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Hormonal management of mares (Proceedings)


Hormonal management of mares is used to manipulate the onset of the breeding season, the timing of ovulation and in performance mares the prevention of estrus. Pasture breeding, hand breeding using teasing with estrus detection by a stallion, and artificial insemination (AI) of cooled and frozen semen are still commonly used in Canada.

Hormonal management of mares is used to manipulate the onset of the breeding season, the timing of ovulation and in performance mares the prevention of estrus. Pasture breeding, hand breeding using teasing with estrus detection by a stallion, and artificial insemination (AI) of cooled and frozen semen are still commonly used in Canada. In Europe it has been estimated that about 80% of all mares are bred using AI. The future of the equine breeding industry will likely continue to include expansion of artificial insemination (AI) which requires an understanding of the mare's physiology and options for breeding management. This paper will briefly review: the physiologic (seasonal) changes in the mare and how to advance the breeding season of the anestrus or transitional mare; the basis for hormonal therapy, the hormones used and the success rates obtained; and breeding strategies for use with cooled and frozen semen programs.

The physiologic cycle – seasonal changes and transitions in mares

The mare is a seasonal polyestrous breeder and she senses changes in day length through retinal receptors and the pineal gland, which influences her secretion of melatonin. The secretion of melatonin increases when darkness increases. Melatonin is a hormone which lowers GnRH secretion and decreases ovarian activity. Input from other hypothalamic factors such as Kisspeptins may also be important in regulation GnRH secretion. When day length shortens, and GnRH decreases a reduced amount of FSH and LH are secreted. Photoperiod has a strong effect on mares and up to 85% of mares will enter a fall transition in October, and cease all ovarian activity to become anestrus by November. The anestrus period typically lasts from November to February. Spring transition may begin in February or March. Once days lengthen after the spring Equinox in March the effect is to reduce melatonin secretion and to induce increased GnRH secretion, which leads to FSH secretion and a recrudescence of follicle waves. Three anovulatory follicular waves occur in the spring transitional phase, then finally a mature wave will occurs resulting in an ovulation that begins the physiologic breeding season usually in late April or early May (Ginther, 1992). If a mare owner wishes to breed outside of the natural season, such as often occurs with breeds such as the Thoroughbred, or halter type Quarter horses, one option is to make a plan in the fall for stimulating the mares to cycle outside of the natural breeding season. January1st is the arbitrary birth date for all foals in these breeds. There is an idea that older yearlings and 2 yrs perform better athletically. The photoperiod of mares foaling in winter may also have to be manipulated if the owners wish to breed the mare back right away after foaling. Options for advancing the breeding season include a manipulation of the mare's photoperiod, a use of hormones to stimulate ovarian activity or a combination of both methods.

Manipulation to hasten the onset of the breeding season: artificial photoperiod

The traditional method was to use supplemental light at the end of the day so the mare experienced 16 hours of total light. A 200-watt incandescent bulb or two 40-watt fluorescent bulbs are recommended in each box stall to give at least 100 lux of light in all corners of the stall. Exposure of the mare to less light will have some effect, but will decrease the response rate. The mare needs to sense light during her photosensitive window which is metered from the onset of darkness. "Flash Lighting" protocols exploit this fact and a single one-hour exposure of artificial light, given 9-10 hours after sunset in the photosensitive window, has been shown to stimulate follicular development in anestrous mares. Following winter solstice, light increases at about 5 minutes per day, hence if you are using flash lighting the time for lighting needs to be adjusted weekly to make suer the supplemental light is in the photosensitive window. Mares need at least 60 days of supplemental light exposure to become stimulated and the additional light must be continued at least until the spring Equinox.

Hormonal therapy for anestrus

Not all farms or management situations are suitable for artificial lighting. The simplest protocols to advance the breeding season hormonally include the use of injectable deslorelin. The Deslorelin is a GnRH agonist which in seasonally anestrous mares stimulates the secretion of FSH and LH synthesis. Deslorelin is available as Ovuplant or as a compounded liquid product. A new injectable product is soon to be released from Bioniche. Ovuplant pellets may be placed under the skin every 72 hours for up to 10 treatments until ovulation, with hCG being given when a follicle (F) reaches 35mm. The injectable deslorelin is administered at a dose of 62.5ug per mare BID until ovulation (about 10 days) with an additional treatment of 2000 IU hCG administered when a follicle (F) reaches 35mm. Mares with 20mm F are more likely to respond than mares with no ovarian activity. Treatment of anestrus mares may result in 30-50% ovulation rates. Mares may not experience a second cycle and may need to retreated if not in foal (Johnson A, 1987; Raz, 2009).

