Effects of gonadotrophin-releasing hormone and prostaglandin F-2 alpha on corpus luteum function and timing of the subsequent ovulation in the mare

Standard bred mares that were cycling normally were treated beginning on Days 9 or 10 of the oestrous cycle with repeated pulses of GnRH (20 micrograms/h) and/or a single injection of prostaglandin (PG)F-2 alpha (alfaprostol, 3 mg), and were subsequently bled and palpated daily until the next ovulation. GnRH treatment increased serum concentrations of LH and progesterone at 4 days after the start of treatment compared to controls. The combination of PGF-2 alpha + GnRH treatment resulted in an immediate decline in serum progesterone values, and subsequently decreased the interval to next ovulation by 4.5 days compared to controls. Mean serum concentrations of FSH were not different among treatment groups 4 days after the start of treatment, and there was a consistent trend among all treatment groups for decreasing concentrations of FSH within the 6 days before ovulation. We conclude that, under our experimental conditions, pulsatile administration of GnRH provides a short-term luteotrophic stimulus, probably by the elevation in serum LH, but that this stimulus cannot indefinitely prevent the luteolytic effects of exogenously administered PGF-2 alpha. Although GnRH treatment combined with PGF-2 alpha injection hastened the impending ovulation, this regimen was no more effective than PGF-2 alpha treatment alone.     

Regulation of estrus and ovulation in mares with progesterone or progesterone and estradiol biodegradable microspheres with or without PGF2alpha

A single injection of a microsphere preparation, designed to deliver 1.25 gm progesterone and 100 mg estradiol-17beta at a controlled rate, for a duration of 12 to 14 days, produces accurate control of estrus and fertile ovulations in mares. Theatment is followed by PGF(2)alpha injection 14 days after steroid injection. The objectives of the present study were to determine whether estradiol added to the progesterone treatment or PGF(2)alpha administered at the end of the steroid treatment regimen, would improve synchronization of estrus and ovulation. A total of 45 cyclic horse mares was randomly assigned to 1 of 5 treatment groups as follows: Group 1 (control, n=9) sterile microsphere vehicle + sterile PGF(2)alpha vehicle 14 days after treatment with microsphere vehicle; Group 2 (n=9) progesterone and estradiol microspheres + PGF(2)alpha 14 days after treatment with microspheres; Group 3 (n=9) progesterone and estradiol microspheres + PGF(2)alpha vehicle 14 days after treatment with microspheres; Group 4 (n=9) progesterone + PGF(2)alpha 14 days after treatment with microspheres; and Group 5 (n=9) progesterone + PGF(2)alpha vehicle 14 days after treatment with microspheres. Addition of estradiol (P<0.05) or PGF(2)alpha (P<0.05) to the treatment regimen increased synchronization efficary by reducing variation in days to ovulation. All treatments significantly reduced variation in days to estrus compared with that of the controls; however, mares in the progesterone groups had an increased incidence of silent or shortened estrous behavior (<- 2 days) following treatment. Estradiol added to the treatment regimen increased (P<0.05) the number of mares with post treatment estrus > 2 days in duration compared with mares treated with progesterone (78 vs 33%, respectively). Therefore, estradiol and PGF(2)alpha each appear to reduce variation in days to ovulation while estradiol seems to promote better expression of posttreatment estrous behavior.     

Reproductive hormone secretions in pony mares subsequent to ovulation control during late winter

