Effects of deslorelin or hCG administration on reproductive performance in first postpartum estrus mares

A tendency for deslorelin implants to suppress subsequent follicular growth and delay return to estrus following induced ovulation has been documented in nonlactating mares. To investigate this phenomenon in lactating mares, 22 broodmares in southeast Texas were administered either deslorelin or hCG to induce ovulation in the first postpartum estrus during February and March 2001. Mares were teased daily and examined twice weekly (Tuesdays and Thursdays) by transrectal ultrasonography. When a follicle >35 mm diameter was detected on Tuesday, mares were treated with either 2,500 U hCG administered intravenously or with one implant (2.1 mg) deslorelin administered subcutaneously. Mares were bred every other day until ovulation was detected or until they ceased behavioral estrus, and were examined 16 days after treatment to detect pregnancy. Follicular measurements were recorded for all mares during each examination, and interestrous intervals were recorded for mares not becoming pregnant. Treatment of mares with either hCG or deslorelin resulted in similar ovulatory responses and pregnancy rates. Deslorelin-treated mares had fewer ovarian follicles >20 mm in diameter 16 days after treatment than hCG-treated mares (P < 0.01). Interestrous intervals for mares failing to become pregnant on foal heat breeding were prolonged in deslorelin-treated compared to hCG-treated mares (P < 0.01). Date of treatment was negatively correlated with length of the interestrous interval in deslorelin-treated mares (P < 0.01), but was not correlated with length of interestrous interval in hCG-treated mares (P > 0.10). All mares failing to become pregnant from foal heat breedings became pregnant from later breedings, but the parturition to conception interval was prolonged in deslorelin-treated compared to hCG-treated mares that did not become pregnant on foal heat (P < 0.01).     

Effect of dose of GnRH analog on ovulation in mares

Proper timing of insemination for optimal conception is accomplished by frequent palpations per rectum, by ultrasonography of the preovulatory follicle and/or by treatment with hCG or GnRH. Sustained release of GnRH from implants has been shown to hasten ovulation. Therefore, 2 studies were conducted to evaluate the efficacy of a GnRH analog, deslorelin, for hastening ovulation in nonlactating cyclic mares. The GnRH implant was 2.3 x 3.7 mm and released deslorelin for 2 to 3 days. In Experiment 1, 60 nonlactating, cycling mares were assigned to 1 of 5 doses: 0, 1.2, 1.7, 2.2 and 2.7 mg per implant. Mares were assigned sequentially on the first day of estrus (Day 1). Ovaries were examined per rectum and with ultrasonography every 12 h until ovulation. Once the mares obtained a follicle >30 mm, they were injected subcutaneously with a GnRH implant. The mares were inseminated every other day during estrus with semen from 1 of 3 stallions. Pregnancy was determined with ultrasonography. Experiment 2, 40 nonlactating, cyclic mares were assigned to 1 of 5 treatments (same treatments as in Experiment 1). Data were obtained on interval to ovulation, duration of estrus and pregnancy rates at 12, 18 and 35 d after ovulation. Time to ovulation was shorter (P<0.05) in GnRH-treated mares than in control mares in the Experiment 1. Mean time to ovulation was 68, 49, 48, 47, 44 h in Experiment 1, and 91, 66, 58, 46, 58 h in Experiment 2 for mares given 0, 1.2, 1.7, 2.2 and 2.7 mg/mare in the 2 trials. Averaged for both experiments, the proportion of mares ovulating within 48 h of treatment was 40, 75, 85, 90 and 90% for 0, 1.2, 1.7, 2.2 and 2.7 mg/mare. For both experiments, there was no effect of GnRH on pregnancy rate. In summary, a subcutaneous implant containing GnRH analog induced ovulation in most mares by 48 h of injection, and there was no advantage of doses higher than 2.2 mg/mare.     