Anestrus and transitional mares: hormonal and light regimes

Antagonists such as sulpiride or domperidone will increase prolactin, because dopamine is the putative PRL inhibiting substance. It has been suggested that lowering dopamine will increase the number of gonadotropin receptors in the ovary, or dopamine may act directly through ovarian receptors or at the hypothalamic level, or may increase in receptor number or sensitivity may result in heightened responsiveness to the gonadotropins LH and FSH. Prolactin normally increases as daylight increases. Protocols describe the use of increased photoperiod with variable periods of supplemental lighting and hormonal manipulation. One protocol combines 2 months of additional light with pretreatment with estradiol benzoate 11mg / mare for 10 days before adding a dopamine antagonist, sulpiride 250mg/mare SQ (Kelley et al., 2006). Another protocol uses supplemental light beginning in January for 2 weeks and the administration of dopamine antagonists (sulpiride 1 mg/kg IM BID) or domperidone (1.1 mg / kg PO SID), for 3 weeks or until ovulation. Sulpiride appears to be more potent than domperidone, however there are both D- and L-enantiomers and may have different biological effects (Mari et al., 2009). The L-form of sulpiride has been used in Europe. The use of these agonists points to a potential role that prolactin or dopamine regulation may play a role in the control of deep anestrus and seasonality in the horse. Dopamine is a prolactin inhibiting substance. Hormonal regimes early transition: We have found the dopamine agonists to be less effective than sulpiride, and both to have a longer treatment to response time than treatment with either deslorelin, or eFSH in mares in early transition. Several recent studies have demonstrated that deslorelin (64μg per day BID IM) or eFSH (12.5 mg BID IM) can be used successfully to induce ovulations in early transitional mares (mares that have at least one 20 mm follicle) when administered as twice daily treatments, or serial insertion of slow release deslorelin implants (2.1 mg each) (Raz, 2009). The mares receive a new implant every 3 days for up to 3 weeks or until they ovulate. Using these protocols the hCG is still administered to induce ovulation. Ovulations occur in approximately 78% of early transitional mares treated, and eFSH stimulates more ovulations per mare. The duration from the onset of treatment to ovulation is approximately 7 – 8 days. Hormonal treatment of early transitional mares may result in additional cycles following treatment however a proportion of mares (around 15%) will return to anestrus following the end of treatment, even if ovulation was induced, if they fail to become pregnant. Pregnancy rates following ovulations induced by hormonal means in early transitional mares are not significantly different than pregnancy rates following the first spontaneous ovulation of the year. Hormonal

Manipulation of mares in late transition

The treatment options for mares in late transition (April) usually are progestagen (Regumate) or an ovulation induction agent such as hCG or deslorelin or both. The progestagen (altrenogest 0.044mg/kg/day PO) is administered orally to the late transitional mare (>20 mm follicles in April) for 10 – 14 days. The treatment is proposed to alter the secretion of gonadotropins or change the sensitivity to them such that withdrawal of treatment results in an LH surge and ovulation, however most transitional mares are treated with hCG. Statistically progestagen treatment only advances the breeding season by 2 weeks. Mares with a >30mm follicle may simply be treated with deslorelin (1.5 mg injectable, or 2.1 mg implant) or hCG (2000 IU IM) to stimulate ovulation. Transitional mares tend to be more LH rather than FSH deficient. Ovulation rates of around 50% should be expected.

The breeding season

The 21 day estrous cycle of the mare can be divided into 2 phases based on sexual receptivity: estrus (receptive) and diestrum (non-receptive). The duration of estrus is usually 5-7 days and is a consequence of the estrogen produced by developing follicles. Common behavioural responses of mares in estrus when teased with a stallion include raising of the tail, responsive urination, repeated eversion and exposure of the clitoris (winking), and assuming a mating posture. The strongest expression occurs in mid to late estrus where the mare will posture, and stand to be mounted. Behavioural signs are however highly variable and vary by mare and therefore are not considered highly reliable. Mares with foals at side are notorious for only showing estrus briefly, usually when they are about to ovulate. Diestrus refers to the period during which mares are influence of the hormone progesterone and reject the advances of the stallion. As little as 12 hours post ovulation mares will reject the stallion. Progesterone production by the corpus luteum controls the behavioural responses of the mare during the 14-16 day diestrous period. When teased to a stallion, mares in diestrus will pin their ears back, kick, switch their tail, vocalize and attempt to actively move away from the stallion.

