and friable. The recently gravid uterus will be heavy, and difficult to retract caudally to permit surgical accessibility. Some healing of the lacerated cervix will occur spontaneously and no medical therapy other than routine postpartum care is required.

Confirmation of a cervical tear is made during a reproductive exam approximately 30 days postfoaling. Manual examination of the cervix is required because the outline of the cervical os during speculum examination may appear normal if the mucosa is intact. This is frequently the case. Digital examination is accomplished by a combination of the thumb and index finger. Typically the clinician inserts the index finger into the cervical canal and palpates the thumb against it circumferentially around the cervix. The goal is to detect any defect of the cervical musculature as evidenced by thinning of the cervical wall. The optimal time to perform this examination is during diestrus. Progesterone supplementation can be used for 2 to 3 days before the examination to ensure maximal cervical closure. The external cervical os should provide an initial level of resistance to the forward progression of the index finger. The location of lacerations is usually reported in reference to the face of a clock, and the length is reported in centimeters. The length of the defect must be estimated because some tears may not warrant repair. Defects that involve only a small percentage of the length of the cervical canal may still provide a functional barrier to the gravid uterus. Cervical defects that involve less than 25% of the length of the cervix probably do not require surgical intervention. Defects involving between 25% to 50% of the length of the cervix may benefit from repair. Defects of 50% or greater do require repair if the mare is to have a chance of carrying a foal to term. Obviously surgical repair is only indicated if the mare is to be used for breeding. The prognosis for future fertility following surgical repair of a lacerated cervix is reported to be approximately 60%. Owners should be advised that the cervix is unlikely to dilate normally after surgical repair, and that it is likely to tear again. Close monitoring of mammary secretions (electrolyte changes) can be used to select the most optimum time for an elective cesarean section.


Stallion Behavior Problems

This post briefly outlines several of the most common behavior problems of breeding stallions. These problems include self-mutilation, inadequate libido, rowdy breeding behavior, specific erection dysfunction, mounting and thrusting difficulties, frenzied hyperactive behavior, and specific ejaculation dysfunction. Also briefly outlined is the common problem of residual stallionlike behavior in geldings.

Inadequate Libido

Specific stallion libido problems include slow starting novices, slow or sour experienced stallions, and specific aversions or preferences. Although certain genetic lines tend to be shy or quiet breeders, the majority of inadequate libido in stallions is man-made in the sense that it is the result of domestic rearing, training, or breeding conditions. Stallions that have been disciplined for showing sexual interest in mares during their performance career, discouraged from showing spontaneous erection and masturbation, or mishandled during breeding under halter are at risk of libido problems. When exposed to a mare for teasing, stallions such as these may simply stand quietly, may appear anxious and confused, or may savage the mare.

Most stallions with such experience-related libido problems respond well to behavior therapy alone or in combination with anxiolytic medication. These stallions typically respond best to continued exposure to mares, initially with minimal human presence, and then with gradual introduction of quiet, respectful, patient, positive reinforcement-based handling. These stallions appear to respond favorably to reassurance for even small increments of improvement. Tolerance of minor misbehavior rather than punishment is often the most effective strategy with low-libido stallions. The anxiolytic diazepam (0.05 mg/kg through slow IV 5-7 min before breeding) is useful in about half of such cases as an adjunct to behavior modification.

Some libido problems are hormone-related, with androgens on the low side of the normal range. These stallions will likely improve with management aimed at increasing exposure to mares and reduced exposure to other stallions. This will typically increase androgen levels, general confidence, as well as sexual interest and arousal. Gonadotropin-releasing hormone (GnRH; 50 μg SQ 2 hr and again 1 hr before breeding) can be useful to boost libido in stallions, particularly in those with low normal levels. In rare cases when more rapid improvement is required to rescue a breeding career, treatment with testosterone can effectively jump-start a slow novice without apparent significant adverse effects on spermatogenesis. Current recommendations are 0.1 to 0.2 mg/kg aqueous testosterone SQ every other day for as long as 2 weeks, with frequent assay of circulating testosterone not to exceed 4 ng/ml.

Specific Erection Dysfunction

Libido-independent erection dysfunction is rare in stallions. The majority of erection dysfunction that does occur is related to traumatic damage of the corpora cavernosa that results in insufficient or asymmetric tumescence (lateral or ventral deviations) that impairs insertion. In some instances, penile injury appears to impair sensory and or proprioceptive feedback from the penis, delaying ejaculation, coupling, or organized thrusting. Common causes include stallion ring injuries, drug-related paralyzed penis and paraphimosis, kick injuries, and self-serve breeding dummy accidents.

An interesting and often confusing type of erection dysfunction involves the folding back of the penis within the prepuce. The behavioral hallmark of this situation is a stallion that appears aroused and ready to mount, without a visible erection. The stallion may also appear uncomfortable or intermittently distracted, pinning the ears, kicking toward the groin, and/or stepping gingerly on the hind legs. Close observation reveals a rounded, full-appearing prepuce, with the skin stretched taut. Resolution usually requires removal of the stallion from the mare until the penis detumesces. Once the penis is fully withdrawn, application of a lubricating ointment to the prepuce facilitates subsequent normal protrusion. This situation tends to repeat occasionally over time, particularly in stallions with profuse smegma production or with dryness of the penis and sheath from frequent cleansing.

Mounting And Thrusting Difficulties

A significant percentage of breeding dysfunction appears to involve neurologic or musculoskeletal problems that affect the stallion’s ability to mount and thrust. Many such stallions can continue breeding with therapy aimed to reduce discomfort and accommodate disabilities during breeding, including adjustments to the breeding schedule aimed at reducing the total amount of work. This author has found that long-term treatment with oral phenylbutazone (2-3 mg/kg orally twice daily) often works well to keep such stallions comfortable for breeding. Certain debilitated stallions can benefit from semen collection while standing on the ground.

Specific Ejaculation Dysfunction

Although any libido, erection, or mounting and thrusting problem can result in failure to ejaculate, stallions also exist in which the dysfunction seems to be specific to ejaculation. Specific ejaculation problems can include apparent

failure of the neural ejaculatory apparatus, physical or psychologic pain associated with ejaculation, and genital tract pathology. Goals of therapy are to address as many contributing conditions as possible, as well as to optimize handling and breeding conditions and maximize musculoskeletal fitness and libido to enhance the stallion’s ability to overcome ejaculatory difficulty. Imipramine hydrochloride (0.5-1.0 mg/kg orally 2 hr before breeding) can effectively reduce the ejaculatory threshold.

