Anestrous Mares Research Articles

Use of gonadotropin-releasing hormone for hastening ovulation in transitional mares.

Natural GnRH and its analog have potential for hastening ovulation in mares. A study was conducted to evaluate the efficacy of a GnRH agonist given either as an injectable or s.c. implant for induction of ovulation in mares. Forty-five seasonally anestrous mares (March) were assigned to one of three groups (n = 15/group): 1) untreated controls; 2) i.m. injection of the GnRH agonist buserelin at 12-h intervals (40 micrograms/injection for 28 d or until ovulation) and 3) GnRH agonist administered as a s.c. implant (approximately 100 micrograms/24 h for 28 d). Six mares per group were bled on d 0, 7, 14 and 21 after injection or insertion of implant. Samples were taken at -1, -.5 and 0 h and at .5, 1, 1.5, 2, 4, 6 and 8 h after GnRH. Additional daily samples were drawn for 28 d after injection or until ovulation. Samples were assayed for concentration of LH and FSH. Progesterone concentrations were determined in samples collected on d 4, 6 and 10 after ovulation. Number and size of follicles and detection of ovulation were determined by ultrasonography. Number of mares induced to ovulate within 30 d was 0 of 15, 7 of 15 and 9 of 15 for groups 1, 2 and 3, respectively. During treatment, follicle sizes were smaller for mares in group 3 (implant). The LH response to GnRH agonist (area under curve) was similar among groups at d 0 but was greater (P less than .05) for mares in group 3 on d 7 and 14 and groups 2 and 3 on d 21 than for controls. A similar pattern was detected for peak concentrations of LH after GnRH on d 0, 7, 14 and 21. Daily concentrations of LH remained low in untreated control mares compared with GnRH-treated mares throughout the sampling period. Concentrations of LH for mares in group 3 that ovulated were elevated greatly above those for group 2 mares, whereas concentrations of FSH were similar in both treatment groups prior to ovulation.

Use of gonadotropin-releasing hormone (GnRH) treatment to induce multiple ovulations in the anestrous mare

A two-year study was conducted to evaluate the effectiveness of pulsatile gonadotropin-releasing hormone (GnRH) administration to induce multiple ovulations in the anestrous mare. With one exception, all doses of GnRH utilized (2, 20 and 100 g GnRH/hour) induced ovulation within a period of 11.4 ± 1.0 days from the start of infusion (overall mean ± SEM), whereas control mares (saline-infused and uninfused) did not show follicular development or ovulate within the experimental period. The exception was a single mare in the 2 g GnRH/hour treatment group which showed significant follicular development by Day 15 of infusion, but did not ovulate. This mare was subsequently induced to ovulate a single follicle by the administration of 1000 U human chorionic gonadotropin. Mares in the 20 and 100 g GnRH/hour treatment groups ovulated 2.9 ± 0.5 and 3.5 ± 0.9 follicles, respectively and this response was greater (P<.05) than in the group treated with 2 g GnRH/hour (1.3 ± 0.2 follicles ovulated). Peak pre-ovulatory concentrations of FSH, following the initiation of GnRH infusion, occurred 5 to 6 days prior to ovulation, and peak concentrations in mares across treatments were correlated with the number of follicles subsequently ovulated (R=.51, P=.013). By contrast, serum LH and estradiol-17β levels peaked one day prior, to 2 days following ovulation, and one to 2 days prior to ovulation, respectively, and these levels were unrelated to the number of follicles ovulated (P>.50). We conclude that the use of GnRH may be useful for inducing multiple ovulations in the seasonally anestrous mare, however, the extent of response is much less than that observed using conventional superovulation techniques in other domestic animal species.

