Transitional Mares Research Articles

Effect of short-term artificial light and transvaginal progesterone device on first ovulation in late transitional mares.

In study I, plasma progesterone concentrations were evaluated in anoestrous mares that received an intravaginal progesterone release device (IPRD) for 10 days. Mares were divided into 3 groups based on the dosage of progesterone (0 g, n=3; 1.38 g, n=5; and 1.9 g, n=5). No statistical differences were found in plasma progesterone concentrations between the two doses tested. In study II, the effects of a protocol based on a short program of artificial light combined with an IPRD containing 1.38 g of progesterone on oestrous behaviour and onset of ovulation were evaluated. IPRDs were inserted into 31 late transitional mares (10 days of treatment). The mares were divided into a control group (n=9, IPRD with 0 g of progesterone) and two treatment groups (T1, n=10, IPRD with 0 g of progesterone and artificial light; T2, n=12, IPRD with 1.38 g of progesterone and artificial light). The percentages of mares in heat within the first 14 days after treatment were 100%, 70%, and 100% in the control, T1, and T2 groups, respectively (P=0.097), and their ovulation rates were 44%, 60%, and 100%, respectively (P≤0.01). In conclusion, a protocol based on artificial light and an IPRD containing 1.38 g of progesterone for 10 days could be considered to advance the first ovulation of the year in late transitional mares, as it ensures a higher rate of ovulation within the first 14 days after treatment.

HCG is more effective than the GnRH agonist buserelin for inducing the first ovulation of the breeding season in mares.

BackgroundHuman Chorionic Gonadotropin (hCG) and Gonadotropin Releasing Hormone agonists (GnRHa) are routinely used to induce ovulation in mares. However, GnRHa efficacy in transitional mares has been suggested to be low.ObjectivesThe aims of this study were as follows: (a) to compare the efficacy of hCG and GnRHa in inducing the first ovulation of the breeding season and (b) to evaluate the correlation between ovulatory response, uterine oedema and teasing score at the time of treatment during the early or late transitional phase.Study designRandomised controlled superiority trial.MethodsMares in winter anoestrus were treated with sulpiride when at least two follicles reached a diameter of 25 mm. The day after the follicle reached 35 mm in diameter, mares in oestrus were treated with GnRHa buserelin (N = 29) or hCG (N = 33) and checked daily for ovulation.ResultsMore mares (30/33, 90.1%) ovulated when the first ovulation after winter anoestrus was induced with hCG, than with GnRHa, (11/29, 38.0%) (P = .0001). Ovulation rate was lower in mares that did not show uterine oedema and full acceptance of the teaser stallion for at least three days before the treatment (32/41, 78% vs 9/21, 42.9%) P = .01.Main limitationsPlasma LH and oestrogen concentrations were not performed.ConclusionsThese results demonstrate that hCG was more effective than GnRHa for inducing ovulation in the first cycle after winter anoestrus. Uterine oedema and behavioural signs of oestrus, for at least three days before the treatment, were predictors for a positive response to ovulation induction.

Reproductive management of the transitional mare

The mare is a seasonally polyoestrous long-day breeder with a physiological breeding season lasting from April–October in the Northern Hemisphere. The hypothalamic-pituitary-gonadal axis in the mare is subject to a circannual endogenous rhythm that is primarily regulated by day length. Increasing ambient photoperiod in the spring alters the pattern of melatonin secretion. The resulting stimulation of hypothalamic gonadotropin-releasing hormone secretion triggers pituitary follicular stimulating hormone release and follicular growth. Exposure of mares in deep anoestrus to a stimulatory photoperiod remains the most successful method of advancing the first ovulation of the season. The most commonly used lighting regimen is providing a fixed length of 15–16 hours of light exposure and 8–9 hours of dark, with a minimum light intensity in a stable of 100-lux (100–200 watt incandescent bulb). Other methods include using an additional 2.5 hours of light beginning at sunset and a pulse lighting system, providing 1 hour of light, 9.5–10.5 hours after the onset of darkness, during the photosensitive phase. Alternatively, the EquilumeTMlight masks provide a unilateral LED light source emitting 50 lux of blue-light directly to the eye during the hours after dusk (until 11 pm). Mares that have not been maintained under lights, or that have been exposed to ineffective light therapy, may require therapeutic hormonal intervention to advance the onset of the first ovulation of the season. Many hormone protocols involving progestins, GnRH, dopamine agonists and recombinant luteinising hormone/follicle stimulating hormone have been studied with variable results. Therapy is typically more effective when started either in late transitional mares or following a period of stimulatory artificial photoperiod.