Examination of mares in the breeding season

Veterinarians are asked to get mares ready for breeding, typically on farms where there are no stallions. A few examinations may be required to determine where in the mares is in her estrus cycle. When examining a mare it is best to use both palpation and ultrasound to determine if she is in diestrus or estrus. Recorded data should include palpation, ultrasound and teasing information.

Getting mares ready for breeding

Teasing mares with a stallion is the natural way to determine when a mare is ready to breed. Rectal palpation and ultrasound are also used to confirm or to determine the mare's status. Early in the season (Feb – April) it is important to determine if mares are transitional or if they are cycling. This is done by evaluation of the uterine / cervical tone (which will be atonic or flaccid) and determination of the activity on the ovaries. Active luteal tissue and progressive follicle growth culminating in an ovulation means a mare is cycling. Transitional mares will have no progesterone, some follicle growth but will not ovulate. Therefore monitoring follicle growth on 2 or 3 visits spaced 3 days apart will allow a determination of the mare's status if careful records are kept.

Breeding strategies: hormonal manipulation of mares

Hormonal therapy has become an integral part of routine equine breeding strategies practice. Hormones commonly used to manipulate the equine estrus cycle include: prostaglandin (PG), human chorionic gonadotropin, estradiol 17-beta, deslorelin (gonadotropin releasing hormone analogue), progestagen / progesterone.

Induction of ovulation

Induction of ovulation is used to time the breeding of a mare with the availability of the semen. The 2 products most commonly administered are hCG and deslorelin. Human Chorionic Gonadotropin (hCG) is used for the induction of ovulation in cycling mares (McCue et al., 2007). Estrus mares treated with hCG with a follicle 30 ≥ 35 in diameter will induce ovulation of that follicle 75% of the time within 24-48 hours. The average time to ovulation post injection is 36 hours. The use of hCG improves the efficiency of a breeding program by reducing the duration of estrus, decreasing the number of breedings at each estrus and provides a means of synchronizing ovulation with a stallion's breeding schedule, or the availability of the semen. Forms, Doses and Routes of Administration of hCG - Chorulon, 1,000 IU per ml dose 750 - 2,000 I.U. I.M. or I.V. Ginther recently suggested that mares that tend not to ovulate in response to hCG may have higher anti-hCG antibody levels (Siddiqui et al., 2009). Deslorelin - is available as a 2.1 mg implant delivered SQ (Ovuplant) and as a liquid injectable compounded product. It is as effective as hCG with the mean time to ovulation is 40 hours, which is longer than hCG at 36 hours. If the implant is not removed in 48 hrs some mares will have a delayed return to estrus, so removal of the implant at 48 hrs is common. The vulvar edge is a common location for insertion. Delsorelin was reported to have few earlier ovulations compared with hCG.

Prostaglandin and hcg or deslorelin

An easy and simple means of inducing estrus is to use PG to bring a mare into heat. The mean time from PG to the onset of estrus is 3 days, and the mean time to ovulation if using ovulation induction (hCG (Chorulon) or deslorelin (Ovuplant) is 8 days. The status of the follicles on the mare's ovaries at the time of PG treatment determines how fast the she will come into heat. Calculate that a follicle grows at around 3 mm a day and determine how many days it will take a mare to reach a follicle size of about 30 - 35 mm which is when most mares show early signs of heat, and typically will respond to ovulation induction. Note the wide variability in the size of the follicular structures on the mare's ovary is the underlying basis for the range of response to PG from 1 – 7 days. Generally when shipping semen if you are playing the odds PG is given on Thursday to expect the mare in heat by Monday. Therefore you have a Monday and Wednesday semen shipment to access and Friday if your carrier and customs people allow for Saturday delivery. Please note a reason the character of the CL (new mid or old) is observed is that a single shot of Prostaglandin is only effective between days 5 – 16. If a very young CL is present it may not respond to the PG, and may not undergo luteolysis. The mare may require a second treatment. Mares that have a large follicle (>40mm) at the time of PG treatment pose additional challenges. Most commonly they have a short heat (3 days), less common is a full 6 day heat, and least likely is that they ovulate within 24 hrs after treatment with PG, thus making a new CL with 24 hours. These mares do not show heat but will conceive if bred. Mares that are in heat are managed by palpating and noting uterine, cervical and follicle tone, using ultrasound to measure follicle diameter, and score the uterine edema. Administer an ovulation induction agent at >30-35 mm, or follow to the end of natural estrus (most light mares ovulate a follicle around 42- 45 mm; draft and warm bloods closer to 55 mm).