Rowdy Breeding Behavior

Rowdy, misbehaved breeding stallions in most cases represent a human-animal interaction problem. Most problems can be overcome with judicious, skillful, respectful re-training. Even strong, vigorous, and misbehaved stallions can be brought under control by using consistent positive and negative reinforcement, with very little or no severe punishment. Re-training can be done in a safe and systematic manner without abuse or commotion, usually within a few brief sessions. Some of the most challenging, rowdy stallions may benefit from vigorous exercise under saddle or ground work immediately before breeding. This practice not only fatigues the stallion but also establishes a pattern of the stallion taking direction from a handler. For similar reasons, this author recommends an intensive schedule for breeding shed retraining, with as many as several breedings per day. With fatigue and reduced urgency to breed, many stallions seem more able to abide direction and learn a routine. With rapid repetition, stallions seem to more readily understand the routine. Tranquilization is generally not recommended. Levels of sedation that improve controllability without compromising musculoskeletal stability or ejaculatory function are difficult to achieve. Tranquilizing agents commonly used in stallions, such as xylazine or detomidine, can both facilitate and inhibit erection and ejaculation depending on dose.

Frenzied Behavior

Distinct from simple rowdiness, some stallions are hyperactive or even frenzied. This is typically greater during the breeding season. Some will spend nearly their entire time budget frantically “climbing the walls,” or running a fence line. In general frenzied breeding stallions can benefit from more roughage and less grain in the diet, organized physical work and pasture exercise, and consistent housing in a quiet area. Careful observation (particularly video surveillance) can be useful to identify environmental conditions and events that set off episodes or tend to quiet a stallion. In extreme cases, pasturing directly with mares can effectively quiet or sensibly occupy a frenzied stallion. L-Tryptophan supplementation (1-2 g twice daily in feed) can have a calming effect on such stallions. Tranquilization for this purpose is not recommended in breeding stallions because of risk of paralyzed penis and paraphimosis.


Although not unique to stallions, self-mutilation is a severe and relatively uncommon fertility limiting and/or life-threatening problem. This behavior typically takes the form of self-biting of the flank, chest, or limbs, with violent spinning, kicking, and vocalization. Self-mutilation in horses appears to occur in two distinct forms. One appears to be a severe reaction to irritation or pain, and would be similar in males or females. The self-biting is typically targeted toward the site of discomfort. Another form occurs in males and is reminiscent of stallion intermale aggression. The behavior is targeted at the typically intermale sites of aggression — the groin, flank, knees, chest, and hocks. The sequence of the behavior follows closely to that of two males fighting, with sniffing and nipping of the groin, vocalization, stamping with a fore leg, kicking out with a hind leg, and then taking occasional larger bites from anywhere on the opponent’s body.

Episodes often appear to be stimulated by sight, sound, or smell (feces or oily residues) of another stallion. For some stallions, episodes are set off by sniffing their own excrement or oily residues on stall walls or doorways. Current recommendations to control episodes are as follows: (1) physically protect the stallion from injury by padding walls or limbs, blanketing, and muzzling as effective; (2) aggressively evaluate the housing and social environment to identify exacerbating and ameliorating conditions that may be manipulated for greatest relief; (3) reduce concentrates and increase grass and hay in the diet to increase feeding time and eliminate highly palatable meals (feeding tends to distract and occupy the stallion; concentrate meals tend to increase stereotypic behavior); (4) apply odor-masking agents (Vicks or Acclimate) around the nares; and (5) provide as much organized exercise as possible, also to distract the stallion.

Residual Stallionlike Behavior In Geldings

Castration, regardless of age or previous sexual experience, does not always eliminate stallionlike behavior in horses. If given the opportunity, as many as half of geldings will show stallionlike behavior to mares, many will herd mares, and even mount and appear to breed. Similarly, although castration does tend to “mellow” most horses, it does not eliminate general misbehavior. Traditional behavior modification is usually much more effective in the control of sexual and aggressive behavior in a gelding under saddle or in-hand than it is with an intact stallion. Also, treatment aimed at quieting sexual and aggressive behavior, such as progesterone (e.g., altrenogest, 50-75 mg orally daily), is typically more effective in geldings than in intact stallions. Elimination of stallionlike herding and teasing at pasture is difficult. Separation from mares is recommended.


Placentitis: Clinical Signs and Diagnosis

Clinical signs include those observed in mares with pending abortion. Udder development, premature lactation, and cervical softening are often seen before the mare aborts. Vaginal discharge may or may not proceed abortions. Once clinical signs develop, the disease has reached an advanced stage and treatment may not always be successful.

Evaluation of the equine placenta should routinely be performed after abortion or parturition. In aborting mares with an ascending placentitis, the pathologic lesions are characteristic. An area of the chorion adjacent to the cervical star is depleted of chorionic villi and is thickened, discolored, and covered by fibronecrotic exudate (). Placentitis caused by N. actinomycete causes characteristic lesions at the ventral aspect of the base of the gravid horn and nongravid horn at the junction between the body and the horn of the placenta. The chorionic surface is thickened and covered with brown-red, thick, mudlike material. Placentitis caused by a hematogenous route of infection shows less characteristic multifocal lesions of the chorionic surface of the placenta. A thorough inspection of the placenta is necessary to ensure that any existing lesions are found. Examination of the placenta postpartum provides excellent information on disease processes or dysfunctions that could have affected the well being of an aborted fetus, or that may potentially cause illness in the neonatal foal. However, this examination does not aid the clinician in decisions that are aimed to prevent abortion or neonatal diseases of the foal. Evaluation of the placenta in the pregnant mare must be performed by the use of ultrasonography and endocrine tests.