Reproductive management of the transitional mare

The mare is a seasonally polyoestrous long-day breeder with a physiological breeding season lasting from April–October in the Northern Hemisphere. The hypothalamic-pituitary-gonadal axis in the mare is subject to a circannual endogenous rhythm that is primarily regulated by day length. Increasing ambient photoperiod in the spring alters the pattern of melatonin secretion. The resulting stimulation of hypothalamic gonadotropin-releasing hormone secretion triggers pituitary follicular stimulating hormone release and follicular growth. Exposure of mares in deep anoestrus to a stimulatory photoperiod remains the most successful method of advancing the first ovulation of the season. The most commonly used lighting regimen is providing a fixed length of 15–16 hours of light exposure and 8–9 hours of dark, with a minimum light intensity in a stable of 100-lux (100–200 watt incandescent bulb). Other methods include using an additional 2.5 hours of light beginning at sunset and a pulse lighting system, providing 1 hour of light, 9.5–10.5 hours after the onset of darkness, during the photosensitive phase. Alternatively, the EquilumeTM light masks provide a unilateral LED light source emitting 50 lux of blue-light directly to the eye during the hours after dusk (until 11 pm). Mares that have not been maintained under lights, or that have been exposed to ineffective light therapy, may require therapeutic hormonal intervention to advance the onset of the first ovulation of the season. Many hormone protocols involving progestins, GnRH, dopamine agonists and recombinant luteinising hormone/follicle stimulating hormone have been studied with variable results. Therapy is typically more effective when started either in late transitional mares or following a period of stimulatory artificial photoperiod.

Interferon-γ aids in protection against equine protozoal myeloencephalitis

Advanced reproductive technologies have been developed to enhance fertility in mares and stallions. Some of these technologies in mares include superovulation, embryo transfer (ET), intracytoplasmic sperm injection (ICSI), oocyte transfer (OT), gamete intrafallopian transfer (GIFT), and cloning. Superovulation can provide multiple oocytes for these techniques. This review will focus on how recombinant equine follicle-stimulating hormone (reFSH) and recombinant equine luteinizing hormone (reLH) are important for superovulation and ET and may be useful for ICSI, OT, GIFT, and cloning. Superovulation would increase pregnancy rates in normal and subfertile mares and enhance reproductive efficiency when using semen from subfertile stallions. Superovulation depends on a timely interaction of gonadotropins and gonadal feedback in the mare. Historically, several hormone protocols have been used to manipulate follicular waves to increase development and ovulations in cycling, anestrous, and transitional mares. Attempts to superovulate cyclic mares or induce the first ovulation of the year in anestrous or transitional mares using preparations of equine chorionic gonadotropin, gonadotropin-releasing hormone (GnRH), GnRH agonists, porcine FSH, domperidone, sulpiride, equine pituitary extracts, native equine FSH, human chorionic gonadotropin, progesterone, and immunization against inhibin have produced variable results. The use of recombinant technology has improved the ability to produce a reliable product in substantial quantities that is free of other hormones and possible contaminants. Several studies using reFSH and reLH that demonstrate their efficacy to superovulate the mare and induce the first ovulation of the year will be discussed in this review.

A preliminary investigation into the use of a “high altitude chamber” in racing Thoroughbreds

Advanced reproductive technologies have been developed to enhance fertility in mares and stallions. Some of these technologies in mares include superovulation, embryo transfer (ET), intracytoplasmic sperm injection (ICSI), oocyte transfer (OT), gamete intrafallopian transfer (GIFT), and cloning. Superovulation can provide multiple oocytes for these techniques. This review will focus on how recombinant equine follicle-stimulating hormone (reFSH) and recombinant equine luteinizing hormone (reLH) are important for superovulation and ET and may be useful for ICSI, OT, GIFT, and cloning. Superovulation would increase pregnancy rates in normal and subfertile mares and enhance reproductive efficiency when using semen from subfertile stallions. Superovulation depends on a timely interaction of gonadotropins and gonadal feedback in the mare. Historically, several hormone protocols have been used to manipulate follicular waves to increase development and ovulations in cycling, anestrous, and transitional mares. Attempts to superovulate cyclic mares or induce the first ovulation of the year in anestrous or transitional mares using preparations of equine chorionic gonadotropin, gonadotropin-releasing hormone (GnRH), GnRH agonists, porcine FSH, domperidone, sulpiride, equine pituitary extracts, native equine FSH, human chorionic gonadotropin, progesterone, and immunization against inhibin have produced variable results. The use of recombinant technology has improved the ability to produce a reliable product in substantial quantities that is free of other hormones and possible contaminants. Several studies using reFSH and reLH that demonstrate their efficacy to superovulate the mare and induce the first ovulation of the year will be discussed in this review.

