Effect on fertility of human chorionic gonadotrophin and uterine lavage with oxytocin performed after mating in Arabian barren mares

The objective of the present study was to evaluate the beneficial effect of hCG injected immediately after mating in Arabian barren mares treated with uterine lavage and oxytocin. Arabian barren mares (n = 36) with PMIE were subjected to detailed clinical examinations including palpation per rectum, vaginoscopy, and cytological examination. After mating the 36 mares were randomly divided into four groups. The mares in group 1 (n = 10) were immediately after breeding injected with hCG 3000 IU IM. Uterine lavage with 1 L of N-saline containing 4 million IU of crystalline penicillin and 4 g of streptomycin sulphate was performed 4 h after breeding. Then mares received two injections of oxytocin 40 IU IM 2 h apart after 6 h of mating. Mares in group 2 (n = 10) treated with uterine lavage and oxytocin as group 1. While mares in group 3 (n = 10) received uterine lavage only. A control group (n = 6) as group 4 did not received any treatment. The results of clinical examination indicated that 69.4% of PMIE mares were harboring severe endometritis and 30.6% with a moderate form of endometritis. Significant (P < 0.01) increase in lymphocytes were founded in barren mares included in this study. Higher pregnancy rate (P < 0.01) was founded in Arabian barren mares 80% injected with hCG immediately after breeding and uterine lavage and oxytocin. No significant difference was found in mares received uterine lavage and oxytocin and uterine lavage only. In a conclusion, administration of hCG immediately after mating and intrauterine lavage containing antibiotics performed 4 h and two injections of oxytocin 40 IU IM 2 h apart after 6 h of mating had improved fertility of Arabian barren mares.     

Use of mesenchymal stem cells or autologous conditioned serum to modulate the inflammatory response to spermatozoa in mares

Current treatments for Persistent mating-induced endometritis such as uterine lavage and oxytocin therapy focus on aiding the uterus in removal of inflammatory products, but these treatments do not modulate the inciting inflammatory response. Biological treatments, such as autologous conditioned serum (ACS) and mesenchymal stem cells (MSCs), have been used in human and veterinary medicine for immunomodulation for over 10 years. The objectives of this project were to evaluate the ability of ACS or MSCs to modulate the inflammatory response to spermatozoa after breeding. Two experiments were performed with six normal mares in each study to evaluate the effects of intrauterine administration of ACS, dexamethasone, or a placebo (experiment 1), or allogeneic MSCs or a placebo (experiment 2) on the inflammatory response to spermatozoa using clinical and biochemical endpoints. Treatment with ACS and MSCs significantly (P < 0.05) reduced the number of neutrophils in the uterine lumen 6 hours after the sperm challenge. An increase (P < 0.05) in the anti-inflammatory cytokine IL-1Ra was observed after treatment with MSCs before exposure to spermatozoa. There was no difference in IL-1Ra concentration in mares treated with ACS, dexamethasone, or a placebo. Mesenchymal stem cells and ACS were able to modulate the immune response to spermatozoa in normal mares. The effect may be due to an increase in IL-1Ra in MSCs-treated mares, but other bioactive molecules may be responsible for the decrease in neutrophils in ACS-treated mares. Autologous conditioned serum and bone-derived culture expanded MSCs were able to modulate the uterine inflammatory response to spermatozoa in normal mares. Treatment with allogeneic stem cells may be beneficial if a similar modulation in inflammatory cytokines occurs in mares affected by persistent mating–induced endometritis.     

Does artificial insemination with chilled, extended semen reduce the antigenic challenge to the mare's uterus compared with natural service?

Uterine response in infection-resistant mares (n = 5) at 48 h after AI was compared with that following natural service in these same mares, and after AI in infection-susceptible mares (n = 6). In the resistant mares, small amounts of uterine fluid were detected and bacteria were isolated infrequently at 48 h after breeding, but cytological examination of uterine flushes revealed that a significant degree of endometritis was present. There was no difference in the degree of inflammatory response by 48 h after AI or natural mating. In 4 of the 6 susceptible mares moderate to large accumulations of intrauterine fluid were detected at 48 h after AI, and massive uterine neutrophilia was present in all 6 mares. It was concluded that there was no evidence that using conventional AI techniques limited the inflammatory response of the mare's uterus.     