Beginning in December, pony mares were placed under a schedule of increasing light. Starting in February, onset of estrus was checked by daily teasing with a stallion. Mares were randomly assigned to one of three treatments (6 mares per group) administered in March. Treatments were: Group I — 75 mg progesterone injected intramuscularly every day for 10 days in combination with a 1.25 mg injection of PGF₂α on day 7 of progesterone treatment and a 2,000 IU injection of HCG on day 2 of estrus; Group II — a norgestomet ear implant inserted for 10 days in combination with 1.25 mg PGF₂α given 7 days after insertion and 2,000 IU HCG administered on day 2 of estrus; and Group III — same as II except that 2 mg of GnRH rather than HCG were administered on day 2 of estrus. Blood plasma for radioimmunoassay of progesterone, LH and estradiol was collected from the first day of treatment until 14 days after the end of estrus. Also in March, 6 mares were bled daily from the first day of estrus until subsequent estrus or day 21 (control estrus). Although estrus was detected in all mares, 14 of 18 mares ovulated subsequent to treatments and four of the six control estrus mares ovulated. Only among HCG treated mares was the ovulation rate higher (P < .05) than it was in the control estrus group. The interval from last progesterone injection or norgestomet implant removal to estrus did not differ between treatment groups. Concentrations of estradiol and LH were increased for several days around the time of ovulation and tended to be positively correlated with each other. In the mares that did not ovulate, concentrations of LH and estradiol appeared to be lower than in mares that ovulated. In summary, progestins in combination with PGF₂α and increasing light will synchronize estrus in mares during late winter and HCG will hasten ovulation in some mares.     

Effect of xylazine on interovulatory interval and serum progesterone concentrations in the horse mare

The objective of this study was to determine the effect of the alpha(2)-adrenergic agonist, xylazine, on interovulatory interval and progesterone concentrations in the horse mare. Mares were assigned to one of four treatments: Group 1 (controls) received an intramuscular injection (i.m.) of 5 cc saline (n=6), Group 2 received 10 mg prostaglandin F(2alpha) (PGF(2alpha)) i.m. (n=5), Group 3 received 500 mg xylazine i.m. (n=6) and group 4 received an intravenous injection (i.v) of 350 mg xylazine (n=6). Treatment was administered on Day 10 of the estrous cycle (Day 0 = Day of detected ovulation). There was no difference in length of interovulatory interval between PGF(2alpha)-treated mares and control mares (mean +/- SEM; 18.8 +/- 1.0 versus 21.7 +/- 1.6 d). When compared with either xylazine-treated group, PGF(2alpha)-treated mares had a shorter interovulatory interval (18.3 +/- 1.0 d versus 22.2 +/- 0.6 and 22.8 +/- 1.3 d, respectively; P < 0.05). There was no difference in the length of interovulatory interval between control mares and either xylazine-treated group. At the time of treatment all mares had progesterone concentrations > 10 ng/ml, therefore the onset of luteolysis was defined as the day of the estrous cycle when progesterone concentrations decreased below 10 ng/ml. In PGF(2alpha)-treated mares, this event occurred earlier than in any other group (Day 11.2 +/- 0.2 of the estrous cycle versus 16.0 +/- 1.3 for control, Day 15.7 +/- 0.2 for Group 3 and Day 15.2 +/- 0.6 for Group 4; P < 0.002). It was concluded that a single treatment with xylazine, either by an intramuscular or intravenous route, had no significant effect on interovulatory interval or progesterone concentrations in horse mares.     

Antiluteogenic effects of serial prostaglandin F2α administration in cycling mares