An opioid antagonist, either alone or in combination with a GnRH agonist, alters LH and ovarian responses in transitional mares

An opioid antagonist, either alone or in combination with a GnRH agonist, alters LH and ovarian responses in transitional mares

An opioid antagonist, either alone or in combination with a GnRH agonist, influences FSH and ovarian responses in transitional mares

An opioid antagonist, either alone or in combination with a GnRH agonist, influences FSH and ovarian responses in transitional mares

Administration of 2.5 mg of estradiol followed by 1,500 mg of progesterone to anovulatory mares promote similar uterine morphology, hormone concentrations and molecular dynamics to those observed in cyclic mares

To test the hypothesis that the administration of 2.5 mg of estradiol benzoate (EB) followed by 1500 mg of long acting progesterone (LA P4) causes similar uterine changes and molecular dynamics in anovulatory mares to those observed in cyclic ones, we evaluated the changes of estrogen (ERα and ERβ) and progesterone receptors (PR) in anestrous, transitional and cyclic mares by RT-qPCR and immunohistochemistry. In addition, we evaluated uterine edema, tonus and estrogens and progesterone plasma profile. Endometrial biopsies were taken from anestrous and transitional mares immediately before EB injection, 48 h after EB administration and five days after LA P4 was given. In cyclic mares, biopsies were collected at estrus and at five days after ovulation. Similar estrogen peaks were achieved after the injection of the single EB dose between treated and cyclic groups, as well as maximum uterine edema. Uterine tone was increased to diestrus levels after administration of 1500 mg of LA P4. Changes in relative abundance of transcripts for PR, ERα and ERβ when progesterone stimulated endometrium was compared to estrogen stimulated endometrium were similar between cyclic and non-cyclic treated mares. However, apparent decreased PR in the endometrial glandular epithelium was not observed in non-cyclic mares five days after LA P4 administration as observed at five days after ovulation in cyclic mares. The protocol produced similar endometrial edema, uterine tonus and changes in relative abundance of PR, ERα and ERβ transcripts to those observed in cyclic mares during late estrus and early diestrus, as well as similar estradiol and estrogen conjugate plasma concentrations.

What Are the Options for Induction of Ovulation in the Mare in Europe? Buserelin as an Alternative to Human Chorionic Gonadotropin

A pharmaceutical survey conducted among veterinary surgeons practising in Europe (n = 46) revealed that the first choice drug to induce ovulation in the mare is human chorionic gonadotropin (hCG) (91%) and the second choice was a deslorelin implant (Ovuplant; 64.7%). The reason for not using the second choice drug more often was because, although more effective, it was more expensive (66.7%) or more difficult to obtain (20% of responses). In a parallel retrospective study, the efficacy of four different dose rates of buserelin acetate (Suprefact, 1 mg/mL, Sanofi-Aventis; 1 mg, 0.5 mg, 0.25 mg, and 0.125 mg) to induce timely ovulation (within 48 hours of treatment) was compared in 94 mares examined for the occurrence of ovulation three times a day during 225 estrous cycles in 1 year. The ovulatory responses in mares (84.8%, 87.9%, 88.5, and 90%) were similar (P > .05) for all four buserelin doses, 0.125, 0.25, 0.5, and 1 mg, respectively. Transitional mares failed to respond more frequently (34.5% follicles regressed) and took longer to ovulate (36.8% of cycles had an interval to ovulation >48 hours) than cyclic mares (0.0% of follicles regressed and 8.9% of intervals to ovulation >48 hours) following treatment of 0.5 mg of buserelin (P < .05). Furthermore, 21 mares (median age 15.5 years old) were treated with 1,500 IU of hCG in 54 cycles and with buserelin in 137 cycles. The ovulatory response to hCG (25.6%) was lower (P < .05) in these mares than that to buserelin (85.8%).

