Topic:Estrus
Estrus in horses refers to the recurring reproductive cycle in mares during which they exhibit signs of sexual receptivity and fertility. This phase is part of the larger estrous cycle, which includes several stages: estrus, metestrus, diestrus, and proestrus. During estrus, mares experience physiological changes, such as follicular development and ovulation, which are regulated by hormonal fluctuations, primarily involving estrogen and luteinizing hormone. Behavioral changes, including increased vocalization, restlessness, and receptivity to stallions, are also observed. This page compiles peer-reviewed research studies and scholarly articles that explore the physiology, endocrinology, and behavioral aspects of estrus in horses, as well as its implications for breeding management and equine reproductive health.
Estrous cycle characteristics, luteal function, secretion of oxytocin (OT) and plasma concentrations of 15-keto-13,14-dihydro-PGF2alpha (PGF2alpha-metabolite) after administration of low doses of prostaglandin F2alpha (PGF2alpha) in pony mares. In the present study, the kinetics of the prostaglandin F2alpha (PGF2alpha)-metabolite 15-keto-13,14-dihydro-PGF2alpha after a single intramuscular application of various doses of the natural PGF2alpha dinoprost at Day 7 of the cycle in the mare were investigated. Effects of low doses on estrous cycle length and life span of corpus luteum were examined, because release of PGF2alpha is still under discussion to have detrimental influence on success rates of transcervical transfer of equine embryos. Eight Shetland pony mares were each randomly assigned to each of four treatments: (a) 0.8 mg/100 ...
Ultrasonic characteristics of preovulatory follicle and ovulation in Caspian mares. The Caspian breed of horses is believed to be the direct descendant of the earliest equine animals. Some special characteristics of Caspian horse differentiate this breed of horses from other breeds. In the current study the ultrasonically observed characteristics of a preovulatory dominant follicle and the lengths of estrus, diestrus as well as some related parameters were studied during 42 interovulatory intervals in 11 healthy Caspian mares. The preovulatory dominant follicle deviated from subordinate follicles and became the largest follicle in the ovaries at Day -8.7 +/- 0.53 (Day 0=ovula...
Successful non-surgical transfer of horse embryos to mule recipients. Mules, hybrids resulting from the mating of a horse mare (Equus caballus, 2n = 64) to a Jack donkey (E. asinus, 2n = 62), are generally infertile. Five horse embryos were transferred non-surgically to two cyclic and one acyclic recipient mules. In the mares and cycling mules, oestrus and ovulation were induced with, respectively, d-cloprostenol and human chorionic gonadotrophin (hCG). The acyclic mule, on the other hand, received oestradiol benzoate when the embryo donor was showing oestrus and progesterone after the donor had ovulated and until pregnancy diagnosis. Non-surgical embryo collect...
A light microscopic and ultrastructural study on the presence and location of oxytocin in the equine endometrium. It has been reported that oxytocin is produced not only in the hypothalamus and posterior pituitary but also in outside the classical hypothalamo-neurohypophyseal axis such as the ovary, testis, placenta and in some nonreproductive sites. In the mare, oxytocin-mRNA has been identified in the endometrium, and oxytocin and its neurophysin have been identified in the uterus. In the present study, oxytocin was localised in the endometrium of the mare at the light microscopic and ultrastructural level by immunostaining and immunogold labelling of endometrial biopsy specimens collected during estrus...
Proteins and enzymes in uterine lavage fluid of postpartum and nonparturient mares. Uterine lavage fluids from postpartum and nonparturient mares were compared to determine when the normal secretory capacity of the postpartum uterus is restored. Lavage fluids were obtained from cyclic nonparturient mares on the second, fourth or fifth day of oestrus, and 3, 8, or 14 days after ovulation (seven mares/sampling day). Twelve intact postpartum mares were sampled 1 to 28 days postpartum (group A: 1, 6, 12 and 20; group B: 2, 8, 14 and 24; group C: 4, 10, 16 and 28 days postpartum; four mares/group). Three ovariectomized (OVX) postpartum mares were sampled as mares in group C. Sampl...
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 admini...
Free and conjugated estrogens and androgens in stallion semen. The steroid content of semen from a total of 11 mature fertile stallions was studied during two breeding seasons and one winter. The levels of free and conjugated substrates (testosterone and androstenedione), and products (estradiol and estrone), of aromatase were measured by radioimmunoassay with a validated method. The results were seasonally and monthly highly variable with characteristic peaks. The concentrations of free and conjugated estrogens were always higher in the gel-free ejaculate than in the gel except in one subfertile stallion used as comparison. Furthermore, the steroid produ...