Effects of a new injectable short-term release deslorelin in foal-heat mares

Mares treated with subcutaneous deslorelin implants on the first postpartum estrus early in the breeding season had significant reductions in the number of large follicles at early pregnancy examinations and delayed return to estrus (in mares that failed to become pregnant); these adverse effects were attributed to a prolonged release of the drug from the implant. In 2003, an injectable short-term release (<24 h) deslorelin product became available. The objective of this study was to determine if this product would hasten ovulation in early foaling first postpartum estrus mares without reducing the number of large follicles at early pregnancy examination (14-15 days postovulation). Beginning 5-6 days postpartum, first postpartum estrus (foal-heat) mares were teased daily and examined thrice weekly (Tuesday, Thursday and Saturday) by transrectal ultrasonography. Mares in estrus with a follicle > or = 34 mm diameter on Tuesdays or Thursdays were alternately assigned to: Treatment 1, n = 17; 1.5 mg injectable short-term release deslorelin, or Treatment 2, n = 16; Control (no treatment). The schedule allowed accurate determination of the number of mares ovulating within 2 days of treatment (i.e., ovulations detected on Thursday or Saturday). Mares were mated on the day of treatment and at 2-day intervals until either ovulation was confirmed or until behavioral estrus ceased. Transrectal ultrasonography was done 14-15 days after ovulation to assess ovarian follicles and pregnancy status. Fewer covers were required and more mares ovulated within 2 days of treatment in deslorelin-treated versus Control mares (P < 0.01). Pregnancy rates were normal (69%) in deslorelin-treated mares. The number of large follicles 14-15 days after ovulation did not differ between deslorelin-treated and Control mares (P > 0.10), suggesting follicular suppression did not occur with this formulation of deslorelin.     

The effect of GnRH on induction of follicular development and ovulation in anovulatory and ovulatory mares

The effects of estradiol cypionate (ECP) and GnRH injections were tested on mares during January and February. Sixteen mares were blocked on their ovarian status and equally allotted to two groups. Group one received daily injections of 500 μg ECP (im) for 14 days followed by a 21 day period of twice daily injections of 200 μg GnRH (im). Group two received the carrier vehicle.Mean length of diestrus of ovulatory mares was 14.3 ± 1.6 days and 17.8 ± 3.5 days for treated and control groups respectively. Corresponding estrus lengths were 8.0 ± 1.4 days and 6.3 ± 2.1 days. Plasma LH levels, number of follicles < 20 mm, number of follicles > 20 mm and diameter of the largest follicle in ovulatory mares were not significantly affected by treatment with ECP or GnRH.Anovulatory mares treated with ECP and GnRH exhibited estrus more frequently (54% and 70% of the time) than sham injected controls (17% and 15% of the time). Plasma LH levels were significantly elevated (P<.05) in anovulatory mares treated with GnRH. Also more follicles < 20 mm (P<.09) were detected on the ovaries of GnRH treated mares than on those of control mares. Effects of the treatment were transient since LH levels and ovarian activity were similar in both mare groups after cessation of treatment.     

Deslorelin on Day 8 or 12 postovulation does not luteinize follicles during an artificially maintained diestrous phase in the mare

Practical estrus synchronization schemes are needed for mares. The Ovsynch synchronization protocol for cattle involves the administration of gonadotropin-releasing hormone (GnRH) to induce ovulation or luteinization of dominant follicles during the luteal phase and prostaglandin 7 days later to cause regression of any luteal tissue and development of a preovulatory follicle. An Ovsynch-type synchronization program potentially could be developed for horses if luteinization or ovulation of diestrous follicles occurred in response to GnRH treatment. The objective of this study was to determine if administration of the GnRH agonist, deslorelin acetate, on Day 8 or 12 postovulation would induce luteinization or ovulation of diestrous follicles in the mare. The model used was cycling mares maintained in an artificial luteal phase by administration of a synthetic progestin following prostaglandin-induced luteal regression. On the day of ovulation, 21 light horse mares were randomly assigned to one of three groups: (1) no GnRH, altrenogest from Days 5 to 15 postovulation with prostaglandin on Day 15; (2) GnRH on Day 8, altrenogest from Days 5 to 15 with prostaglandin given on Day 6 to induce luteolysis of the primary corpus luteum, an implant containing 2.1mg of deslorelin acetate inserted on Day 8 and removed on Day 10, with a second prostaglandin treatment on Day 15; (3) GnRH on Day 12, altrenogest from Days 9 to 19, prostaglandin on Day 10, a deslorelin acetate implant injected on Day 12 (subsequently removed on Day 14), and a second dose of prostaglandin administered on Day 19. Follicular development was monitored every other day from Day 5 until a 30-mm sized follicle was observed, and then daily to detection of ovulation. Serum progesterone concentrations were determined daily for 12 consecutive days. Progesterone concentrations in Group 1 remained elevated until approximately Day 12 postovulation. Prostaglandin administration on Day 15 resulted in complete luteolysis in all seven mares. In Group 2, progesterone concentrations in six of seven mares declined to baseline after prostaglandin treatment. No increase in serum progesterone was noted in any of the six mares that were given GnRH on Day 8, including three mares that had diestrous follicles > or =30mm in diameter at the time of treatment. Similarly, progesterone concentrations in six of seven mares in Group 3 declined to baseline after prostaglandin and there was no increase in progesterone after administration of GnRH on Day 12. No ultrasound evidence of luteinization or ovulation of diestrous follicles were noted after GnRH administration in any mares of Group 2 or 3. In conclusion, administration of the GnRH agonist deslorelin acetate to mares failed to induce luteinization or ovulation of diestrous follicles. Consequently, the Ovsynch program (as used in cattle) has little efficacy for synchronization of estrus in mares.     