Susceptibility to persistent breeding-induced endometritis in the mare: relationship to endometrial biopsy score and age, and variations between seasons

The objectives were to: (1) investigate the associations of age and endometrial biopsy score with uterine fluid retention after insemination; and (2) determine if a strict classification of susceptibility to persistent breeding-induced endometritis (PBIE) based on biopsy score, endometrial cytology, and fluid retention after inseminations, is consistent over subsequent breeding seasons. In Experiment 1, 57 mares were inseminated with 10⁹ freeze-killed sperm during estrus and evaluated for uterine fluid retention 48 h and 96 h after insemination. Comparisons were made between fluid retention and biopsy score or age. In Experiment 2, a subset of 14 mares was classified for susceptibility to persistent breeding-induced endometritis in two subsequent breeding seasons. Biopsy score and age were associated with fluid retention (P < 0.001). In addition, age was related to biopsy score (P < 0.001). Of the mares examined for susceptibility, 36% (5 of 14) changed status during subsequent seasons. Three mares changed to a more severe classification (intermediate to susceptible, or resistant to intermediate), whereas two mares changed to a less severe classification (susceptible to intermediate).     

The functional competence of uterine-derived polymorphonuclear neutrophils (PMN) from mares resistant and susceptible to chronic uterine infection: a sequential migration analysis

The functional competence of uterine-derived polymorphonuclear neutrophils (PMNs) from 28 mares was measured for migration responsiveness by use of a chamber (filter) assay. Uterine infection was induced with Streptococcus zooepidemicus in mares considered resistant to chronic uterine infection (Grade I). In sequential analysis of uterine flushings obtained from these mares 5, 12, 15, 20, and 25 h after infection was induced, PMNs showed an initial rise at 12 h (from 5), then a general decline in migration response and in concentration of cells per ml from 12 through 25 h post-inoculation. In contrast, PMNs obtained from the uterine flushings from mares considered susceptible to chronic uterine infection (Grade III) demonstrated premature migration dysfunction 12 h after infection. Subsequent increases in functional competence of the PMNs were demonstrated at 15 and again at 25 h after induced infection. The concentration of uterine PMNs per ml from mares considered susceptible to chronic endometritis remained elevated from 12 through 25 h after inoculation, which suggests a possible continued recruitment of new PMNs from the peripheral circulation. The results of this study suggest that uterine-derived PMNs obtained from mares susceptible to chronic uterine infection have a compromised ability to migrate. This dysfunction may play an important role in rendering the endometrium (uterus) susceptible to chronic endometritis.     

Endometritis in the mare: A comparison between reproductive history and uterine biopsy as techniques for predicting susceptibility of mares to uterine infection

Thirty mares with no clinical signs of endometritis were categorized as being susceptible or resistant to uterine infection depending on whether or not they had a history of recurrent endometritis. The same mares were then independently classified as susceptible or resistant on the basis of their uterine biopsies; those with significant endometrial degeneration were considered to be susceptible to endometritis. The mares then received an intrauterine inoculation of pathogenic Streptococcus zooepidemicus . Those mares which eliminated bacteria by 10 d after inoculation were considered truly resistant to endometritis, whereas those still infected at 10 d were considered susceptible. The original classifications based on history or biopsy were compared to the inoculation results. A history of recurrent endometritis provided a more sensitive (0.90) and specific (0.95) indication of susceptibility to uterine infection than a uterine biopsy with significant endometrial degeneration (sensitivity 0.5, specificity 0.75).     

Uterine clearance and resistance to persistent endometritis in the mare

The objective of this article is to review the role of uterine defense mechanisms in natural resistance to chronic or persistent endometritis. A breakdown of uterine physical clearance mechanisms is currently believed to play a major role in susceptibility to persistent endometritis. Mares with increased susceptibility to persistent endometritis have impaired myometrial contractility in response to an acute inflammation, resulting in an accumulation of fluid and inflammatory products within the uterine lumen. The origin of this defect remains unknown. Recent studies have demonstrated that spermatozoa trigger PMN chemotaxis into the uterine lumen. This observation suggests that a transient endometritis is a normal physiological response to breeding. However, in mares with impaired uterine defense mechanisms, the condition may develop into a persistent endometritis and subsequent subfertility. In contrast to spermatozoa, seminal plasma has a suppressive effect on complement activation and PMN chemotaxis (65). The exact role of seminal components in breeding-induced inflammation needs further investigation.     

