Periparturient events in ovariectomized embryo transfer recipient mares

Ovariectomized mares treated with progesterone have established and maintained pregnancy after embryo transfer. This study evaluated the ability of ovariectomized embryo transfer recipients to successfully undergo parturition, raise a foal, and return to a useful reproductive status. Periparturient events in three ovariectomized embryo transfer recipient mares and three intact mares were compared. All mares foaled normally. Mammary scores were similar for both groups and all mares produced sufficient colostrum and milk to allow normal growth of healthy foals. Plasma progesterone levels decreased to < 5 ng/ml by Day 4 post partum in both groups. Progesterone concentrations continued to decrease and remained at <1 ng/ml in ovariectomized mares, but increased after the first postpartum ovulation (Day 9 to 15) in intact mares. Endometrial involution as determined by histological evaluation was complete in ovariectomized mares by Day 10 post partum and in intact mares by Day 11 post partum. As assessed by palpation per rectum and clearance of bacteria from the uterus, uterine involution was similar in all mares. The three ovariectomized mares subsequently received embryos by transcervical transfer and two of them established pregnancy. These results indicate that normal parturition, lactation, maternal behavior and uterine involution are independent of ovarian function.     

Age of the Jeju pony (Equus caballus) at first reproduction: potential costs for foals and mothers that first reproduce early in life

In a related paper, we showed that mares that reproduced early in life tended to have higher fecundity because of a decrease in the duration of inter-birth intervals relative to mares that reproduced later in life. However, we know that young mares are subordinate to older mares. Hence, costs associated with low dominance rank might offset the benefits of earlier reproduction. We compared harassment of foals of female Jeju ponies that first reproduced at three years of age with that of foals of females that first reproduced at five years of age. As a consequence of their positions in the linear dominance hierarchy, foals born to and guarded by young mares were harassed more frequently than foals whose mothers were older when they first reproduced. A mare that reproduced early in life guarded her offspring more closely and intervened between her foal and neighboring mares more frequently than those mares which first reproduced when older. This need to guard their foals and the harm that might ensue from frequent harassment might counter-balance selection towards earlier reproduction in mares.     

Amniotic fluid and blood lactate concentrations in mares and foals in the early postpartum period

Amniotic fluid (AF) lactate concentration and time-dependent changes in blood lactate concentration in mares after parturition have never been evaluated. In this study, the venous blood lactate concentration of mares and foals during the first 72 h of the postpartum period was assessed, and the concentration of lactate in the AF collected during delivery and the utility of its measurement for evaluating the foal's health were investigated. This prospective observational study was carried out on mares attended at delivery. They were divided into mares delivering healthy (Group 1) and sick (Group 2) foals. The following samples were collected: AF and umbilical blood at delivery, mare's and foal's jugular blood every 12 hours from parturition until 72 h postpartum (T0-T72). Sixty-two mares were enrolled in Group 1 and 19 in Group 2. In Group 2, the survival rate was 68.4%. The median blood lactate of the foals at T0 was 3.60 mmol/L in Group 1 and 5.05 mmol/L in Group 2. The monitoring of the blood lactate concentration showed a significant time-dependent decrease from T24 in the foals (P < 0.01) and from T12 in the mares (P < 0.01). Lactate concentration over time was significantly different between healthy and sick foals (P < 0.01) but not between mares with normal and dystocic delivery (P = 0.08). A significant difference (P = 0.04) was detected as regards AF lactate concentration between Group 1 (median 14.99 mmol/L) and Group 2 (median 12.61 mmol/L). For the first time, AF lactate concentration was evaluated during parturition, and significantly higher levels were found in mares delivering healthy foals. This was an unexpected and very interesting result which warrants further investigation involving a larger number of mares. Additional studies are needed before either mare's blood or AF lactate concentration can be used in a clinical setting.     