Control of estrus in the lactating postpartum mare with fluprostenol (ICI-81,008)

During the 1976 breeding season 68 mares (56 Thoroughbred and 12 Quarter Horse) were allotted to 4 treatment groups. On day 7, 8 or 9 post-ovulation or day 6, 7 or 8 after foal heat each of 28 mares was injected intramuscularly with 250 μg of the prostaglandin analogue ICI-81,008 (generic name fluprostenol). In the second group, 32 mares were bred at foal heat. Group 3 consisted of 9 mares, which were passed at foal heat and bred at the second postpartum estrus. In group four, 12 of the mares which had failed to conceive at foal heat were bred at the second estrus. Twenty-four prostaglandin treated mares returned to estrus 4.2±.4 days post-injection. The interestrual interval was shorter (P<.05) for prostaglandin treated mares (11.4±.6 days) than for previously mated second heat control mares (19.2±2.3). Also, the interval between parturition and second estrus was decreased (P<.05) in treated mares (24.5±.8 days) compared to group three (32.8±2.5 days). Duration of the second postpartum estrus was similar for both treated and control mares. Plasma progesterone levels in prostaglandin treated mares as determined by RIA were 7.5±.5 and 6.9±.5 ng/ml before treatment and at l hour post-injection, then dropped precipitously to near undetectable levels at 48 hours after injection. Pregnancy rate at 45 days was significantly higher (P<.05) in prostaglandin treated mares (78%) as compared to foal heat mares (48%) and mares which were passed at foal heat and bred at the second postpartum estrus (44%).     

Sexual behavior following introduction of a stallion into a group of mares

A rested stallion was introduced daily for 30 days into each of three herds of 20 mares. Observations of sexual and mating behavior were made for one hour. The stallion remained with a herd until another stallion was introduced the following day. Other mares were isolated from stallions and were bred by artificial insemination. The diameter or growth rate of the preovulatory follicle for the six days preceding ovulation and the length of the interovulatory interval for mares which did not become pregnant were not affected significantly by the presence of a stallion. The number of breedings per hour of observation (2.4 ±0.2) and the length of the interval from introduction of a stallion into the herd of mares to first breeding (12 ±1 min) were significantly different among stallions, but the length of the interval between breedings (17 ±2 min) was not. The mean number of breedings per stallion per hour was not affected significantly by the number of posturing (estrous) mares. The number of times that the stallion rebred the same mare when more than one mare postured during the observation hour (49%) was greater (P<0.01) than what would be expected to occur by chance (30%). The hypothesis that breeding occurs preferentially in those estrous mares that are closest to ovulation was not supported, except for significantly lower breeding activity in posturing mares on days 8 and 7 before ovulation and on days 0 (day of ovulation) and ?1 (26% bred) than on days 6 to 1 (52%).     

Use of deslorelin short-term implants to induce ovulation in cycling mares during three consecutive estrous cycles