Biodegradable estradiol microspheres do not affect uterine involution or characteristics of postpartum estrus in mares

Quarterhorse mares were used to investigate effects of estradiol-17beta on uterine involution, duration of estrus, interval to ovulation, and fertility achieved by breeding on the first postpartum estrus. On the day of foaling, mares were injected with biodegradable poly (DL-lactide) microspheres containing either 100 mg estradiol-17beta (25 mares) or no drug (27 mares). The treatment period was considered to last for 12 to 15 d. Estrus was determined by teasing mares (n=16) with a stallion. Ovulation was detected by transrectal ultrasonographic examination of ovaries (n=48). On Days 6, 11 and 16 post partum, transrectal ultrasonography was used to measure cross-sectional diameters of the uterine body, uterine horns, and fluid within the uterine lumen (n=28). Uteri were swabbed for bacteriologic culture, and uterine biopsies were obtained from the previously gravid uterine horn on Days 11 and 16 post partum, for assessment of endometritis and morphometric analysis of endometrial histioarchitecture (n=19). Twenty-two mares were bred on foal-heat, and pregnancy was determined by transrectal ultrasonography on 14 to 16 and 30 to 35 d after breeding. With only one exception (diameter of previously gravid uterine horn on Day 11), mean values for all measures of uterine involution did not differ between treatment groups (P > 0.05). No differences were detected between treatment group means for length of estrus or interval to ovulation (P > 0.05). No differences were detected between treatment group liklihoods for recovery of potential bacterial pathogens, presence of endometritis, or presence of intrauterine fluid at 11 or 16 d post partum (P > 0.05). Pregnancy rate of mares treated with estradiol (5 11 ; 45%) was not different from that of control mares (9 11 ; 82%; P > 0.05). Estradiol treatment did not hasten uterine involution, increase duration of estrus, delay ovulation, or increase fertility in these postpartum mares.     

Uterine contractility is necessary for the clearance of intrauterine fluid but not bacteria after bacterial infusion in the mare

Bacteria were infused into the uteri of 5 estrous mares resistant to persistent mating-induced endometritis, first during a control cycle, and then during treatment with clenbuterol, a beta 2 agonist. Uterine cellular response was evaluated 48 h later by transrectal ultrasonography, followed by uterine lavage. During clenbuterol treatment all mares accumulated intrauterine fluid, whereas in the control cycle none of the mares retained fluid. There was no significant difference between the 2 cycles in the cloudiness of the lavage fluid, number of cells per milliliter, percentage of neutrophils and frequency of bacterial growth from the recovered fluid. We conclude that uterine contractility is important in the clearance of uterine fluid, but not necessarily for the elimination of bacteria, thus supporting the published evidence that impaired uterine contractility contributes to the pathogenesis of persistent mating-induced endometritis.     

Effect of susceptibility to endometritis on specific antibody in the endometria of mares

Endometrial biopsies were collected on two occasions from mares resistant to (n = 3) and once from mares susceptible to persistent endometritis (n = 6). The endometrial tissue was minced and cultured in vitro for 24 h. No hemolytic complement activity was detected in the endometrial culture supernatant. Endometrial culture supernatant from mares with persistent endometritis contained titers of antibodies to Streptococcus zooepidemicus similar to those from resistant mares. However, the culture supernatant of biopsies from mares with endometritis was less effective (P < 0.05) at opsonizing S. zooepidemicus in vitro.     

Effects of oral treatment with N-acetylcysteine on the viscosity of intrauterine mucus and endometrial function in estrous mares

