Follicle-stimulating hormone pulse amplitude decreases with the onset of the breeding season in the mare.

The relationship between daily mean FSH concentrations in serum and the pattern of FSH detected by frequent sampling for 12-h periods (samples every 15 min) was examined in five mares during the transition into the breeding season. The five mature anestrous mares were exposed to a natural increase in daylength. Blood samples were collected daily from February 1 until the first ovulation of the breeding season (April 14 +/- 3.7 days, Mean +/- SEM). Periods of frequent blood collection were performed every two weeks. Blood samples were obtained daily by jugular venipuncture or jugular cannula (frequent samples). Mean daily concentrations of FSH in serum determined by RIA decreased during seasonal transition. Patterns of FSH in serum detected by frequent sampling were pulsatile. FSH pulse amplitude decreased during seasonal transition, and the decrease in amplitude was associated with the decrease in mean serum FSH concentrations. This decrease in FSH pulse amplitude may reflect an involvement of a follicular product from developing follicles or a change in hypothalamic stimulation of pituitary FSH release.

Changes in LH pulse frequency and amplitude in intact mares during the transition into the breeding season.

Two groups of mares were exposed to an abrupt, artificial increase or a natural increase in daylength. In both groups, mean LH pulse frequency increased with time of year and was accompanied by a reciprocal decrease in LH pulse amplitude. A non-pulsatile pattern of LH secretion was observed in some mares sampled close to the day of ovulation. Maximum mean LH pulse frequency and the onset of the breeding season occurred earlier in those mares exposed to an abrupt artificial increase in daylength. In blood samples collected frequently, mean serum LH concentrations increased in relation to time of year. However, during 60 days before ovulation, when LH pulse frequency increased, mean daily serum LH values only increased on Day -3 before ovulation. The magnitude of the periovulatory LH rise was greater before the second than the first ovulation of the breeding season. These results support the hypothesis that, in the mare, a photoperiod-induced seasonal alteration in LH pulse frequency and/or amplitude may play a role in the onset of the breeding season.

Effect of pulsatile gonadotrophin release on mean serum LH and FSH in peri-parturient mares.

Changes in the pattern of LH and FSH in serum were studied in 6 mares foaling during the summer. Samples were collected frequently (every 15 min) for 24 h twice before foaling, -33 +/- 2 and -12 +/- 2 days, and for 12 h after foaling, on Day 0 and Day 4. Simultaneous pulses of FSH and LH were observed before foaling (r2 = 0.99). Before foaling, gonadotrophin pulses were infrequent (6 in 264 h of observation). On the day of foaling, LH and FSH pulse frequency increased (P less than 0.005) with 2-4 pulses per mare. The amplitudes of pulses of LH and FSH were higher before parturition than for those observed on Day 0 (P less than 0.001). The presence of FSH pulses was associated with an increased mean value of FSH for those days, including Day 0 (P less than 0.02). The presence of resolvable pulses of LH was not associated with an increase in mean LH values on those days (P greater than 0.6). The present results demonstrate that LH and FSH release is pulsatile in the periparturient mare. In addition, mean values of FSH in serum were elevated when pulses were detected, whereas mean concentrations of LH remained low. These results are consistent with the concept of one hypothalamic releasing hormone for LH and FSH in the mare.

Inhibition of ovulation in the mare by active immunization against LHRH.