Alternatives to human chorionic gonadotropin (hCG) for inducing ovulation in cycling mares over several consecutive cycles were explored. Placebo, one, three or five short-term implants each containing 2.2 mg of gonadotropin-releasing hormone (GnRH) analogue (deslorelin) were administered to cycling mares after identification of a follicle over 30 mm. Mares were treated over three consecutive cycles, and artificially inseminated during the third cycle only. Serum was assayed for concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and progesterone during each cycle. All deslorelin doses decreased the interval to ovulation (4.0 days, 2.6 days, 2.4 days and 2.0 days), increased the proportion of mares ovulating within 48 h (20.0%, 83.3%, 73.3% and 85.7%), and decreased the diameter of the largest follicle at ovulation (45.8 mm, 38.0 mm, 41.0 mm and 37.2 mm) for the placebo, 2.2 mg, 6.6 mg and 11.0 mg groups, respectively (P < 0.05). The interovulatory interval was lengthened in the 11.0 mg group compared with all other groups (21.2 days, 21.8 days, 26.4 days and 32.7 days for the placebo, 2.2 mg, 6.6 mg and 11.0 mg groups, respectively, P < 0.05). No differences (P > 0.05) were detected in serum concentrations of progesterone or pregnancy rate among the groups. Cycle number had no effect (P > 0.05) on the reproductive parameters or serum concentrations of progesterone. Concentrations of LH were higher in Cycle 3 in the 6.6 mg and 11.0 mg groups, and FSH concentrations were decreased in Cycles 2 and 3 in treated mares. Deslorelin was efficacious for inducing ovulation in cycling mares with no diminished activity over three consecutive cycles. However, ovarian suppression and changes in serum concentrations of gonadotropins were noted at higher doses.     

Induction of estrus and ovulation: why some mares respond and others do not

The two most common procedures for breeding management of mares involve induction of luteolysis and induction of ovulation. Although both of these events are usually achieved, physiologic conditions affect the timing of the response. In a diestrus mare treated with prostaglandin F(2alpha) (PGF), or a PGF analogue, it is well documented that, on average, the interval from treatment to the onset of estrus is 3-4 days, whereas ovulation occurs 8-10 days after treatment. However, the diameter of the ovulatory follicle, as well as its status at the time of PGF treatment, determines the intervals from treatment to onset of estrus and to ovulation; these intervals can range from 48h to 12 days. Ovulation is routinely induced with human chorionic gonadotropin (hCG), recombinant LH (rLH), or the GnRH analogue Deslorelin. On average, ovulation occurs approximately 36h after treatment, but the effectiveness of any of these treatments can be affected by the stage of the estrus cycle, follicle size and maturity.     

Synchronization of ovulation in yaks (Poephagus grunniens L.) using PGF(2alpha) and GnRH

The objective of this study was to test the efficacy of estrus synchronization in yaks using the Ovsynch protocol. To eight non-lactating cycling yaks were administered GnRH analogue followed by PGF(2alpha) analogue treatment 7 days later and further injected with a second injection of same GnRH analogue 2 days after the PGF(2alpha) analogue administration. Ovulation was detected by rectal palpation at 2 h intervals from the initial signs of estrus till ovulation. For LH and progesterone the blood samples were collected at 15 min intervals starting from 1 h prior to the second injection of GnRH analogue until 6 h later and further at 2 h intervals till 2 h after the ovulation. Ovulation was detected in seven out of eight yaks after Ovsynch treatment. The mean time interval from the second GnRH injection to ovulation was 24.8+/-1.95 h with a range of 20-34 h and the mean interval from the LH peak and ovulation was 19.96+/-1.91 h with a range of 14-29 h. The high degree of ovulation synchronization could be attributed to the highly synchronized LH peaks in the treated animals. It was concluded that this estrus synchronization protocol could be applied for fixed time AI in yak.     

Management and fertility of mares bred with frozen semen.

Semen quality, mare status and mare management during estrus will have the greatest impact on pregnancy rates when breeding mares with frozen semen. If semen quality is not optimal, mare selection and reproductive management are crucial in determining the outcome. In addition to mare selection, client communication is a key factor in a frozen semen program. Old maiden mares and problem mares should be monitored for normal cyclicity and all, except young maidens, should have at least a uterine culture and cytology performed. Mares with positive bacterial cultures and cytologies should be treated at least three consecutive days when in estrus with the proper antibiotic. With frozen semen, timing the ovulation is highly desirable in order to reduce the interval between breeding and ovulation. The use of ovulation inducing agents such as human chorionic gonadotropin (hCG) or the GnRH analogue, deslorelin, are critical components to accurately time the insemination with frozen semen. Most hCG treated mares ovulate 48h post-treatment (12-72h) while most deslorelin (Ovuplant) treated mares ovulate 36-42h post-treatment. However, mares bred more than once during the breeding cycle appear to have a slight but consistent increase in pregnancy rate compared to mares bred only once pre- or post-ovulation. In addition, the "capacitation-like" changes inflicted on the sperm during the process of freezing and thawing appear to be responsible for the shorter longevity of cryopreserved sperm. Therefore, breeding closer to ovulation should increase the fertility for most stallions with frozen semen. Recent evidence would suggest that breeding close to the uterotubal junction increases the sperm numbers in the oviduct increasing the chances of pregnancy. Post-breeding examinations aid in determining ovulation and uterine fluid accumulations so that post-breeding therapies can be instituted if needed. Average pregnancy rates per cycle of mares bred with frozen semen are between 30 and 40% with a wide range between sires. Stallion and mare status are major factors in determining the success of frozen semen inseminations. Pregnancy rates are lower for barren and old maiden mares as well as those mares treated for uterine infections during the same cycle of the insemination. To maximize fertility with frozen semen, a careful selection of the stallions and mares, with proper client communication is critical. Dedication and commitment of mare owner and inseminator will have the most significant impact on the pregnancy rates.     