Dinoprostone induced cervical dilation in the ewe

In the present study, we investigated the effects of reproductive status, size of follicles and plasma progesterone concentrations of mares at PRID™ insertion on the efficacy of the treatment, estrous cycle patterns, plasma concentrations of progesterone and LH. The progesterone-releasing device (PRID™) was administered intravaginally to 28 Haflinger mares for 11 days at different reproductive stages: anestrus (n = 6), estrus (n = 11) and diestrus (n = 11). Plasma concentrations of progesterone at insertion (Day 1) of PRID™ differed among treatment groups (anestrus: 0.2–0.6 ng mL−1, estrus: 0.2–0.5 and diestrus: 1.6–10.8 ng mL−1; P < 0.001). Total secretion of progesterone (area under curve (AUC)) during treatment period revealed highest values in diestrus (38.2 ± 3.1 ng mL−1 h−1) followed by estrus (25.1 ± 2.7) and anestrus (21.0 ± 0.4 ng mL−1 h−1; P < 0.05). Progesterone area under curve (AUC) was positively correlated with initial progesterone concentrations (R = 0.5; P < 0.05), but it did not correlate with the interval from PRID™ removal to ovulation. Plasma concentrations of LH during treatment period, were significantly lower in anestrous mares (184.6 ± 28.6 ng mL−1 h−1) when compared to estrous and diestrous mares (349.7 ± 53.3 and 370.5 ± 40.3 ng mL−1 h−1; P < 0.05). Follicular size at PRID™ insertion had no effects on the intervals from PRID™ removal to subsequent estrus and ovulation. Follicle diameters at removal of PRID™ were significantly correlated with the interval from coil removal to estrus (R = −0.55, P < 0.05) and ovulation (R = −0.72, P < 0.0004) in cyclic mares. In anestrus 0 of 6 (0%) mares, in estrus 5 of 11 (45.5%) and in diestrus 6 of 11 (54.5%) mares ovulated within a defined interval of 1 day before to 1 day after mean interval from PRID™ removal to ovulation. In cyclic mares, response to treatment was significantly higher when compared to anestrous mares: almost all mares responded with estrus and ovulation independent from the stage of the estrous cycle at the start of treatment. However, accuracy of synchronization was still unsatisfactory. In cyclic mares, the plasma progesterone concentrations at insertion of PRID™ seem to be more important for the efficacy of the treatment than the assignment to estrous cycle stages.

Use of hyperimmune serum in treatment of endometritis in mares

Endometritis was induced in four progesterone-treated ovariectomized mares and in two seasonally anestrous mares by intrauterine infusion of Streptococcus zooepidemicus. The bacteria were suspended in phosphate buffered saline (PBS), or in nonimmune or hyperimmune serum. Uterine lavage was performed after 24 h. Significantly fewer (P < 0.05) bacteria were recovered from mares which received hyperimmune serum than from mares which received nonimmune serum or PBS. There was no significant difference between treatment with nonimmune serum or PBS. It is therefore suggested that increasing the availability of specific antibody reduced the severity of intrauterine infection.

Gonadotropin-releasing hormone treatment induces follicular growth and ovulation in seasonally anestrous mares.

A study was conducted to evaluate the effectiveness of gonadotropin-releasing hormone (GnRH) pulse infusion to stimulate follicular development and induce ovulation in seasonally anestrous standardbred mares. Seventeen mares were selected for use in this experiment, on the basis of a previous normal reproductive history, and were housed under a photoperiod of 8L:16D beginning one week prior to the start of the experiment (second week in January). Mares were infused with 20 micrograms (n = 7) or 2 micrograms (n = 6) GnRH/h, or were subjected to photoperiod treatment only (controls, n = 4). Serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and progesterone did not vary, and neither significant follicular development nor ovulation was observed in any control mare throughout the experimental period (greater than 60 days). By contrast, both groups of GnRH-treated mares showed elevated serum concentrations of LH and FSH within one day after the start of infusion. Mares infused with 20 micrograms GnRH/h had at least one follicle greater than or equal to 25 mm in 7.4 +/- 1.3 (mean +/- SEM) days following the start of infusion, and ovulated in 12.0 +/- 0.7 days. In the 2-microgram-GnRH/h treatment group, a 25-mm follicle was detected in 5.7 +/- 0.7 days, and ovulation occurred after 10.0 +/- 0.3 days of infusion. Ovulation in every instance was followed by a functional luteal phase, as indicated by the profiles of progesterone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary responsiveness of mares challenged with GnRH at various stages of the transition into the breeding season.

