The Effect of Sulpiride Treatment During the Periovulatory Period on Prolactin Concentration and Ovulation in the Mare

Sixteen estrous cycles from 10 cyclic mares were randomly assigned to a control or sulpiride group (n = 8 each). All mares received 1,500 IU of human chorionic gonadotropin (hCG) (hour 0) during estrus with a follicular diameter ≥32 mm. Mares were scanned every 12 hours until ovulation. In the treatment group, beginning at hour 0, each mare received 1.5 mg/kg of sulpiride every 12 hours intra-muscularly until ovulation or formation of a luteinized unruptured follicle (LUF). Concentrations of luteinizing hormone (LH) and prolactin (PRL) were measured by radioimmunoassay. In each group, there were 10 preovulatory follicles for the eight cycles. The ovulation rate (9/10, 90%) was similar in the control and sulpiride groups. Two mares formed an LUF, which was first detected at hours 48 and 72 for the sulpiride and control mares, respectively. The interval from hCG to ovulation was 49.5 ± 11.1 and 43.5 ± 5.8 hours, for the control and sulpiride groups, respectively (P > .5). LH followed the typical preovulatory surge pattern, with no difference between groups (P > .5). Sulpiride administration increased PRL concentration in treated mares at 24 (P < .1), 36, and 48 hours (P < .05) after treatment. In conclusion, sulpiride administration every 12 hours increased PRL concentration in treated mares after 24 hours of the beginning of treatment. However, at this time window and concentration, PRL did not have any effect on ovulation. The control mare that developed an LUF had a PRL concentration similar to other ovulatory control mares (always ≤10 ng/mL).     

Clinical experience with native GnRH therapy to hasten follicular development and first ovulation of the breeding season

Breeding records of 48 Thoroughbred and Standardbred mares treated with native GnRH (500μg im, bid) during February—April, 1999 or 2000, on 7 farms in central Kentucky were retrospectively examined. Treated mares were classified as being in anestrus or early transition (n=42; if no signs of estrus occurred within 31/2 weeks and the largest follicle remained ≤25 mm in diameter or the first larger follicle(s) of the season regressed without ovulating), or were classified as being in late transition (n=6; if follicular growth achieved 30-40 mm diameter but ovulation had not yet occurred during the breeding season). Thirty-eight mares (38/48; 79%) ovulated in 13.7 ± 7.4 days. Interval to ovulation was negatively associated with size of follicles at onset of native GnRH therapy (P < 0.01). Per cycle pregnancy rate was 53% (19/36 mares bred). Ovulation inducing drugs were administered to 32 of the native GnRH treated mares (2500 units hCG intravenously, n = 20; deslorelin implant [Ovuplant™] subcutaneously, n=12), while 6 mares were not administered any additional drugs to induce ovulation. Per cycle pregnancy rate did not differ among mares treated only with native GnRH (2/5 mares bred; 40% PR), mares treated with native GnRH plus hCG (12/19 mares bred; 63% PR), or mares treated with native GnRH plus Ovuplant™ (5/12 mares bred; 42% PR) (P > 0.10). Additional treatment with either hCG or Ovuplant™ did not alter mean follicle size at ovulation or interovulatory interval (P > 0.10). The proportion of interovulatory intervals > 25 days was not different between mares receiving no additional treatment to induce ovulation (0/4; 0%) compared to mares receiving hCG to induce ovulation (3/8; 38%) (P > 0.10), but the proportion of interovulatory intervals > 25 days was greater for mares receiving Ovuplant™ to induce ovulation (5/7; 71%) compared to mares receiving no additional treatment to induce ovulation (P < 0.05). The proportion of mares with extended interovulatory intervals (i.e., > 25 days) did not differ between mares with follicles < 15 mm diameter (4/8, 50%) and those with follicles > 15 mm diameter (3/11, 27%) at onset of native GnRH treatment (P > 0.10). While concurrent untreated controls were not used in this study, the 79% response rate to twice daily administration of native GnRH is in agreement with other reports using pulsatile or constant infusion as methods of administration, confirming therapy can hasten follicular development and first ovulation of the breeding season. As with previous reports, follicle size at onset of treatment is an important determinant of interval from onset of native GnRH therapy to ovulation. Use of hCG or Ovuplant™ did not enhance ovulatory response in native GnRH treated mares. Use of Ovuplant™ during native GnRH therapy may increase the incidence of post-treatment anestrus in mares not becoming pregnant.     