Response of plasma LH and FSH to gonadotropin-releasing hormone in pony foals and ovariectomized pony mares

Plasma FSH and LH response to a synthetic GnRH analog was measured in adult ovariectomized pony mares (OVX) and in pony foals (<70 days of age) during late spring (May-June). FSH and LH responded in a similar fashion (200% increase) in the OVX mare, which is different from other reports for intact mares. There was a greater mean response to a comparable dose of GnRH in the prepubertal foal for both FSH (500%) and LH (900%) than in the OVX mare. There was a positive correlation between age and the maximum FSH response to GnRH in male and female foals. The LH response was positively correlated with age in male foals, but not in females. The response to GnRH in the prepubertal foals was consistent with the previously observed patterns of gonadotropin secretion during this age period.     

Removal of deslorelin (Ovuplant) implant 48 h after administration results in normal interovulatory intervals in mares

Deslorelin implants, approved for use in inducing ovulation in mares, have been associated with prolonged interovulatory intervals in some mares. Administration of prostaglandins in the diestrous period, following a deslorelin-induced ovulation, has been reported to increase the incidence of delayed ovulations. The goals of the present study were: (1) to determine the percentage of mares given deslorelin that experience delayed ovulations with or without subsequent prostaglandin treatment, and (2) to determine if removal of the implant 48 h after administration would effect the interval to subsequent ovulation. We considered interovulatory intervals to be prolonged if they were greater than the mean +/- 2 standard deviation (S.D.) of the control group in study 1 and the hCG group in study 2. In study 1, we retrospectively reviewed reproduction records for 278 mares. We either allowed the mare to ovulate spontaneously or induced ovulation using deslorelin acetate implants or hCG. We administered prostaglandin intramuscularly, 5-9 days after ovulation in selected mares in each group. A higher percentage of mares which were induced to ovulate with deslorelin and given prostaglandins had a prolonged interovulatory interval (23.5%; n = 16), as compared to deslorelin-treated mares that did not receive prostaglandins (11.1%; n = 5). In study 2, we induced ovulation in mares with hCG (n = 47), a subcutaneous deslorelin implant via an implanting device provided by the manufacturer (n = 28), or a deslorelin implant via an incision in the neck (n = 43) and we removed the implant 48 h after administration. We administered prostaglandin to all mares 5-9 days after ovulation. In study 2, mares from which the implant was removed had a normal ovulation rate and none had a prolonged interval to ovulation. Administration of prostaglandin after deslorelin treatment was associated with a longer interval from luteolysis to ovulation than that found in mares not treated with deslorelin. Prostaglandin administration during diestrus may have exacerbated the increased interval to ovulation in deslorelin-treated mares. We hypothesize that prolonged secretion of deslorelin from the implant was responsible for the extended interovulatory intervals.     

Induction of ovulation during the postpartum period in the thoroughbred mare with a prostaglandin analogue,Synchrocept™