Ultrasonographic Evaluation of the Equine Placenta

Transabdominal Ultrasonography

Ultrasonographic examinations of the placenta in mares that are considered to be at risk for abortion during late gestation can be performed by a transabdominal approach (Figure 5.24-2). With a 5- or 7.5-MHz sector scanner, four quadrants of the placenta should be examined; right cranial, right caudal, left cranial, and left caudal. With this technique, mares with normal pregnancies should have a rrunimal combined thickness of the uterus and the placenta (CTUP) of 7.1 +/-1.6 mm, and a maximal CTUP of 11.5 +/-2.4 mm. Pregnancies with an increased CTUP have been associated with the delivery of abnormal foals. The caudal portion of the allantochorion cannot be imaged with transabdominal ultrasonography, which prevents the clinician from diagnosing ascending placentitis in its early stages. However, placental thickening and partial separation of the allantochorion from the endometrium may be observed with transabdominal ultrasonography in mares that have placentitis originating from a hematogenous infection. In addition, a pocket of hyperechoic fluid can be seen at the base of the lowest area of the gravid uterus in mares with placentitis caused by N. actinomycete. Mares that graze on endophyte-infected fescue often experience retained placenta, premature separation of the allantochorion, and increased allantochorion weight and thickness. A significant increase in uteroplacental thickness and premature separation of the allantochorion has been found on transabdominal ultrasonographic examination of endophyte-infected mares. However, the thickness was not observed until an average of 8 hours before the onset of labor.

Transrectal Ultrasonography

Transrectal ultrasonography of the caudal allantochorion in late gestational mares provides an excellent image of the placenta close to the cervical star (). A 5-MHz linear transducer should be positioned 1 to 2 inches cranial of the cervical-placental junction, and then moved laterally until the middle branch of the uterine artery is visible at the ventral aspect of the uterine body. The CTUP should then be measured between the middle branch of the uterine artery and the allantoic fluid (). The clinician has to make sure that the amniotic membrane is not adjacent to the allantochorion, because this may result in a falsely increased CTUP. The CTUP should be measured in the ventral part of the uterine body. The CTUP in the dorsal part of the uterus is often thicker than in the ventral part of the uterus. In addition, placental parts of the dorsal uterus have often been found to be edematous in normal pregnant mares during the last month of gestation (). Normal values for CTUP are illustrated in Table Normal Upper Limits for the Combined Thickness of the Uterus and the Placenta during Late Gestation. An abnormal thickness and partial separation of the allantochorion from the endometrium has been observed on ultrasonographic examination in mares with clinical signs of ascending placentitis (). A CTUP greater than 8 mm between day 271 and 300, less than 10 mm between day 301 and 330, and greater than 12 mm after day 330 has been associated with placental failure and pending abortion. In advanced stages, the space between the uterus and the placenta is filled with hyperechoic fluid.

Table Normal Upper Limits for the Combined Thickness of the Uterus and the Placenta during Late Gestation

Cestational Period CTUP
151-270 days <5 mm
271-300 days <7 mm
301-330 days <9 mm
331 + <12 mm

Although transrectal and transabdominal ultrasonographic examination of the placenta is useful to detect early signs of some placental pathology, the clinician should keep in mind that placental changes resulting in periparturient problems can be subtle and not readily detected on ultrasonographic examination.

Endocrine Monitoring of the Placenta


The equine placenta is part of an endocrine fetal-placental interaction that synthesizes and metabolizes progestogens. This endocrine function of the placenta is important for maintenance of pregnancy after the endometrial cups and the secondary corpora lutea disappear at approximately day 150 of gestation. Mares with advanced stages of placentitis or placental separation may have increased plasma concentrations of progestogens as a result of stress to the fetal placental unit. In contrast, circulating progestogens have been reported to fall below normal after fetal hypoxia and infection with equine herpesvirus. Although increased concentrations of plasma progesterone during mid and late gestation would suggest placentitis, therapeutic decisions should not be made on the basis of one sample. Serial blood samples need to be obtained from an individual mare in order to detect a clinically useful trend in progesterone concentrations. Fetal-placental progesterone is rapidly metabolized to 5-pregnanes, and the metabolites may not be recognized by commercial progesterone assays. Therefore maternal serum progesterone concentrations in late pregnant mares do not accurately reflect the conditions in the uterus. Monthly blood sampling of mares at risk of abortion showed no differences in plasma progesterone concentrations in mares with impending abortion and mares with normal pregnancies.


Both estradiol and conjugated estrogen (estrone sulfate) are elevated during late pregnancy in mares. Estrone sulfate in maternal serum is thought to be a marker of fetal well-being. However estrogens have not been useful to detect early signs of placentitis.


Relaxin is produced by the equine placenta and can be detected in peripheral blood plasma from day 80 of gestation and throughout the pregnancy. The role of relaxin during pregnancy is not fully understood, but some evidence exists that placental relaxin production is compromised in mares at risk of aborting their fetuses. No commercial test for equine relaxin is currently available, and more research needs to be performed to evaluate the usefulness of plasma relaxin as a clinical tool to diagnose placentitis and to monitor the efficacy of treatment strategies.


Oocyte Transfer

Oocyte transfer is the placement of a donor’s oocyte into the oviduct of a recipient. The recipient can be inseminated within the uterus or within the oviduct. Placement of the oocyte and sperm within the recipient’s oviduct is more accurately termed gamete intrafallopian transfer (GIFT).

The first successful oocyte transfer was done in 1989; however, the technique was not used for commercial transfers until the late 1990s. Oocyte transfer is currently used to produce offspring in subfertile mares in which embryo transfer is not successful because of various reproductive problems. These problems include ovulatory failure, oviductal blockage, recurrent or severe uterine infections, and cervical tears or scarring. In some cases, the cause of reproductive failure cannot be diagnosed; however, oocyte transfer can be successful.

Sychronization Of Donors And Recipients

Oocytes are collected from preovulatory follicles between 24 and 36 hours after the administration of human chorionic gonadotropic (hCG; 1500-2500 IU, IV) to a donor mare or between 0 and 14 hours before anticipated ovulation. Criteria for hCG administration are as follows:

• Follicles greater than 35 mm in diameter

• Relaxed cervical and uterine tone

• Uterine edema or estrous behavior present for 2 or more days

Some mares, especially older mares, do not consistently respond to hCG. In these cases, this author uses a combination of the gonadotropin-releasing hormone (GnRH) analog, deslorelin acetate (2.1 mg implant; Ovuplant), followed by an injection of hCG (2000 IU, IV) between 4 and 5 hours later.