Four groups of mares, representing anestrus (AN; n = 8), early transition (ET; n = 7), late transition (LT; n = 8) and estrus (EST; n = 12) were used to examine release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) after a bolus injection of gonadotropin releasing hormone (GnRH) during the transition from anestrus into the breeding season. Estrous mares received GnRH on d 2 or 3 of estrus in the cycle immediately preceding slaughter. Anestrous, ET and LT mares received GnRH exactly 1 wk prior to slaughter. A single injection of GnRH (Sigma LHRH, L-0507, 2.0 micrograms/kg body weight in .9% saline, iv) was given to each mare. Blood samples were collected at -2, h, -1 h, directly prior to GnRH, then 15, 30, 45, 60, 90, 120, 150, 180, 210, 240, 300, 360, 420 and 480 min post-injection. Maximum release of LH and FSH was observed within 30 min after injection of GnRH. Except for the LH response in EST mares, concentrations of both hormones had returned to pre-injection baseline levels within 8 h. Group means for area under the curve (AUC) of concentrations of LH in serum, and the maximum amount (MAX) of LH quantified in serum, post-GnRH, increased (P less than .05) progressively from AN to the breeding season. The AUC and MAX responses for FSH showed a reverse pattern, decreasing (P less than .05) from AN to the breeding season.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretion of luteinizing hormone and follicle stimulating hormone in intact and ovariectomized mares in summer and winter.

Sequential samples of blood were drawn via jugular catheters every 15 min for 24 h from four mares in each of five reproductive states: intact anestrous mares in winter, intact diestrous mares in summer, intact estrous mares in summer, ovariectomized mares in winter and ovariectomized mares in summer. Estrous mares were sampled on d 4 or 5 of estrus and diestrous mares on d 10 or 11 of diestrus. Each sample of plasma was assessed for concentrations of luteinizing hormone (LH) and follicle stimulating hormone (FSH) in two independent radioimmunoassays. A computer program was developed that determined peak hormone concentrations based on assay sensitivity, assay variability and repeatability of peaks in both independent assays. Peaks in LH and FSH were observed for mares in all five reproductive states, except for FSH concentrations in estrous mares. High frequency peaks of short duration were observed only in ovariectomized mares. Low frequency peaks of relatively long duration were observed in both intact and ovariectomized mares in both seasons. With the exception of estrous mares, there was variation among mares in the patterns of LH and(or) FSH within any one group; all estrous mares exhibited high, variable LH concentrations and low, constant FSH concentrations. In general, peaks in both gonadotropins occurred simultaneously. Ovariectomized mares exhibited more (P less than .05) peaks/24 h than intact mares for both gonadotropins. Ovariectomized mares also exhibited more (P less than .05) FSH peaks/24 h in summer than in winter.

Measurement of equine prolactin with an equine-canine radioimmunoassay: seasonal effects on the prolactin response to thyrotropin releasing hormone