Use of leukocytes as treatment for endometritis in mares experimentally infected with Streptococcus equi subsp. zooepidemicus

This study compared four treatments for bacterial endometritis in mares experimentally infected with Streptococcus zooepidemicus. Twenty-five mares were used, 20 resistant and five susceptible to endometritis. Mares would be in estrus when infected. Twenty-four hours after inoculation, clinical, bacteriological and cytological examinations were performed and repeated until the first occurrence: negative cytology and no Streptococcus growth or the seventh day post-infection. All mares showed clinical signs of endometritis and were assigned to one of the following treatments: (1) intrauterine infusion of fresh leukocytes; (2) intrauterine infusion of frozen-thawed leukocytes; (3) intrauterine infusion of lysed leukocytes; (4) intrauterine infusion of recombinant human interleukin-8 (rhIL-8); (5) control. Mares were submitted to all treatments, with at least a 14-day interval between treatments in a Latin square design. Treatment did not affect (P=0.121) time needed for resistant mares to eliminate bacteria. Time needed for elimination of bacteria was similar in susceptible mares treated with fresh and frozen leukocytes (P=0.333). Susceptible mares treated with frozen leukocytes also did not differ from those treated with lysed leukocytes (P=0.227) for time to eliminate bacteria, but were significantly different (P>0.02) from those treated with rhIL-8 and control. In resistant mares, physical clearance ability was probably the responsible for bacterial elimination. Intrauterine infusions in susceptible mares with viable or lysed leukocytes associated or not to opsonizing factors, reduced the time to elimination of bacteria. Infusions with bactericidal effect (functional neutrophils and granules) was likely effective and responsible for the more rapid elimination of bacteria in susceptible mares.     

The cosmopolitan maternal heritage of the Thoroughbred racehorse breed shows a significant contribution from British and Irish native mares

The paternal origins of Thoroughbred racehorses trace back to a handful of Middle Eastern stallions, imported to the British Isles during the seventeenth century. Yet, few details of the foundation mares were recorded, in many cases not even their names (several different maternal lineages trace back to 'A Royal Mare'). This has fuelled intense speculation over their origins. We examined mitochondrial DNA from 1929 horses to determine the origin of Thoroughbred foundation mares. There is no evidence to support exclusive Arab maternal origins as some historical records have suggested, or a significant importation of Oriental mares (the term used in historic records to refer to Middle East and western Asian breeds including Arab, Akhal-Teke, Barb and Caspian). Instead, we show that Thoroughbred foundation mares had a cosmopolitan European heritage with a far greater contribution from British and Irish Native mares than previously recognized.     

Evaluation of androgenized mares as an estrus detection aid

Ten pony mares that had developed stallion-like sexual behavior as the result of anabolic steroid treatment (boldenone undecylenate, 0.55 mg/kg intramuscularly (i.m.), once weekly for 12 injections) were evaluated for ability to aid in detecting estrus in cycling mares. In across-the-fence estrus detection trials, androgenized mares failed to elicit signs of estrus or diestrus. In 10-min pasture trials, in which each androgenized mare was placed in a group of 10 cycling mares (six of which were estrous), seven of the 10 androgenized mares elicited estrous behavior from one or two mares. Observations of the 10 androgenized mares among a pasture group of 21 cycling mares indicated that approximately one third of the mares in estrus could be identified on the basis of their response to androgenized mares.     

Electropuncture therapy trial for treating infertility in mares

Four acupuncture points, BAIHUI, LUANCHOSHU, YANCHI and YOUQUAN were used as electropuncture therapy for infertility in mares; each of these is described. A total of 114 mares on several state breeding stations in Xin-Jiang Uyger Zizhique, China, were checked for infertility by palpation per rectum. Thirteen infertile mares were selected for treatment. As a result of the treatment 12 mares showed normal oestrus and ovulated. The physiological functions and pathological changes of the channels and their collaterals were studied with regard to the mechanism of the theory of acupuncture. This study pays very special attention to the physiological and pathological connection between the internal and external organs and structures.     