The reproductive performance of Thoroughbred mares treated with intravaginal progesterone at the start of the breeding season

The objective of this study was to investigate the effects of intravaginal progesterone on the reproductive performance of transitional Thoroughbred mares on commercial stud farms. Two hundred twenty-seven (227) non-lactating transitional Thoroughbred mares aged between 4 and 18 y (mean 9.4 ± 3.2 y) located on three stud farms in the Waikato region of New Zealand were used in the study performed during four consecutive breeding seasons (2007–10). Mares were age-matched in pairs and either treated with an intravaginal progesterone releasing device (Cue-Mare, 1.72 g progesterone, 10% w/w) for up to 10 d (Treated; n = 126) or left untreated (Control; n = 101). In both groups, 1,667 iu of hCG was given IV when an ovarian follicle ≥35 mm was detected (in conjunction with estrous behavior) and each mare was bred by natural service. Treated mares were served earlier in the breeding season (mean ± SD interval to first service was 13.9 ± 3.0 vs 26.7 ± 13.2 d for Treated and Control groups, respectively; P < 0.001). In the Treated and Control groups, 95.2 and 42.6% of mares were served within the first 21 d of the season (P < 0.001). Treated mares conceived earlier in the breeding season (mean number of days to conception 37.5 ± 14.2 vs 50.8 ± 21.3 d, P = 0.01). There was no difference between groups in the first service pregnancy rates (53.9 and 50.5% for Treated and Control mares, P = 0.89). Treatment with an intravaginal device increased the number of mares conceiving by the end of the breeding season (91.3 vs 82.3% for Treated and Control groups, P = 0.04). Therefore, this treatment protocol appeared to offer a convenient, economical and reliable method for managing transitional mares on commercial Thoroughbred stud farms.

Effect of a Single Injection of Long-acting Progesterone on the First Ovulation in Early and Late Spring Transitional Mares

Since 1966, exogenous progestins have been used in equine practice for pregnancy maintenance, estrous suppression, and control of erratic sexual behavior. This study was designed to investigate the use of a new compounded controlled-release progesterone preparation (BioRelease P4 LA 300) in early and late spring transitional mares. In the first experiment, the pharmacodynamic properties of the preparation were studied in five geldings. In the second experiment, the use of a single intramuscular injection (600 mg) was tested in 68 embryo-recipient mares maintained under natural photoperiod in the Southern Hemisphere. Experiment 1 demonstrated elevated serum concentrations of progesterone (>1 ng/mL) for 7.6 ± 2.2 days. In experiment 2, there was no effect of treatment in mares that were treated on September 18, independent of their follicular status at day of treatment (10 to 15 mm; 20 to 25 mm, respectively). When mares with a follicular size of 20 to 25 mm were treated on October 14, significantly more progestin-treated mares (10/12; 83%) ovulated between 10 and 24 days after treatment than untreated controls (3/12; 25%) ( P < .05). Additionally, there was a trend in mares treated on October 14 for a shorter treatment to ovulation interval (mean ± SD, 18.6 ± 8.7 days) compared with untreated controls (mean ± SD, 26.7 ± 14.7 days) ( P = .07). Administration of one single injection of long-acting progesterone is a simple and effective method of controlling the first ovulation of the season in late transitional mares.

Supplementation of equine early spring transitional follicles with luteinizing hormone stimulates follicle growth but does not restore steroidogenic activity

This study was conducted to test the hypothesis that supplementation of growing follicles with LH during the early spring transitional period would promote the development of steroidogenically active, dominant follicles with the ability to respond to an ovulatory dose of hCG. Mares during early transition were randomly assigned to receive a subovulatory dose of equine LH (in the form of a purified equine pituitary fraction) or saline (transitional control; n = 7 mares per group) following ablation of all follicles >15 mm. Treatments were administered intravenously every 12 h from the day the largest follicle of the post-ablation wave reached 20 mm until a follicle reached >32 mm, when an ovulatory dose of hCG (3000 IU) was given. Saline-treated mares during June and July were used as ovulatory controls. In a preliminary study, injection of this pituitary fraction (eLH) to anestrus mares was followed by an increase in circulating levels of LH (P < 0.01) but not FSH (P > 0.6). Administration of eLH during early transition stimulated the growth of the dominant follicle (Group x Day, P < 0.00001), which attained diameters similar to the dominant follicle in ovulatory controls (P > 0.1). In contrast, eLH had no effect on the diameter of the largest subordinate follicle or the number of follicles >10 mm during treatment (P > 0.3). The numbers of mares that ovulated in response to hCG in transitional control, transitional eLH and ovulatory control groups (2 of 2, 3 of 5 and 7 of 7, respectively) were not significantly different (P > 0.1). However, after hCG-induced ovulation, all transitional mares returned to an anovulatory state. Circulating estradiol levels increased during the experimental period in ovulatory controls but not in transitional eLH or transitional control groups (Group x Day, P = 0.013). In addition, although progesterone levels increased after ovulation in transitional control and transitional eLH groups, levels in these two groups were lower than in the ovulatory control group after ovulation (Group, P = 0.045). In conclusion, although LH supplementation of early transitional waves beginning after the largest follicle reached 20 mm promoted growth of ovulatory-size follicles, these follicles were developmentally deficient as indicated by their reduced steroidogenic activity.