A radioimmunoassay (RIA) based on anti-equine prolactin antiserum and radioiodinated canine prolactin was used to assess the dose response of plasma prolactin to thyrotropin releasing hormone (TRH) in mares in the nonbreeding season (winter) and in mares in estrus in the breeding season (summer). Mares were administered TRH intravenously and blood samples were collected via jugular catheters at −15, 0, 15, 30, 45, 60, 90, 120, 180 and 240 min relative to injection. Doses of TRH were 0, .08, .40, 2.0 and 10.0 mg per mare (n = 3 per dose within each season). The prolactin response was assessed by absolute hormonal concentrations before and after TRH injection and by net area under the curve. Prolactin concentrations in plasma before injection of TRH were higher (P < .01) in estrous mares in summer than in anestrous mares in winter (4.8 vs 1.3 ng/ml). Moreover, there was a greater (P < .01) response to TRH injection in estrous mares than in anestrous mares. Based on areas under the curve, there was an effect of season (P < .01) and of TRH dose (P < .01) as well as a season-dose interaction (P < .01). In general, there was little or no prolactin response to any dose of TRH in anestrous mares in winter when pre-TRH concentrations were low. In contrast, there was an increase in the prolactin response with increasing doses of TRH up to 2.0 mg in estrous mares in summer; 2.0 and 10.0 mg of TRH resulted in similar prolactin secretion. We conclude 1) that prolactin secretion in the horse is stimulated by TRH as has been reported for other species and 2) that prolactin concentrations and the TRH-induced secretion of prolactin are greater in estrous mares in summer than in anestrous mares in winter.

Daily rhythm of cortisol, and evidence for a photo-inducible phase for prolactin secretion in nonpregnant mares housed under non-interrupted and skeleton photoperiods.

Studies were conducted in anestrous mares to characterize daily rhythms of cortisol in non-interrupted [ambient and 16 h light (L): 8 h dark (D)] and skeleton (10L:4D:2L:8D, 10L:6D:2L:6D and 10L:8D:2L:4D) photoperiods, and to determine if there exists a photosensitive phase for the secretion of prolactin. Neither peak or nadir concentrations of cortisol, nor the time of peak or nadir concentrations differed among photoperiod treatments. Highest concentrations (66 +/- 4.4 ng/ml, mean +/- SE) occurred between 0700 and 0900, whereas lowest concentrations (31 +/- 3.6 ng/ml) were found from 1900 to 2300. Mean daily concentrations of serum prolactin were significantly higher in mares housed under the 16L:8D and the 10L:8D:2L:4D photoperiods as compared with the remaining photoperiod treatments, and were lowest in the ambient photoperiod treatment. The mean daily concentration of prolactin in February among photoperiod treatments was inversely related to the number of days (from December 1) to first seasonal ovulation (r = -.92, P = .027). The results were interpreted to: 1) suggest that mares in the 10L:8D:2L:4D skeleton photoperiod do not phase-shift to interpret the 2-h light pulse as the beginning of their subjective day; and 2) provide further evidence that the photo-inducible phase for both prolactin secretion and the stimulation of seasonal reproductive activity occurs 8 to 10 h following the onset of the dark period (scotophase).

Androgen and progesterone effects on follicle-stimulating hormone and luteinizing hormone secretion in anestrous mares.

Anestrous lighthorse mares were treated in December with dihydrotestosterone (DHT; 150 micrograms/kg of body weight), progesterone (P; 164 micrograms/kg), both DHT and P (DHT+P), testosterone (T; 150 micrograms/kg), or vehicle (n = 4/group). Daily blood sampling was started on Day 1, and on Day 4 all mares were administered a pretreatment injection of gonadotropin-releasing hormone (GnRH) and were bled frequently to characterize the responses of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) concentrations. Treatment injections were given on Day 4 and then daily through Day 17. On Day 18, all mares were again administered GnRH and were bled frequently. Treatment of mares with DHT, P, or T increased (p less than 0.01) plasma concentrations of these steroids to approximately 1.5 ng/ml during the last 10 days of treatment. There was no effect (p greater than 0.10) of treatment on LH or FSH concentrations in daily blood samples. Relative to the pretreatment GnRH injection, mares treated with T or DHT+P secreted approximately 65% more (p less than 0.01) FSH in response to the post-treatment GnRH injection; FSH response to the second GnRH injection was not altered (p greater than 0.10) in control mares or in DHT- or P-treated mares. There was no effect of any steroid treatment on LH secretion after administration of GnRH (p greater than 0.10). Averaged over all mares, approximately 94 times more FSH than LH was secreted in response to injection of GnRH.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of interrupted photoperiods on the induction of ovulation in anestrous mares.