A single dose of PGF2α does not consistently induce luteolysis in the equine CL until at least 5 days after ovulation, leading to the erroneous assumption that the early CL is refractory to the luteolytic effects of PGF2α. We hypothesized that serial administration of PGF2α in early diestrus would induce a return to estrus similar to mares treated with a single injection in mid-diestrus, and fertility of the induced estrus would not differ. The objectives of the study were to evaluate the effects of the 2 approaches as reflected by: (1) concentrations of plasma progesterone; (2) interovulatory and treatment-to-ovulation intervals; (3) the proportion of mares pregnant after artificial insemination. The study consisted of a balanced crossover design in which 10 reproductively normal Quarter Horse Mares were exposed to 2 treatments on 2 consecutive reproductive cycles. At detected ovulation (Day 0), mares were randomly allotted to 1 of 2 treatment groups: I, mid-diestrus treatment, administration of a single 10-mg dose of dinoprost tromethamine (PGF2α) im on Day 10; II, early diestrus treatment, administration of 10-mg PGF2α im twice daily on Days 0, 1, and 2 and once daily on Days 3 and 4. Mares in estrus and with a follicle 35 mm or greater in diameter were artificially inseminated with at least 2 billion motile sperm from a fertile stallion. Pregnancy was defined as detection of a growing embryonic vesicle on 2 consecutive examinations approximately 14 days after ovulation. Serial plasma samples were collected throughout the study period, and concentration of plasma progesterone was determined by RIA. A mixed-model ANOVA for repeated measures was used to analyze hormonal data. Interovulatory and treatment-to-ovulation intervals were compared by a paired t test and fertility by a McNemar chi-square analysis. All mares in group I underwent luteolysis after PGF2α administration denoted by mean (±SD) concentration of plasma progesterone of 0.25 ± 0.21 ng/mL detected 2 days after treatment. In group II, mean concentration of plasma progesterone remained below 1.0 ng/mL during treatment and until the onset of the next estrus. The mean interovulatory interval in group I was 18.5 ± 2.0 days compared with 13.1 ± 3.7 days in group II (P < 0.01). Treatment-to-ovulation intervals were 8.5 ± 2.0 days and 13.1 ± 3.7 days for groups I and II, respectively (P < 0.05). In both groups, 9 of 10 mares were pregnant (P = 1.0). Serial PGF2α administration beginning at ovulation consistently prevented luteal function in 10 of 10 mares in the present study without adversely affecting pregnancy rate of post-treatment cycles.     

Progesterone and estradiol in biodegradable microspheres for control of estrus and ovulation in mares

Progesterone and estradiol 17-beta in poly (DL-lactide) microspheres were used to control estrus and ovulation in mares after luteolysis was induced by prostaglandin F(2)infinity. Mares were given a single intramuscular injection of biodegradable poly (DL-lactide) microspheres, 1 day following prostaglandin treatment, containing no hormones (control), 0.625 g progesterone and 50 mg estradiol (low dose), 1.25 g progesterone and 100 mg estradiol (medium dose), or 1.875 g progesterone and 150 mg estradiol (high dose; n=15 mares per group). Mares treated with the low dose had significantly longer intervals (P<0.05) to estrus and ovulation than the control mares; however, low dose mares had shorter intervals (P<0.05) to estrus than high dose mares and shorter intervals to ovulation than medium and high dose mares. Regression analysis indicated that the medium dose was sufficient for maximizing interval to ovulation while the high dose maximized interval to estrus. All groups of mares exhibited similar (P>0.05) post-treatment estrus lengths. A clinical response scoring system based on synchrony of both estrus and ovulation within a treatment group was also used to measure the effectiveness of treatments on control of estrus and ovulation. Clinical response scores did not differ (P>0.05) among treatment groups. Mares were randomly assigned for insemination at the beginning of the first post-treatment estrus. Rates for embryo recovery performed by uterine lavage 7 days post-ovulation did not differ (P>0.05) among groups. Concentrations of serum progesterone increased in mares receiving progesterone and estradiol microspheres. At 10 to 14 days post-injection of microspheres, progesterone concentrations were higher (P<0.05) and remained above 1 ng/ml in the mares receiving the high dose. Progesterone concentrations were also higher (P<0.05) on Days -3 to -1 (Day 0 = day of post-treatment ovulation) in mares receiving the high dose when compared to control mares. Gonadotropin concentrations were suppressed (P<0.05) in the medium and high dose groups.     