Oocytes collected 36 hours after hCG administration to the donor are transferred immediately into a recipient’s oviduct. Oocytes collected 24 hours after drug administration to the donor are cultured in vitro for 12 to 16 hours before transfer. The advantage of collection of oocytes between 32 and 36 hours after hCG administration to the donor is that the oocytes do not require culture in vitro. However, donors could ovulate follicles before oocytes are collected. The collection and culture of oocytes at 24 hours after hCG administration to the donor are often easier to schedule; the oocyte can be collected in the afternoon and transferred the next morning. This method requires expensive equipment and training for tissue culture, however. In a modification of these procedures, oocytes are collected 24 hours after hCG and immediately transferred into the recipient’s oviduct to allow maturation to complete within the oviduct. With this latter method, recipients are inseminated 16 hours after transfer.

Oocyte Collection

Oocytes are usually collected by one of two methods. In one method, the ovary is held per rectum against the ipsilateral flank of the mare. A puncture is made through the skin and a trocar is advanced into the abdominal cavity. The ovary is held against the end of the trocar while a needle is advanced through the trocar and into the follicular antrum.

In this author’s laboratory oocytes are collected by using transvaginal, ultrasound-guided follicular aspirations. For this procedure, a linear or curvilinear ultrasound transducer is used with the transducer housed in a casing with a needle guide. Before the procedure, the rectum is evacuated and the vulvar area is cleaned. The mare is sedated (xylazine HC1, 0.4 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV) and a substance to relax the rectum (propantheline bromide, 0.04 mg/kg, IV) is administered. A twitch is applied. The probe is covered with a nontoxic lubricant and placed within the anterior vagina lateral to the posterior cervix and ipsilateral to the follicle to be aspirated. The follicle is positioned per rectum and stabilized with the apex of the follicle juxtaposed to the needle guide. A needle is advanced through the needle guide to puncture the vaginal and follicular walls. In this author’s laboratory, a 12-gauge, double-lumen needle is used (Cook Veterinary Products, Spencer, Ind.). The follicular fluid is aspirated from the follicle by using a pump set at a pressure of -150 mm Hg. After removal of follicular fluid, the lumen of the follicle is lavaged with 50 to 100 ml of flush (typically modified Dulbecco’s phosphate-buffered solution or Emcare [ICP, Auckland, New Zealand]) that contains fetal calf serum (1%) or bovine serum albumin (0.4%) and heparin (10 IU/ml).

Equipment used to handle the oocyte is warmed to 38.5° C before use because the oocyte is sensitive to temperature changes. On collection, the follicular aspirate and flush are poured into large search dishes and examined under a dissecting microscope to locate the oocyte. Aspirations of preovulatory follicles are often bloody because the follicle has increased vascularity as ovulation approaches. The oocyte is approximately 100 μm in diameter and is surrounded by a large mass of nurse ceils — the cumulus complex. Cumulus cells, or the corona radiata, appear as a ring surrounding the oocyte. When the oocyte matures, the cumulus complex becomes less distinct. The corona radiata appears clear in the bloody flush solution and can be observed by the naked eye.

Oocyte Evaluation And Culture

On collection, cumulus oocyte complexes (COC) are evaluated for cumulus expansion (graded from compact to fully expanded) and for signs of atresia. Oocytes are determined to be in a stage of atresia when the COC is clumped and/or sparse, the corona radiata is fully expanded, or when the ooplasm is shrunken and dark or severely mottled. Oocytes with a fully expanded cumulus (marked separation of cumulus cells with expansion of the corona radiata) are considered mature and are transferred as soon as possible into a recipient’s oviduct. Oocytes with a moderately expanded cumulus complex (translucent COC with moderate separation of cumulus cells and incomplete expansion of corona radiata) are cultured before transfer. On occasion, the donor does not respond to hCG and the follicle does not begin to mature. Consequently, the granulosa cells that line the follicle are collected in small, compact sheets, and the oocyte is frequently not retrieved. If an immature (compact COC with little or no separation of cumulus cells) oocyte is collected, special culture conditions are required, including a maturation medium with additions of hormones and growth factors.

On identification and evaluation, the oocyte is washed and placed in a transfer or collection medium. A commonly used medium for the culture of maturing oocytes is tissue culture medium (TCM) 199 with additions of 10% fetal calf serum, 0.2 mM pyruvate, and 25 mg/ml gentamicin sulfate. A carbon dioxide (C02) incubator must be used to establish the proper culture conditions of 38.5° C in an atmosphere of 5% or 6% C02 and air.

Oocyte Transfer

Mares that will receive oocytes should be young (preferably 4-10 years of age) with a normal reproductive tract. Oocytes are transferred surgically; therefore, adequate exposure of the ovary is essential to facilitate transfers. Mares with short, thick flanks and short broad ligaments are not good candidates for recipients. Both cycling and noncycling mares have been used as oocyte recipients. When cyclic mares are used, they must be synchronized with the donor; thus, hCG is administered to the estrous donor and recipient at the same time of day. Before the mare can be used as a suitable recipient, her own oocyte must be aspirated. Anestrus and early transitional mares are suitable noncyclic recipients. During the breeding season, a high dose of a GnRH analog or injections of progesterone and estrogen (150 mg progesterone and 10 mg estradiol) can be administered to reduce follicular development in potential recipients. Noncyclic recipients are given injections of estradiol (2-5 mg daily for 3-7 days) before transfer and progesterone (150-200 mg daily) after transfer. In mares that are not having estrus cycles, pregnancies must be maintained through the use of exogenous progesterone.

Oocytes are transferred through a flank laparotomy into standing sedated mares. Recipients are placed in a stock and a presurgical sedative (xylazine HC1, 0.3 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV) is administered. The surgical area is clipped, scrubbed, and blocked with a 2% lidocaine solution. Immediately before surgery, additional sedation is administered (detomidine HC1, 9 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV). An incision is made through the skin approximately midway between the last rib and tuber coxae, and the muscle layers are separated through a grid approach. The ovary and oviduct are exteriorized through the incision. The oocyte is pulled into a fire-polished, glass pipette, and the pipette is carefully threaded into the infundibular os of the oviduct and advanced approximately 3 cm. The oocyte is transferred with less than 0.05 ml of medium.

