Postpartum Ewes Research Articles

Suppressed release of LH in ovariectomized post-partum ewes.

The patterns of LH release were studied in 5 ewes, ovariectomized when 3 days post partum, at 4, 14, 33, 47 and 61 days after ovariectomy. These patterns were compared with those in 4 seasonally anoestrous (control) ewes ovariectomized and sampled at the same times. In the control ewes regular pulsatile release of LH was apparent 4 days after ovariectomy: plasma LH concentrations and LH pulse frequencies, but not pulse amplitudes, then declined. In the post-partum ewes all indices of LH release were lower than those in the control ewes at 4 days after ovariectomy, but then they increased: pulse amplitude was greater (P less than 0.05) at 14 days after ovariectomy, and plasma LH concentrations were greater (P less than 0.05, 0.01) at 14 and 33 days than in control ewes. These results indicate that the hypothalamopituitary axis of lactating ewes has a reduced capacity to release LH for only a short period after ovariectomy early in the post-partum period. This probably reflects the time needed for recovery from the inhibitory effects of ovarian and placental hormones, and may reflect reduced secretion of LH-RH.

Pituitary content of gonadotropins and GnRH-receptors in pregnant, postpartum and steroid-treated OVX ewes.

Pituitary content of gonadotropins and GnRH-receptors in pregnant, postpartum and steroid-treated OVX ewes.

Responses of tranquilized post-partum ewes to alien lambs

The objective of this study was to determine the effect of tranquilization on the responses of ewes to alien lambs substituted for own lambs 48–72 h after parturition. Eleven ewes whose young had been taken away, and nine ewes with a lamb of their own, were penned with alien lambs following a single injection of the tranquilizer perphenazine. Seventeen control ewes, including 15 whose only young had been taken away, were also exposed to alien lambs after receiving placebo injections of normal saline. The responses of the ewes to the alien lambs were studied over a four-day period. All ten tranquilized ewes exposed to a single alien lamb without their own young present readily accepted the strange offspring. All of the tranquilized ewes accompanied by one of their own offspring, and a single ewe that was given an alien set of twins in place of her own single lamb, subsequently rejected the strange young. Six of 15 control ewes, with own young removed, accepted alien lambs. The remaining 11 control ewes rejected alien lambs. The success of fostering appeared to be positively related to the persistence of sucking attempts by the lamb, and inversely related to the aggressiveness of the ewe. Tranquilization under the conditions employed can be an efficient technique for fostering single alien lambs on post-partum ewes that have lost their own lambs, but does not facilitate fostering on ewes with their own young present.

Induction of pregnancy in lactating anoestrous ewes

Efforts have been made to control the oestrous cycle of ewes so that breeding can be induced at most times of the year (Gordon, 1963, 1975; Cognie, Hernandez-Barreto & Saumande, 1975; Smith, Boland & Gordon, 1981). Despite a high oestrous response, conception rates have generally been low in lactating anoestrous ewes (Hunter, 1968; Land & McClelland, 1971; Cognie et al. 1975; Shevah, Black & Land, 1975). The ability to achieve conception in post-partum ewes appears to be related to season, breed and lactational status (Cognie et al. 1975; Evans & Robinson, 1980; Rhind et al. 1980). Early research has shown that conception rates of 90% can be achieved in Finnish Landrace × Dorset Horn ewes using exogenous hormones, early weaning and artificial lighting (Robinson, Fraser & McHattie, 1975). Subsequent studies have shown that similar results can be obtained under natural daylight conditions provided that each ewe received pregnant mare serum gonadotrophin (PMSG) at the time of sponge withdrawal (Fraser el al. 1976). Some breeds of sheep may be induced to lamb at 6-month intervals, when maintained under ideal conditions including artificial lighting (Evans & Robinson, 1980). There seems to be some conflict in the literature as to factors affecting conception rate in ewes. Kann & Martinet (1975) have shown that it is the problem of suckling rather than lactation that is responsible for the delay in return to normal reproductive activity. It has been shown that luteinizing hormone (LH) release is quantitatively less in lactating than in cyclic ewes (Cognie & Pelletier, 1976; Evans, 1980). Also the pattern of LH production has been shown to differ in ewes conceiving and those which fail to conceive (Rhind et al. 1980). Cognie & Pelletier (1976) have suggested that in lactating ewes, a proportion of eggs are shed abnormally late in relation to the LH surge and may not be fertilized. The purpose of the present experiment was to investigate some of the factors which may influence the induction of pregnancy in nursing ewes in the non-breeding season.