To investigate the hypothesis that the onset of the breeding season in the mare may be due to a daylength-induced seasonal increase in LHRH pulse frequency, 5 mares were immunized against LHRH. Beginning 1 December, 5 immunized and 5 untreated control mares were exposed to an abrupt, artificial increase in daylength (16L:8D) to advance the onset of the breeding season. In control mares ovulation occurred 49.6 +/- 3.5 (s.e.m.) days later (18 January), whereas in 3/5 immunized mares ovulation had not occurred by 1 April. In the remaining 2 mares, although ovulation occurred once (Mare 79) or twice (Mare 72) during February, a booster immunization restored acyclicity for the duration of the study (No. 72) or to 30 March (No. 79). The absence or occurrence of ovulation in LHRH-immunized mares appeared to be related to antibody titre, such that the highest antibody titres were observed in those mares that remained anovulatory throughout the experimental period, while low titres were seen in the 2 mares that ovulated. In 3 acyclic immunized mares, LH pulses were not observed in blood samples collected frequently at 2-week intervals from November to January. In contrast, LH pulse frequency in control mares, and in the 2 immunized mares that ovulated, increased from 0/12 h (November) to 2-5/12 h (January). These results confirm our previous observation that, in the mare, the onset of the breeding season is associated with an increase in LH pulse frequency. Furthermore, the results suggest that the increase in LH pulse frequency reflects an increase in pulsatile LHRH release from the hypothalamus.

Changes in patterns of luteinizing hormone secretion before and after the first ovulation in the postpartum mare.

To determine whether luteinizing hormone (LH) secretion during the first estrous cycle postpartum is characterized by pulsatile release, circulating LH concentrations were measured in 8 postpartum mares, 4 of which had been treated with 150 mg progesterone and 10 mg estradiol daily for 20 days after foaling to delay ovulation. Blood samples were collected every 15 min for 8 h on 4 occasions: 3 times during the follicular phase (Days 2-4, 5-7, and 8-11 after either foaling or end of steroid treatment), and once during the luteal phase (Days 5-8 after ovulation). Ovulation occurred in 4 mares 13.2 +/- 0.6 days postpartum and in 3 of 4 mares 12.0 +/- 1.1 days post-treatment. Before ovulation, low-amplitude LH pulses (approximately 1 ng/ml) were observed in 3 mares; such LH pulses occurred irregularly (1-2/8 h) and were unrelated to mean circulating LH levels, which gradually increased from less than 1 ng/ml at foaling or end of steroid treatment to maximum levels (12.3 ng/ml) within 48 h after ovulation. In contrast, 1-3 high-amplitude LH pulses (3.7 +/- 0.7 ng/ml) were observed in 6 of 7 mares during an 8-h period of the luteal phase. The results suggest that in postpartum mares LH release is pulsatile during the luteal phase of the estrous cycle, whereas before ovulation LH pulses cannot be readily identified.

Changes in pulsatile LH secretion and the positive and negative feedback actions of oestradiol after administration of a GnRH agonist in the ovariectomized ewe.

Administration of a GnRH agonist (5 micrograms) every 12 h to long-term ovariectomized ewes for 5 or 10 days during the breeding season suppressed mean LH levels from around 6 to 1 ng/ml on Days 1 and 4 after treatment; on Day 1 after treatment LH pulse frequency and amplitude were lower than pretreatment values. On Day 4 after treatment LH pulse frequency was restored to pretreatment levels (1 per h) whereas LH pulse amplitude had only slightly increased from 0.5 to 1 ng/ml, a value 25% of that before treatment. This increase in amplitude was greater the shorter the duration of treatment. Ovariectomized ewes treated with the agonist for 5 days exhibited both negative and positive feedback actions after implantation of a capsule containing oestradiol; however, compared to control ewes treated with oestradiol only, the positive and negative feedback actions of oestradiol were blunted. These results suggest that the recovery of tonic LH concentrations after GnRH agonist-induced suppression is limited primarily by changes in LH pulse amplitude. The results also demonstrate that the feedback actions of oestradiol are attenuated, but not blocked, by GnRH agonist treatment.

Evidence that changes in LH pulse frequency may regulate the seasonal modulation of LH secretion in ovariectomized mares.

To determine whether tonic LH secretion in ovariectomized mares is characterized by pulsatile release, frequent blood samples (every 15 min) were collected from 5 ovariectomized horse mares in nine 8-h periods between February and May. Mean serum LH concentrations increased 9-fold and were associated with a 4-fold increase in mean LH pulse frequency. These results provide the first evidence of pulsatile LH release in ovariectomized mares and suggest that the increasing daylengths of the spring months may increase serum LH by increasing LH pulse frequency.