Breeding of mares at the foal heat is desirable, but the low pregnancy rate and the not infrequent occurrence of subsequent anestrus are a deterrent. A PGF_2a analogue, Synchrocept^? (prostalene), as a single 2 mg subcutaneous injection was used to induce ovulation, either during the corpus luteum phase subsequent to a foal heat (test group 1) or at a fixed interval of 20 days from parturition (test group 2) in mares that also had a foal heat. Mares with no signs of a foal heat were treated 20 days postpartum (test group 3). A total of 80, mostly Thoroughbred, mares were treated in more than 15 breeding centers in Northern Germany between mid-January and mid-May 1975. At the same locations and over the same time periods, 60 control mares (control group 1) were bred at the foal heat and 40 additional control mares (control group 2) were bred later than 25 days postpartum.In groups 1 and 2, treatment induced estrus in 95% of the mares and in group 3 estrus was induced in 68% of the mares. Of these animals, 92% were bred and 81% conceived on the average 28 days after parturition. The number of services per conception in mares treated was 25% lower (2.5) than in controls (3.2). Synchrocept acted as a luteolysin, but seems also to have induced heat and estrus in animals with plasma progesterone levels below 1 ng/ml, as has been reported previously. Neither mares nor foals showed detectable clinical signs of drug intolerance.     

Active immunization of intact mares against gonadotropin-releasing hormone: differential effects on secretion of luteinizing hormone and follicle-stimulating hormone

Five lighthorse mares were actively immunized against gonadotropin releasing hormone (GnRH) to determine the relative importance of this hypothalamic hormone in the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Five mares immunized against the conjugation protein served as controls. Mares were initially immunized in November and received secondary immunizations 4 wk later, and then at 6-wk intervals until ovariectomy in June. All mares immunized against GnRH exhibited an increase (p less than 0.01) in the binding of tritiated GnRH by plasma, an indication that antibodies against this hormone had been elicited. Concentrations of LH, FSH and progesterone in weekly blood samples were lower (p less than 0.05) in GnRH-immunized mares than in controls after approximately 4 mo of immunization. However, the LH concentrations were affected to a greater degree than were FSH concentrations. All five control mares exhibited normal cycles of estrus and diestrus in spring, whereas no GnRH-immunized mare exhibited cyclic displays of estrus up to ovariectomy. All mares were injected intravenously with a GnRH analog (which cross-reacted less than 0.1% with the anti-GnRH antibodies) in May, after all control mares had displayed normal estrous cycles, to characterize the response of LH and FSH in these mares; two days later, the mares were injected with GnRH. The LH response to the analog, which was assessed by net area under the curve, was lower (p less than 0.01) by approximately 99% in mares immunized against GnRH than in control mares. In contrast, the FSH response to the analog was similar for both groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Failure of hCG to support luteal function in bitches after estrus induction using deslorelin implants

Induction of estrus with deslorelin implants was followed by abortions in bitches that conceived during the induced estrus. Lowering the deslorelin dose and choosing a better implantation site prevented the abortions. This study investigated the hypothesis that induction of estrus with deslorelin is followed by reduced serum progesterone concentrations (SPC) during the ensuing diestrus. Assuming that reduced luteal function resulted from reduced LH secretion due to hypophyseal down-regulation of GnRH receptors, the effect of human chorionic gonadotropin (hCG) treatment on the SPC of diestrous bitches was also investigated. In Experiment 1, 10 spontaneously cycling bitches served as controls, whereas estrus was induced with deslorelin implants in 24 others. In Experiment 2, six diestrous bitches were treated with a single dose of hCG between Days 39 and 45 of diestrus. The SPC was lower in deslorelin-induced bitches from Days 35 to 56 of diestrus and hCG increased SPC during the first 24 h after treatment, followed by a dramatic decline thereafter. Although SPC recovered in pregnant bitches, it remained much lower (< or = 1 ng/mL) than in untreated, non-pregnant bitches. The suppression of progesterone secretion after hCG treatment suggested that decreased luteal activity in deslorelin-induced bitches may not be a simple consequence of down-regulation of hypophyseal GnRH receptors.     

Effects of ovarian input on GnRH and LH secretion immediately postovulation in pony mares