Evidence for a change in oestradiol negative feedback and LH pulse frequency in post-partum ewes.

Plasma LH levels and the effects of ovariectomy and of chronic and acute administration of oestradiol-17 beta were compared in post-partum (anoestrous) and cyclic Merino ewes during the breeding season. Plasma LH and LH pulses were detected in more post-partum (anoestrous) than cyclic (dioestrous) ewes (10/10 and 4/10 respectively). The post-partum ewes had a greater pulse frequency (1.8 +/- 0.42 compared with 0.4 +/- 0.16 pulses/6 h) and higher plasma LH levels (1.16 +/- 0.32 compared with 0.17 +/- 0.05 ng/ml). Ovariectomized post-partum ewes demonstrated fewer LH pulses (3.8 +/- 0.36 compared with 5.0 +/- 0.15 pulses/6 h) than did ovariectomized cyclic ewes. The depression of plasma LH following acute i.v. injection of 3 micrograms oestradiol-17 beta was similar (0.05 < P < 0.1) for the two groups (80.5 +/- 3.6 and 66.3 +/- 4.1%). At 7 days after ovariectomy and an oestradiol implant more cyclic ewes than post-partum ewes showed detectable plasma LH and LH pulses (6/10 compared with 1/9 ewes). By 66 h after implant removal pulse frequency was less in the post-partum ewes (3.2 +/- 0.55 compared with 5.5 +/- 0.56 pulses/6 h). These findings indicate that in post-partum ewes there is an increased inhibitory (negative-feedback) effect of oestradiol on LH release and a lower intrinsic frequency of pulsatile release of LH. These factors may be casually related to ovarian acyclicity.

Effects of various hormone treatments on induction of lactation in the ewe.

In Exp. I and II, 52 of 68 ewes were induced into lactation with twice-daily injections of estradiol-17 beta (E2-beta) and progesterone (P4; .5 and 1.25 mg/kg body weight/day) for 7 days. Additional treatments were twice-daily injections (days 18 to 20) of hydrocortisone, growth hormone, thyroxine and thyrotropin releasing hormone alone or in various combinations. In Exp. III, 12 ewes were induced into lactation. In this experiment, all ewes were injected with E2-beta and hydrocortisone, as previously, but four ewes (III-2) had P4 injections extended to day 20, and four ewes (III-3) were not injected with P4. Across experiments, lowest milk yields during lactation and the lowest percentage of ewes induced into lactation (58%) occurred when only E2-beta and P4 were injected. Inclusion of hydrocortisone injections (50 mg/day) induced the highest percentage of ewes into lactation (86%, 38 of 44), the highest peak daily yields of milk and the highest total yields during lactation. Including injections of growth hormone, thyroxine or thyrotropin releasing hormone alone or in combinations did not produce better results than injections of E2-beta and P4 alone. Injections of E2-beta and hydrocortisone without concurrent injections of P4 were less effective. Intramuscular injections of P4 (10 mg/day) from days 8 to 20 did not inhibit lactogenesis or subsequent lactation. Across all experiments, 76% of multiparous (52/68) and 50% of nulliparous (6/12) ewes produced greater than 100 ml milk/day during their lactation (34 to 95 days). However, yields of milk for ewes that lactated were only 25 to 50% of those from postpartum ewes. The importance of including injections of hydrocortisone in the induction procedure was established, but determination of optimum time to inject and potential importance of other hormones requires additional research.