Persistent breeding-induced endometritis is ranked as the third most common medical problem in the adult mare and leads to enormous economic loss in horse breeding. In mares suffering from persistent breeding-induced endometritis, increased amounts of intrauterine (i.u.) fluid or viscous mucus in estrus or after breeding may act as a barrier for sperm and can contribute to low fertility. Current therapies of these mares aim to eliminate i.u. fluid and mucus by uterine lavage and/or administration of ecbolic drugs. Recently, i.u. administration of N-acetylcysteine (NAC) has been shown to support therapy in mares with endometritis. It was the objective of the present study to investigate effects of an oral administration of NAC on the viscosity of i.u. fluid in estrous mares. It was hypothesized that oral treatment with NAC reduces the viscosity of i.u. fluid and has a positive effect on the inflammatory response of the endometrium. Mares (n = 12) were included in the study as soon as estrus was detected (ovarian follicle >3.0 cm and endometrial edema), which was defined as Day 1. They were randomly assigned to a treatment (10 mg/kg NAC on Days 1-4) or a control group (no treatment). On days 1 and 5 i.u. mucus was collected and its rheologic properties were accessed. On Day 5, endometrial biopsies were obtained and evaluated for integrity of the luminal epithelium, number of polymorphonuclear neutrophils (PMN), staining for cyclooxygenase 2 (COX2), staining with Kiel 67 antigen (Ki-67), lectins and periodic acid Schiff (PAS). In the treatment group, viscosity of i.u. mucus increased significantly between Days 1 and 5 (P < 0.05), while no differences were found in control mares (n.s.). At no time were significant differences between treated and control mares seen. Integrity of epithelium was not affected. After NAC treatment the mean number of PMN in endometrial biopsies was significantly lower compared to mares of the control group (1.9 ± 0.3 vs. 4.8 ± 0.4; P < 0.05). Nuclear immunostaining for COX2 was significantly lower after NAC treatment compared to control mares (P < 0.05). Score for PAS and Alcain staining of mucus in deep uterine glands differed significantly between groups (both P < 0.05). We conclude that oral NAC treatment does not reduce viscosity of uterine mucus but has an antiinflammatory effect on the equine endometrium.     

Administration of oxytocin immediately after insemination does not improve pregnancy rates in mares bred by fertile or subfertile stallions

It is probable that reduced pregnancy rates in mares bred to subfertile stallions is attributable, in part, to the reduced number of normal spermatozoa that colonize the oviduct. Administration of oxytocin stimulates both uterine and oviductal contractility. The hypothesis that oxytocin may enhance sperm transport to/into the oviducts, and thereby increase pregnancy rates, was tested in 2 trials. For both trials, fertile estrous mares with follicles > or = 35 mm in diameter were inseminated once at 24 h after administration of 1500 to 2000 U hCG. The inseminate dose was limited to 100 million spermatozoa in order to lower pregnancy rates and thus increase the chance of detecting a treatment effect. Pregnancy status was determined by transrectal ultrasound examination 14 to 16 d after insemination. In Trial 1, 49 mares were inseminated with 4 mL extended semen from 1 of 3 stallions (1 fertile and 2 subfertile males). Immediately after insemination, the mares were administered either 20 U oxytocin or 1 mL saline intravenously. In Trial 2, 51 mares were inseminated with 4 mL extended semen from 1 of 4 stallions (1 fertile and 1 subfertile male used in Trial 1, and 2 additional fertile males). Immediately after insemination, and again 30 min later, mares were administered either 5 U oxytocin or 0.25 mL saline intramuscularly. To test for effects of treatment with oxytocin and for the interaction between semen quality and treatment, a generalized linear mixed regression model was used that accounted for the split-plot design (treatment within stallions), the random effect of stallion, the fixed effect of semen quality, the binary outcome of a single breeding trial, and the varying number of trials per stallion/treatment groups. Three treatment protocols or regimens were used: placebo, 5 U oxytocin injected twice intramuscularly, and 20 units oxytocin injected twice intravenously. Semen was classified as high (fertile stallions) or low (subfertile stallions) quality. No interaction between semen quality and treatment was detected (P > 0.10). The pregnancy rate of mares treated with oxytocin immediately after insemination was 30% (15/50) compared with 50% (25/50) for mares treated with saline immediately after breeding. Administration of oxytocin did not affect pregnancy rates (P > 0.10).     

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.     

Mares with delayed uterine clearance have an intrinsic defect in myometrial function

Persistent, postmating endometritis affects approximately 15% of mares and results in reduced fertility and sizable economic losses to the horse-breeding industry. Mares that are susceptible to postmating endometritis have delayed uterine clearance associated with reduced uterine contractility. Unfortunately, the mechanism for reduced uterine contractility remains an enigma. The present study examined the hypothesis that mares with delayed uterine clearance have an intrinsic contractile defect of the myometrium. Myometrial contractility was evaluated in vitro by measuring isometric tension generated by longitudinal and circular uterine muscle strips in response to KCl, oxytocin, and prostaglandin F(2alpha) (PGF(2alpha)) for young nulliparous mares, older reproductively normal mares, and older mares with delayed uterine clearance. In addition, intracellular Ca(2+) regulation was evaluated using laser cytometry to measure oxytocin-stimulated intracellular Ca(2+) transients of myometrial cells loaded with a Ca(2+)-sensitive fluorescent dye, fluo-4. For all contractile agonists, myometrium from mares with delayed uterine clearance failed to generate as much tension as myometrium from older normal mares. Oxytocin-stimulated intracellular Ca(2+) transients were similar for myometrial cells from mares with delayed uterine clearance and from older normal mares, suggesting that the contractile defect did not result from altered regulation of intracellular Ca(2+) concentration. Furthermore, no apparent age-dependent decline was observed in myometrial contractility; KCl-depolarized and oxytocin-stimulated longitudinal myometrium from young normal mares and older normal mares generated similar responses. However, circular myometrium from young normal mares failed to generate as much tension as myometrium from older normal mares when stimulated with oxytocin or PGF(2alpha), suggesting possible age-related alterations in receptor-second messenger signaling mechanisms downstream of intracellular Ca(2+) release. In summary, for mares with delayed uterine clearance, an intrinsic contractile defect of the myometrium may contribute to reduced uterine contractility following breeding.     