The potential involvement of ovarian factors in regulating GnRH and LH postovulation was studied in ovarian intact (Group 1; n=3) and ovariectomized (OVX; Group 2; n=3) mares (OVX within 12 hr of ovulation). Blood samples were collected every 10 min for 6 hr from jugular vein (JV) and intercavernous sinus (ICS) during estrus and on Day 8 postovulation for LH and GnRH analysis. Additionally, JV samples were collected twice daily (12-hr intervals) for 30 days for LH and progesterone (P4) analysis. A significant treatment x day effect (P<0.0001) describes declining plasma LH concentrations in intact mares, and regression analysis indicated that response curves were not parallel (P<0.001). Plasma LH concentrations remained elevated in OVX mares. LH increased further in OVX mares by Day 8 post-OVX (P<0.06), reflecting the increased (P<0.07) LH episode amplitude. GnRH decreased from estrus to Day 8 in both groups reflecting an effect of sampling period (P<0.03). GnRH episode amplitude declined (P<0.08) from estrus (62.8+/-3.1 pg/mL) to Day 8 (46.3+/-3.1 pg/mL) in OVX mares, but not in control mares (intact estrus, 36.5+/-6.4; intact Day 8, 37.5+/-7.3; OVX estrus, 62.8+/-3.1; OVX Day 8, 46.3+/-3.1 pg/mL). In conclusion, we propose that postovulatory LH decline requires ovarian feedback in mares, and that OVX alters GnRH secretory dynamics such that LH concentrations does not decline postovulation and, in fact, is further elevated with time after OVX.     

Induction of parturition in the mare with prostaglandin F2α

Thirty-one mares of Quarter Horse and Thoroughbred breeding were utilized in two experiments to evaluate the efficacy of prostaglandin F₂α (PGF₂α)_for induction of equine parturition and to monitor the effects of this treatment on viability of the resulting foals.Three of five mares given 5 mg PGF₂α (im) on day 338 of gestation foaled 19.6 ± 8.2 hr postinjection. In the second experiment immediately following 3 daily injections of 10 mg estradiol cypionate (ECP) given on days 326, 327 and 328 of gestation, seven mares were infused (iv) with PGF₂α at the rate of 1.3 mg/hr for 24 hr or until parturition occurred. Four of the seven mares foaled in 8.8 ± 1.8 hr after the start of infusion. Side effects including sweating, hypothermia, increased respiration rate and diarrhea were evident in both injected and infused mares, but effects were transient. Neither the injection, nor infusion route of administration of PGF₁α adversely affected the viability of foals. However, some mares induced to foal 12 days prior to expected parturition had foals with slightly weaker pasterns than those of control mares.     

Luteolysis and cycle synchronization with a new prostaglandin analog for artificial insemination in the mare

Over a period of three years, 165 cyclic or anestrous Hanoverian mares received 177 treatments with 2, 3 or 4 mg of a novel luteolytic prostaglandin analog, K 11941. Heat and ovulations, indicating luteolysis, were observed after an average of 3.98 and 7.62 days, respectively, in 88.04% of 142 treated cyclic, postpartum and anestrous mares, and mares after an early loss of the conceptus. All doses tested were effective in inducing luteolysis as confirmed by determination of progesterone blood levels in samples collected daily, in 70 of 80 mares studied (87.5%). Attempts to achieve control of the cycle by various methods revealed that with K 11941 given once or twice, alone or in combination with hCG and/or an GnRH analog (Hoe 766), one can effectively concentrate estrus periods and follicular growth patterns, but can neither synchronize nor concentrate ovulations. Since most of the mares treated were confirmed problem mares, the pregnancy rate of 40.0% from first insemination at drug induced estrus, was regarded as satisfactory when compared to a pregnancy rate of 43.8% obtained with natural breeding in the same population. No drug related clinical signs of side-effects were observed.     

The effects of prostalene and alfaprostol as uterine myotonics, and the effect on postpartum pregnancy rate in the mare following daily treatment with prostalene

The effects of oxytocin and two prostaglandin (PG) F(2)alpha analogues, prostalene and alfaprostol, on uterine pressure in the mare were measured using balloon-tipped catheters connected to pressure transducers. The PGF(2)alpha analogues caused increased uterine pressure beginning 7 to 15 min postinjection and persisting for the duration of each 60 min recording session. Forty postpartum mares of light-horse breed were used to evaluate the effects of prostalene on postpartum pregnancy rate. Eighteen mares were injected by aseptic technique subcutaneously with 1 mg prostalene twice daily, beginning on the day of foaling (Day 0) and continuing for 10 consecutive days (Day 10) or until the mare was first bred at foal heat. Twenty-two postpartum mares were injected with 1.0 ml sterile saline by the same technique as the controls. Of treated mares, 76.9% were diagnosed pregnant after breeding versus 44.4% of the control mares (P = 0.07). Of treated mares, 66.7% bred at their second postpartum estrus became pregnant versus 28.6% of control mares (P = 0.03). Prostalene, given at 1 mg twice daily for 10 d postpartum, produced an increased pregnancy rate after both foal heat and second postpartum estrus breedings in the mare.