Reproductive potential and endocrinological responses of sheep kept under controlled lighting. III. Prolactin and pituitary responsiveness to gonadotrophin releasing hormone in the post partum period

Five ewes of each of four breed types (pure Merino and crosses with Dorset Horn, South Suffolk and Border Leicester) were kept on a 6-monthly light-cycle in each of two light controlled rooms in an attempt to breed them twice in one calendar year. Four and 17 days following parturition they were treated with gonadotrophin releasing hormone (GnRH) (75, 150, 300 μg) and the pattern of release of luteinising hormone (LH) was studied and compared with that obtained in ‘normal’ cyclic ewes. Weekly plasma prolactin levels were determined at weeks 1–5. Overall, mean and peak levels of LH in the post-partum ewes were only one half those of cyclic ewes. The mean level ran from 8.4 ng/ml at day 4 to 18.2 ng/ml at day 17. This compared with 25.3 ng/ml for cyclic ewes. There was a significant effect of breed type, Merino and Dorset Horn × ewes releasing more LH than did South Suffolk × and Border Leicester × ewes. There were no significant effects of breed or week of sampling on plasma prolactin. None of the observed differences in pattern of release of LH could be related to the observed breed-type differences in fertility and it appears that factors other than endocrinological - e.g. uterine involution - may be involved in the ability to conceive of ewes in which oestrus and ovulation have been induced shortly after parturition.

Pituitary Responsiveness to LH-RH in Post-partum Ewes Treated With Oestradiol-17ß and Failing to Show a Plasma LH Surge

Oestradiol-17 beta (40 micrograms intravenously) failed to elicit a surge in plasma LH levels by 13 h after administration in 64% (16 out of 25) Merino ewes about 30 days post partum in the anoestrous season. LH-RH responsiveness and LH-RH priming effect were significantly greater in these ewes than in similar post-partum (n = 9) and non-parturient ewes (n = 3) not treated with oestradiol. This suggests that the failure of the oestrogen-positive feedback effect on LH release in post-partum ewes is not due to a failure of oestradiol action on the pituitary increasing pituitary responsiveness to LH-RH and LH-RH priming effect, but could be due to inadequate release of LH-RH from the hypothalamus.

Postpartum endocrine function of cattle, sheep and swine.

Reproductive efficiency can be increased in farm animals by decreasing the interval from parturition to conception. At parturition the placentae are expelled and the corpora lutea of pregnancy regress, resulting in dramatic reductions in plasma concentrations of progesterone and estrogens. The early postpartum period usually is characterized by ovarian inactivity and the absence of estrus in cows, ewes and sows. Plasma concentrations of progesterone and estradiol are minimal before the first postpartum ovulation and estrus. The first ovulation may occur before the first estrus. However, occurrence of a "silent ovulation" varies with species and management. The lack of postpartum ovarian activity in most species probably is caused by reduced gonadotropin secretion, since follicular growth can be induced in anestrous postpartum cows, ewes and sows by gonadotropin treatments. Pituitary gonadotropin content and plasma gonadotropin concentrations are usually low at parturition and increase during the postpartum period. Hormonal treatments have resulted in marginal success in decreasing the postpartum anestrous interval. Therefore, the interval from parturition to conception in cows, ewes and sows may be reduced by methods developed from research to elucidate the factors and mechanisms that regulate postpartum gonadotropin secretion.

Postpartum Reproductive Performance of Finnsheep-crossbred Ewes1

Postpartum reproductive potential was evaluated in Finnsheep-crossbred ewes. Luteinizing hormone (LH) concentrations in serum were determined on days 10 and 30 postpartum in lactating and nonlactating ewes that were ovariectomized on day 2 postpartum. The increase in LH secretion after ovariectomy was not affected by the number of lambs (0, 1 or 2) that each ewe nursed, but LH concentrations were higher on day 10 for ewes that lambed in the fall than for ewes that lambed in the spring. Bromocriptine reduced prolactin concentrations in postpartum ewes but had no effect on LH secretion in ewes that were ovariectomized on day 2. Treatment of ovariectomized ewes with estradiol-17β on day 11 postpartum produced increased LH concentrations that were maximal 18 hr after treatment. The magnitude of this effect of estrogen was reduced in lactating ewes, but was not affected significantly by bromocriptine treatment. Estrous activity was not increased in intact lactating ewes by bromocriptine treatment from days 15 to 29 postpartum. The interval from parturition to estrus in ewes that lambed in the fall was longer for lactating than for nonlactating ewes, and this interval was shortened in lactating ewes by a single treatment with estradiol between days 2 to 5 postpartum followed by 15 days of progesterone implantation. Results of the present studies indicate that poor reproductive performance before day 40 postpartum in Finnsheep-crossbred ewes that lamb in the fall is not due to impairment of gonadotropin secretion or ovulation. Rather, the main limitation appears to be the absence of estrous behavior and possibly the ability to maintain pregnancy.