Clinical use of dopamine antagonist sulpiride to advance first ovulation in transitional mares

Artificial photoperiod treatment is currently the best method to hasten the first ovulation of the breeding season in winter anoestrus mares. However, this is not easy to apply in large herds of mares and, to be effective, has to be planned in the northern hemisphere in December at the latest. Pharmacological treatments have been proposed as alternatives: GnRH agonists, progesterone or its synthetic agonist Altrenogest, and dopamino-antagonists, as pherphenazine, domperidone or sulpiride. Dopamino-antagonists protocols, beginning at a given day of the year, gave controversial results in terms of hastening ovulation. The aim of this study was to evaluate the efficacy of an up-to-21-d long dopamine antagonist (sulpiride) treatment on mares at the beginning of the spring transition for its ability to hasten estrous cyclicity. In Study 1, 49 seasonally-acyclic standardbred mares, maintained in paddocks under natural photoperiod, were treated with 1 mg/kg/d sulpiride at the evidence of the first follicle with of 25 mm in diameter until ovulation for a maximum of 21 d (Group S 1; n = 34) or left untreated (Group C 1; n = 15). Group S 1 and C 1 mares showed a follicle of 35 mm in diameter after 8 and 22 d (median; P < 0.05) and ovulated after 18 and 43 d, respectively (median; P < 0.05). Twenty-two/26 and 6/15 mares of the Group S 1 and C 1 ovulated within 30 d from the beginning of the treatment, respectively (P < 0.05). All the mares of the study cycled until Autumn, unless they became pregnant. In Study 2, pregnancy rates after the first ovulation of the year of 22 acyclic standardbred mares maintained in paddocks under natural photo-period, treated following the same protocol as Study 1 (S 2), and 47 untreated mares (C 2) were compared. In Groups S 2 and C 2, 63.6% and 61.7% of the mares became pregnant after the first cycle (P > 0.05) and 50.0% and 61.1% of the remaining became pregnant in the following cycles (P > 0.05), respectively. Beginning with sulpiride treatment when follicles were 25 mm in diameter resulted in a significant advancement of cyclicity in non-photo-stimulated mares. Pregnancy rates after artificial insemination of treated mares were similar to those of untreated animals.

Selected Ovarian Ultrasonographic Characteristics During Vernal Transition are Useful to Estimate Time of First Ovulation of the Year

It is important to get mares pregnant as early as possible after vernal transition and thus, identification signs of impending 1st ovulation of the year are warranted. To identify clinical indicators of an approaching first ovulation of the year, mares were teased with a stallion for oestrous detection starting January 3 and subjected to ultrasonographic examination. Day of first appearance of uterus oedema, follicular wall invagination, intrafollicular echogenicity, double contour of the follicle wall, increase in granulosa thickness, follicular wall hyperechogenicity and appearance of pear-shaped follicles was registered, as well as follicle diameter and number. Seventy per cent of the mares had anovulatory oestrous periods of 4.6 ± 3.6 days, with an interoestroual interval of 12.5 ± 12.2 days. Number of anovulatory oestruses per mare was 2.4 ± 2.3. Uterine oedema occurred in 77% of the mares, 32.4 ± 25.6 days before ovulation. Invagination of the follicular wall appeared in 44.4% of the animals, 24.5 ± 18.4 days before ovulation. Intrafollicular echogenicity was seen in all mares and double contour of the follicle was seen in 77% of the animals. Both last two characteristics appeared 1-72 days before ovulation. Increased thickness of the granulosa occurred in 66% of the mares, 1-19 days before ovulation. Pear-shaped follicles and follicular wall hyperechogenicity were detected 3 or less days before the first ovulation, in 44.4% and 55.5% of mares, respectively. Mean number of follicles > 15 mm decreased at least 16 days before ovulation. We concluded that no isolated characteristic was a reliable indicator. However, increase in granulosa thickness, formation of a pear-shaped follicle and follicular wall hyperechogenicity, associated with the reduction of the number of follicles > 15 mm in diameter to < 3, resulted in the first ovulation of the year in 44-67% of the transitional mares, 1-19 days after the characteristics appeared.