Relationship between pharmacological induction of estrous and/or ovulation and twin pregnancy in the Thoroughbred mares

The aim of this study was to evaluate the possible relationship of pharmacological induction of estrous and/or ovulation with the occurrence of twin pregnancies in Thoroughbred mares. Out of 680 mares, 356 received one of the following treatments during the estrous cycle in which they became pregnant: injection of 0.5mg of cloprostenol at the ultrasonographic detection of a CL (n=86); injection of 5000 IU human chorionic gonadotropin (hCG) immediately before mating (n=221); injection of 0.5mg of cloprostenol at the ultrasonographic detection of a CL plus injection of 5000 IU hCG immediately before mating on cloprostenol-induced estrous (n=49). The other 324 mares, not treated for induction of estrous or ovulation in the estrous cycle resulting in pregnancy, were used as control group. The occurrence of twin and single pregnancies in treated and control mares underlines that the percentage of twin pregnancy in treated mares (16.6%) was statistically significantly higher (P<0.0001; odds ratio, OR=2.87) than the percentage of twinning in the control group (6.5%). Comparison of the occurrence of twins between treatments revealed a statistically significant difference between mares treated with hCG alone compared to animals given prostaglandin F2alpha (PGF2alpha) plus hCG. The results show a statistically significant difference for each treatment compared to controls, with the least difference (P<0.05; OR=2.18) for the comparison between hCG treatment group and controls, a significance of P<0.01; OR=3.05 for the comparison between PGF2alpha treatment and controls, and a highly statistically significant difference (P<0.0001; OR=6.37) for the comparison between PGF2alpha plus hCG-treated animals and controls.     

Superovulation in cycling mares using equine follicle stimulating hormone (eFSH)

Superovulation would potentially increase the efficiency and decrease the cost of embryo transfer by increasing embryo collection rates. Other potential clinical applications include improving pregnancy rates from frozen semen, treatment of subfertility in stallions and mares, and induction of ovulation in transitional mares. The objective of this study was to evaluate the efficacy of purified equine follicle stimulating hormone (eFSH; Bioniche Animal Health USA, Inc., Athens, GA) in inducing superovulation in cycling mares. In the first experiment, 49 normal, cycling mares were used in a study at Colorado State University. Mares were assigned to 1 of 3 groups: group 1, controls (n = 29) and groups 2 and 3, eFSH-treated (n = 10/group). Treated mares were administered 25 mg of eFSH twice daily beginning 5 or 6 days after ovulation (group 2). Mares received 250 (of cloprostenol on the second day of eFSH treatment. Administration of eFSH continued until the majority of follicles reached a diameter of 35 mm, at which time a deslorelin implant was administered. Group 3 mares (n = 10) received 12 mg of eFSH twice daily starting on day 5 or 6. The treatment regimen was identical to that of group 2. Mares in all 3 groups were bred with semen from 1 of 4 stallions. Pregnancy status was determined at 14 to 16 days after ovulation.In experiment 2, 16 light-horse mares were used during the physiologic breeding season in Brazil. On the first cycle, mares served as controls, and on the second cycle, mares were administered 12 mg of eFSH twice daily until a majority of follicles were 35 mm in diameter, at which time human chorionic gonadotropin (hCG) was administered. Mares were inseminated on both cycles, and embryo collection attempts were performed 7 or 8 days after ovulation.Mares treated with 25 mg of eFSH developed a greater number of follicles (35 mm) and ovulated a greater number of follicles than control mares. However, the number of pregnancies obtained per mare was not different between control mares and those receiving 25 mg of eFSH twice daily. Mares treated with 12 mg of eFSH and administered either hCG or deslorelin also developed more follicles than untreated controls. Mares receiving eFSH followed by hCG ovulated a greater number of follicles than control mares, whereas the number of ovulations from mares receiving eFSH followed by deslorelin was similar to that of control mares. Pregnancy rate for mares induced to ovulate with hCG was higher than that of control mares, whereas the pregnancy rate for eFSH-treated mares induced to ovulate with deslorelin did not differ from that of the controls. Overall, 80% of mares administered eFSH had multiple ovulations compared with 10.3% of the control mares.In experiment 2, the number of large follicles was greater in the eFSH-treated cycle than the previous untreated cycle. In addition, the number of ovulations during the cycle in which mares were treated with eFSH was greater (3.6) than for the control cycle (1.0). The average number of embryos recovered per mare for the eFSH cycle (1.9 ± 0.3) was greater than the embryo recovery rate for the control cycle (0.5 ± 0.3).In summary, the highest ovulation and the highest pregnancy and embryo recovery rates were obtained after administration of 12 mg of eFSH twice daily followed by 2500 IU of hCG. Superovulation with eFSH increased pregnancy rate and embryo recovery rate and, thus, the efficiency of the embryo transfer program.

Introduction

Induction of multiple ovulations or superovulation has been an elusive goal in the mare. Superovulation would potentially increase the efficiency and decrease the cost of embryo transfer by increasing embryo collection rates.[1 and 2] Superovulation also has been suggested as a critical requirement for other types of assisted reproductive technology in the horse, including oocyte transfer and gamete intrafallopian transfer. [2 and 3] Unfortunately, techniques used successfully to superovulate ruminants, such as administration of porcine follicle stimulating hormone and equine chorionic gonadotropin have little effect in the mare. [4 and 5]The most consistent therapy used to induce multiple ovulations in mares has been administration of purified equine pituitary gonadotropins. Equine pituitary extract (EPE) is a purified gonadotropin preparation containing approximately 6% to 10% LH and 2% to 4% FSH.[6] EPE has been used for many years to induce multiple ovulations in mares [7, 8 and 9] and increase the embryo recovery rate from embryo transfer donor mares. [10] Recently, a highly purified equine FSH product has become available commercially.The objectives of this study were to evaluate the efficacy of purified eFSH in inducing superovulation in cycling mares and to determine the relationship between ovulation rate and pregnancy rate or embryo collection rate in superovulated mares.