Treatment of unobserved oestrus in a dairy cattle herd with low oestrous detection rate up to 60 days post-partum. The efficiency of treatments for unobserved oestrus and their effect on the reproductive performance of a dairy cattle herd with low oestrous detection rate till 60 days post-partum (dpp), attributed to the declivous and slippery concrete floor were investigated. The herdsman requested advice in order to improve the mean days open of the herd, but no investments were allowed because a new unit was about to be built. Due to the low oestrus detection rate of the herd, the breeding policy was to inseminate at the first detected post-partum oestrus. Cows were examined at 20-30 dpp to assess uterin...
Regulation of ovarian follicular dynamics in farm animals. Implications for manipulation of reproduction. In this review, the main features of folliculogenesis are summarized and compared among species. In the past few years, ultrasonography has clarified follicle growth patterns, and our understanding of follicle maturation has improved considerably. As the follicles develop towards the ovulatory stage, three features appear to be highly conserved across all species: 1) the sequence of events (recruitment, selection and dominance); 2) the sequential need for gonadotropins (FSH for recruitment, LH for dominance) and 3) the large variability of numerical parameters (number of waves per cycle, numbe...
T-cell distribution in two different segments of the equine endometrium 6 and 48 hours after insemination. The T-cell response after the introduction of semen into the uterine cavity in the mare was studied by examining, immunohistochemically, the distribution of helper T-cells (CD4+) and cytotoxic T-cells (CD8+) in endometrial biopsy specimens. Endometrial tissue samples were obtained from twenty-five gynecologically healthy mares during estrus before and 6 or 48 h after deposition of a single dose of stallion semen. An increase (P=0.04) in the number of helper T-cells (CD4+) compared to pre-insemination values was observed in the uterine body in both groups, 6 and 48 h, after insemination. No sig...
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 m...
Factors influencing ovarian activity and sexual behavior of postpartum mares under farm conditions. Management of the postpartum period is one of the most important factors of stud farm medicine. In horses, owing to the long gestation period, the time from parturition to repeat conception needs be short to maintain an optimal yearly foaling interval. For this reason the features of postpartum ovarian activity and sexual behavior were studied under farm conditions. During 2 consecutive breeding seasons, 107 mares on 5 commercial horse farms were monitored after parturition by regular teasing, transrectal ultrasonography and blood sampling for progesterone. Foalings took place from January 1 t...
Estrous cycle characteristics and response to estrus synchronization in mammoth asses (Equus asinus americanus). Breeding records from a herd of mammoth asses (Equus asinus americanus) maintained on pasture in southeast Texas from 1990 to 1998 were reviewed. Jennies were pasture or hand mated, and estrus was either observed while the jennies were on pasture or when exposed to a jack after being penned. Eighty-one estrus periods and 43 diestrus intervals were recorded in 33 jennies over 4 seasons of the year (January-March, April-June, July-September, and October-December). Estrous cycle length and the duration of estrus were similar among seasons. Over all seasons, estrous cycle length was 23.3 +/- 2.6 d...
Dynamics of prostaglandin secretion, intrauterine fluid and uterine clearance in reproductively normal mares and mares with delayed uterine clearance. Two experiments were performed to investigate relationships between oxytocin, prostaglandin release, uterine emptying and fluid accumulation in the uterus. In Experiment 1, the effect of oxytocin on the pattern of prostaglandin release during uterine clearance of radiocolloid was measured in 5 normal mares and 5 mares with delayed uterine clearance. Uterine clearance was measured during estrus by scintigraphy at 0, 60 and 120 min after colloid infusion. After the 120-min reading, 20 IU, i.v., oxytocin were given, and the amount of colloid cleared was measured at 135, 150 and 180 min. Plasma wa...
Changes in intrauterine pressure after oxytocin administration in reproductively normal mares and in those with a delay in uterine clearance. Intrauterine pressure was measured in 4 reproductively normal mares and 4 mares with delay in uterine clearance after administration of oxytocin to determine if intrauterine pressure varied between dosage and group. Changes in intrauterine pressure were measured during estrus, when a follicle was > or =35 mm, using a Millar "Mikro-tip" catheter that had 3 discrete pressure sensors/channels. Mares received 4 different treatments of 10, 5, 2.5 or 0 IU (vehicle) of oxytocin. The protocol for each treatment consisted of a 10-min baseline recording, administration of treatment and measurement of...