Control of ovulation in mares in the early breeding season with ovarian steroids and prostaglandin.

Two trials were conducted to (1) determine the degree of control of ovulation achieved by treating mares in late winter with progesterone and oestradiol-17 beta combined after prior exposure to an artificially increased photoperiod, and (2) to examine the effectiveness of such a procedure incorporated into equine breeding farm management systems. Following a 15-day treatment of 150 mg progesterone and 10 mg oestradiol-17 beta daily with 10 mg PGF-2 alpha on the last day of steroid treatment, 27 of 31 mares ovulated on Days 8-14 after the last injection in one trial. Conception rate for mares mated during that period was 77%, compared with 50% in untreated mares mated at first oestrus following the treatment period. In a second, larger field trial involving the same treatment for a 10-day period, 103 of 128 mares ovulated on Days 9-16 after the last injection and 62% of the mares mated on those days conceived. These results indicate that the combined steroid treatment provided reasonably precise and practical control of ovulation time in mares adequately stimulated by increased photoperiod without detrimental effect on fertility.

Effects of injected ovarian steroids on reproductive patterns and performance in post-partum mares.

Treatment of foaling mares with 150 mg progesterone and 10 mg oestradiol daily beginning within 12 h of parturition resulted in a delay of ovulation (15.6 +/- 2.6 days compared with 10.3 +/- 2.4 days in untreated mares). When mares were mated after this 5.3-day delay there was no advantage in reproductive performance over that of mares mated according to a conventional management system. Gonadotrophin secretion was inhibited during treatment but the following secretory patterns were similar to those of normal oestrous periods.

Effects of exogenous steroids on serum FSH and LH, and on follicular development in cyclic mares.

Cyclic mares were given daily i.m. injections of 150 mg progesterone (Group P, N = 4), 150 mg progesterone and 10 mg oestradiol-17 beta (Group PE, N = 3), 10 mg oestradiol-17 beta (Group E, N = 3) or cottonseed oil vehicle (Group C, N = 4), from the day after ovulation (Day 1) to Day 28. Blood samples were collected daily, and the ovaries were palpated every 1-2 days. Serum FSH and LH concentrations were measured in all samples, and means determined for 7 consecutive 4-day periods throughout treatment. Comparisons within each steroid treatment group between time periods and comparisons between hormone treatment groups within each time period were made to investigate the way in which these ovarian steroids control cyclic gonadotrophin changes in the mare. The increase in LH during oestrus in Group C mares (controls) appeared to be inhibited by progesterone, resulting in low LH concentrations and failure of preovulatory-sized follicles to ovulate. In Group PE LH concentrations were lower than those in Group P, resulting in suppression of the development of the largest follicle. In Group E, treatment had little effect on FSH and LH concentrations, while follicular development was variable (ovulations on Days 25, 33 and 36). None of the steroid treatments appeared to suppress FSH concentrations directly but FSH concentrations showed a reciprocal relationship with the LH-dependent follicular development.

Control of ovulation in cycling mares with ovarian steroids and prostaglandin.

A combined progesterone-estradiol-17beta treatment was given in two experiments conducted to examine its effectiveness in controlling ovulation time in cycling mares. In the first experiment, the combined steroid (150 mg progesterone, 10 mg estradiol-17beta daily for 10 days) alone or combined with prostaglandin on the first and last days of steroid treatment resulted in ovulation in 15 of 16 mares 9-13 days after last injection, 13 of them on days 10-12. A CL present prior to treatment in one mare that received no prostaglandin persisted through and for 14 days after treatment. In the second experiment the combined steroid treatment started on the first or second day of estrus blocked ovulation in only 5 of 13 mares. Thus prostaglandin is necessary at least at the end of treatment. In both experiments a total of 20 mares with no luteal function at the end of steroid treatment ovulated on days 9-13 after last injection, 18 of these on days 10-12. These results indicate that the combined steroid-prostaglandin treatment can result in ovulations in a very restricted interval with apparently a normal distribution.