Materials and methods

Experiment 1

Forty-nine normally cycling mares, ranging in age from 3 to 12 years, were used in a study at Colorado State University. Group 1 (control) mares (n = 29) were examined daily when in estrus by transrectal ultrasonography. Mares were administered an implant containing 2.1 mg deslorelin (Ovuplant, Ft. Dodge Animal Health, Ft. Dodge, IA) subcutaneously in the vulva when a follicle 35 mm in diameter was detected. Mares were bred with frozen semen (800 million spermatozoa; minimum of 30% progressive motility) from 1 of 4 stallions 33 and 48 hours after deslorelin administration. The deslorelin implants were removed after detection of ovulation.[11] Pregnancy status was determined at 14 and 16 days after ovulation.Group 2 mares (n = 10) were administered 25 mg of eFSH (Bioniche Animal Health USA, Inc., Athens, GA) intramuscularly twice daily beginning 5 or 6 days after ovulation was detected. Mares received 250 g cloprostenol (Estrumate, Schering-Plough Animal Health, Omaha, NE) intramuscularly on the second day of eFSH treatment. Administration of eFSH continued until a majority of follicles reached a diameter of 35 mm, at which time a deslorelin implant was administered. Mares were subsequently bred with the same frozen semen used for control mares, and pregnancy examinations were performed as described above.Group 3 mares (n = 10) received 12 mg of eFSH twice daily starting 5 or 6 days after ovulation and were administered 250 μg cloprostenol on the second day of treatment. Mares were randomly selected to receive either a deslorelin implant (n = 5) or 2500 IU of human chorionic gonadotropin (hCG) intravenously (n = 5) to induce ovulation when a majority of follicles reached a diameter of 35 mm. Mares were bred with frozen semen and examined for pregnancy as described above.

Experiment 2

Sixteen cycling light-horse mares were used during the physiologic breeding season in Brazil. Reproductive activity was monitored by transrectal palpation and ultrasonography every 3 days during diestrus and daily during estrus. On the first cycle, mares were administered 2500 IU hCG intravenously once a follicle 35 mm was detected. Mares were subsequently inseminated with pooled fresh semen from 2 stallions (1 billion motile sperm) daily until ovulation was detected. An embryo collection procedure was performed 7 days after ovulation. Mares were subsequently administered cloprostenol, and eFSH treatment was initiated. Mares received 12 mg eFSH twice daily until a majority of follicles were 35 mm in diameter, at which time hCG was administered. Mares were inseminated and embryo collection attempts were performed as described previously.

Statistical analysis

In experiment 1, 1-way analysis of variance with F protected LSD was used to analyze quantitative data. Pregnancies per ovulation were analyzed by x² analysis. In experiment 2, number of large follicles, ovulation rate, and embryo recovery rate were compared by Student,'s t-test. Data are presented as the mean S.E.M. Differences were considered to be statistically significant at p < .05, unless otherwise indicated.

Results

In experiment 1, mares treated with 25 mg eFSH twice daily developed a greater number of follicles 35 mm in diameter (p = .001) and ovulated a greater number of follicles (p = .003) than control mares (Table 1). However, the number of pregnancies obtained per mare was not significantly different between the control group and the group receiving 25 mg eFSH (p = .9518). Mares treated with 12 mg eFSH and administered either hCG or deslorelin to induce ovulation also developed more follicles 35 mm (p = .0016 and .0003, respectively) than untreated controls. Mares receiving eFSH followed by hCG ovulated a greater number of follicles (p = .003) than control mares, whereas the number of ovulations for mares receiving eFSH followed by deslorelin was similar to that of control mares (p = .3463). Pregnancy rate for mares induced to ovulate with hCG was higher (p = .0119) than that of control mares, whereas the pregnancy rate for eFSH-treated mares induced to ovulate with deslorelin did not differ from that of controls (p = .692). Pregnancy rate per ovulation was not significantly different between control mares (54.5%) and mares treated with eFSH followed by hCG (52.9%). The lowest pregnancy rate per ovulation was for mares stimulated with 25 mg eFSH and induced to ovulate with deslorelin. The mean number of days mares were treated with 25 mg or 12 mg of eFSH was 7.8 ± 0.4 and 7.5 ± 0.5 days, respectively. Overall, 80.0% of mares administered eFSH had multiple ovulations compared with 10.3% of control mares.     