Insemination Of Recipients

In a commercial oocyte transfer program, use of stallions with good fertility is essential. Cooled and transported semen is often provided. When fresh semen from fertile stallions and oocytes from young mares was used in different experiments, insemination of the recipient only before (12 hours) or only after (2 hours) oocyte transfers resulted in embryo development rates of 82% (9/11) and 57% (8/14), respectively. In a commercial oocyte program, mares were older with histories of reproductive failure and cooled semen from numerous stallions of variable fertility was used. Pregnancy rates when recipients were inseminated before or before and after oocyte transfer were significantly higher than when recipients were only inseminated after transfer (18/45, 40%; 27/53, 51% and 0/10, respectively). These results suggest that the insemination of a recipient before transfer with 5 X 108 to 1 x 109 progressively motile sperm from a fertile stallion is sufficient. However, if fertility of the stallion is not optimal, insemination of the recipient before and after transfer may be beneficial.

After insemination and transfer, the recipient’s uterus is examined by ultrasonography to detect intrauterine fluid collections. The uterine response to insemination often appears to be more severe when recipients are inseminated after transfer than when they are inseminated only before transfer. The uterus is evaluated and treated once or twice daily until no fluid is imaged. Recipients with accumulations of intrauterine fluid are treated similar to ovulating mares, with administration of oxytocin or prostaglandins to stimulate uterine contractions or with uterine lavage and infusion.

Future Of Oocyte Transfer

Oocyte transfer has proved to be a valuable method of obtaining pregnancies from mares that cannot carry their own foal or produce embryos for transfer. Because the mare does not have to ovulate or provide a suitable environment for fertilization or embryo development, the oocyte donor is only required to develop a preovulatory follicle with a viable oocyte.

The transfer of oocytes and a low number of sperm (200,000 motile sperm) into the oviduct of recipients has been successful. Pregnancies could be produced with GIFT when sperm numbers are limited, such as from subfertile stallions and from sex-selected or frozen sperm.

Through the use of this technique at this author’s laboratory, pregnancies have been recently produced from oocytes that were frozen and thawed and from oocytes that were collected from the excised and shipped ovaries of dead mares. These advances provide excellent methods to preserve the genetics of valuable mares.


Selection And Management Of Recipients

Selection and management of recipient mares for an embryo transfer program is the most important factor affecting pregnancy rates. On farms handling only one or two donors, recipient mares may be purchased from local backyard horse owners who are familiar with the mare’s reproductive history. However, acquiring a large number of recipient mares requires that mares be purchased from local sale barns. Thus the reproductive history of these mares is unknown. In either case the recipient mare should meet the following criteria: 900 to 1200 pounds; 3 to 10 years of age; and broken to halter. The effect of size of recipient on the subsequent size of the foal has not truly been determined. However, the size of the donor mare should be matched with the recipient as nearly as possible. This may be difficult when obtaining embryos from large warmbloods or draft horses.

Typically nonlactating mares are easier to use in an embryo transfer program than a mare that is lactating. If a lactating mare is not being used, the animals should not be used as recipients until at least the second postpartum cycle. Numerous types of recipient mares can be used: ovarian-intact cycling mares; ovariectomized mares; mares in anestrus; and mares during the transitional period. This author prefers to use ovarian intact normal cycling mares. However, pregnancy rates using ovariectomized, progesterone-treated mares have been shown to be similar to ovarian-intact mares.

Occasionally, early in the year a scarcity of normal cycling mares occurs. The alternative at that time of the year is to use either an anestrous mare or a transitional mare. In this author’s experience transitional mares are more appropriate to use than truly anestrous mares. Mares in transition should be selected based on the presence of endometrial folds. This indicates that estrogen is being secreted. Transitional mares can then be placed on progesterone at the time of the donor mares ovulation. The suggested progestin treatment for either ovariectomized mares or transitional mares includes altrenogest (Regumate) daily or 150 mg of progesterone injected daily. With the use of ovariectomized mares, progesterone treatment must continue until the placenta begins to produce progesterone at approximately 100 to 120 days. With transitional mares, progesterone treatment may be terminated once the mare has ovulated and developed secondary corpora lutea during early gestation.

The recipient mare should be examined by rectal palpation and ultrasonography before purchase. The external genitalia are observed for normal conformation. Those mares with poor external conformation that may predispose them to wind sucking are generally rejected. Mares are then palpated per rectum and the size and tone of the uterus, cervix, and ovary are determined. The uterus and ovary are then examined with ultrasonography. Evidence of pathology such as uterine fluid, uterine cyst, ovarian abnormalities, or the presence of air or debris in the uterus would render the mare unsuitable for purchase as an embryo recipient. In addition, any mare found to be pregnant is not purchased unless the pregnancy is less than 30 days.

Approximately 15% to 20% of the mares initially presented are rejected. Mares that pass the initial examination are given a breeding soundness exam similar to the exam of the donor mare. Recipients are vaccinated for influenza, tetanus, and rhinopneumonitis and are quarantined from other mares for at least a period of 30 days. Those mares that are in thin condition are fed a concentrate ration and a free-choice alfalfa hay. The majority of recipients are purchased in late fall and placed on a 16-hour lighting regimen beginning December 1. Starting approximately February 1, mares are palpated and examined with ultrasonography twice per week until a follicle greater than 35 mm is obtained. Mares with follicles greater than 30 mm are examined daily with ultrasonography until ovulation. Ideally, recipient mares should have one or two normal estrous cycles prior to being used as a recipient. Mares are excluded as potential recipients if they consistently have erratic or abnormal estrous cycles.

Hormonal manipulation of the recipient mare’s estrous cycle is an important component of an embryo transfer program. The degree of hormonal manipulation is dependent upon the size of the embryo transfer operation. Smaller operations that deal with only one or two donors use more hormonal manipulation than larger operations that may have a large number of donors and recipients. Small operations should place the donor and one or two recipients on progesterone for 8 to 10 days and then administer prostaglandins on the last day of treatment. The progesterone can either be altrenogest used daily or injectable progesterone at a level of 150 mg daily for the same length of time. It is not uncommon to use a combination of progesterone and estrogen (150 mg progesterone, 10 mg estradiol-17β) daily for 8 to 10 days followed by prostaglandins.

