Plasma progesterone concentrations during early pregnancy in spring- and autumn-bred ewes

The objective of this experiment was to measure blood progesterone concentrations during early gestation to determine if the apparent reproductive failure in ewes bred out-of-season is due to a failure to conceive or embryonic loss. Blood samples were collected from spring- (n=61) and autumn-bred ewes (n=29) from Days 8 to 39 post-oestrus. Serum progesterone concentrations were analysed to ascertain whether ewes were ovulating and failing to maintain pregnancy, or conception was failing. Following pregnancy diagnosis 62 days after ram introduction, ewes were categorised as; no display of oestrus, mated but then identified as non-pregnant, or pregnant. A majority of spring-bred ewes that failed to display oestrus had silent oestrus (86%) and 66% of those ewes had abnormally short-lived corpora lutea. Circulating progesterone concentrations during dioestrus in ewes that had ovulated and displayed oestrus were unaffected by season. Similarly, progesterone concentrations during dioestrus did not differ between pregnant and mated non-pregnant ewes. The results indicated that while early luteylosis, low progesterone secretion from corpora lutea and embryo mortality did occur, these were in only a small proportion of ewes. Progesterone concentrations indicated that a majority of mated non-pregnant ewes had elevated progesterone concentrations necessary for the production of at least one viable embryo/foetus. This may be indicative to the failure of maternal recognition of pregnancy, and it is recommended that events surrounding this stage of pregnancy (Days 12-14) be examined more closely in ewes during the non-breeding season.     

The effect of undernutrition on the establishment of pregnancy in the ewe

The relationship between nutrition and reproduction in sheep has been the subject of research in several international groups. This review will particularly focus on the effects of undernutrition on the potential causes of reproductive failure including abnormalities of the ovum or the embryo, luteal inadequacy and failure of the supply of progesterone to the uterus, or the mechanisms involved in maternal recognition of pregnancy. The level of nutrition and peripheral progesterone concentrations are inversely related, and increased rates of embryo loss, associated with higher progesterone concentrations in ewes with low levels of nutrition have been reported. Undernutrition may act through changes in the distribution of progesterone in the endometrium. Thus, lower endometrial levels on day 5 of the cycle in ewes fed half of their maintenance requirements have been observed, providing a link between the known role of progesterone in embryo survival by the modulation of uterine function and the higher embryo losses found in undernourished ewes. The evidence of an effect of maternal nutrition on IFNtau secretion from the conceptus and of PGF2alpha production from the uterus is presented. Moreover, undernutrition provokes a reduction in the sensitivity of the endometrium to progesterone that may affect embryo survival. Finally, a state of undernutrition induces changes in the endometrial sensitivity to steroid hormones at early stages of pregnancy that could adversely alter uterine environment to the detriment of embryo survival.     

Embryo survival in dairy cows managed under pastoral conditions

Efficient pasture-based milk production systems require a compact calving pattern aligned to the onset of the grazing season, a 365-day calving interval and low culling rates for infertility. Achievement of these targets requires high herd reproductive performance. While high genetic merit Holstein cows produce more milk in grass-based systems their fertility is compromised. Management of the modern high genetic merit Holstein dairy cow presents a major challenge in pasture-based systems of production. It appears that the extent of early embryo loss is greater (up to 20% points greater) in the modern high-producing dairy cow and that a much higher proportion of the embryos die before day 7 following insemination in contrast to heifers and lower yielding cows. About 7-8% of pregnancies are lost between days 30 and 90 of gestation with no evidence that loss rate is related to cow genetic merit, parity or level of production. Systemic concentrations of progesterone during both the cycle preceding and following insemination affect embryo survival rate with evidence that too low or indeed too high a concentration of progesterone been negatively associated with embryo survival rate. Peripheral concentrations of both progesterone and oestradiol are lowered by increased plane of feed intake due to increased metabolic clearance rate of the steroids, which is related to liver blood flow. It appears that high producing dairy cows have an increased risk of embryo death as a result of lowered peripheral concentrations of progesterone as a consequence of increased hepatic metabolism of progesterone. Uterine expression of mRNA for progesterone receptor, oestradiol receptor and retinol binding protein mRNA appears to be sensitive to changes in peripheral concentrations of progesterone during the first week after AI. It would appear that energy balance and dry matter intake during the 4 weeks, immediately after calving are critically important in determining conception rate when cows are inseminated at 70-100 days post-calving. Concentrate supplementation of cows at pasture during the breeding period has minimal affects on conception rates though sudden reduction in dietary intake should be avoided. For pasture-based systems of milk production more balanced breeding strategies, with greater emphasis on fertility and feed intake must be developed.     