Effects of repeated daily injections of prostaglandin F2α on ovaries in mares

In experiment 1, seven groups of pony mares (2 or 3/group) were given either no injections (controls), or 5(5X) or 10(10X) daily subcutaneous (SC) injections of 1.25 mg PGF_2α beginning on days 1, 7 or 13 post-ovulation. Compared to controls (24.5 days), the interovulatory interval was longer (P<.05) for day 7, 10X (33.5 days) and day 13, 10X mares (49.0 days) but was not different for the remaining groups. In experiment 2, nine groups of pony mares (4/group) were given either no injections (controls) or 1(1X) or 10(10X) daily SC injections of 1.25 mg PGF_2α beginning on day 2 of estrus or on days 1, 7 or 13 post-ovulation. Compared to controls (25.0 days), the interovulatory interval was longer (P<.05) for day 13 post-ovulation, 10X mares (40.0 days) and shorter (P<.05) for day 1 post-ovulation, 10X mares (14.5 days). The interovulatory interval for the remaining groups was not different (P>.05) from that for controls. In day 13 post-ovulation, 10X mares, the longer interovulatory interval did not appear to be related to a depression in either peripheral LH concentration (no effect of treatment on LH) or on follicular development (no effect of treatment on diameter of largest follicle). This suggests that circulating levels of gonadotropins were adequate for ovarian follicular development and ovulation and the effect of repeated daily injections of PGF_2α in preventing ovulation was likely exerted at the ovarian level directly on the follicle.     

Estrous cycle characteristics and response to estrus synchronization in mammoth asses (Equus asinus americanus)

Breeding records from a herd of mammoth asses (Equus asinus americanus) maintained on pasture in southeast Texas from 1990 to 1998 were reviewed. Jennies were pasture or hand mated, and estrus was either observed while the jennies were on pasture or when exposed to a jack after being penned. Eighty-one estrus periods and 43 diestrus intervals were recorded in 33 jennies over 4 seasons of the year (January-March, April-June, July-September, and October-December). Estrous cycle length and the duration of estrus were similar among seasons. Over all seasons, estrous cycle length was 23.3 +/- 2.6 d, duration of estrus was 5.9 +/- 2.1 d, and diestrus length was 17.4 +/- 2.6 d (mean +/- SD). During these same 9 yr, 58 injections of PGF2 alpha (5 mg, i.m.) were administered to 38 jennies without regard to stage of estrous cycle. Seventy-six percent (44/58) of the jennies showed signs of estrus after PGF2 alpha treatment, with an interval to estrus of 4.4 +/- 1.6 d and a duration of estrus of 5.6 +/- 1.7 d. Two estrus synchronization schemes were also assessed. Trial 1 was performed in October to November 1996, and Trial 2 was performed in February to March 1998. In Trial 1 (Group PE + PGF, n = 10), each jenny was injected intramuscularly once daily for 10 d with 150 mg progesterone and 10 mg estradiol-17 beta in sesame oil, and PGF2 alpha (10 mg) was injected intramuscularly on the last day of treatment. In Trial 2 (Group PGF-2X, n = 11), each jenny was injected intramuscularly twice, 16 d apart, with 10 mg PGF2 alpha. All Group PE + PGF jennies responded to treatment. One jenny in Group PGF-2X did not respond to either injection of PGF2 alpha, while 2 jennies responded to the first but not the second PGF2 alpha injection (8 of 11 jennies returned to estrus and ovulated after the second PGF2 alpha injection). Duration of estrus was 6.8 +/- 1.9 d for Group PE + PGF and 7.1 +/- 1.8 d for Group PGF-2X jennies. Interval to estrus and interval to ovulation following the last treatment were 9.0 +/- 0.9 d and 14.5 +/- 1.7 d, respectively, in Group PE + PGF jennies, and 4.5 +/- 0.9 d and 10.4 +/- 1.8 d, respectively, for Group PGF-2X jennies. In summary, estrous cycle characteristics of mammoth asses are similar to those reported for standard jennies, and estrus synchronization schemes used in horses are effective in mammoth asses.     

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.     