The donors and recipients will ovulate 7 to 10 days after prostaglandin treatment. Generally, having the recipient ovulate either 1 day before or up to 3 days after the donor mare is desirable. This can be accomplished by using hCG (Chorulon) or GnRH (Ovuplant) to induce ovulation in either the recipient or donor mare to provide optimal synchrony of ovulation. In a larger embryo transfer station it is common to manipulate the cycle by using only prostaglandins, hCG, or GnRH. Typically the ovulation dates of the recipient are recorded and once a donor mare ovulates then a recipient is selected that has ovulated either 1 day before or up to 3 days after the donor. If a mare is not used as the recipient she is then given prostaglandins 9 or 10 days after her ovulation and induced to return to estrus.

Each recipient mare is given a final examination 5 days after ovulation before to being used as the recipient. Mares are classified as acceptable, marginal, or nonacceptable based on this 5-day exam. The 5-day exam includes palpation per rectum for uterine and cervical tone, and ultrasonography of the uterus and ovaries. An acceptable recipient should have a round, tubular, firm uterus and a closed cervix. She also would have the absence of endometrial folds, a normal sized uterus, and the presence of a visible corpus luteum. Mares generally are placed in the marginal category based on a decrease in uterine tone or cervical tone or perhaps the presence of grade 1 endometrial folds. Unacceptable recipients typically have poor uterine tone, a soft-open cervix, or presence of endometrial folds and/or fluid in the uterus. A retrospective examination of this author’s commercial embryo transfer program has revealed that the 5-day check is the best predictor of whether or not a recipient mare will become pregnant.

Embryos are transferred either surgically by flank incision or nonsurgically. Most of the embryo transfer stations are now using nonsurgical transfer methods. The details of the transfer methods are presented in the subsequent chapter. Mares are examined with ultrasonography for pregnancy detection 4 or 5 days after transfer. Mares that are diagnosed pregnant are reexamined on days 16, 25, 35, and 50. Mares not confirmed pregnant on the initial examination (day 12) are reexamined 2 days later. If the ultrasound scan continues to be negative the mare is considered not pregnant and given prostaglandin to induce estrus. Unless the embryo was extremely small (<150 microns) the majority of mares that are to be pregnant have a visible vesicle at 12 days of gestation. Those mares in which the vesicle does not appear until 14 or 16 days of gestation have delayed embryonic development and are more likely to suffer early embryonic loss. The initial ultrasound examination allows the breeder to decide whether to rebreed the donor and attempt a second embryo recovery. The ultrasound exam at 25 days determines whether a fetus is present with a viable heartbeat. The majority of losses that do appear in embryo transfer recipients occur between days 12 and 35. However, early embryonic loss before 50 days of gestation appears to be no greater in embryo transfer recipients than other pregnant mares that are inseminated with either fresh or cooled semen. Mares that fail to become pregnant after an embryo transfer are generally used a second time but not a third. The pregnancy rates on mares receiving an embryo on a second attempt are no different than those that receive an embryo only one time and become pregnant.

Pregnant recipients should be fed a maintenance ration similar to other broodmares during the first two thirds of gestation and then administered extra energy in the form of concentrate rations during the final one third of pregnancy. Recipients should be monitored closely around the time of impending parturition. Management procedures identical to those used for foaling broodmares should be used. No greater difficulty in foaling embryo transfer recipients than normal broodmares has been found. The influence of the size of the recipient versus size of donor on ease of foaling has not been adequately studied, although this does not appear nearly as critical in horses as it does in cattle.

In summary, a relatively high pregnancy rate can be anticipated in an embryo transfer program if management of the donor and recipient mares are maximized. Attention should be given to selection of both donor and recipient, nutrition, proper monitoring of the donor and recipient with palpation per rectum and ultrasonography, careful assessment of the recipient, and management of the recipient after embryo transfer. Day 12 pregnancy rates for either fresh or cooled semen should be 75% to 80% and those at 50 days of gestation should be 65% to 70%.


Mare Behavior Problems

This post briefly outlines the four most common complaints concerning reproductive behavior in mares: (1) failure to show estrus or to stand for breeding, (2) maternal behavior problems, (3) stallionlike behavior, and (4) estrus cycle-related performance problems in mares.

Failure To Show Estrus Or To Stand For Breeding

Research and clinical experience consistently indicate that most mares show estrus, or some detectable and reliable change in behavior consistent with estrus, in association with ovulation. A stallion given free access to the mare probably would have no difficulty detecting estrus and proceeding with normal breeding. A trained and careful observer would see changes in response to prolonged interaction with a male. Therefore “failure to show estrus” or “silent ovulation” in most cases represents management failure to adequately elicit and/or detect estrus under farm conditions. Difficulty detecting estrus also is complicated in certain individual mares that may naturally show good estrus for only a few hours.

The recommendation for detection of estrus in mares is teasing for at least 5 minutes, preferably with the mare at liberty to approach the stallion, along a fence line or with the stallion in a teasing pen. This enables a fuller range of mare estrus behavior and avoids submissive behavior evoked by forced encounter with the stallion. Sometimes it helps to tease with two or more stallions (sequentially for at least 5 min each).

Some mares show estrus during teasing and then fail to stand for mounting. Normal fertile mares pastured with stallions often are observed to go through periods of alternating solicitation and rejection of the stallion. This natural tendency for ambivalence may account for some of failure to stand for breeding in hand-breeding. Another factor in failure to stand for breeding appears related to severe restraint of the mare and limited precopulatory interaction with the stallion at the time of breeding.

Maternal And Foal Behavior And Problems

Stallionlike Behavior

Heterotypical behavior, that is, abnormal behavior typical of the opposite sex, in mares includes fighting with stallions; elimination-marking behavior (olfactory investigation, flehmen, and marking of excrement); herding teasing; and mounting mares. It is caused by exposure to androgens or high levels of estrogens that convert to androgen. The most common source of androgens in mares are granulosa cell tumors and administered steroids. Removal of the source of androgens generally leads to cessation of stallionlike behavior within weeks to months.

Stallionlike behavior occasionally is observed during mid pregnancy. At one time this was attributed to androgens in a male fetus, but it has been observed since in mares carrying females.