Immunization of ewes to oestrogens: Effects on the oestrous cycle and parturition

Four treatments, designed to increase plasma concentrations of LH, were assessed for efficacy at inducing oestrus and ovulation in anoestrous progestagen-pretreated Merino ewes that were 20–37 days post partum during the ovulatory season. The treatments involved the introduction of rams to ewes that had been isolated from rams, GnRH administration, and immunisation against androstenedione or oestrone. Ram introduction resulted in a higher (P<0.001) proportion of ewes showing oestrus and ovulating within 7 days (1619 ewes, flock R-+) than in ewes having continuous contact with rams (420 ewes, flock R++). Treatment with GnRH (125 ng/h s.c. for 7 days) inhibited the ram effect on the number of ewes showing oestrus, but not on the number of ewes ovulating. GnRH treatment, and immunisation against androstenedione or against oestrone did not induce oestrus or ovulation. Immunisation against oestrone inhibited the occurrence of ovulation in ewes in both flocks (R++, R-+). Pregnancy rates were lower (P<0.001) in ewes mated less than 40 days post partum than in ewes mated 41–70 days post partum (17.2% vs. 57.3%). The number of services/pregnancy were lower (P<0.05) in the R-+ flock than in the R++ flock (1.3±0.1 vs. 1.6±0.1), but pregnancy rates over a 32-day joining period were similar (63.7%, 59.7%). The study showed that ram introduction can be an effective method of advancing and synchronising oestrus and ovulation and of advancing the onset of pregnancy in progestagen-pretreated anoestrous post-partum ewes.

Response of the ewe to synthetic gonadotropin releasing hormone.

Several experiments were performed to evaluate the response of the ewe to synthetic gonadotropin releasing hormone, pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 (GnRH). The intramuscular injection of GnRH during early- or mid-anestrus effected serum LH changes which were both quantitatively and qualitatively similar. Peak serum concentrations occurred at 120 min. and were directly related to the dose of GnRH. The LH response in cycling ewes following GnRH (days 3 and 10) was also dose related, but not influenced by the stages of the estrous cycle. The peak levels of serum LH were 2.6 ± 0.3, 8.1 ± 1.8, 14.6 ± 2.9 and 41.2 ± 8.9 ng/ml, respectively, for GnRH doses of 0.2, 1, 5 and 25 micrograms. The times of the LH peaks after injection were 7.5, 15, 60 and 90 min., respectively. In another group of mid-luteal phase ewes (day 10), doses of 25 and 100 µg GnRH induced ovulation in four and six of six ewes, respectively. In contrast, GnRH did not consistently ovulate the anestrous ewe although the release of LH was as great or greater than during the estrous season. The LH response in the ovariectomized ewe following GnRH was more abrupt than in intact ewes. Maximal serum concentrations of LH occurred within 20 to 40 min. and tended to remain elevated for a longer period. Moreover, the normal endogenous episodic releases of LH had not resumed within 9 hr. of GnRH treatment. In contrast, LH-responsiveness of the early postpartum ewe to GnRH was considerably less than that observed in nonlactating animals. Peak serum concentrations of LH were 13 to 17 ng/ml compared to mean levels of 40 to 90 ng/ml in cycling ewes following a dose of 25 µg GnRH. In the postpartum ewe serum levels of LH were maximized within 20 to 45 min. and returned to baseline within 4 hours. In each of the experiments in this report, the time required for LH to return to baseline following injection of GnRH was greater than expected based on the half-disappearance time for LH in ovine serum. This may reflect a continual release of LH from the pituitary, but at a decreasing rate.

Proceedings: Comparison of LH levels in post-partum anoestrous and cycling ewes and the effects of synthetic gonadotrophin-releasing factor.

Proceedings: Comparison of LH levels in post-partum anoestrous and cycling ewes and the effects of synthetic gonadotrophin-releasing factor.

Some factors affecting response of anestrous ewes to hormone treatment.