Effect of Dose of Cloprostenol on the Interval to Ovulation in the Diestrous Mare: A Retrospective Study

Although the ovulatory effects of prostaglandins are well documented in several domestic species including horses, there has been little attention paid to the use of this ovulatory effect for clinical purposes. Mares often grow large follicles during the luteal phase that may or may not ovulate before progesterone levels decline. Clinical observations of administering prostaglandins in diestrous mares with large follicles suggest that there may be a negative correlation between follicular diameter and interval from treatment to ovulation. The objectives of this study were twofold: to investigate the cloprostenol dose rate effect on interval to ovulation and to confirm the negative correlation between follicular diameter and interval to ovulation. The hypothesis tested was that high doses of cloprostenol given in diestrus to mares with larger follicles would induce ovulation more rapidly than in mares given lower doses or with smaller follicles. To test the hypothesis, a total of 1,234 estrous cycles were induced with different doses of cloprostenol (ranging from 8.75 to 625 μg). All mares had at least one follicle of 28 mm or larger. Dominant follicles were followed by transrectal ultrasound examinations every other day until ovulation was detected. There was a significant effect of dose (P < .000) and follicular diameter (P < .000) on the interval from treatment to ovulation. The shortest mean interval (2.4 days) was observed after administration of 625 μg in mares with follicles 36 mm or larger, whereas the longest (4.9 days) occurred after 8.75 μg in follicles of 28 to 31 mm.     

Cloprostenol in Equine Reproductive Practice: Something More Than a Luteolytic Drug

Prostaglandin F_2α and its analogues (PGF) are widely used in equine reproductive practice. The interval from PGF treatment to ovulation (ITO) varies greatly with a range from 2 to 16 days. Clinical observation suggests that mares mated and ovulated soon after PGF treatment may have poor fertility. Reproductive records of 329 cyclic Thoroughbred mares were analysed retrospectively. The following parameters were analysed: (i) use of cloprostenol; (ii) ITO and (iii) number of ovulations per cycle. According to these parameters, mares were classified into four groups. (i) mares with spontaneous ovulations, n = 57; (ii) mares induced with cloprostenol and ITO = 4–7 days, n = 77; (iii) ITO = 8–10 days, n = 89 and (iv) ITO = ≥11 days, n = 106. Differences in pregnancy (PR) and multiple ovulation (MO) rates among groups were tested using chi‐squared test. PR rates for groups 1–4 were: 73.7%, 46.7%, 64% and 71.7% respectively (p < 0.05). Groups 1 and 2 had lower (p < 0.05) MO rate (24.6% and 20.8%) than groups 3 and 4 (40.4% and 44.3%). It appears that ovulation soon after PGF‐induced luteolysis is detrimental to PR rates. It was found highly significant that in cloprostenol‐treated mares, the MO rate was enhanced without subsequent increase in multiple pregnancies.     

Preovulatory changes in follicular prostaglandins and their role in ovulation in cattle.

Indomethacin (INDO, n = 5) or vehicle (CONTROL, n = 4) was injected into superovulated heifers at 48 and 60 h following a luteolytic cloprostenol injection (0 h). One heifer from each group was ovariectomized (OVX) at 48, 56, 64 and 72 h. The fifth heifer of the INDO group was OVX at 80 h. Blood samples were collected at 0 h, every 2 h between 37 and 47 h, and at the time of each OVX to monitor plasma progesterone (P4) and luteinizing hormone (LH) concentrations. Following each OVX, the number and size of follicles were recorded and the incidence of ovulation determined. Follicular fluid (FF) was aspirated from follicles greater than or equal to 8 mm to determine the concentration of prostaglandins E2 (PGE2) and F2 alpha (PGF2 alpha). The highest PG concentrations were measured in both groups at 24-25 h following the preovulatory LH surge and the PGF2 alpha concentration at this time was significantly greater (p less than 0.01) in the CONTROL group compared to the INDO group. By 35-36 h after the LH surge, 75% (25/34) of the CONTROL follicles had ovulated, whereas there were no ovulations (0/50) on either ovary of the INDO treated heifer. These preliminary results suggest that the preovulatory rise of PGs in FF, particularly PGF2 alpha, is essential for ovulation and that suppression of this rise with indomethacin will inhibit ovulation in heifers.     

Observations on the influence of exogenous gonadotrophins and cloprostenol on ovulation in gilts

We studied the effects of gonadotrophins and prostaglandin (PG) F_2α on ovulation in gilts. Twenty-eight gilts were induced to ovulate using 750 IU pregnant mares serum gonadotrophin (PMSG) and 500 IU human chorionic gonadotrophin (hCG), administered 72 h apart. At 34 and 36 h after hCG, gilts received injections of either 500 μg or 175 μg PGF_2α (cloprostenol), or had no injections. Laparotomies were performed at 36 h (cloprostenol gilts) or 38 h (controls) after hCG injection. The ovaries were examined and the proportion of preovulatory follicles that had ovulated (ovulation percent) was determined at 30 min intervals for up to 6 h. The number of gilts in which ovulation was initiated and the ovulation percent increased (p<0.001) with time, but was not affected by treatment. Many medium sized follicles (≤6 mm) were also observed to ovulate, or to exhibit progressive luteinization without overt ovulation, during the surgical period. A discrepancy between numbers of preovulatory follicles and corpora lutea suggests that luteal counts may not be an accurate assessment of ovulation rate following gonadotrophic stimulation.     