Influence of age and parity on the distribution of cells expressing major histocompatibility complex class II, CD4, or CD8 molecules in the endometrium of mares during estrus. To evaluate effect of age and parity on distribution and number of cells expressing major histocompatibility complex (MHC) class II, CD4, or CD8 molecules in the endometrium of mares during estrus. Methods: 32 gynecologically healthy mares, categorized as young (3 to 8 years; n = 17) or old (9 to 16 years; 15) and nulliparous (n = 6), nulliparous embryo donors (16), or parous (10). Methods: Endometrial specimens collected from the uterine body and horns during estrus were stained by use of the avidin-biotin-peroxidase method, using monoclonal antibodies against equine MHC class II, CD4, and CD...
Modulation of oviductal transport in mares by local application of prostaglandin E2. Equine embryos enter the uterus 144-156 h after ovulation, before which time the passage of embryos through the oviduct is halted in the region of the ampullary-isthmic junction. It is thought that further onward movement of embryos to the uterus is facilitated by secretion of prostaglandin E2 (PGE2) by the embryos, which relaxes the smooth muscle of the isthmus. In the present study, the effect of a single local application of PGE2 on oviductal transport was examined in 22 Thoroughbred and Pony mares that were inseminated on alternate days during oestrus. On day 4 after ovulation, the ipsilat...
A detailed study of hormonal profiles in mares at luteolysis. Jugular blood samples were collected at 4 h intervals from six mares during an oestrous cycle to study the hormonal events that occur around the time of luteolysis. Blood samples from day 10 (day 0 = ovulation) until day 3 of oestrus were assayed for prostaglandin metabolite 13,14-dihydro-15-keto PGF2alpha (PGFM), oxytocin, prolactin, progesterone and oestrogen conjugates. PGF2alpha (0.5 or 1.5 mg) was administered to six mid-dioestrous mares and the oxytocin and prolactin responses were measured. One to five large (peak > or =2 x nadir) pulses of PGFM, oxytocin and prolactin were detected ...
Ultrasonic evaluation for the time of ovulation in ewes treated with norgestomet and norgestomet followed by pregnant mare’s serum gonadotropin. Progestogens and follicular stimulants have proved reasonably successful for estrus synchronization, but time of ovulation relative to removal of the progestogen is not clearly established. We monitored time of ovulation in ewes following synchronized estrus. Ovaries of 40 Dorset and Rambouillet x Dorset ewes were evaluated during the spring and fall (20/replicate). Ewes were randomly assigned to one of two treatment groups (n = 20/group): implant-only (I) ewes received a norgestomet implant for 10 d; and implant + PMSG (PI) ewes received a norgestomet implant for 10 d with an i.m. injection o...
Progesterone determination in equine plasma using different immunoassays. Several assay systems (3H radioimmunoassay (RIA) with and without extraction; microplate enzyme-linked immunoassay (ELISA); qualitative ELISA (tube test)] were used to measure plasma progesterone concentration in mare plasma. The direct RIA showed a close correlation (R = 0.94) with the extraction RIA. The direct RIA and the microplate ELISA were compared in two different studies. In the first study 1155 samples of postpartum mares were used for progesterone determination with both assays. The ELISA resulted in more elevated values both in oestrus and dioestrus (0.19+/-0.3 and 2.44+/-3.62 nmol...
Follicular dynamics in Mangalarga mares. Ovarian follicular activity was studied by ultrasonography during 17 oestrous cycles in 9 Mangalarga mares during the second half of the ovulatory season. Sixteen oestrous cycles were considered normal and one 3-wave cycle showing a prolonged luteal phase was considered atypical. Daily ultrasonographic examinations were performed and the compiled data on follicular dynamics were studied retrospectively. One major wave of follicular growth was observed in 13 of the 16 normal cycles (81.25%), whereas 2 major waves occurred in 3 cycles (18.75%). The mean (+/- s.d.) days of emergence of the primar...
Distribution of glycoconjugates in the uterine tube (oviduct) of horses. To examine glycoconjugates in the isthmic and ampullar regions of the uterine tube (oviduct) of horses during estrus, diestrus, and pregnancy. Methods: Oviductal samples from 17 mares. Methods: Oviducts were collected during estrus (n = 3), diestrus (n = 3), or pregnancy (n = 3), embedded, and snap frozen in liquid nitrogen. Frozen sections (5 to 6 microns in thickness) were stained with 100 micrograms/ml of fluorescein-isothiocyanate-conjugated lectin (30 min at 38.5 C) and were evaluated by use of epifluorescence microscopy and video image analysis. Specificity of lectins was established by ...