Validation of a new pregnancy-associated glycoprotein radioimmunoassay method for the detection of early pregnancy in ewes

The aim of the current study was to describe the use of a pool of different antisera raised against pregnancy-associated glycoproteins (PAGs; purified from both ovine and caprine placentas) for early pregnancy diagnosis in ovine species. Sixty-three pluriparous Sarda ewes (Ovis aries) were synchronized. Blood samples were withdrawn on Days 18, 24, 26, 28, 30, and 50 after mating. These samples were assayed for progesterone (radioimmunoassay [RIA] including an extraction step) and for pregnancy-associated glycoproteins (RIA-706 and RIA-srPool). Progesterone concentrations were under 1.0 ng/mL in all nonpregnant Sarda ewes. In pregnant ewes, mean progesterone concentrations ranged from 2.4 ng/mL (Day 24, single pregnancies) to 4.4 ng/mL (Day 28, multiple pregnancies). During all periods of examination, PAGs remained lower than 0.8 ng/mL in nonpregnant ewes. On Day 18 of pregnancy, PAG concentrations could be detected in 26 of 43 (60.5%) and in 41 of 43 (95.3%) pregnant ewes using the RIA-706 and RIA-srPool methods, respectively. From Day 24 to Day 50, using both RIA methods, PAGs could be detected in all pregnant ewes. On Day 24, the best threshold for pregnancy diagnosis was obtained by use of RIA-srPool, maximal concentration in nonpregnant ewes being 0.3 ng/mL and minimal concentration in pregnant ewes being 4.8 ng/mL. In general, progesterone and PAG concentrations were higher in multiple pregnancies than in single pregnancies. However, because of large individual variations, single pregnancies could not be differentiated from multiple pregnancies.     

First postpartum ovulations and corpora lutea in ewes which lamb in the breeding season

Two experiments involving crossbred ewes which lambed during the breeding season were performed to determine whether: (a) the interval to first postpartum ovulation could be reduced by weaning or mastectomy; (b) there are differences in luteal structure and luteinizing hormone (LH) receptor concentration between first postpartum corpora lutea induced with GnRH and normal cycling corpora lutea and (c) pretreatment of postpartum ewes with progesterone would affect luteal LH receptor concentration and luteal phase serum progesterone concentration.In experiment I, the mean interval (±SEM) to the first postpartum ovulation was 22.3 ± 1.1 days and was not significantly altered by weaning or mastectomy. More than half of the ewes had small, short-lived peaks of serum progesterone associated with short-lived corpora lutea prior to the normal luteal phase rise of serum progesterone. In experiment II, 2 h after GnRH injection on day 18 postpartum, serum LH concentrations were higher in ewes which received progesterone treatment on days 13 and 14 than in control ewes. Progesterone treatment did not affect mean corpus luteum weight (157 mg) or concentration of LH receptors (0.95 fmol/mg) in first postpartum corpora lutea, but progesterone-treated ewes had significantly higher endogenous serum progesterone concentrations on days 21–24. GnRH-induced corpora lutea from postpartum ewes were lighter in weight, paler in color, had lower LH receptor concentrations and had a more regressed histological appearance than corpora lutea of a similar age from normal, cycling ewes.     

Embryonic survival in the uterus of ewes inseminated at the uterotubal junction on Day 32 post partum

Experiments were conducted to determine 1) if pregnancy initiated on Day 32 post partum would be maintained until lambing, 2) if there is a difference in the ability of the previously gravid or nongravid uterine horn to maintain pregnancy, and 3) if season has an effect on embryo loss. Estrus was induced in ewes on Day 32 post partum. At estrus, ewes were inseminated surgically at the uterotubal junction and assigned to the following groups: 1) inseminated at estrus and laparotomized on Day 3 to collect embryos for determination of fertilization rate (C), 2) inseminated in the previously gravid uterine horn (PG), 3) inseminated in the previously nongravid uterine horn (NG), and 4) inseminated when both horns were previously gravid (BG). Ewes pregnant in the PG, NG and BG groups were allowed to lamb. Conception rate in Group C at embryo collection was 70%. Embryo loss, based on concentrations of progesterone at Day 18 post insemination, was 43, 19 and 18% in the BG, NG and PG group, respectively. The high embryo loss in Group BG occurred only during the breeding season. Only 24% of the ewes that had been inseminated lambed. This was due to the prepartum loss of embryos and fetuses (47, 48 and 33% in Group BG, NG and PG, respectively. In conclusion, the detrimental effects of the uterus on embryo survival was evident within 18 d post insemination in Group BG (breeding season), and embryo loss prior to lambing was high in all the treatment groups (both seasons).     