The effect of the gonadotrophin-releasing hormone analog, buserelin, administered in diestrus on pregnancy rates and pregnancy failure in mares

Two trials involving 578 mares were performed to investigate the effect of a single intramuscular treatment of 40 microg buserelin, an analog of gonadotrophin releasing hormone, on pregnancy rate in mares. All mares were bred by natural mating and were allocated into pairs One mare in each pair was injected with buserelin either on Day 10 or 11 (Trial 1) or on Days 8 to 10 (Trial 2) after ovulation. Pregnancy status of mares was determined by transrectal ultrasonographic examination on Day 14 or 15 after the day of ovulation and was repeated between Days 28 and 30 of pregnancy. In Trial 1, buserelin treatment increased the pregnancy rate at Days 14 and 15 (72.5 vs 66.6%, P < 0.01). At the second pregnancy examination, pregnancy losses were lower in the treated group of mares (4.1 vs 7.4%; P < 0.05). In Trial 2, buserelin also improved the pregnancy rate (57.2 vs 53 5%; P < 0.05) at Days 14 and 15 Pregnancy losses between the first and second examinations were lower in the treated group of mares (6.5 vs 12.0%; P < 0.05). Buserelin increased pregnancy rates after breeding at the first estrus in both trials. In addition, buserelin treatment increased the pregnancy maintenance rate at Days 28 to 30.     

The effect of isosorbide dinitrate on uterine and ovarian blood flow in cycling and early pregnant mares: A pilot study

Poor uterine perfusion has been proposed as a cause of infertility in mares. The objective of this study was to investigate the effect of isosorbide dinitrate (ISDN), a nitric oxide donor, on uterine and ovarian blood flow resistance during diestrus and early pregnancy in mares. Six Trotter mares, aged 7 to 14 years, were examined daily during the first 11 days of three diestrous periods, and five of those mares were also examined during the first 11 days of two pregnancies. Six mares randomly received a placebo, a low dose (30 mg, ISDN30), or a high dose of ISDN (60 mg, ISDN60) through three nonconsecutive cycles. The treatments were administered orally, every 12 hours from Day 1 to 11 of the cycle (Day 0 = ovulation). Five of the 6 mares received a placebo or 60 mg of ISDN orally every 12 hours from Day 1 to 11 of pregnancy. The mares were short cycled on Day 12 of each trial. Transrectal color Doppler was used to determine blood flow resistance semiquantitatively and expressed as pulsatility index. Mean pulsatility index of both uterine arteries combined and of the dominant (ipsilateral to the CL) ovarian artery was lower (treatment effects: P ≤ 0.01; time effects: P ≤ 0.002) in mares receiving 30 mg or 60 mg of ISDN compared with placebo-treated mares. Blood flow resistance in the dominant ovarian artery was lower in ISDN-treated pregnant mares than in placebo-treated pregnant and cycling mares (treatment effect: P = 0.04; time effect: P = 0.003). Isosorbide dinitrate increases uterine and ovarian perfusion in cycling mares and ovarian perfusion in early pregnant mares. Further studies are needed to investigate these effects in relation to fertility of the mare.     