Estrus Cycle-Related Performance Problems

Temperament and performance of mares can vary with the ovarian cycle, with some mares showing more or less desirable behavior during diestrus, estrus, or anestrus. Complaints require careful, detailed analysis of the specific desirable and undesirable behavior in relation to ovarian activity. Careful evaluation of complaints may reveal a physical, handling, or training problem that may be either unrelated to the ovarian cycle or that may worsen with estrus as many physical problems do. In evaluating complaints, a common finding is that owners and trainers are unaware of the specific behavioral elements of estrus and diestrus, often confusing the two states, and sometimes assuming estrus equals bad behavior. When it is confirmed that problem behaviors are associated with the ovarian cycle, improvement can be achieved with suppression or manipulation of the cycle using progesterone, hCG, and prostaglandin as recommended in “Induction of Ovulation.” A large percentage of such complaints involve submissive cowering, leaning away, and urine squirting that are easily misinterpreted as estrus. This pattern of behavior often is called “starting gate estrus” because it is common in young anxious race fillies in the starting gate.


Enlarged Ovaries

Anovulatory Follicles

Large, anovulatory follicles are a normal finding during the spring and fall transition periods. Anovulatory follicles can exceed 10 cm in diameter and may persist for several weeks. The cause is likely to be abnormal estrogen production by the follicle and/or insufficient release of pituitary gonadotropin to induce ovulation. Often the ultrasonographic image reveals scattered free-floating echogenic spots as a result of the presence of blood in the follicular fluid (hemorrhagic follicles). In others are echogenic fibrous bands resulting from gelatinization of the hemorrhagic fluid. Although human chorionic gonadotropin (2500 IU IV) or a GnRH implant may induce ovulation, in most cases the treatment is ineffective. Fortunately most of these anovulatory follicles spontaneously regress within 1 to 4 weeks. Breeding a mare in anticipation of ovulation of a persistent follicle is unwise because fertility of the aged oocyte is likely to be poor.

Clinicians should be aware that not all palpable and ultrasonographically imaged structures around the ovary have to be follicles. Fossa cysts and parovarian (fimbrial) cysts can be found in many mares as an incidental finding.

These structures tend to arise from remnants of the embryonic (mullerian and wolffian) duct systems. If they are of a significant size they should be noted on the mare’s breeding records, but they generally are not associated with any reduction in fertility. Theoretically an excessively large cyst could interfere with ovulation or oocyte transport.


During the physiologic breeding season in a healthy, non-pregnant mare, a surge of luteinizing hormone from the anterior pituitary results in rupture of the mature follicle (ovulation). Normally some hemorrhage from blood vessels in the theca layer occurs, and this results in a soft, intermediate structure — the corpus hemorrhagicum. Immediately after ovulation a depression may be palpable, but this is soon replaced by the developing corpus luteum. The theca cells and invading granulosa cells become luteinized such that the serum progesterone level is elevated until endometrial prostaglandin brings about luteolysis.

A hematoma is the most likely explanation for a unilateral ovarian enlargement during the physiologic breeding season. Excessive postovulatory hemorrhage is not uncommon. The former follicle can become distended markedly. Treatment is not indicated because the structure is essentially an abnormally large corpus hemorrhagicum. Behavior will be normal. The mare continues to have regular estrous cycles, and the opposite ovary remains functional. Serum hormone levels are normal. The hematoma resolves over a period of several weeks, and normal ovarian function can be expected to return in most cases.


Although an ovarian tumor could begin development during pregnancy, the most likely explanation for ovarian enlargement and abnormal behavior during this time is normal physiologic events. Secondary corpora lutea tend to cause bilateral ovarian enlargement after approximately day 40 of gestation. Expressions of estrus and stallion-like or just aggressive behavior can occur during pregnancy. The large fetal gonads are a significant source of testosterone. Obviously progesterone from the corpora lutea and progestins from the placenta are present. By 2 to 3 months of gestation, testosterone levels can exceed 100 pg/ml and then continue to rise until about 6K months. The testosterone concentrations then gradually decline to basal levels at parturition.

Granulosa Cell Tumors

In a normal ovary the granulosa cells line the inside of follicles, whereas the theca cells surround the outside of the follicle. The theca cells produce testosterone. Both the granulosa and theca cells are involved in the steroidogenic pathway that leads to estradiol production. The granulosa cells also produce the protein hormone, inhibin.

The granulosa cell tumor (GCT) is the most common tumor of the equine ovary. These tumors tend to be unilateral, slow growing, and benign. In fact, they can develop during pregnancy. If a GCT is detected at the foal heat, it may be possible to remove the ovary and have the mare bred back later that season. This depends on the time of year that the mare foals and also the degree of follicular suppression present in the contralateral ovary.

Although GCTs are steroidogenically active, the hormonal milieu can vary from case to case. This affects the amount of follicular activity on the contralateral ovary and the type of behavior being exhibited. Typically the opposite ovary is small and inactive, but occasionally a GCT presents on one ovary while a corpus luteum is on the other. Owners may report that the mare has failed to exhibit estrous behavior (prolonged anestrus) or that it is continuously displaying signs of being in estrus (nympho-mania). A dangerous side effect in some mares is aggressive behavior towards the handler. These mares tend to exhibit stallion-like behavior and may develop a crested neck and clitoral hypertrophy if the tumor has been present for some time.

Loss of the characteristic kidney-bean shape is usually a good indication that a tumor may be present in a small ovary (). Often the ovary is too large to be palpated thoroughly. In both instances the characteristic multicystic (honeycomb) image on an ultrasound examination can support the diagnosis (). Occasionally the GCT may present as a large unilocular cyst ().

The ultrasonographic diagnosis can be supported by hormonal assays if necessary (Table Hormonal Concentrations in Mares with a Granulosa Cell Tumor). Most GCT appear to secrete sufficient inhibin to suppress pituitary release of follicle-stimulating hormone (FSH), and this probably explains the typical suppression of follicular activity on the contralateral ovary. If a significant theca cell component exists in the tumor then the serum testosterone level is elevated, and these mares are more likely to be aggressive and exhibit stallion-like behavior. Although progesterone levels tend to be low (<1 ng/ml) in affected mares, in some instances cyclic activity may continue in the presence of a GCT.