Seasonal variations in prolactin, growth hormone and thyroid hormones and the prolactin surge at ovulation do not affect litter size of ewes during pregnancy in the oestrous or the anoestrous season

Injection of bromocriptine from 5 days before until 5 days after mating clearly suppressed the periovulatory prolactin surge in ewes in the anoestrous and oestrous season but did not change the litter size significantly. Progesterone, GH, TSH or thyroid hormone concentrations were not influenced by the bromocriptine treatment. The progesterone concentrations were lower during the first weeks after mating in the anoestrous season compared to the oestrous season, while there was no difference between pregnant and non-pregnant ewes. During later gestation this seasonal difference was only observed in the non-pregnant ewes. At the same time there was a clear difference between pregnancy and non-pregnancy in both seasons. The prolactin, GH and thyroid hormone values also varied significantly during gestation. Since these patterns are identical in pregnant and non-pregnant ewes, the fluctuations are due to environmental factors and not to pregnancy or altered progesterone concentrations. In the anoestrous season prolactin, GH, T4 and T3 levels were higher than in the breeding season, while rT3 showed the opposite pattern. The TSH concentration did not differ between the two seasons. These results suggest that seasonal variations in prolactin, GH and thyroid hormones or the periovulatory prolactin surge do not affect litter size of ewes during pregnancy in the oestrous or the anoestrous season.     

Influence of haemoglobin type and selenium status on peripheral plasma progesterone and corticosteroid concentration in ewes grazing oestrogenic pastures.

Peripheral plasma progesterone and corticosteroid concentrations were studied in ewes grazing oestrogenic pastures during oestrus, pregnancy and parturition in 1974 and during pregnancy in 1975. Haemoglobin (Hb) type A ewes had significantly higher mean corticosteroid concentrations than HbB ewes in both years. In 1974 the concentration of progesterone did not differ significantly between Hb types whilst in 1975 the concentration was significantly (P less than 0.001) higher in HbA ewes. The mean corticosteroid concentrations were higher in selenium-supplemented ewes of both Hb types than in unsupplemented ewes in 1974 but lower in 1975. Selenium treatment did not significantly influence the progesterone concentration in ewes of both Hb types in 1974 but significantly (P less than 0.01) increased in 1975. It is concluded that the higher concentration of plasma corticosteroids or factors regulating the relationship between corticosteroids and progesterone could be responsible for the higher estimates of embryonic mortality reported in HbA ewes.     

The effect of active immunization against progesterone on plasma concentrations of total and free progesterone, estradiol-17beta and LH in the cyclic ewe

Nine mature cyclic ewes were actively immunized against progesterone which was rendered immunogenic by conjugation to bovine serum albumin (BSA). Seven control ewes were immunized with BSA. In ewes immunized against progesterone, the concentration of total plasma progesterone increased to 24.3 ng/ml vs 2.8 ng/ml in control animals (P<0.001). However, immunization did not affect the plasma levels of free, unbound progesterone. The correlation coefficient between total plasma progesterone concentrations on Days 4 to 11 of the estrous cycle and antibody titer was r=0.983. Estradiol-17beta concentrations in immunized ewes were higher than in controls on Days 6 to 15 of the estrous cycle (P approximately 0.05). Frequent sampling for LH over a 6-h period on Days 2, 5, 8, 11 and 14 of the cycle revealed no significant differences in the frequency and amplitude of LH pulses between immunized and control ewes. The immunized ewes had estrous cycles of normal length and maintained normal pregnancies. It is suggested that the immunized cyclic ewe is capable of maintaining adequate levels of free progesterone by greatly increasing progesterone synthesis, thus neutralizing the effect of the antibodies.     