The ability of interrupted photoperiods to induce early estrus and ovulation was examined. Horse mares were exposed to long (16 h light) or short (10 h light), noninterrupted photoperiods, ambient light, or various interrupted photoperiod treatments from December 1 to April 15 (135 d). Follicular development was assessed by rectal palpation and estrous behavior was determined by teasing with a stallion. Serum concentrations of progesterone were used as an indicator of corpus luteum function. Differences among the light treatment groups were compared for the following behavioral and ovarian characteristics: days to first detectable 3-cm follicle, days to first estrous behavior, days to first ovulation, the number of mares ovulating within the treatment period, and the number of ovulations within the treatment period per mare. Compared with the ambient and 10L:14D (L = h of light and D = h of darkness) photoperiod treatments, ovulation was advanced to the greatest extent by a photoperiod of 16L:8D and the interrupted photoperiod 10L:8D:2L:4D. These two stimulatory photoperiod treatments were characterized by the presence of light 8 to 10 h after dusk. Therefore, the present data are consistent with an external coincidence model for the induction of seasonal breeding in horses, with the photoinducible phase occurring within the period 8 to 10 h after dusk.

Thyroid stimulating hormone and prolactin secretion after thyrotropin releasing hormone administration to mares: Dose response during anestrus in winter and during estrus in summer

The response of thyroid stimulating hormone (TSH) and prolactin (PRL) concentrations to administration of thyrotropin releasing hormone (TRH) was determined in light-horse mares during the anestrous season (winter) and during estrus (standing heat) in the summer. Within each season, mares (4/group) were treated with either saline (controls) or one of four doses of TRH (80, 400, 2,000 or 10,000 ug) intravenously. Samples of blood were drawn at −15, −.5, 15, 30, 45, 60, 90, 120, 180 and 240 min relative to TRH injection. Concentrations of TSH and PRL in pre-TRH samples were greater (P<.05) in anestrous mares during winter than in estrous mares during summer. Concentrations of TSH increased (P<.05) within 30 min after administration of TRH and remained elevated during the 4-hr sampling period. The maximal net change in TSH concentrations and the area under the response curve were greatest for 2,000 ug of TRH; 80 ug did not produce a significant TSH response. There was no interaction (P >.10) between reproductive state and TRH dose for TSH concentrations. Concentrations of PRL were not significantly affected by any TRH dose during either season. It appears that mares differ from many mammalian species in that they do not respond to an injection of TRH with increases in both TSH and PRL.

Effects of placement of intravaginal sponges on LH, FSH, estrus and ovarian activity in mares during the nonbreeding season.

Eight seasonally anestrous mares were administered intravaginal polyurethane sponges on December 15 and then weekly thereafter until February 1. Control mares received no sponges or genital contact. Sponge insertion caused an immediate surge in follicle-stimulating hormone (FSH) concentrations in jugular plasma in 50% of treated mares whereas no control mares had surges in FSH (P less than .05). The effect of treatment on luteinizing hormone (LH) concentrations was much less dramatic and only three treated mares appeared to have positive responses. Sponge-treated mares exhibited positive responses in FSH concentrations 11 times out of 32 mare-days and control mares zero out of 28 (P less than .05). The magnitude of the FSH response decreased rapidly with successive responses. Sponge insertion induced estrus in four of eight treated mares; no control mares exhibited estrus (P less than .05). Sponge insertion also increased ovarian size and the incidence of large follicles. When all mares were fed altrenogest for 14 d beginning February 1, there was no beneficial effect of sponge treatment on number of mares exhibiting estrus or on pregnancy rate. These data confirm earlier speculations that sponge treatment causes surges in gonadotropins and increased ovarian size in approximately 50% of anestrous mares. However, sponge treatment does not appear to provide a practical means of preparing mares for progestogen synchronization during the nonbreeding season.

Effects of melatonin and thyrotropin releasing hormone on mares during the nonbreeding season.