Regression and resurgence of the CL following PGF2alpha treatment 3 days after ovulation in mares

The present study was designed to characterize and compare the physiology and ultrasonographic morphology of the corpus luteum (CL) during regression and resurgence following a single dose of native prostaglandin F2alpha (PGF) given 3 days after ovulation, with a more conventional treatment given 10 days after ovulation. On the day of pre-treatment ovulation (Day 0), horse mares were randomly assigned to receive PGF (Lutalyse; 10 mg/mare, i.m.) on Day 3 (17 mares) or Day 10 (17 mares). Beginning on either Days 3 or 10, follicle and CL data and blood samples were collected daily until post-treatment ovulation. Functional and structural regression of the CL in response to PGF treatment were similar in both the Day 3 and 10 groups, as indicated by an abrupt decrease in circulating concentrations of progesterone, decrease in luteal gland diameter and increase in luteal tissue echogenicity. As a result, the mean +/- S.E.M. interovulatory interval was shorter (P < 0.0001) in the Day 3 group (13.2 +/- 0.9 days) than in the Day 10 group (19.2 +/- 0.7 days). Within the Day 3 group, functional resurgence of the CL was detected in 75% of the mares (12 of 16) beginning 3 days after PGF treatment, as indicated by transient major (6 mares) and minor (6 mares) increases (P < 0.05 and < 0.1, respectively) in progesterone. Correspondingly, mean length of the interovulatory interval was longer (P < 0.03) in mares with major resurgence (15.8 +/- 1.6 days) than in mares with minor (11.2 +/- 1.2 days) and no resurgences (13.5 +/- 0.3 days) in progesterone. Structural resurgence of the CL in the Day 3 group and functional and structural resurgence in the Day 10 group were not detected. In conclusion, PGF treatment 3 days after ovulation resulted in structural and functional regression of the CL and hastened the interval to the next ovulation, despite post-treatment resurgences in progesterone.     

Luteal function in jennies following PGF(2alpha) treatment 3 days after ovulation

Native PGF(2alpha) and its analogs have been used in the horse mare to manipulate ovarian activity, primarily as luteolytic agents to induce estrus. Despite numerous studies on the effects of these luteolysins in the mare, to date only a single investigation has been conducted in the jenny. The aim of this study was to evaluate the response of the corpus luteum (CL) to a single dose of PGF(2alpha) given 3 days (72h) after ovulation and to establish the plasma progesterone (P4) profile from pre-treatment to post-treatment ovulation in the Martina Franca donkey. Twenty-two jennies were ultrasonographically monitored and treated 72h after the detection of ovulation with 0.075 mg i.m. of R-cloprostenol. From the day of ovulation until ovulation post-treatment, blood was collected daily for P4 determination by enhanced luminescence immunoassay. All the jennies except one, exhibited behavioral signs of PGF(2alpha)-induced estrus within 4 days of treatment lasting 5.4+/-1.16 days. Post-treatment ovulation was also hastened, reducing the interovulatory interval (9.6 days). In response to treatment, plasma P4 concentrations fell to estrus levels and then remained constant until the next ovulation in all but the non-responding animal. Our findings indicate that PGF(2alpha) treatment on Day 3 post-ovulation causes the functional regression of the CL in the jenny, reflected both by the rapid induction of estrus and ovulation and by an abrupt drop in circulating P4 concentrations.     

Effect of beta-carotene supplementation on reproductive performance of dairy heifers

Three trials were conducted to determine the effect of feeding supplemental beta-carotene on reproductive performance of Holstein heifers. In each trial, the animals were randomly assigned to either a control or treatment group. Animals in the treatment groups received 300 mg supplemental beta-carotene daily for the test periods which varied from 6 to 8 weeks in length. To facilitate sample collections and observations, estrus was synchronized with two injections of 25 mg PGF(2alpha) at 11 day intervals. The first injection was after 2 weeks of feeding supplemental beta-carotene. Blood serum beta-carotene concentrations were higher in the heifers fed supplemental beta-carotene as compared to concentrations in control heifers after 1 week of feeding and this difference increased throughout the test periods. The interval from the second injection of PGF(2alpha) to onset of estrus was shorter in the control heifers as compared to that interval in heifers supplemented with beta-carotene (trial 1,44.0 vs 56.0 hr; trial 2, 51.3 vs 70.8 hr; trial 3, 40.7 vs 62.5 hr, respectively). The intervals from PGF(2alpha) administration to the preovulatory LH peak (43.3 vs 61.5 hr) and ovulation (69.3 vs 85.9 hr) were also shorter in the control heifers in trial 3. No significant differences were found among treatments in the number of heifers that expressed estrus, the blood serum progesterone concentrations or the conception rates in any of the three trials.     