Passive immunoneutralization of endogenous inhibin increases ovulation rate in miniature Shiba goats

We reported previously that passive immunization against inhibin enhances follicular growth and increases the ovulation rate. However, the ovulation rate was not comparable to the number of follicles. Therefore, the aim of this study was to attempt to increase the ovulation rate by increasing the interval between inhibin immunization and PGF2alpha injection. Five miniature Shiba goats were treated with 10 ml inhibin antiserum (inhibin-AS) developed against [Tyro30]-inhibin alpha (1-30). A control group (n=5) was treated with normal goat serum. All animals were injected intramuscularly with 125 microg PGF2alpha 72 h after treatment to induce estrus and ovulation. Blood samples were collected for hormonal assay and the ovulation rate was determined by laparotomy. In contrast to the control group, there was a significant increase in plasma concentrations of FSH in the immunized group. After luteolysis, plasma concentrations of estradiol-17beta increased markedly to a preovulatory peak about 2 folds higher (P<0.01) than that of controls. In addition, the ovulation rate was greater in the immunized group (14.4 +/- 2.2) than in the control group (2.2 +/- 0.6), and the mean number of follicles > or = 4 mm in diameter was 10.0 +/- 0.8 in the inhibin-AS group compared with 2.4 +/- 0.3 in control group. The present results demonstrate that immunoneutralization of endogenous inhibin increased FSH secretions in miniature shiba goats. The increased FSH secretion enhanced follicular growth and increased the ovulation rate. Additionally, increasing the interval between inhibin-AS and PGF2alpha injections (to 72 h) resulted in a greater ovulation rate compared with the previous protocol (48 h). Therefore, inhibin-AS treatment proved to be an effective alternative to exogenous gonadotropin methods for induction of superovulation in goats.     

Effect of exogenous progesterone administration on luteal sensitivity to PGF during the early development of the corpus luteum in mares and cows

The objective of this study was to determine the effect of exogenous progesterone administration at ovulation and during the early development of the CL, on its future sensitivity to a single administration of PGF2a in mares and cows. Horse Retrospective reproductive data from an equine clinic in the UK during three breeding seasons were used. Mares were divided into: control group, cycles with single ovulations; double ovulation group cycles with asynchronous double ovulations; and PRID group: cycles with single ovulations and treatment with intravaginal progesterone device (CIDR) immediately after the ovulation. All mares were treated with d‐cloprostenol (PGF) at either: (i) 88 hr; (ii) 96 hr; (iii) 104 hr; or (iv) 112 hr after the last ovulation. Cattle A total of nine non‐lactating Holstein cows were used. All cows were administered PGF14 d apart and allocated to one of two groups control group GnRH was administered 56 hr after the second PGF administration. CIDR group CIDR was inserted at the same time of GnRH administration. All cows were administered PGF at 120 hr post‐ovulation. The complete luteolysis rate of mares with double ovulation (66.7%) and those treated with exogenous progesterone (68.4%) was significantly higher than the rate of mares with single ovulation (35.6%) at 104 hr. In the cow, however, the treatment with CIDR did not increase the luteolytic response in cows treated at 120 hr post‐ovulation. In conclusion, the degree of complete luteolysis can be influenced by increasing the concentration of progesterone during the early luteal development in mares.     

Ultrasonographic characteristics of the preovulatory follicle preceding and during ovulation in mares

Changes in appearance of preovulatory follicles were observed with real-time ultrasonography prior to and during ovulation in mares. Preovulatory follicles of 15 mares were scanned at < 1 hr intervals for 12 hr or more frequently if displaying signs of impending ovulation. If ovulation was not imminent at the end of 12 hr (n = 2), mares were removed from the trial. Mean follicular diameter decreased 13% from 30 minutes prior to ovulation until the beginning of ovulation. Fifteen to 77 minutes (mean = 41 min) prior to ovulation, a break in or a protrusion of the follicular wall toward the ovulation fossa was visualized in all follicles and was a consistent indicator of impending ovulation. A rapid decrease in size of follicles (ovulation) occurred within a period of 5 to 90 seconds (mean = 42 sec). Little or no fluid remained in the antrum following ovulation. An increase in echogenicity (whiteness) of the follicular wall and echogenic “spots” within the follicle were frequently visualized (13/13, 100% and 7/13, 54% respectively) prior to ovulation; however, prediction of time of ovulation could not be based solely on these individual changes.     