Forms, doses and routes of administration of prostaglandin

Lutalyse (PGF2-α) 5 mg SQ estrus induction, or low dose to reduce side effects: 0.5 - 2.5 mg IM given twice 12 hours apart. Estrumate (clopostenol) 250 μg, IM or SQ. Side effects of PG's include: often causes profuse sweating, increased heart rate and respiratory rate, abdominal discomfort, locomotor incoordination and lying down. Signs occur within 10 - 20 minutes of injection and typically abate in another 20 - 30 minutes. Caution should be used when handling these products, especially pregnant women and asthmatics. Avoid inhalation and self injection.

Progesterone and estradiol 17β

This hormonal method is highly suitable for rural practitioners, and those with limited experience in ultrasound and palpation. Mare are administered a combination of 150 mg Progesterone and 10 mg Estradiol – 17β also called "P&E" daily for 8 – 14 days. The product is compounded at 50 mg per ml progesterone and 3.3 mg/ml estradiol 17-beta. The mares receives 3cc IM per day. The product is obtained through a compounding pharmacy. These steroids causes follicular suppression. Prostaglandin is given on the last day of P&E treatment. Withdrawal of the treatment allows a new follicular wave to emerge. Eight - 10 days after the last treatment the mare is usually showing estrus and is ready to induce ovulation. Breeding is usually at 10 – 12 days post P&E. Advise evaluating the mare at 6 -8 days after stopping the P&E to monitor follicle growth and plan ovulation induction and ordering / timing of breeding. Ovulation induction is used to further tighten the window so when a 30 - 35 mm follicle is present the mare is given hCG 2000 IU IM, or deslorelin. The total duration of the protocol is about 18 days. The treatment may be started on any day of the mare's cycle. The protocol is very, very reliable. The longer the mare is on the P&E the larger the tendency for her follicle wave to emerge more slowly. Temperament of the mare may be a consideration as some mares do not cooperate with this many injections.

Breeding strategies for mares bred with cooled transported semen

Mares should be bred so ovulation occurs within the next 48 hours, or not more than 12 hours after ovulation. Semen of acceptable quality may provide good fertility for at least 48 hours in the mare, and some provide fertility longer than that. Therefore a single timed insemination based on ovulation induction is widely used. Mares may be bred by following their cycle, inducing estrus and then ovulation, or they may be allowed to have a natural heat and then are bred. Due to the expense of shipping and collecting cooled semen, a single insemination is optimally used with this breeding method.

Frozen semen is usually sold by the breeding dose. Therefore either fixed time insemination after ovulation induction is used or a single periovulatory insemination is used (Crowe et al., 2008). Recently deep horn insemination has gained popularity however if the semen is of acceptable quality this is not necessary and may lower fertility if the uterus is traumatized. Fertility is a function of the quality of the mare's reproductive tract, the timing of the insemination in relationship to ovulation and the dose and quality of the semen. Generally the most limiting factor is the stallion's fertility in terms of the frozen semen. Frozen semen of acceptable quality (>100 million normal and motile sperm post thaw) will remain fertile for around 12 hours before and at least 8 hours after ovulation. Less than optimal timing of insemination may still result in pregnancy, however the likelihood of pregnancy decreases. One strategy is to divide the semen dose in half and use fixed time insemination of a half dose of semen at 28 and 40 hours after hCG induction of ovulation (for example treat with hCG at noon (Time 0), breed the following day at 4 pm (28 hrs) and then the day after that at 8 am (44 hours). If using deslorelin administer fixed time is often done at 32 and 44 hours. Give deslorelin at 10 am breed at 32 hrs (6 pm next day) and 44 hrs (6 am ) the day after that. One author reported deslorelin induced ovulation between 38 and 42 hours in 94% of mares (Hemberg et al., 2006).

Other hormones: progestagen- regumate

Altrenogest is a synthetic progestagen. It is commonly used for the Suppression of Estrous Behavior. Administer to mares beginning 2-3 days prior to show or race to suppress estrous behavior that may detract from performance. Reliable and safe for use in mares. It is also indicated in select cases to assist with the maintenance of Pregnancy. Administered to mares with a history of repeated embryonic loss, poor tone after breeding, or low mean plasma progesterone concentrations. Synchronization of estrus but not ovulation is achieved by administering progestagen to mares for 10-14day with mares coming into estrus within 2-3 days of drug withdrawal. This and other progestagens do not suppress the follicle waves. The lack of follicular regulation leads to wide variability in time to estrus and ovulation following progestagen treatment. Progestagen does NOT cause luteolysis, nor does in interfere with luteolysis, it does NOT suppress follicle development, mares will ovulate while on progestagen without showing behavioural estrus, and they may undergo luteolysis.