Several experimental, range and farm flocks were used to study the effects of various environmental factors on response of the anestrous ewe to hormone therapy. Lactating ewes started on estrogen, progesterone and repeat PMS treatment beginning 18 to 24 days post-partum were significantly more fertile than ewes started on treatment 2 to 8 days post-partum. Ewes which lambed in June following the treatment described above were more fertile than ewes which lambed in February. However, both those lambing in June and February were more fertile than those lambing in April. Anestrous ewes suckling single lambs proved generally to be more fertile than ewes suckling twin lambs. In another study dry ewes (failed to lamb) were more fertile than ewes which lambed but did not raise their lambs. Ewes which lambed but failed to raise their lambs were in turn more fertile than ewes raising singles which in turn more fertile than ewes raising twins. A treatment including an intramuscular injection of 2 mg estradiol-17β at the time of progesterone implantation resulted in an apparent improvement in fertility in one flock and a significant improvement in fertility in another flock over other hormone treated ewes not receiving this estrogen pretreatment. Estradiol-17β (.025 mg) reduced fertility at the first estrus when it was given at the same time as the first PMS. The overall treatment effect did not, however, differ from other treatments. Young early post-partum ewes (1, 2 or 3 years old) whether lactating or nonlactating were significantly more fertile following hormone therapy than mature ewes of the same status. Dorset × Targhee crossbred ewes appeared to be more fertile following hormone therapy than Rambouillet, Targhee or Columbia ewes of the same ages. The response of Suffolk ewes to hormone treatment was significantly lower than other breeds studied in the same flock. Hormone treated lactating anestrous ewes given a high level of nutrition appeared to be more fertile than those on a standard level of nutrition.

The contribution of the conceptus to weight change in pregnant Merino ewes at pasture

SUMMARYSeventy-nine pregnant Merino ewes grazing pasture at two intensities were slaughtered in groups at intervals from 60 days after conception to post-partum. Sixty-one nonpregnant ewes were also slaughtered. The weights of various organs including uteri and their contents were recorded and the energy content of each ewe was determined.The weight of uterine contents (conceptus) increased rectilinearly with increasing foetal age to 100 days: differences between singles and twins were not significant. From 100 days to term, the relationship was also rectilinear but the slope was greater: a more rapid increase occurred with twins than with singles (P &lt; 0·05). The nutritional difference imposed did not affect the relationship.A regression of total energy reserve on live weight (less fleece and conceptus) was established. Differences between this relationship and those established separately for non-pregnant, pregnant and post-partum ewes were not significant.The contribution of changes in conceptus weight to live-weight changes in gravid ewes is discussed.

Progesterone in blood. V. Progesterone and 20alpha-hydroxypregn-4-en-3-one in the placenta and blood of ewes.

SUMMARY Progesterone has been identified in the placenta of intact and ovariectomized ewes in concentrations of 4–9 μg/kg tissue. In addition to progesterone, a closely related metabolite, 20-αhydroxypregn-4-en-3-one, has also been found, in concentrations of 9–12 μg/kg tissue. The concentrations of both these steroids were highest in the placenta from ovariectomized animals, suggesting that the placenta might be able to compensate for the removal of the ovaries. Progesterone and 20α-hydroxypregn-4-en-3-one levels were also measured in the peripheral blood of pregnant, non-pregnant and post-partum ewes; the mean concentrations of both hormones during pregnancy were 0·41 and 0·37 μg/100 ml. plasma respectively. These levels were not significantly higher than those found in non-pregnant animals, and there was no significant difference (progesterone: P &gt; 0·02; 20α-hydroxypregn-4-en-3-one: P &gt; 0·05) between the concentrations in ewes with single and multiple pregnancies. The ratio of the two hormones did not appear to change during the course of pregnancy, and there was no sign of a fall in the blood levels prior to parturition. After birth, however, the levels seemed to fall rapidly. It is concluded that the sheep placenta is capable of producing small amounts of progesterone, and that some homeostatic mechanism exists for the maintenance of a relatively constant blood progesterone level throughout pregnancy. However, it is still not possible to assess the relative importance of the adrenals and the placenta as extra-ovarian sources of progesterone in the ewe.