Effects of a gonadotropin-releasing hormone antagonist and altrenogest on luteinizing hormone concentration and ovulation in the mare

The objective of Experiment 1 was to determine a dose and frequency of gonadotropin-releasing hormone (GnRH) antagonist administration to effectively suppress serum luteinizing hormone (LH) concentration and to delay ovulation when administered to mares. The objectives of Experiment 2 were 1) to determine the effects of subcutaneous or intravenous administration of a GnRH antagonist or oral altrenogest on serum LH concentration in the estrual mare; and 2) to determine the effectiveness of human chorionic gonadotropin (hCG) in inducing ovulation in mares with suppressed LH concentrations. In Experiment 1, mares (N = 20) were randomly assigned and treated with either 5% mannitol (control, single subcutaneous injection, 1 mL, at time 0; n = 5); low-dose GnRH antagonist (single subcutaneous injection, 0.01 mg/kg, at time 0; n = 5); frequent low-dose GnRH antagonist (subcutaneous injections, 0.01 mg/kg, at 0, 6, 18, and 24 hours; n = 5); or high-dose GnRH antagonist (single subcutaneous injection, 0.04 mg/kg, at time 0; n = 5). Both the frequent low-dose and high-dose GnRH antagonist treatments resulted in significantly lower LH concentrations compared with controls at 90, 102, and 114 hours after treatment (P < .05). In Experiment 2, mares (N = 38) were randomly assigned and treated with subcutaneous sterile saline (control), altrenogest (oral), subcutaneous GnRH antagonist, or intravenous GnRH antagonist. LH concentration for the altrenogest group was lower than the control group at 3, 4, 18, and 30 hours after treatment (P < .05). LH concentration for both the subcutaneous and intravenous GnRH antagonist groups were lower compared with the control group at several time points (P < .05). Based on these data, dose but not frequency of administration of a GnRH antagonist lowered LH concentration in the estrous mare but did not delay ovulation. In addition, serum LH concentrations can be lowered and ovulation effectively postponed in mares treated with altrenogest followed by administration of hCG. This indicates that serum LH concentrations can be lowered and ovulation effectively postponed in mares treated with altrenogest followed by administration of hCG.     

Efficacy of Deslorelin Acetate (SucroMate) on Induction of Ovulation in American Quarter Horse Mares

Equine clinicians rely on ovulation induction agents to provide a timed ovulation in mares for optimal breeding management. Numerous studies have been performed on the efficacy of human chorionic gonadotropin (hCG) to induce ovulation in the mare, but limited clinical data are available for the new deslorelin acetate product SucroMate. This study was designed to evaluate the efficacy of SucroMate (deslorelin) in comparison with hCG to induce ovulation. American Quarter horse mares (n = 256) presented to Colorado State University for breeding management were used in this study. Mares received either deslorelin or hCG when a follicle ≥35 mm was detected by transrectal ultrasound in the presence of uterine edema. Ultrasonographic examinations were subsequently performed once daily until ovulation was detected. Deslorelin was administered to 138 mares during168 estrous cycles, and hCG was given to 118 mares during 136 estrous cycles. Mares administered deslorelin had a similar (P < .05) higher ovulation rate (89.9%) within 48 hours following drug administration than mares administered hCG (82.8%). There are no effects of season or age on ovulation rates in either treatment group. Twenty-one mares administered deslorelin and 11 mares administered hCG were monitored by transrectal ultrasound every 6 hours to detect ovulation as part of a frozen semen management program. Average intervals from deslorelin or hCG administration to ovulation were 41.4 ± 9.4 and 44.4 ± 16.5 hours, respectively. Results of this study indicate that SucroMate is effective at inducing a timed ovulation in the mare.     

hCG‐Induced Ovulation in Thoroughbred Mares Does Not Affect Corpus luteum Development and Function During Early Pregnancy

Our aim was to compare Corpus luteum (CL) development and blood plasma concentration of progesterone ([P₄]) in thoroughbred mares after spontaneous (Control: C) or human chorionic gonadotrophin (hCG)‐induced ovulation. Lactating mares (C = 12; hCG = 21) were daily teased and mated during second oestrus post‐partum. Treated mares received 2500 IU hCG i.v. at first day of behavioural oestrus when dominant follicular size was >35, ≤42 mm and mated 12–24 h after. Control mares in oestrus were mated with dominant follicular size ≥45 mm. Dominant follicle before ovulation, CL and gestational sac were measured by ultrasound and [P₄] by radioimmunoassay (RIA). Blood sampling and ultrasound CL exams were done at days 1, 2, 3, 4, 8, 12, 16, 20, 25, 30, 35, 40, 45, 60 and 90 after ovulation and gestational sac from day 12 after ovulation in pregnant (P) mares; non‐pregnant (NP) were followed until oestrus returned. Data analyses considered four subgroups: hCG‐P, hCG‐NP, C‐P and C‐NP. Preovulatory follicular size was smaller in hCG mares than in C: 39.2 ± 2.7 mm vs 51.0 ± 1.8 mm (p < 0.0001). All hCG mares ovulated 24–48 h after treatment and presented similar oestrus duration as controls. C. luteum size in P mares showed the same pattern of development through days 4–35, presenting erratic differences during initial establishment. Thus, on days 1 and 3, CL was smaller in hCG‐P (p < 0.05); while in hCG‐NP, CL size was greater than in C‐NP on day three (p = 0.03). Corpus luteum size remained stable until day 90 in hCG‐P mares, while in C‐P a transient and apparently not functional increase was detected on days 40 and 45 (p < 0.05) and the decrease from day 60 onwards, made this difference to disappear. No differences were observed in [P₄] pattern between P, or between NP subgroups, respectively. So, hCG‐induced ovulation does not affect CL development, neither [P₄] during early pregnancy. One cycle pregnancy rate tended to be lower in hCG mares while season pregnancy rates were similar to controls.     

