Induction of ovulation with gonadotrophin releasing hormone and progesterone in seasonally anestrous mares

Five seasonally anestrous mares were treated with a regimen of gonadotrophin releasing hormone and progesterone in an attempt to induce estrus and ovulation. The treatment induced follicular activity and estrus in all mares. Two of the five mares ovulated but none conceived.     

Comparison of two dosage regimens of the GNRH agonist deslorelin acetate on inducing ovulation in seasonally anestrous mares

This study investigated the efficacy of two dosage regimens of a potent GnRH analogue (GnRHa), deslorelin acetate, in inducing ovulation in seasonally anestrous mares. Forty-five seasonally anestrous mares were randomly assigned according to follicular size to one of three treatment groups: control, increasing GnRHa dose, and constant GnRHa dose. Treatment began on February 28 and continued until ovulation or for a maximum of seven treatments. Mares were palpated every other day until a 35 mm follicle was detected, then every day until ovulation or regression of the follicle occurred. Blood samples were taken from five randomly chosen mares in each treatment group and analyzed for LH levels.Twenty percent of mares in both deslorelin treatment groups ovulated, while no control mares ovulated during the treatment period. There was no difference in the number of mares that ovulated between treatment groups. Four of the six mares that ovulated were in transitional anestrus at the initiation of treatment, while only two were in deep anestrus.Concentrations of LH were greater (p=0.0008) in both GnRH-treated groups than in the control mares. Concentrations of LH did not differ between the two GnRH-treated groups until day 12 of treatment, when mares treated with a constant dosage had higher (p=0.0358) levels of LH than those treated with an increasing dosage. It is possible that administration of larger amounts of the GnRH agonist lowered the sensitivity of the pituitary to stimulation by GnRH.Deslorelin acetate did stimulate follicular growth and ovulation in a limited number of anestrous mares. Further investigation into the potential of this short-term implant to shorten the onsent of the breeding season is recommended.     

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.     

Induction of ovulation in anestrous mares with pulsatile administration of gonadotropin-releasing hormone

Four seasonally anestrous mares (Standardbred), housed under a nonstimulatory photoperiod of 8 hours light:16 hours dark, were administered gonadotropin-releasing hormone (GnRH) in a pulsatile pattern (50 or 250 micrograms of GnRH/hour) for 8 to 18 days during February and March 1985. Treatment with GnRH, irrespective of dose or month, induced an increase in serum luteinizing hormone from a mean pretreatment value typical of anestrus (0.58 +/- 0.02 ng/ml +/- SE) to 10.84 +/- 1.27 ng/ml on day 8 of GnRH treatment. Ovulation in the 4 mares occurred 8.8 +/- 0.7 days after the initiation of pulsatile GnRH administration. In each instance, ovulation was followed by a functional corpus luteum, as indicated by a luteal phase (defined as the number of days on which serum levels of progesterone were greater than 1.0 ng/ml) which lasted 14.5 +/- 0.6 days. These results indicate that infusion of GnRH in a pulsatile pattern is effective in inducing follicular development and ovulation in anestrous mares in the absence of a stimulatory photoperiod.     

Effect of melatonin on induction of estrous cycles in anestrous ewes

To examine the influence of melatonin on seasonality of reproduction, 97 multiparous Suffolk and Suffolk-cross ewes were randomly assigned to one of four treatment (TRT) groups in a 2 X 2 factorial arrangement. Treatments were as follows: 1) control (C); 2) melatonin (M); 3) progestogen (P) implant of norgestomet plus pregnant mare serum gonadotropin (PMSG) injection (P + PMSG) and 4) M plus TRT 3 (M + P + PMSG). From April 3 to June 24 an oral dose of 2 mg M was administered once daily at 1600 to each ewe in TRT groups M and M + P + PMSG. On April 30, ewes in groups P + PMSG and M + P + PMSG were implanted in the ear with 2 mg norgestomet for 13 d. Immediately following implant removal, each ewe was injected with 500 IU PMSG. Blood samples were collected from all ewes twice weekly from March 22 through June 24. Number of estrous cycles per ewe during the TRT period of 82 d (April 3 to June 24) was higher (P less than .05) for M + P + PMSG (2.1 +/- .2) than for C (.3 +/- .2), M (1.5 +/- .2) and P + PMSG (1.1 +/- .2). Control ewes had fewer (P less than .05) estrous cycles per ewe than either M or P + PMSG. Following the induced estrus, 40% of ewes in the M TRT had one estrous cycle; 32% had two or more cycles. For ewes treated with M + P + PMSG, 24% had one cycle, and 32% had two estrous cycles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Uterine influences on the formation of subnormal corpora lutea in seasonally anestrous ewes