Prolactin administration to seasonally anestrous mares: reproductive, metabolic, and hair-shedding responses. Eight pony mares received 4 mg of recombinant porcine prolactin (rpPRL) daily for 45 d beginning on January 15; eight control mares received vehicle. Reproductive end points and various indicators of metabolism, hair shedding, and thyroid activity were monitored. Prolactin concentrations peaked in mares treated with rpPRL at 94 +/- 19 ng/mL 2 h after injection and were 5.1 +/- 1.7 ng/mL 24 h after injection. Treatment with rpPRL increased (P < .01) hair shedding within 14 d, which peaked at 28 d and then dropped precipitously. Binding of 125I-equine prolactin confirmed that antibodies were ...
Tissue and serum concentrations of amikacin after intramuscular and intrauterine administration to mares in estrus. Concentrations of amikacin in endometrial tissue and plasma were studied in mares in estrus after intrauterine infusion of 1.0 or 2.0 g once a day for 3 consecutive d, and after 9.7 or 14.5 mg/kg body weight (BW) had been injected intramuscularly once a day for 3 consecutive d to determine concentrations of amikacin sulfate in plasma and endometrial tissues, and whether parenteral administration provides any advantages over intramuscular infusion. No amikacin was detected in serum at the 1.0 g dose. At the infusion dose of 2.0 g once a day, very low levels of serum amikacin were detected at 1 ...
Ultrastructure of the secretory endometrium during oestrus in young maiden and foaled mares. Cyclical accumulation of uterine fluid occurs during oestrus and is often seen in excessive volumes in mares considered susceptible to endometritis. Since the mechanisms behind the formation of free uterine fluid remain to be clarified, the fine structure of the secretory equine endometrium was studied in biopsies collect during videoendoscopy from 14 endometritis-free, 4-9-year-old mares during oestrus. A distinct oedema of the tunica mucosa was evident. The surface epithelium had both ciliated and nonciliated cells and, particularly at the uterine body, often presented intra-epithelial macro...
Oxytocin in mares: lack of evidence for oxytocin production by or action on preovulatory follicles. It is not known whether the equine preovulatory follicle produces oxytocin or is a target tissue for oxytocin, as has been reported for other species, especially ruminants. Bovine granulosa cells secrete oxytocin, and oxytocin modulates the production of progesterone by granulosa cells in vitro. We examined whether oxytocin plays a comparable role in the equine preovulatory follicle. To test the hypothesis that the equine preovulatory follicle produces oxytocin during estrus and that its production increases in late estrus, preovulatory follicles were isolated during early (Days 1 to 2; n = 4)...
Histological changes in the uterus during postpartum in the mare. An histological study of the postpartum period in 29 mares was carried out. Uterine biopsies were taken daily during the first 10 days postpartum in a total of 87 samples. At day 0, equine endometrium was characterized in the surface by the presence of regularly ordered microcaruncles; the stratum spongiosum was oedematous and contained distended and scarce glands. Degenerative changes in microcaruncles and endometrial glands were present on day 1 postpartum. The epithelium of the microcaruncles from 2 to 5 days postpartum showed cytoplasmic vacuolization, karyorrhexis and an inflammatory reac...
Characteristics of cyclicity in maiden thoroughbred mares in the United Kingdom. The characteristics of the cyclicity of 12 maiden thoroughbred mares kept in two groups were studied over a total of 58 cycles. On average, oestrus lasted 5.3 days and in 60 per cent of the cycles ovulation occurred in the last two days of oestrus. Oestrus and ovulation tended to be synchronised in each group of mares. The mean diameter of single-ovulating preovulatory follicles on the day before ovulation was 41.5 mm and during the seven days before ovulation they grew 2.5 mm/day. More than one follicle ovulated in 19 (33 per cent) of the cycles (seven double ovulations and 12 dioestrous ovul...
MHC Class II positive cells and T cells in the equine endometrium throughout the oestrous cycle. The quantity and distribution of MHC Class II positive cells and T cells in the equine endometrium was investigated throughout the oestrous cycle. Significantly more MHC Class II positive cells were detected in the stratum compactum and stratum spongiosum of endometria from naturally cycling mares during the follicular than during the luteal phase of the oestrous cycle. Significantly more T cells were also detected in the stratum compactum, but not stratum spongiosum, of these mares during the follicular phase. Furthermore, there was a marked increase in the number of MHC Class II positive cel...
Electrophoretic characterization of equine oviductal fluid. To characterize further the events involved in fertilization and early embryonic development in the mare, effect of the estrous cycle on oviductal fluid proteins was investigated. Five mares had indwelling cannulas placed in their oviducts so that fluid could be collected throughout the estrous cycle. Daily fluid volumes were recorded and mares were monitored for signs of standing estrus. Oviductal fluid samples were pooled across mares according to stage of cycle (either estrus or nonestrus) for further analysis. Two-dimensional polyacrylamide gel electrophoresis (PAGE) was used to determine ...