Postfixation embryo reduction in unilateral and bilateral twins in mares

Multiple ovulations were induced with a pituitary extract in mares, and the development of multiple conceptuses was monitored daily by ultrasound on days 11 to 40. The incidence of abortion (loss of all embryos) was not significantly different between mares with multiple embryos (5 38 mares; 13%) and mares with singletons (4 36 ; 11%). Embryo reduction (elimination of excess embryos) was not detected during the embryo mobility phase (days 11-15) or on the day of fixation of embryos (day 16) in any of 38 mares with multiple embryos. The incidence of postfixation embryo reduction for mares with twins was 64% (18 28 ); however, the incidence for unilateral twins (17 19 ; 89%) was greater (P<0.01) than for bilateral twins (1 9 ; 11%). Reduction of unilateral twin embryos seemed to occur earlier (53% before day 20 and 82% before day 30) than for the set of bilateral twins (day 36). The remaining embryo in all mares in which embryo reduction occurred seemed normal in size and appearance on the last day of examination. However, in four of eighteen mares in which unilateral reduction occurred, the umbilical cord of the remaining embryo was attached in the ventral hemisphere of the all antochorion. This apparent disorientation was not seen in any of 16 bilaterally located embryos or in 16 singletons.     

Comparison of the effects of two GnRH antagonists on LH and FSH secretion,follicular growth and ovulation in the mare

The effects of two GnRH antagonists were tested in order to delay and/or synchronise ovulation in mares. Five mares received Antarelix (0.01 mg.kg(-1)), 5 mares received Cetrorelix (the same dose), 5 mares (control mares) received the vehicle intravenously, twice daily, for 8 days from the day the largest follicle reached 22 mm following prostaglandin administration. Ovulation was postponed in all mares injected with Antarelix (19.4 +/- 1.2 days after the beginning of the treatment) and in 2/5 mares injected with Cetrorelix (20 +/- 1 days) vs. 6.2 +/- 0.4 days in control mares. During the treatment, LH concentrations were strongly depressed in Antarelix and in Cetrorelix mares (1.6 +/- 0.1 and 3.8 +/- 0.5 ng.mL(-1) respectively vs. 21 +/- 2.5 ng.mL(-1) in control mares). In the 3 Cetrorelix mares which ovulated during the treatment. 2 initiated their LH surge at this moment. FSH concentrations were not affected in Antarelix or in Cetrorelix mares during the treatment (11.4 +/- 1.3 and 7.9 +/- 0.8 ng.mL(-1) respectively vs. 10.5 +/- 0.8 ng.mL(-1) in control mares). In conclusion, Antarelix seems more efficient than Cetrorelix for postponing ovulation in mares. The role of LH in antral follicular development before the preovulatory stage is confirmed.     

Induction of male sex behavior in pony mares with testosterone propionate

Two pony mares were administered 150 mg of testosterone propionate every other day for 20 days (ten injections) and every ten days there-after. An additional two mares and one stallion were not treated and served as controls. Testosterone propionate was dissolved in absolute ethanol and administered subcutaneously. Sex behavior tests were conducted 26 and 40 days after the first injection. Control mares exhibited very little male sex behavior. Both testosterone propionatetreated mares, however, exhibited mounting, sniffing, flehmen, biting and vocalization behavior in the presence of an estrous mare. The testosterone propionate-treated mares mounted and bit estrous mares more frequently than the stallion but exhibited less sniffing, flehmen and vocalization behavior in the presence of an estrous mare than the stallion. Testosterone propionate-treated mares and the stallion mounted an estrous mare 23.3 +/- 9.7 seconds and 172.5 +/- 22.5 seconds, respectively, after being introduced into the pen. Mares in estrus were mounted by the testosterone propionate-treated mares and the stallion an average of 4.0 +/- 1.3 and 1.0 +/- 0 times, respectively, during a ten-minute test. None of the non-estrous mares was ever mounted by the testosterone propionate-treated mares. In summary, testosterone propionate induced male sex behavior in intact mares and the testosterone propionate-treated mares effectively detected estrous mares.     