Effects of cervical dilation and intrauterine infusions on the timing of oviductal transport of equine embryos

Equine embryos spend 5 to 6 d in the oviduct before entering the uterus as expanded blastocysts, and cannot be consistently collected nonsurgically until Day 7. Technologies such as cryopreservation and embryo splitting, which are most successful with embryos at the morula or early blastocyst stage, have not been used in mares because equine morulae and early blastocysts are located in the oviduct and cannot be recovered nonsurgically. These experiments test the hypothesis that transport of equine embryos through the oviduct can be hastened by cervical dilation or by acute, sterile endometritis induced by intrauterine oyster glycogen treatment. Cervical dilation with or without intrauterine infusion of 0.5 ml PBS on Day 4 did not appear to hasten the transport of embryos into the uterus since Day 5 uterine embryo recovery rates were not higher (P > 0.1) for mares with cervical dilation or cervical dilation plus PBS infusion vs mares receiving no treatments (0 of 5 and 0 of 5 vs 0 of 10, respectively). Intrauterine infusions of 40 ml of 1% oyster glycogen or 40 ml of PBS on Day 3 did not appear to hasten the transport of embryos into the uterus since Day 5 uterine embryo recovery rates were not higher (P > 0.1) for oyster glycogen- or PBS-treated vs untreated mares (2 of 12 and 3 of 11 vs 0 of 10, respectively). Cervical and uterine treatments on Day 3 or Day 4 and uterine lavages on Day 5 decreased (P < 0.05) Days 11 to Day 15 pregnancy rates compared with that of untreated mares. Day 11 to Day 15 pregnancy rates were 1 of 5 for mares with Day 4 cervical dilation and Day 5 uterine lavage, 1 of 5 for mares with Day 4 PBS infusion and Day 5 uterine lavage, 2 of 12 for mares with Day 3 oyster glycogen infusion and Day 5 uterine lavage, and 3 of 11 for mares with Day 3 PBS infusion and Day 5 uterine lavage vs 7 of 10 for mares that received no treatment or lavage. Cervical and uterine manipulations on Day 3 or 4 and uterine lavage on Day 5 appeared to decrease pregnancy rates by Days 11 to 15. The results of these experiments do not support the hypothesis that cervical dilation or uterine infusion hasten oviductal transport, since neither cervical manipulation nor transcervical infusion of oyster glycogen or PBS into the uterus significantly hastened the rate of embryo transport into the uterus.     

Effect of intrauterine treatment with prostaglandin E2 prior to insemination of mares in the uterine horn or body

Two trials were conducted to investigate the effects of intrauterine infusion of PGE2 and uterine horn insemination on pregnancy rates in mares achieved by breeding with a suboptimal number of normal spermatozoa. Estrus was synchronized and mares were teased daily with a stallion to detect estrus. Mares in estrus were examined by transrectal palpation and ultrasonography to monitor follicular status. On the first day a 35-mm diameter follicle was present, hCG (1500 IU, iv) was administered and the mares were bred the next day. Mares (Trial 1, n = 34; Trial 2, n = 28) were inseminated with 25 million total spermatozoa from either a stallion with good semen quality (Trial 1) or poor semen quality (Trial 2). In each trial, mares were assigned to 1 of 4 treatment groups as follows: Group PGE-HI - infusion of 0.25 mg PGE2 into the proximal end of the uterine horn ipsilateral to the dominant follicle 2 h prior to insemination in the proximal end of the same uterine horn; Group PGE-BI - infusion of 0.25 mg PGE2 into the proximal end of the uterine horn ipsilateral to the dominant follicle 2 h prior to insemination in the uterine body; Group SAL-HI - infusion of 1 mL sterile saline into the proximal end of the uterine horn ipsilateral to the dominant follicle 2 h prior to insemination in the proximal end of the same uterine horn; or Group SAL-BI - infusion of 1 mL sterile saline into the proximal end of the uterine horn ipsilateral to the dominant follicle 2 h prior to insemination in the uterine body. After breeding, mares were examined daily by transrectal ultrasonography to confirm ovulation, and were re-examined 14 to 16 d after ovulation for pregnancy status. Data were analyzed by Chi-square. Overall pregnancy rates were 59% for stallion 1 and 29% for stallion 2. Group pregnancy rates did not differ for mares bred by either stallion (P > 0.10). Pregnancy rates were not altered by horn insemination for either stallion (P > 0.10). Intrauterine infusion of PGE2 improved pregnancy rate in mares bred by the stallion with good quality semen (P < 0.05), but did not alter pregnancy rate in mares bred by the stallion with poor quality semen (P > 0.10). Further research is warranted to determine if intrauterine infusion of PGE2 will enhance spermatozoal colonization of the oviduct and pregnancy rates in mares, and if PGE-treatment will improve pregnancy rates achieved by subfertile stallions.