The hypothesis that subnormal luteal function after induced ovulation in anestrous ewes was the result of uterine influences exerted during the periovulatory period was tested. Crossbred ewes (n = 27) in seasonal anestrus were induced to ovulate by administration of 12 doses of 250 ng of LHRH at 2-h intervals, followed immediately by a bolus injection of LHRH (250 micrograms; d 0). Ewes were unilaterally hysterectomized on either d -3 (PRELHRH) or 2 (POSTLHRH). Daily blood samples were collected and assayed for progesterone (P4) and 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM). All ewes were slaughtered on d 10, and corpora lutea (CL) were collected, weighed, and assayed for concentration of P4. All ewes that ovulated exclusively in the ovary ipsilateral to the remaining uterine horn had a transient increase in plasma P4 of 2 to 3 d (short luteal phase). In ewes with at least one CL in the isolated ovary, elevated plasma P4 was maintained after hysterectomy but was consistently lower (P less than .05) in POSTLHRH ewes than in PRELHRH ewes. Concentrations of PGFM did not differ between treatments. The CL ipsilateral to the remaining uterine horn weighted less (P less than .01) and contained less P4 (P less than .01) than contralateral CL. These data confirm the hypothesis that premature regression of subnormal CL is uterine-dependent in a local fashion. Presence of the uterus during the follicular and(or) early luteal phase inhibited subsequent luteal function in seasonally anestrous ewes.     

Fertility at the first post partum estrous compared with fertility at the following estrous cycles in foaling mares and with fertility in nonfoaling mares

A retrospective study on the reproductive performance of 401 artificially inseminated trotter mares during six breeding seasons is presented. Mares, 279 post partum (PP) and 122 maiden and barren, or nonlactating (NL), were inseminated with fresh semen obtained from four fertile stallions of the same breed. Pregnancy rate (PR) of mares inseminated at the foal heat (182/253, 71.9%) was lower, but not significantly different, than the PR (22/26, 84.6%) of mares inseminated for the first time at the second post partum cycle and similar to the PR at the first and second cycle of NL mares (95/112, 77.8% and 25/33, 75.7%, respectively). PR of mares inseminated at the foal heat was higher, but nonsignificantly different, from PR of the post partum mares not pregnant after artificial insemination (AI) at foal heat and inseminated again at the following estrous cycle. The PR after AI at the foal heat was significantly higher than the PR when the AI was performed at the third or later cycle in NL mares (71.9% vs 22.2%, P<0.01). The estrus cycle/pregnancy ratio for the PP mares inseminated, for the first time at the foal heat or at the second heat and for NL mares was, respectively, 1.4, 1.2, 1.3 at first cycle and 1.4, 1.3 and 1.4 at the end of the season or when mares left the stud. The proportion of open mares at the end of the season or when leaving the stud was 7.1% (18/253), 3.8% (1/26) and 4.1% (5/122) for PP mares first inseminated at the first or second post partum cycle and for NL mares, respectively; the total rate of open mares was 6% (24/401). The foaling rate (FR) following conception at the foal heat (72.1%) was not statistically different from the FR following conception at any other cycle (50–100%). Based on the absence of significant differences in fertility at the first post partum estrus cycle versus any other estrus cycle, we conclude that breeding at the foal heat should be advisable.     