Age and pasture effects on vernal transition in mares

The objectives of the present study were to determine if follicular activity was less in old than in young mares during the spring transition and if green pasture would hasten onset of the ovulatory season. Experiments were conducted over 2 sequential years using young mares (3 to 7 yr) and old mares (> or =14 yr). In Experiment 1, growth of the largest and second-largest follicles were compared for young mares (5 to 7 yr) and old mares (> or =14 yr) for 21 d prior to the first ovulation of the year. More follicular activity was noted in young than in old mares. Main effect of age was significant for diameter of the largest follicle, and interaction of day-by-age was significant for diameter of the second-largest follicle. Prior to the beginning of the breeding season, the mares were randomly divided into dry-lot and pasture groups. The interval from May 2 to ovulation was shorter (P < 0.005) for mares put on pasture on May 2 than for mares kept in dry lot (means +/- SEM, 14.5 +/- 2.7 and 21.3 +/- 3.2 d, respectively). In Experiment 2, follicular activity was compared among 3 age groups (3 to 7, 17 to 19, and > or =20 yr). The total number of follicles > or =10 mm was higher (P < 0.05) for young mares and lower (P < 0.05) for old mares than for mares of an intermediate age. Main effect of age and interaction of day-by-age were significant for diameter of largest and second-largest follicles, being smaller for mares > or =20 yr than for younger mares. The interval from development of a follicle > or =30 mm to ovulation was shorter (P < 0.05) for mares placed on pasture when a > or =30 mm follicle developed than the interval for mares kept in dry lot (5.7 +/- 0.7 and 8.2 +/- 0.9 d, respectively). In summary, less follicular activity occurred in old than in young mares during the transitional period, and mares pastured on green grass ovulated sooner in the spring than mares housed on dry lot and fed hay.     

Aberrations in uterine contractile patterns in mares with delayed uterine clearance after administration of detomidine and oxytocin

An experiment was conducted to determine whether the uterotonic effects of oxytocin, a drug used to treat mares that have a delay in uterine clearance were affected by the sedative detomidine (an alpha2-agonist), a drug used to treat fractious mares. An additional objective was to identify propagation patterns of uterine contractions and determine whether these patterns differed between normal mares and mares with delayed uterine clearance (DUC). Intrauterine pressure was measured in five reproductively normal mares and four mares with DUC during estrus using an 8-F Milar catheter with two discrete pressure sensors. Mares received one of three treatments in random order: detomidine (0.001 mg/kg; i.v.); detomidine followed in 10 min by oxytocin (10 IU; i.v.); and saline (0.9% NaCl 0.5 ml; i.v.) followed in 10 min by oxytocin. All treatments induced waves of contractions; however, only three mares with DUC exhibited contractions after administration of detomidine. Normal mares experienced more uterine contractions (P < 0.01) that tended to last longer (P < 0.06), and were of greater intensity (P < 0.04) than mares with delayed clearance. Administration of detomidine before oxytocin increased the number of contractions (P < 0.02) and increased the maximum intrauterine pressure in the uterine horn (P < 0.05) in normal mares as compared to response after administration of saline and oxytocin. Detomidine had no effect in mares with delayed clearance. All mares had more propagating than non-propagating uterine contractions (74 +/- 8 versus 25 +/- 8%, respectively). Normal mares exhibited a normal propagation pattern more frequently (P < 0.0001) than mares with DUC. Simultaneous (P < 0.05) and inverted (P < 0.03) contractions occurred more frequently in mares with DUC. Administration of detomidine increased the number (P < 0.01), and tended to increase the percentage (P < 0.07) of normal propagating uterine contractions in normal mares, but did not affect propagation patterns in mares with DUC. In conclusion, detomidine augmented the uterotonic effect of oxytocin in normal mares but not in mares with DUC. Data suggest that mares with DUC have a defect in myoelectrical signaling and a decrease in the contractile strength of the uterine muscle.     

Equine Abortion (Herpes) Virus: Evaluation of Markers in a Field Vaccination Trial

Twelve mares were vaccinated with attenuated equine abortion virus (EAV) strain RAC-H. Two nonvaccinated mares served as controls. In at least three mares the vaccination appeared to coincide with a natural infection. This was indicated by characterization of the EAV isolated from nasal secretions of six vaccinated mares, a nonvaccinated control, and also from the lung, spleen, and liver of a fetus aborted by a vaccinated mare. The relative sensitivity of the isolated EAV to dithiothreitol was used to distinguish the RAC-H strain and wild-type virus. Of the 10 EAV isolates, four were recognized as being the vaccine strain while six were recognized as being wild-type strains. Three of the latter originated from two vaccinated mares and a nonvaccinated control, and three were recovered from the fetus. The ability of the EAV strains to form plaques in a cloned line of L cells proved to be unsuitable for use as a marker in this study.     