Synchronization of estrus in post-partum mares with progesterone and estradiol 17β

Thirty-six mares which foaled over a 10-day period were given 1 to 10 daily intramuscular injections of a combination of 150 mg. progesterone and 10 mg. estradiol 17β. The first injection was given within 18 hours after parturition. Because individual mares foaled on different dates during the 10 day period, commencement of treatment varied, but treatment for all mares ceased on the same day. Teasing and breeding began seven days after the final treatment. The mares were teased daily for 10 days and artifically inseminated every second day until ovulation occurred. The mean interval from the end of treatment to beginning of estrus was 9.4 days (range 7 to 14) and 33 of 26 mares (94.7%) ovulated 10 to 16 days after the final treatment. Both estrus and ovulation were effectively synchronized, resulting in a first estrus pregnancy rate of 80.6% (29 of 36).     

Persistence of the luteal phase following ovulation during altrenogest treatment in mares

Two experiments were conducted to test the efficacy of altrenogest treatment in mares. The response to 15-d altrenogest treatment (Experiment 1) was characterized in 20 mares that were given 22 mg daily of altrenogest in oil (n = 10) or in gel (n = 10) from Day 10 to 25 after ovulation. In 17 mares, luteolysis occurred during altrenogest treatment (Day 17.7 +/- 0.5), while 2 mares retained their corpus luteum (CL), and 1 mare had a diestrous ovulation on Day 16, resulting in a prolonged luteal phase. Ten of the 17 mares in which the CL had spontaneously regressed returned to estrus after the end of treatment, and ovulated 5.7 +/- 0.8 d after the end of altrenogest treatment. Two of these 17 mares ovulated 2 and 3 d after the end of altrenogest treatment but ovulation was not accompanied by estrous behavior, and 5 mares ovulated during altrenogest treatment resulting in an interovulatory interval of 22.4 +/- 1.1 d (range: 20 to 25d). Five mares which ovulated during altrenogest treatment and 2 mares which ovulated during silent estrus after the end of altrenogest treatment failed to regress the CL around 14 d post ovulation, and had a prolonged luteal phase. In Experiment 2, the effect of altrenogest administered from luteolysis to ovulation on duration of the subsequent luteal period was analyzed. In 6 mares altrenogest was begun on Day 14 post ovulation and continued until the hCG-induced ovulation. The interval from ovulation during altrenogest treatment to spontaneous luteolysis was 45.6 +/- 2.4 d (range: 40 to 54d) in altrenogest-treated mares and was significantly longer than in 10 untreated control mares (14.5 +/- 0.3 d, range: 13 to 16d). The results suggest that the oil and gel altrenogest preparations are equally effective in modulating estrous behavior and time to estrus and ovulation. Altrenogest treatment started late in diestrus appears to result in a high incidence of ovulation during treatment and when luteolysis and ovulation occur during treatment; the subsequent luteal phase is frequently prolonged due to failure of regression of the CL.     

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.     

Temporal interrelationships among luteolysis, FSH and LH concentrations and follicle deviation in mares