Table Hormonal Concentrations in Mares with a Granulosa Cell Tumor

Hormone Diagnostic Level Incidence
Testosterone More than 50 to 100 pg/ml 50%-60% of cases
Inhibin More than 0.7 ng/ml -90% of cases

Indications for removal of these benign tumors include breeding purposes, behavioral problems, and in some cases colic episodes. Diagnosis must be certain because a histopathologic diagnosis of normal ovarian tissue can be difficult to explain to an owner once the ovary has been removed. Veterinarians must explain to owners that not all behavioral problems are ovarian in origin. An endometrial biopsy and cervical evaluation are recommended if the mare is to be used for breeding purposes. Although the abnormal hormonal environment can cause reversible changes in the density of the endometrial glands, chronic degenerative changes including fibrosis limit the mare’s ability to carry a foal to term. The affected ovary can be removed by several surgical approaches, depending on the size of the GCT and the preference of the surgeon. Options for ovariectomy include laparoscopy, colpotomy, and flank and ventral midline laparotomy. The time until subsequent ovulation on the remaining ovary can vary tremendously, and owners should be advised that it might take up to 6 to 8 months.

Other Ovarian Tumors

Although they are rare, teratomas are the next most common ovarian tumor after a GCT. They are also unilateral but are not hormonally active and do not alter the mare’s behavior. The opposite ovary remains active and the mare exhibits normal estrous activity during the physiologic breeding season. A teratoma is a germ cell tumor and may contain cartilage, bone, hair, mucus, and other tissues. The surface of the ovary tends to be sharp and irregular on palpation, and the varying density of the aberrant tissues causes abnormal shadows on the ultrasound image (). Although an ovarian teratoma generally is thought of as being benign, this author has reported on one malignant case that had metastasized to several organs.

Even more rare tumors of the equine ovary include cystadenomas and dysgerminomas. Cystadenomas tend to be benign, whereas dysgerminomas may be malignant. They are both unilateral and hormonally inactive. Thus the contralateral ovary and behavior are normal. The ultrasonographic image of a cystadenoma can resemble that of multiple follicular activity. The same considerations for surgical removal apply as for GCT.


Foal Heat-Breeding

Commercial horse breeding farms operate under the same economic rules as other forms of intensive livestock production. Use of stallions must be efficient, and mares must produce the greatest number of foals possible. This requires intensive management. The primary reason for foal heat-breeding is to increase the efficiency of the production unit.

The performance of a herd of mares should be measured not only by the final conception rate but also by how efficiently that pregnancy rate was achieved. This can be measured by the number of breedings per conception and by noting when in the breeding season each mare becomes pregnant. A set of criteria can be established such that the performance of individual mares and the herd can be measured against the ideal. The criteria can be flexible and should be tailored to meet the economic objectives of the operation. For example, the time at which the farm would like the earliest foal to be born is determined by the management practices of that farm and by the requirements of a particular breed registry. Once these criteria are established, a management goal can be set. In central Kentucky the main commercial business is the production and sale of Thoroughbred yearlings. Thus a workable criterion is that no mare is bred before the fifteenth of February. Optimal fertility dictates that no mare is bred before the tenth day after foaling. That means that all maiden, barren, and January foaling mares should be bred as close to the fifteenth of February as possible. All other foaling mares are bred on or soon after the tenth day from foaling. Of course, this schedule is not always possible, nor in many instances is it advisable. However, if management strives to meet these goals, the net result will be that the farm can push foaling dates to the front of the season. This shift will result in increased production from the mare herd. It can be readily seen that foal heat-breeding plays an important part in such a strategy because it keeps the interval between foaling dates to a minimum.

Selection Of Mares

Determining An Appropriate Time To Breed The Mare

Mares that are normal and eligible to be bred on the foal heat can then be monitored for follicular development and bred at the optimal time. Several studies have shown that time of ovulation is critical to the success of foal heat-breeding. Mares that ovulate before ten days after foaling have a much lower pregnancy rate than do mares that ovulate at 10 days or later. Therefore if ovulation occurs too early, it is better not to breed the mare even if she is deemed suitable for mating. The performance of the herd will be better if these mares are not bred at the foal heat and then are managed so that they can be bred at the earliest time possible after this first ovulation.

If a decision has been made to skip the foal heat, these mares may be bred earlier by using prostaglandin to cause regression of the corpus luteum. The prostaglandin can be given at day six or seven postovulation, and the mare will often be ready to breed six or seven days later. This will shorten the time from foaling to breeding by about a week compared to allowing the mare to return to heat naturally. Mares that have a uterine infection, have fluid in their uteri, or have poor uterine involution after foaling often will also benefit from prostaglandin therapy. The early return to estrus appears to have a cleansing effect on the reproductive tract of these mares. It also gives the veterinarian a chance to continue therapy if necessary.

Some managers use hormonal therapy to delay the onset of the first estrus and thus ensure that the ovulation will occur ten days or more after foaling. Two methods are used to achieve this delay. One is with the use of oral altrenogest, and the other with the use of injectable progesterone and estradiol. Although the altrenogest (0.044 mg/kg) is the simplest and most readily available product, it is also the least precise. The progesterone (150 mg) estradiol-17β (10 mg) in oil combination provides more precise control of follicular development but must be given by daily injections and thus can cause some muscle soreness. Irrespective of the method chosen, beginning treatment on the first day postfoaling is important. If treatment is begun later — after follicular development has commenced — it may be difficult to suppress this growth, and some mares will continue through the therapy and ovulate. Mares that are successfully managed in this manner can be made to ovulate at day ten, twelve, or even later from foaling. Some investigators believe that this can be helpful. However, a controlled study in which this author participated showed very little benefit over the intensive management of foal heat alone.

Care Of The Mare After Breeding

Mares bred on foal heat should be examined the day after breeding — not only to confirm ovulation but also to ensure that the uterus has not retained a significant amount of fluid. If an ultrasound examination reveals an echogenic fluid accumulation, the mare should be treated with oxytocin to help promote elimination of this fluid. If a large volume of fluid is present, it may be advisable to lavage the uterus. Postbreeding antibiotic infusion may also be indicated in foal heat mares — more so than at other times. If the mare requires a Caslick operation, it should be performed at this time. The mare is less forgiving at the foal heat than during later estrous periods. Thus attention to detail is especially important.

Foal heat-breeding is a useful management tool, but it must be done carefully and with thought. Indiscriminate foal heat-breeding can often be detrimental to the mare and thus make the overall management program less efficient.