Two hormonal treatments, chosen for their effectiveness in other seasonally breeding species, were tested in mares during the nonbreeding season to determine if they could induce ovarian activity and estrus during the winter. Of 15 functionally anestrous (anovulatory) mares, five received intravaginal, polyurethane sponges containing .75 g of melatonin on December 16; fresh sponges containing melatonin were inserted weekly until February 3. These mares also received daily injections of saline. Five other mares received daily im injections of 100 micrograms of thyrotropin-releasing hormone (TRH) and control sponges during the treatment period. The remaining five mares were given control injections and control sponges throughout the experiment. Intravaginal sponges containing melatonin increased (P less than .05) concentrations of melatonin in systemic plasma for at least 7 d to levels at least 10-fold higher than those expected during the nighttime hours. The TRH significantly increased concentrations of thyroid-stimulating hormone within 60 min after injection, whereas there was no detectable increase in concentrations of prolactin after TRH. Ovarian size in all three groups of mares was increased (P less than .05) shortly after the onset of the treatment regimens. Moreover, there were surges in concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in plasma closely associated with sponge insertion and(or) injection of TRH or saline in mares of all groups. Due to the temporal correlation of gonadotropin surges and sponge insertion, we suspect that placement of intravaginal sponges may have caused the release of LH and FSH, perhaps through a neuroendocrine reflex. These surges in gonadotropins may have mediated the ovarian response. Alternatively, ovarian activity may have been stimulated by an unknown environmental factor, a possibility that was not examined in this study. Melatonin or TRH did not augment or inhibit this nonspecific response.

Determination of clinical and luteolytic effectiveness of a prostaglandin analog in mares by a dose response study

In trials covering two seasons, 124 thoroughbred and 40 quarterhorse mares with either normal cycles (55 diestrous mares, 33 postpartum mares after foal heat) or in anestrus during the breeding season (76) were treated with either a novel PGF analog K 11941 ∗ ∗ K 11941 (Alfaprostol) 18, 19, 20-trinor-17-cyclohexyl-13, 14 didehydro-PGF 2α-methylester: VETEM S.p.A., Milan, Italy. One ml of the solution contains 1 mg K 11941. or with the PGF analog prostalene (Synchrocept ™). K 11941 was used over a range of doses from 0.5 to 4.5 mg, while prostalene was applied at the recommended dose level of 2 mg. Blood progesterone determinations, clinical observations and the results of breeding confirmed that K 11941, at doses of 2 mg or larger, and prostalene, were effective and safe luteolysins; heat and ovulations occurred within the expected time intervals and fertility was satisfactory. Clinical data were converted into a response score (CRS) and an added fertility score (CRSF) for statistical evaluation and the establishment of a dose response curve. In both scores, 0.5 to 1.5 mg were significantly less effective than the higher dose levels (p<.0001). No significant differences were found for the 2 and 3 mg dose of K 11941 in diestrous and anestrous mares. In both indications, prostalene scored less (p<.05). Data analysis and establishment of a dose response curve point to 3 mg of K 11941 as the optimal dose.

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 (a) (a) K 11941 (Alfaprostol): 18, 19, 20-trinor-17-cyclohexyl-13,14 didehydro-PGF 2α-methylester: VETEM S.p.A., Milan, Italy. . 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.

Effectiveness of a novel prostaglandin analog in postpartum mares and mares with early season anestrus