Ovarian mass in three mares with regular estrous cycles.

Three mares with regular estrous cycles and a large ovary were examined. In each case, the ovary was composed of a single, fluid-filled cavity with a thick capsule. The ovarian mass was surgically removed from each mare. Histologic diagnosis of each mass was different.     

Use of melengestrol acetate and gonadotropins to induce fertile estrus in seasonally anestrous ewes.

Ewes of three genotypes (Hampshire, n = 59; Rambouillet, n = 36; crossbred, n = 57) were used to determine the efficiency of melengestrol acetate (MGA) and(or) PG-600 (a combination of pregnant mare's serum gonadotropin and human chorionic gonadotropin) in inducing fertile estrus in seasonally anestrus ewes. Ewes were assigned randomly, within genotype, to treatments in a 2 x 2 factorial arrangement. Treatments were control, .125 mg of MGA given twice per day for 9 d (MGA), a single 5-mL injection of PG-600 (PG-600), and the combination of treatments MGA and PG-600 (MGA/PG-600). Feeding of MGA began on May 14, 1990, and ended on May 23. Injections of PG-600 were given immediately after the last feeding of MGA or vehicle on May 23. All ewes were exposed to fertile, brisket-painted rams on May 24 (d 0) for 40 d. Ewes were checked for estrus twice daily for 9 d. Laparoscopy was performed, to assess ovulation rate (OR), on d 6 for ewes that were not detected in estrus and on d 12 for ewes that exhibited estrus. Percentage of ewes mated was increased by MGA (P less than .001). Ovulation rate of ewes exposed to rams was increased by PG-600 (P less than .01) and this effect was enhanced by MGA (P less than .05), whereas MGA alone tended to decrease OR (P less than .10). Melengestrol acetate decreased the interval to lambing by 6.5 d (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

N-methyl-D, L-aspartate induces a transient increase in LH secretion in the seasonally anestrous ewe

The primary objective of this study was to determine the LH response to an excitatory amino acid agonist, N-methyl-D, L-aspartate (NMA) in the seasonally anestrous ewe. In experiment 1, 3 i.v. injections of NMA were given; doses of 0.5, 1.5 and 4.5 mg/kg BW were tested. LH response to NMA depended on the dose. There was little response to the lowest dose. All animals responded to the first injection of the intermediate and the highest doses (mean pulse amplitude: 9.2 +/- 0.4 and 6.8 +/- 1.2 ng ml, respectively). The responses to the second or third injections of both doses were variable and were either absent or reduced compared to that of the first. In experiments 2 and 3, ewes were given 3 injections of normal saline (NS) followed by 3 injections of NMA (1.25 and 4.5 mg/kg BW, respectively) at 2 hr intervals. The last injection of NMA was followed 2 hr later by an injection of GnRH (3.0 ng/kg BW). In experiment 2, the first NMA injection induced an immediate LH pulse (mean pulse amplitude: 8.0 +/- 1.6 ng/ml) in all ewes, however, the second and third injections induced LH pulses in only 25% and 75% (mean pulse amplitude: 2.2 and 2.4 +/- 0.6 ng/ml) of the ewes, respectively. In experiment 3, NMA increased mean LH release (P less than 0.05) after all injections, but responsiveness to the third injection was reduced in some ewes. GnRH injections induced LH release in all ewes in experiments 2 and 3 (mean pulse amplitude: 6.9 +/- 1.8 and 6.4 +/- 2.2 ng/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship between pharmacological induction of estrous and/or ovulation and twin pregnancy in the Thoroughbred mares