Effect of Deslorelin and/or Human Chorionic Gonadotropin on Inducing Ovulation in Mares During the Transition Period Versus Ovulatory Season

The objective of this study was to compare the rate of ovulation when deslorelin and/or human chorionic gonadotropin (hCG) was administered in mares in both the transition period and the ovulatory season. A total of 200 Paint Horses, Quarter Horses, and crossbred mares were used during the transition season (July to September) and the ovulatory season (October to February) of the southern hemisphere. The animals were divided into four groups. In the control group (n = 72), mares received 1 mL of saline; in deslorelin group (n = 171), 1.5 mg of deslorelin was administered by intramuscular (IM) injection; in hCG group (n = 57), 1,667 IU of hCG was administered IV; and in hCG + deslorelin group (n = 438), 1.5 mg of deslorelin (IM) and 1,667 IU of hCG (IV) were administered. The drugs were administered after follicles ≥35 mm in diameter were identified and grade III uterine edema was observed. At 48 hours after application, ultrasonography was performed to detect ovulation. During the transition period, the ovulation rates were 4.3% (control), 78.6% (deslorelin), 50% (hCG), and 73.3% (hCG + deslorelin). During ovulatory season, the ovulation rates were 16.4% (control), 68.8% (deslorelin), 60% (hCG), and 73% (hCG + deslorelin). There was no significant difference (P > .05) in the ovulation rate between the groups or the periods, except that the control group was lower than all others. Furthermore, both hCG and deslorelin are viable options for inducing ovulation during the transition period before ovulation season.     

Luteoprotective Role of Equine Chorionic Gonadotropin (eCG) During Pregnancy in the Mare

The effects of repeated cloprostenol administration were compared in mares impregnated by horses and mares impregnated by donkeys in order to assess the role of eCG on the development of pregnancy‐associated resistance to the luteolytic and abortifacient effects of PGF2α. Eleven mares impregnated by donkey (mule pregnancy) and 9 mares impregnated by horse (horse pregnancy) were used. Six mares with mule pregnancy and four with horse pregnancy were injected with cloprostenol (0.25 mg) when they were between day 65 and day 75 of pregnancy, and the treatment was repeated 48, 72 and 96 h latter. The rest of the mares remained as controls. Concentrations of eCG were 10 times higher (p < 0.001) in mares impregnated by horses than in mares impregnated by donkeys, and they were not affected by cloprostenol treatment. Luteolysis was completed 30 h after the first cloprostenol injection in mule pregnancies, while mares with horse pregnancies required 96 h and three cloprostenol injections to complete luteolysis. Regression analysis revealed significant associations between eCG concentrations at time 0 and the time required for completion of luteolysis (p < 0.001), foetal death (p < 0.01) and foetal expulsion (p < 0.05). It is concluded that high eCG concentrations in mares impregnated by horses protect the corpora lutea of pregnancy against the luteolytic effects of PGF2α. Low eCG concentrations in mares carrying mule foetuses afford them less protection against the luteolytic effect of PGF2α, and this may be a cause of the increased foetal mortality that occurs between days 60 and 90 of pregnancy in these mares.     

Effect of the administration of PGF2 alpha synchronously with insemination on the pregnancy rate in mares in an insemination program

Investigations in different species including the horse have demonstrated that prostaglandin F2 alpha (PGF2 alpha) is involved in initiating uterine contractions occurring during mating and artificial insemination (A.I.). Uterine contractions play an important role with respect to the sperm transport within the female genital tract. The objective of the present investigation was to evaluate whether the administration of PGF2 alpha (Dinoprost) synchronously to A.I. could have a positive effect on the pregnancy rate in mares. A field study including 346 warmblood-mares (age two to 20 years) belonging to a private studfarm was conducted during the breeding season 1996. The mares were assigned to two groups, group A: mares with spontaneous ovulation, group B: mares in which the ovulation was induced by a GnRH-analog-implant (Deslorelin). PGF2 alpha (Dinoprost) was administered either intramusculary (i.m., 5.0 mg) or intrauterine (i.ut., 0.5 mg diluted in 1.9 ml isotonic NaCl-solution and added to the semen dosis). The study was carried out in a double-blind fashion using isotonic NaCl-solution as a placebo. The mares of each group were randomly assigned to one of the two treatments (i.m. vs. i.ut.). The following first cycle pregnancy rates (day 18) were obtained in mares treated and inseminated once per oestrus: group A1 (PGF2 alpha, i.m.): 54.5% (n = 33); group A2 (placebo, i.m.): 69.7% (n = 33); group A3 (PGF2 alpha, i.ut.): 65.4% (n = 26); group A4 (placebo, i.ut.): 69.8% (n = 32); group B1 (PGF2 alpha, i.m.): 56.5% (n = 46); group B2 (placebo, i.m.): 29.6% (n = 27); group B3 (PGF2 alpha, i.ut.): 66.7% (n = 45); group B4 (placebo, i.ut.): 60.0% (n = 30). The pregnancy rates did not differ between the different groups with the exception of group B2 (p < 0.05). In mares treated repeatedly during the oestrus period (group A, n = 88; group B, n = 23), the pregnancy rates did not differ significantly between treatment and control groups. From the results obtained it is concluded that the PGF2 alpha-application did not show an effect on the pregnancy rate. Further factors influencing the results to a small degree were the stallions, semen age and quality and frequency of insemination per oestrus.     