Contents Fifty thoroughbred mares, located at Central-Southern Brazil near Sao Paulo (13 anestrous maiden, 18 anestrous barren mares and 19 postpartum mares) were treated with one to three injections, 48 hrs apart, of 2 mg of the novel PGF analog K 11941*, during the early part of the breeding season. Early season anestrus in these problem mares was compounded by the adverse effects of a drought. Plasma progesterone determinations revealed unexpectedly elevated plasma progesterone levels in 61.5% of the maiden mares, in 77.8% of the barren mares and in 68.4% of the postpartum mares (20 to 25 days postpartum). Treatments with K 11941 initiated luteolysis, heat and ovulations in 91.4, 45.5 an 63.3% of these mares, respectively, but initiated also heat and ovulation in 13% and 33% of mares with baseline progesterone levels. Treatments initiated covert and overt cyclic functions in 17 and 12 of those animals which did not conceive immediately. Fifteen animals with recurrent anestrus received further treatment with K 11941. Ten mares cycling silently were force bred when palpations indicated ovulatory follicles, and 7 conceived. Of the 50 mares treated, 38 became pregnant (76%: maiden mares: 76.9%; barren mares: 77.8% and postpartum mares: 73.7%) with breeding indices of 1.9, 1.36 und 1.57, respectively; and 37 mares gave birth to a live foal (74%), At the same stud, 138 control mares had a foaling rate of 82.6%.

Measurement of equine follicle stimulating hormone and luteinizing hormone: response of anestrous mares to gonadotropin releasing hormone.

Heterologous radioimmunoassays using human systems which were developed for the measurement of serum concentrations of equine follicle-stimulating hormone (eFSH) and luteinizing hormone (eLH) are described. These radioimmunoassays meet the criteria of specificity, precision and repeatability. The equine FSH assay is the first described for the measurement of this hormone. Serum levels of FSH and LH following the injection (im) of 1 mg of gonadotropin releasing hormone (GnRH) into 7 anestrous mares were measured using these methods and LH was also measured using an ovine system. Within 0.5 h following the administration of the GnRH, serum FSI-I reached a peak of 3.7 times the mean pretreatment concentration. The concentration of FSH then declined towards baseline for 12 h posttreatznent. These changes in FSH are consistent with those occurring in normal cycling mares. Serum LH concentrations were not significantly elevated above baseline during the 12 h after injection of GnRH as measured in the human system but increased to 2.35 times the pretreatment level by 0.5 h after GnRH when measured in the ovine system. We bdieve this is the first species to show a significantly greater release of FSH than LH following administration of GnRH, and gives support for the existence of a single hypothalamic gonadotropin releasing hormone.

Stimulation of follicular activity and estrous behavior in anestrous mares with light and temperature.

SUMMARY Two groups of pony mares were housed for 120 days beginning October 17 in controlled environment rooms in the University Biotron. Temperature and daily photoperiod were programmed to simulate outdoor conditions in southern Wisconsin for either October 17 to February 15 (control group, six mares) or March 1 to July 1 (treated group, seven mares). All mares were given a maintenance diet of a commercial feed. Ovarian follicular development was monitored by rectal palpation and the incidence of estrus was monitored daily by teasing with a stallion. After 63 days of treatment, there were more (P 10 mm diameter) in the treated mares. By day 60, diameter of the largest follicle was greater (P 20 mm, and average follicle diameter) were significandy greater in treated mares than in controls. The changes for each ovarian end point in treated mares were best characterized by a significant cubic regression curve with a steep central portion (sigmoid curve). In treated mares, estrous behavior was shown in four of seven by the 94th day, and in seven of seven by the ll2th day. None of the control mares exhibited estrus during the entire project. Two of seven treated mares ovulated by the project termination date; no control mares ovulated.

Efficacy of a prostaglandin analogue in reproduction in the anestrous mare

A prostaglandin F analogue was studied in anestrous mares: a dose-response study; a study in mares presumed pregnant; and a field evaluation of effective doses in breeding establishments. A dose of 2.0mg given by single subcutaneous injection to mares with initial plasma progesterone levels greater than 1.0ng/ml, caused luteolysis on the basis of decline in plasma progesterone concentrations. Follicle maturation leading to ovulation, accompanied by estrus, was observed, and fertility at mating either by natural service or artificial insemination was satisfactory. A dose of 1.0mg was generally effective for luteolysis, but pregnancy rates were lower than after 2.0mg. A proportion of mares which had less than 1.0mg of plasma progesterone at the time of injection ovulated and became pregnant.