The aim of this study was to evaluate the possible relationship of pharmacological induction of estrous and/or ovulation with the occurrence of twin pregnancies in Thoroughbred mares. Out of 680 mares, 356 received one of the following treatments during the estrous cycle in which they became pregnant: injection of 0.5mg of cloprostenol at the ultrasonographic detection of a CL (n=86); injection of 5000 IU human chorionic gonadotropin (hCG) immediately before mating (n=221); injection of 0.5mg of cloprostenol at the ultrasonographic detection of a CL plus injection of 5000 IU hCG immediately before mating on cloprostenol-induced estrous (n=49). The other 324 mares, not treated for induction of estrous or ovulation in the estrous cycle resulting in pregnancy, were used as control group. The occurrence of twin and single pregnancies in treated and control mares underlines that the percentage of twin pregnancy in treated mares (16.6%) was statistically significantly higher (P<0.0001; odds ratio, OR=2.87) than the percentage of twinning in the control group (6.5%). Comparison of the occurrence of twins between treatments revealed a statistically significant difference between mares treated with hCG alone compared to animals given prostaglandin F2alpha (PGF2alpha) plus hCG. The results show a statistically significant difference for each treatment compared to controls, with the least difference (P<0.05; OR=2.18) for the comparison between hCG treatment group and controls, a significance of P<0.01; OR=3.05 for the comparison between PGF2alpha treatment and controls, and a highly statistically significant difference (P<0.0001; OR=6.37) for the comparison between PGF2alpha plus hCG-treated animals and controls.     

Effect of transportation on the estrous cycle and concentrations of hormones in mares

Effect of transportation on estrous behavior, duration of the estrous cycle, ovulation, pregnancy rates and concentrations of serum cortisol, plasma ascorbic acid (AA), LH, estradiol and progesterone in mares was investigated. Fifteen mares were transported for 792 km (12 h) during the preovulatory stage of estrus. Transported mares were bled immediately before transport (baseline), at midtrip and 0, 12, 24, 48 and 72 h post-transport and twice daily from d 1 before transport to d 1 (estrogen) or 3 (LH) post-ovulation. Blood samples also were taken for progesterone on d 0, 2, 6, 10, 15, 16, 17, 18, 19 and 20 post-ovulation. Nontransported control mares (n = 15) were bled on the same schedule as transported mares. There was no difference (P greater than .05) in number of mares ovulating, estrous behavior, duration of the estrous cycle or pregnancy rate between groups. Cortisol in transported mares increased to concentrations greater (P less than .05) than those in control mares at midtrip and 0 h post-transport. Concentrations of AA in transported mares also increased (P less than .05) at midtrip, then decreased (P less than .05) below baseline at 24 h post-transport. Concentrations of LH and estradiol increased (P less than .05) above baseline throughout the blood-sampling period. Increases apparently were due to preovulatory surges of these hormones. Increase in LH concentrations in transported mares, however, was greater (P less than .05) than that in control mares at 0 h post-transport.(ABSTRACT TRUNCATED AT 250 WORDS)     

Microvascular development and growth of uterine tissue during the estrous cycle in mares

OBJECTIVE: To document uterine growth and microvascular development in the endometrium of uteri with differing degrees of fibrosis as well as uterine growth throughout the estrous cycle of mares. ANIMALS: 30 mares. PROCEDURE: Uterine tissue was obtained during the breeding season from a slaughter facility. Stage of estrous cycle of the mares was assessed on the basis of ovarian structures and plasma progesterone concentrations. Endometrium was characterized by use of light microscopy, and blood vessel walls were marked by histochemical techniques. Microvascular development was evaluated by a computerized image analysis system. Growth of uterine tissue was based on cellular content of DNA and RNA, RNA:DNA, and protein:DNA. RESULTS: Significant differences in vascular density were not observed in the endometrium of uteri obtained from mares euthanatized during the follicular or luteal phase of the estrous cycle, regardless of whether endometrial classification of degree of fibrosis was considered. There was a 3-fold increase in amount of DNA and RNA of endometrial cells in the follicular phase when compared to myometrium. Hypertrophy of endometrial tissue during the luteal phase was reflected by a significant increase in cell protein content and protein:DNA. CONCLUSIONS AND CLINICAL RELEVANCE: Endometrial growth of vascular tissues during the estrous cycle may be coordinated with development of nonvascular tissue. Estrogen and progesterone may play a role in regulation of uterine growth and angiogenesis.