Equine retained placenta: technique for and tolerance to umbilical artery injections of collagenase

Under laboratory conditions and in clinical experiments, bacterial collagenase has proven to be effective in hydrolyzing placenta and detaching cotyledon from caruncle in the bovine species. Laboratory studies in which placental samples were incubated with collagenase have also demonstrated that collagenase is 3.7 times more effective in hydrolyzing equine placenta than bovine placenta. This led to the hypothesis that collagenase may be a potential treatment for mares with retained placenta. However, that collagenase may hydrolyze the uterine wall and perforate the uterus was a concern. It was the purpose of this study thus to determine any adverse effects of collagenase on the equine uterus and to develop a method for intraplacental injection of collagenase. Three normally expelled intact placentas from Arabian mares, 10 cyclic mixed-breed mares, and 4 mares of various breeds with retained placenta were used. Fluoroscein dye and latex were used to study the placental vasculature and to determine a suitable dose of collagenase; placentas were hydrolyzed by collagenase solution in vitro. Bacterial collagenase solution (40,000 units, 200 ml) was infused into the uterine lumen of each cyclic mare. Uterine biopsies were obtained from the mares before collagenase infusion and again at 16 h and 26 d after infusion. In the mares with retained placenta, each placenta was infused via its umbilical cord vessels with 200,000 units of bacterial collagenase in 1 L of saline. Results showed that none of the uteri from cyclic mares were damaged by collagenase treatment. During a 4-wk period of monitoring (including endoscopy) mares with retained placenta did not show any abnormalities. Retained placentas were expelled in less than 6 h after collagenase treatment. It was concluded that intraplacental injections of collagenase are a safe and potentially effective treatment for retained placenta in mares.     

Emergence and selection of the dominant follicle and gonadotropin dynamics in postpartum lactating versus non-postpartum cycling mares

Among female livestock, the mare has the shortest interval from parturition to first ovulation. Due to the scarcity of research on postpartum mares, little progress has been made on the characterization of the resumption of ovarian cyclicity after parturition. This study compared follicular and gonadotropin dynamics during follicle emergence and deviation in postpartum lactating (PP Lactating) versus non-postpartum cycling (N-PP Cycling) mares. On the day of parturition, every PP Lactating mare was paired with a N-PP Cycling mare. Comparisons were made by considering the partum-ovulation interval and the postpartum interovulatory interval for the PP Lactating mares, and two interovulatory intervals for the N-PP Cycling mares. The results presented herein demonstrate that during the postpartum period, lactating mares have some similarities in follicular and hormonal profiles around emergence and deviation when compared with non-postpartum cycling mares. However, some peculiar and important characteristics were noticed during the postpartum period in lactating mares: (1) The emergence of the DF occurs around the day of parturition; (2) follicle deviation in the ovulatory wave occurs earlier during the foal heat than in other intervals; (3) lower FSH and LH systemic concentrations were not detrimental enough to prevent the rapid resumption of ovarian activity just after parturition; and (4) the association between parturition and season can have an additional and confounding effect during postpartum ovarian activity in mares. The novel findings of this study provide better knowledge of the resumption of ovarian activity after parturition and may help provide insight into the reproductive management of this species.     