Fetal growth and reproductive performance in ewes administered GnRH agonist on day 12 post-mating

Reproductive performance and fetal growth was determined in GnRH (4 microg synthetic GnRH agonist, Receptal) administered (i.m.) to ewes on day 12 post-mating (n = 103) compared to control ewes (n = 97) during the breeding season. Plasma progesterone and LH concentrations were analyzed. A total of 13 ewes was slaughtered on day 45 of pregnancy (six from control, seven from GnRH treated groups). GnRH administration on day 12 post-mating increased plasma progesterone concentration (4.39+/-0.25 ng/ml) compared to control group (3.43+/-0.15 ng/ml) on days 13-15 post-mating (P < 0.01). GnRH administration also increased plasma LH concentration between 1 and 4 h after GnRH administration (P < 0.01). Pregnancy rate was higher in GnRH treated group (84%) than control (66%) group (P < 0.05). The ewes in GnRH administered group had more twins (P < 0.05) than those in control group. The ovarian weights (P < 0.05) and the number of corpora lutea (CL) (P < 0.01) were greater in ewes slaughtered on day 45 of pregnancy in GnRH treated group than those in control group. GnRH administration on day 12 post-mating did not have any effect on products of conception at day 45 of pregnancy except on crown-rump length (CRL) of fetuses and cotyledon weight. CRL of fetuses and cotyledon weight in GnRH treated group was higher than those in control group (P < 0.05). In conclusion GnRH administration improved reproductive performance of ewes when administered on day 12 post-mating probably through its beneficial effect on embryo survival by enhancing luteal function, but not through stimulating fetal growth.     

Effect of exogenous melatonin on the ovary, the embryo and the establishment of pregnancy in sheep

Administration of melatonin to advance the breeding season in sheep has been widely used, since this hormone conveys the photoperiodic signal to the reproductive neuroendocrine axis. An increased lambing percentage has been reported following such treatment during anoestrus, which could be mediated through a higher rate of embryonic survival, either by an improvement in luteal function or by a reduction in the antiluteolytic mechanisms. The aim of this article is to review the body of knowledge on the effect of melatonin on the ovine ovary, the embryo and the establishment of pregnancy. Some studies using synchronized ewes have found that melatonin treatments during anoestrus do not improve ovulation rate by modifying the timing of follicle emergence, but increasing the number of ovulatory follicles by decreasing the atresia of medium and large follicles. On the other hand, the addition of melatonin to the in vitro maturation medium does not improve oocyte maturation rate in oocytes from sheep ovaries recovered either in anoestrus or in the breeding season. However, a luteotrophic effect of melatonin at either short or medium term has been reported. We have recently observed that melatonin implants tend to improve the survival of embryos collected from ewes after superovulation in anoestrus. More specifically, melatonin induced a significant reduction of the number and rate of non-viable (degenerate and retarded) embryos. Preliminary data from our laboratory suggest that the uterine sensitivity to progesterone - in terms of progesterone receptor expression - of superovulated ewes could be reduced by melatonin treatment. It can be concluded that the success of exogenous melatonin as a means to improve lamb production of sheep is due, at least in part, to an improvement of luteal support and embryonic survival.     

Ovarian function in ewes at the onset of the breeding season

Transrectal ultrasonography of ovaries was performed each day, during the expected transition from anoestrus to the breeding season (mid-August to early October), in six Western white-faced cross-bred ewes, to record ovarian antral follicles > or = 3 mm in size and luteal structures. Jugular blood samples were collected daily for radioimmunoassay (RIA) of follicle-stimulating hormone (FSH), oestradiol and progesterone. The first ovulation of the breeding season was followed by the full-length oestrous cycle in all ewes studied. Prior to the ovulation, all ewes exhibited a distinct increase in circulating concentrations of progesterone, yet no corpora lutea (CL) were detected and luteinized unovulated follicles were detected in only three ewes. Secretion of FSH was not affected by the cessation of anoestrus and peaks of episodic FSH fluctuations were associated with the emergence of ovarian follicular waves (follicles growing from 3 to > or = 5 mm). During the 17 days prior to the first ovulation of the breeding season, there were no apparent changes in the pattern of emergence of follicular waves. Mean daily numbers of small antral follicles (not growing beyond 3 mm in diameter) declined (P < 0.05) after the first ovulation. The ovulation rate, maximal total and mean luteal volumes and maximal serum progesterone concentrations, but not mean diameters of ovulatory follicles, were ostensibly lower during the first oestrous cycle of the breeding season compared with the mid-breeding season of Western white-faced ewes. Oestradiol secretion by ovarian follicles appeared to be fully restored, compared with anoestrous ewes, but it was not synchronized with the growth of the largest antral follicles of waves until after the beginning of the first oestrous cycle. An increase in progesterone secretion preceding the first ovulation of the breeding season does not result, as previously suggested, from the ovulation of immature ovarian follicles and short-lived CL, but progesterone may be produced by luteinized unovulated follicles and/or interstitial tissue of unknown origin. This increase in serum concentrations of progesterone does not alter the pattern of follicular wave development, hence it seems to be important mainly for inducing oestrous behaviour, synchronizing it with the preovulatory surge of luteinizing hormone (LH), and preventing premature luteolysis during the ensuing luteal phase. Progesterone may also enhance ovarian follicular responsiveness to circulating gonadotropins through a local mechanism.     