The effect of altered LH concentrations on the deviation in growth rates between the 2 largest follicles was studied in pony mares. The progestational phase was shortened by administration of PGF2alpha on Day 10 (Day 0=ovulation; n=9) or lengthened by daily administration of 100 mg of progesterone on Days 10 to 30 (n=11; controls, n=10). All follicles > or = 5 mm were ablated on Day 10 in all groups to initiate a new follicular wave. The interovulatory interval was not altered by the PGF2alpha treatment despite a 4-day earlier decrease in progesterone concentrations. Time required for growth of the follicles of the new wave apparently delayed the interval to ovulation after luteolysis. The FSH concentrations of the first post-ablation FSH surge were not different among groups. A second FSH surge with an associated follicular wave began by Day 22 in 7 of 11 mares in the progesterone group and in 0 of 19 mares in the other groups, indicating reduced functional competence of the largest follicle. A prolonged elevation in LH concentrations began on the mean day of wave emergence (Day 11) in the prostaglandin group (19.2 +/- 2.2 vs 9.0 +/- 0.7 ng/mL in controls; P<0.05), an average of 4 d before an increase in the controls. Concentrations of LH in the progesterone group initially increased until Day 14 and then decreased so that by Day 18 the concentrations were lower (P<0.05) than in the control group (12.9 +/- 1.6 vs 20.2 +/- 2.6 ng/mL). Neither the early and prolonged increase nor the early decrease in LH concentrations altered the growth profile of the second-largest follicle, suggesting that LH was not involved in the initiation of deviation. However, the early decrease in LH concentrations in the progesterone group was followed by a smaller (P<0.05) diameter of the largest follicle by Day 20 (26.9 +/- 1.7 mm) than the controls (30.3 +/- 1.7 mm), suggesting that LH was necessary for continued growth of the largest follicle after deviation.     

Factors influencing ovarian activity and sexual behavior of postpartum mares under farm conditions

Management of the postpartum period is one of the most important factors of stud farm medicine. In horses, owing to the long gestation period, the time from parturition to repeat conception needs be short to maintain an optimal yearly foaling interval. For this reason the features of postpartum ovarian activity and sexual behavior were studied under farm conditions. During 2 consecutive breeding seasons, 107 mares on 5 commercial horse farms were monitored after parturition by regular teasing, transrectal ultrasonography and blood sampling for progesterone. Foalings took place from January 1 to June 15. Body condition scoring was carried out within 5 d and at 60 to 65 d after parturition. The first ovulation occurred within 20 d after foaling in 84.1% (90/107) of the mares. The mean intervals from foaling to the first and second ovulations were 17.8 +/- 1.6 d (+/- SEM) and 40.9 +/- 2.7 d (+/- SEM), respectively. The mean intervals from parturition to the first and second ovulation (P < 0.001), the interovulatory interval (P < 0.01), the second follicular phase (P < 0.001), and the time until the first overt estrus (P < 0.01) were significantly longer in mares foaling before the vernal equinox. In the beginning of the breeding season the intervals from parturition to the first ovulation (P < 0.01), to the second ovulation (P < 0.01), and to the first overt estrus (P < 0.001) were significantly longer for primiparous mares than for multiparous animals. There was a tendency for an increased interovulatory interval and for a longer second follicular phase in mares with decreased body condition after parturition (P = 0.069, P = 0.089, respectively). Suckling and breed had no effect on postpartum ovarian activity. We concluded that under field conditions the resumption of cyclic ovarian activity and sexual behavior in mares after foaling are strongly affected by the season of parturition and parity. In some cases, body condition change and other factors may also play a role in influencing postpartum reproductive function.     

Reproductive function of mares given daily injections of prostaglandin F2alpha beginning at day 42 of pregnancy

Mares at Day 42 of pregnancy received daily intramuscular (i.m.) injection of 5 mg of prostaglandin F2alpha (PGF(2alpha)) until the beginning of the first (Group I, n = 3) or second estrous cycle (Group II, n = 2). All mares aborted 3 to 4 d after the first injection; they displayed estrus 2 to 6 d after this injection. As determined by palpation per rectum and serum progesterone levels, each estrus was accompanied by an ovulation. Endometrial cups did not regress after PGF(2alpha) treatment since serum samples from the mares contained pregnant mare serum gonadotropin (PMSG) for at least 30 d after first injection, as determined by mare immunopregnancy test. After the first estrus, two of three mares in Group I displayed a prolonged diestrus (> 25 d). In contrast, the first estrous cycle was short (8 to 12 d) for mares in Group II. Serum progesterone levels in the first 6 d postovulation were lower (P < 0.05) for Group II than for Group I, indicating that formation of the corpus luteum was impaired by daily injections of PGF(2). Results indicate that 1) daily injections of PGF(2alpha) can induce abortion in mares at Day 42 of pregnancy, 2) abortion is followed by estrus and ovulation, 3) the endometrial cups do not regress as a result of this treatment, and 4) daily injections of PGF(2) can impair early corpus luteum development.     