Reproductive performance of Friesian mares after retained placenta and manual removal of the placenta

Because the incidence of retained placenta in Friesian mares is estimated to be high, and no reports have been published on the reproductive performance of Friesian mares after retained placenta, we studied postpartum reproductive performance in Friesian brood mares with (n = 54) and without (n = 50) retained placenta. We defined a retained placenta as the failure to expel all fetal membranes within 3 h after the delivery of a foal. We subdivided the group of mares with retained placenta into mares in which the placenta had been removed manually (n = 30) and mares in which it had not (n = 24). Within each group, we compared reproductive performance after breeding in the foal heat and breeding in a subsequent heat. We also recorded the age of the mares, number of mares treated with antibiotics after insemination, and number of mares treated with prostaglandins. The interval between delivery and conception, efficacy rate (number of served cycles divided by the number of mares that had a positive pregnancy diagnosis), seasonal pregnancy rate, pregnancy rate after first insemination, pregnancy loss rate, and foaling rate did not differ between mares with and without retained placenta or between mares with and without manual removal of the retained placenta. Within each group, the pregnancy rate after first insemination did not differ between breeding for the first time in the foal heat and breeding for the first time in a subsequent heat. We concluded that reproductive performance did not differ between (1) Friesian mares with and without retained placenta and (2) Friesian mares with and without manual removal of the placenta. With regard to reproductive performance, retained placenta and manual removal of the placenta are not valid reasons to avoid foal heat breeding in Friesian mares.     

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).     

Embryonic loss in pony mares induced by intrauterine infusion of Candida parapsilosis

Pony mares which were detected pregnant by transrectal ultrasonography received a single intrauterine infusion of either sterile saline (control, n = 12 mares) or 10(6)Candida parapsilosis (treated, n = 12 mares) between Days 11 to 14 postovulation. Subsequent embryonic loss was studied by daily ultrasonography of the mare's uterus, by serum progesterone levels, by endometrial swabs for cytologic and microbiologic examination and by endometrial biopsies that were taken after embryonic loss was detected. Significantly fewer (P<0.01) embryonic losses occurred in control than in treated mares (4 12 vs 12 12 ). The mean interval from intrauterine infusion until embryonic loss was 5.8 +/- 2.8 d for control mares (n = 4) and 2.1 +/- 0.2 d for treated mares (n = 12). Prior to embryonic loss, moderate to marked edema of the endometrial folds in 12 of 12 treated mares and free fluid in the uterine lumen of 5 of 12 treated mares were detected by ultrasonography. After embryonic loss, Candida parapsilosis was cultured from the uteri of 8 of 12 treated mares, and E . coli was cultured from the uteri of 2 of 4 control mares. Postloss endometrial smears had cytologic evidence of inflammation in 10 of 12 treated mares and 3 of 4 control mares. Intrauterine inoculation of C. parapsilosis consistently induced embryonic loss and may provide a basis to further study the relationship between endometritis and embryonic loss in mares.     

Effect of GnRH treatment during the anovulatory season on multiple ovulation rate and on follicular development during the ensuing pregnancy in mares

Seasonally anovulatory mares were injected, i.m., twice daily with a GnRH analogue (GnRH-A), and hCG was given when the largest follicle reached 35 mm in diameter. In Exp. 1, treatment was initiated on 23 December when the largest follicle per mare was less than or equal to 17 mm. An ovulatory response (ovulation within 21 days) occurred in 17 of 30 (57%) GnRH-A-treated mares on a mean of 15.8 days. The shortest interval to ovulation in control mares (N = 10) was 57 days. The diameter of the largest follicle first increased significantly 6 days after start of treatment. In Exp. 2, treatment was begun on 15 January and mares were categorized according to the largest follicle at start of treatment. The proportion of mares ovulating within 21 days increased significantly according to initial diameter of largest follicle (less than or equal to 15 mm, 9/25 mares ovulated; 15-19 mm, 13/21; 20-24 mm, 20/24; greater than 25 mm, 3/3). The multiple ovulation rate was greater (P less than 0.01) for treated mares (27/86 mares had multiple ovulations) than for control mares (2/35). Treated mares in which the largest follicle at start of treatment was greater than or equal to 25 mm had a higher (P less than 0.01) multiple ovulation rate (9/14) than did mares in which the largest follicle was less than 25 mm (18/72). The pregnancy rate for single ovulators was not different between control mares (26/30 pregnant mares) and treated mares (43/54).(ABSTRACT TRUNCATED AT 250 WORDS)