Stimulation of Ovulation in Seasonally Anovulatory and Vernal Transitional Mares with Estradiol and Domperidone: Dose and Combination Studies

Three experiments were conducted to test the efficacy of different doses of estradiol cypionate (ECP) and domperidone for inducing ovulation in seasonally anovulatory (January; Experiments 1 and 2) and transitional period (March; Experiment 3) mares. In the first two experiments, mares in Kentucky and Louisiana were administered domperidone (3 g in biodegradable particles) alone or after pretreatment with 100 or 150 mg of ECP; another group received ECP, domperidone, and progesterone, and a fifth group received ECP and progesterone only (the latter two in Kentucky). Control mares in both states received no treatment. The proportion of mares ovulating within 35 days (for mares treated in January) was greater ( P = .0002) for those receiving any combination of ECP plus domperidone relative to mares not receiving the combined treatment. Addition of progesterone to ECP plus domperidone did not enhance ( P = .7) the response relative to the combination alone; domperidone by itself, or ECP plus progesterone, did not alter the response relative to controls ( P > .24). Experiment 3 was conducted in Louisiana as a 2 × 3 factorial, with two doses of domperidone (1.5 or 3 g) and three doses of ECP (0, 75, or 150 mg). There was no main effect of domperidone or ECP dose; a greater proportion ( P = .055) of mares receiving any combination of ECP plus domperidone ovulated in 21 days compared with those receiving no ECP. In conclusion, pretreatment with ECP before injection of domperidone 10 days later can be used to increase the proportion of mares ovulating early in the year; within the limits of the present experiments, there appears to be no difference in doses of ECP of 75, 100, or 150 mg, and no difference in doses of domperidone of 1.5 or 3 g.

The effects of intravaginal progesterone on follicular dynamics and FSH, LH and progesterone concentrations in transitional mares

The effects of intravaginal progesterone on follicular dynamics and FSH, LH and progesterone concentrations in transitional mares

Administration of equine luteinizing hormone promotes the development of ovulatory follicles in early transitional mares

Administration of equine luteinizing hormone promotes the development of ovulatory follicles in early transitional mares

Effect of a single injection of long-acting progesterone on first ovulation in early and late spring transitional mares

Effect of a single injection of long-acting progesterone on first ovulation in early and late spring transitional mares

The first ovulation of the breeding season in the mare: The effect of progesterone priming on pregnancy rate and breeding management (hCG response rate and number of services per cycle and mare)

The mare is a seasonally polyestrus breeder. In early spring, the mare enters a “transition period” between the anovulatory season and the first ovulation of the year. This period is characterized by irregular estrus cycles and high incidence of regressing dominant follicles. There is a belief that pregnancy rates resulting from the first ovulation of the season is lower than in subsequent ovulations, however this has never been studied critically. Progestagens are often used as an aid to manage the transition period. The objective of this study was to compare pregnancy rates of mares from the first ovulation of the year with: (a) mares on their second or subsequent ovulations and (b) mares with progesterone-primed first ovulations. A total of 136 thoroughbred mares were used in the study. The mares were classified into four groups: (1) mares mated at the first ovulation of the year ( n = 46); (2) mares mated in the first ovulation of the year after removal of a previously inserted progesterone device (CIDR) ( n = 29); (3) mares mated at the second or more ovulations of the year after prostaglandin-induced estrus ( n = 50); and (4) mares mated after spontaneous return to estrus ( n = 11). Pregnancy rates were not different in any of the groups studied: 65.2, 75.9, 76 and 72.7 for groups 1–4 respectively ( P > 0.05). Group 1 mares had the lowest response to hCG treatment which resulted in the highest number of services per cycle ( P < 0.05). In conclusion, although the use of progesterone priming did not affect pregnancy rates, it did improve the breeding management of transitional mares by enhancing the hCG response rate and consequently decreasing the number of services per cycle.