Sources of variation in response intervals after prostaglandin treatment in mares with functional corpora lutea.

Sixty-two non-cycling mares were classified according to the size of largest follicles at the time of treatment with Prostalene, an analogue of prostaglandin (PG) F-2 alpha. Although oestrus occurred in only 77.4% of mares, 98.4% ovulated at an average of 6.8 days after treatment. Greatest variance of interval to ovulation was observed in mares having follicles greater than or equal to 40 mm at the time of treatment. This was due to regression of large follicles about one-third of the time and later ovulation of a succeeding follicle. This resulted also in greatest uncertainty of prediction of ovulation time based on ovarian palpation. Ninety foaling mares were given Prostalene at various days following the first ovulation post partum. Ovulation less than 6 days after treatment was strongly associated with the presence of a large follicle on the day of treatment. Otherwise most (72%) ovulations occurred 6--10 days after treatment. The distribution of interovulatory periods resulting from Prostalene on Day 6 after ovulation differed from that of Day 8 treatment.

Oestrogen excretion in mares in relation to various reproductive states.

The rate of urinary oestrogen excretion was studied in four ovariectomized mares. In two animals ovariectomized when urinary oestrogen concentration was high, levels fell to a minimun within 48 hr. Intramuscular injection of oestradiol-17beta produced maximum urinary oestrogen levels within 3 to 6 hr and a return to minimum detectable levels within 24 hr. The major urinary oestrogen in mares was oestrone. In nine maiden mares studied between September and June, the cycle, the durations of dioestrus and oestrus, and the time of ovulation were similar in autumn (September to December) and spring (January to June). All nine mares had periods of anoestrus and 'shallow' and 'deep' forms of this condition were observed. In all cyclic mares, urinary oestrogen excretion showed a gradual rise from dioestrous levels beginning 6 to 7 days before ovulation. A rapid and highly significant (P less than 0-001) fall in oestrogen excretion occurred on the day of ovulation and the day after. Maximum urinary oestrogen levels were twice as high in spring than in autumn cycles.

Effects of progesterone on reproductive function in mares after parturition.

The injection of 100 mg of progesterone/day from Days 5 to 14 after foaling blocked or delayed oestrus and ovulation in 6/9 mares; the remainder ovulated during treatment without showing oestrus. Urinary oestrogen concentrations in these mares were highest between Days 9 and 15 after foaling but the peak levels were only about half those seen in untreated mares during the post-partum oestrus and ovulation. The injection of 200 mg progesterone/day blocked oestrus and ovulation in two mares but did not depress oestrogen excretion maxima any further. Follicles prevented from developing beyond a cricital stage lost their ability to ovulate after progesterone withdrawal. No significant histological differences were found in endometrial biopsies taken from progesterone-treated and control mares on Days 5, 10 and 15 after foaling. Injection of 100 mg progesterone between Days 1 and 10 after foaling delayed oestrus and ovulation in all six mares. Conception occurred in 8/12 progesterone-treated mares as the result of mating at oestrous periods which were delayed until Days 15 to 28 after foaling.

The relation of infection to infertility in the mare and stallion.

Many normally fertile stallions harbour bacteria in and on the genital organs. Many mares served by such stallions are unaffected by the bacteria to which they are thus exposed; however, some mares so exposed will become infected and diseased. Presumably, the genital defenses of such mares had been compromised. Strain differences in pathogenicity of bacteria do exist. Some mares affected with pyometra had irregular ovarian activity and some had normal ovarian cycles. In the former group, destruction of the endometrium many have prevented the production of endogenous luteolysin. The leukopenia which occurs in both groups is due to neurtropenia.