Forms, doses and routes of administration of progesterone/progestagen

Progesterone-in-oil (50 mg/ml), may be used to suppress estrus behavior and at 150 mg (3 mls) administered as an intramuscular (IM) injection once per day. Altrenogest (Regumate) (Synthetic progestagen) 0.044 mg/kg (1 ml per 110 lbs) administered orally once/day. Wear gloves. The CIDR device has been used in mares. It contains native progesterone (1.9g). The device is inserted into the vagina of the mare. Cut the plastic string off the end of the CIDR to avoid irritation of the vulvar mucosa. In transitional mares a 14 day exposure is recommended, in cycling mares a shorter exposure may be effective (8 to 14 days) if combined with prostaglandin at withdrawal of the CIDR. The progesterone products take a number of days to suppress estrus. May be used in mares during competitions to prevent estrus behavior but should be inserted at least 3 days prior to the event. Mild vaginal discharge is present in most mares treated with a CIDR. Shorter exposure is associated with less discharge. The use of the CIDR device in subfertile mares has not been investigated.

Other hormonal protocols for estrus synchronization

The main options for estrus synchronization are: Progestagen (altrenogest = Regumate) for 10 -14 days. Give PG on the last day. Stop treatment and expect mares to come into heat in 2 – 3 days. Follow into heat and use ovulation induction with hCG or deslorelin. However this product does not synchronizing the follicular wave, so the time frame to estrus and ovulation is variable. Prostaglandin – mares receive 2 treatments with prostaglandin (5mg SQ) 14 days apart. Mares will come into heat around 3 days after the last treatment. This is a long protocol and not very reliable. Follicular wave is not synchronized. Mares are followed into heat and usually given an ovulation induction agent to tighten up the synchrony. Must use this long interval due to the long length of estrus in the mare. Mares are given prostaglandin and are on the average coming into heat in 3 days, then they are given an ovulation induction agent. Due to lack of synchronization of the wave there is a variable time to estrus based on the follicle size on the mare's ovary at the time of treatment with PG.

Prostaglandin E1

Misoprostil Cytotec Prostaglandin E, PGE1 tablets 100 μg each. Grind 2 tablets into a powder add a small amount of sterile sleeve lubricant, apply to cervical canal and external cervical os. May find a mild blood or serum tinged discharge in vagina in association with the treatment. Cervix will enlarge and swell. PGE induces an inflammatory response and effect lasts for around 24 – 36 hours.

Meclofenamic acid

1 g daily (compounded powder) has been used to increase endometrial receptivity in embryo recipients starting on day 9 post-ovulation.


Crowe, C.A.M., Ravenhill, P.J., Hepburn, R.J., Shepherd, C.H., 2008. A retrospective study of artificial insemination of 251 mares using chilled and fixed time frozen-thawed semen. Equine Veterinary Journal 40, 572-576.

Ginther, O.J. (Ed.), 1992. Reproductive Biology of the Mare: Basic and Applied Aspects. Equiservices, Cross Plains, WI

Hemberg, E., Lundeheim, N., Einarsson, S., 2006. Successful timing of ovulation using deslorelin (Ovuplant) is labour-saving in mares aimed for single AI with frozen semen. Reproduction in Domestic Animals 41, 535-537.

Johnson A, L., 1987. Gonadotropin-releasing hormone treatment induces follicular growth and ovulation in seasonally anestrous mares. Biol Reprod 36, 1199-1206.

Kelley, K.K., Thompson, D.L., Jr., Storer, W.A., Mitcham, P.B., Gilley, R.M., Burns, P.J., 2006. Estradiol interactions with dopamine antagonists in mares: prolactin secretion and reproductive traits. Journal of Equine Veterinary Science 26, 517-528.

Mari, G., Morganti, M., Merlo, B., Castagnetti, C., Parmeggiani, F., Govoni, N., Galeati, G., Tamanini, C., 2009. Administration of sulpiride or domperidone for advancing the first ovulation in deep anestrous mares. Theriogenology 71, 959-965.

McCue, P.M., Magee, C., Gee, E.K., 2007. Comparison of compounded deslorelin and hCG for induction of ovulation in mares. Journal of Equine Veterinary Science 27, 58-61.

Raz, T., Card, C., 2009. Comparison of the effects of eFSH and deslorelin treatment regimes on ovarian stimulation and embryo production of donor mares in early vernal transition. Theriogenology. 71 1358-1366.

Siddiqui, M.A.R., Gastal, E.L., Gastal, M.O., Beg, M.A., Ginther, O.J., 2009. Effect of hCG in the presence of hCG antibodies on the follicle, hormone concentrations, and oocyte in mares. Reproduction in Domestic Animals 44, 474-479.

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