Uterine involution, day and variance of first postpartum ovulation in mares treated with progesterone and estradiol-17beta for 1 or 2 days postpartum

The effects of a single or double regimen of exogenous progesterone and estradiol-17beta (P/E, total dose 300 mg P/20 mg E) were investigated in 50 postparturient Quarter Horse mares. In Trial 1, at 1 and 24 h after foaling, mares were injected with progesterone (150 mg) and estradiol-17beta (10 mg) (n = 7) or 0.9% NaCl (control, n = 13). In Trial 2, within 12 h after foaling, mares were injected with progesterone (300 mg) and estradiol-17beta (20 mg) (n = 13) or 0.9% NaCl (control, n = 17). Mares were examined daily by palpation per rectum and transrectal ultrasonography to determine the day of ovulation. The largest cross sectional diameters of each uterine horn and uterine body were measured ultrasonographically on Day 15 postpartum. Mean uterine diameters did not differ between treatment groups (P > 0.05) in Trial 1, Trial 2 or for combined data for both Trials 1 and 2. For mares bred on the first postpartum estrus pregnancy rates did not differ (P > 0.05) between treatment groups (16/18, 89%) and controls (22/30, 81%) nor was there a difference in mean day to first postpartum ovulation (P > 0.05) between treated and control groups in Trial 1, Trial 2 or Trials 1 and 2 combined. However, fewer (P < 0.05) total P/E treated mares (0/20) ovulated prior to Day 10 postpartum than did control mares (6/30). Variance in days to ovulation was lower (P < 0.05) for P/E treated mares (var = 3.73 days) than for control mares (var = 7.64 days) for data combined from Trials 1 and 2.     

Luteolytic activity of a synthetic prostaglandin and PGF2alpha in heifers.

Two experiments involving 44 cycling heifers were conducted to evaluate the luteolytic activity of a synthetic prostaglandin, AY 24366, and PGF2alpha. Activity was assessed by the decline in progesterone level of peripheral blood and occurrence of estrus. Progesterone concentrations of jugular blood plasma were quantified by radioimmunoassay. In the first experiment, 36 heifers were treated during diestrus with AY 24366 (A-10mg intrauterine, B-30mg intramuscular and C-60mg im) or with PGF2alpha (D-5mg iu, E-15mg im and F-30mg im). Mean progesterone 0, 24 and 48 hours after treatment were A-6.33, 5.55 and 5.06; B-6.35, 2.79 and 3.92; C-5.23, 2.69 and 3.91; D-5.19, 1.50 and 1.51; E-4.69, 0.85 and 0.61; F-6.66, 0.80 and 0.48 ng/ml. Standing estrus was observed in 1, 1, 1, 4, 5 and 6 females in groups A, B, C, D, E and F respectively within 72 hours of treatment. PGF2alpha resulted in significantly (P less than 0.01) lower progesterone at 24 and 48 hours than AY 24366. However, in administration of the latter did significantly (P less than 0.05) lower progesterone at 24 hours. In the second trial six heifers were treated with either 120 or 180mg of AY 24366 im on day 12 of the cycle. Mean progesterone declined from 3.84 to 2.12 ng/ml (P less than 0.01) by 6 hours and to 1.59 ng/ml by 12 hours. Thereafter the decline was gradual and reached a level of 0.65 ng/ml at 72 hours. All six heifers showed standing estrus at 78 +/-2 hours and were inseminated. Two in each group conceived. Doses of 15mg PGF2alpha and 120mg AY 24366 were effective in causing luteal regression, however, the latter caused respiratory discomfort for 5 to 10 minutes post treatment.