Effect of purified equine follicle-stimulating hormone on follicular development and ovulation in transitional mares

Horse owners want to have their mares bred as early as possible in the breeding season after February 1. Numerous medical treatments, such as progesterone, dopamine antagonists, and gonadotropin-releasing hormone have been administered to anestrous or transitional mares in an attempt to induce follicular development. Some of these treatments are ineffective or impractical, so there is a need in the horse industry to develop alternative techniques to stimulate follicular development and ovulation early in the breeding season. Twenty transitional mares were assigned to one of two treatment groups. Mares in group 1 (n = 10) served as untreated controls, and mares in group 2 (n = 10) were administered 12.5 mg of purified equine follicle-stimulating hormone (eFSH) (Bioniche Animal Health USA, Inc., Athens, Ga) intramuscularly twice daily for a maximum of 15 consecutive days. Mares were considered to be in transition when the diameter of the largest follicle was ≥25 mm. Once one or more follicles >35 mm were detected, eFSH treatment was discontinued and human chorionic gonadotropin was administered intravenously. The percentage of mares ovulating during the 15-day observation period was compared by means of chi-square analysis. The interval to ovulation and the number of ovulations per mare were compared between the two groups by Student t test. In 8 of 10 mares treated with eFSH follicles developed and ovulation occurred during the 15-day observation period, compared with 0 of 10 control mares. Interval from onset of treatment to ovulation was 7.6 ± 2.4 days for these eight mares. The eight mares were treated for an average of 5.2 ± 1.3 days with eFSH. Thus, the eFSH treatment was effective in advancing the first ovulation of the year in transitional mares.     

Effect of Follicle Size and Follicle-Stimulating Hormone on Ovulation Induction and Embryo Recovery in the Mare

The timing of ovulation is an important component to many equine breeding strategies. The action of luteinizing hormone on ovulation induction has been recognized; however, potential effects of follicle-stimulating hormone (FSH) have been less defined. Objectives of this study were to determine whether (1) mares could be induced to ovulate follicles ≤30 mm; (2) equine FSH (eFSH) has a positive effect on ovulation induction, and (3) ovulation of small follicles would affect embryo recovery. Light-horse mares (n = 12) between 4 and 10 years of age were assigned to treatments when they had a dominant growing follicle with a mean diameter of 24, 28, or 35 ± 2 mm and endometrial edema. Treatments were (1) H35, human chorionic gonadotropin (hCG) at 35 ± 2 mm; (2) F35, eFSH at 35 ± 2 mm; (3) H28, hCG at 28 ± 2 mm; (4) FH28, eFSH and hCG at 28 ± 2 mm; (5) D28, deslorelin (gonadotropin-releasing hormone [GnRH] analog) at 28 ± 2 mm; (6) FH24/H24, hCG or eFSH and hCG at 24 ± 2 mm. Mares’ reproductive tracts were scanned at 24 ± 2-hour intervals after treatment to detect ovulation. Mares were inseminated, and embryos were collected. Numbers of mares that ovulated within 48 ± 2 hours after treatment were: H35, 8/8 (100%); F35, 8/14 (57%); H28, 7/12 (58%); FH28, 9/12 (75%); D28, 3/7 (43%) and FH/H24, 4/14 (29%). The number of mares that ovulated in 48 ± 2 hours for H35 was not different from that for FH28 but was higher (P < .05) than all other groups. Embryo recovery rates, diameters, developmental stages, and morphology scores were not different for mares ovulating 48 hours or less versus more than 48 hours after treatment or among treatment groups. Results of this study demonstrate that follicles ≤30 mm can be induced to ovulate with no effect on embryo recovery or quality, as assessed by stereomicroscopy.     

Increased Frequency of Double Ovulations after Induction of Luteolysis with Exogenous Prostaglandin F2α

The effect of induction of luteolysis by intramuscular treatment with prostaglandin F2α (PGF) on the frequency of double ovulations and formation of hemorrhagic anovulatory follicles (HAFs) was studied. The PGF (5 mg) was given 10 days after ovulation (n = 47 estrous cycles). No treatment or sham injection was used for control estrous cycles (n = 39). After treatment, the mares were scanned by transrectal ultrasonic imaging every 2 days until the largest follicle reached 25 mm and every day thereafter until the outcome of all follicles of at least 25 mm was determined. The frequency of two ovulations during the posttreatment ovulatory period was greater (P < .03) in the treated group (17%) than in the controls (3%). The combined frequency of two ovulations or one ovulation and one HAF also was greater (P < .002) in the treated group (30% vs. 5%). Equine veterinarians should be aware that PGF induction of luteolysis may increase the frequency of double ovulations or HAFs.