Donor and recipient ewe factors affecting in vitro development and post-transfer survival of cultured sheep embryos

Donor and recipient factors were assessed during development of embryos following superovulation, collection at the pronuclear and two-cell stage, culture in Synthetic Oviduct Fluid medium for 5 days and twin transfer into synchronised recipients to elucidate what factors affect embryo development and post-transfer survival. In particular, the administration of exogenous progesterone to recipients using an intravaginal CIDR^TM device immediately following embryo transfer was investigated.

From 138 embryos collected from 30 donor ewes, 75% (103) were of transferable quality following culture, of which 100 were transferred to 50 recipients. There was significant variation (P < 0.001) in embryo development to the blastocyst stage between different donor ewes, but this was not related to the donor ovulation rate. At ultrasound sonography (approximately Day 60 of pregnancy), 58% of recipients were pregnant and 42% embryos had survived. Donor ovulation rate was related to embryo survival (P < 0.05) after transfer; the survival rate of embryos from ewes with high ovulation rates was lower than that of embryos from ewes with low ovulation rates. Exogenous progesterone supplementation following transfer did not affect embryo survival, rate of embryo development or plasma progesterone levels. In general, the results from this study suggest that factors other than efficacy of embryo culture can affect the outcome of embryo survival following transfer and that, where possible, these factors should be considered and balanced in experimental designs.     

Associations between milk progesterone concentration on different days and with embryo survival during the early luteal phase in dairy cows

The relationships between the concentration of milk progesterone and early embryo survival on Days 4-8 inclusive and between the concentration of progesterone on different days from Days 0-8 inclusive following ovulation and insemination were examined in dairy cows. The relationships were examined following 77 randomly chosen artificial inseminations to cows in standing oestrus. There was a significant (P < 0.05) linear and quadratic relationship between the concentration of milk progesterone on each of Days 4-6 after ovulation and the probability of embryo survival. There was no association (P > 0.05) between milk progesterone concentration and probability of embryo survival on Days 7 and 8 after ovulation. There were no associations between milk progesterone concentration on Days 0-2 and the concentrations on Days 4-7, however, progesterone concentrations on Days 4 and 5 were highly predictive of the concentration on Days 6 and 7, respectively. Overall, the results indicate that suboptimal progesterone support during the early luteal phase is likely to deleteriously affect embryo viability and in addition, that it is possible to predict milk progesterone concentrations during the early luteal phase based on earlier stage concentrations and thus identify cows at risk of early embryo loss.     

Role of the corpus luteum of pregnancy in controlling pituitary gonadotrophin secretion during the early post-partum period in the ewe

No difference was found between 5 intact ewes and 5 ewes from which the CL had been excised at Day 70 of pregnancy in the plasma concentration of progesterone at Day 140, and concentrations of progesterone remained below 0.2 ng/ml during the first 20 days post partum. Plasma concentrations of LH, frequency and amplitude of LH pulses were low at Day 140 and increased considerably, particularly in the CL-excised ewes, as early as Day 5 post partum. No significant differences were found between the two groups of ewes in the mean plasma concentrations of FSH for any of the 5 stages examined. Taken together, these results suggest that some factor, other than progesterone, associated with the CL of pregnancy is involved in the inhibition of pulsatile LH secretion during the early post-partum period.     

Effects of adrenalectomy on serum concentrations of progesterone during the luteal phase of sheep

An experiment was conducted to test the hypothesis that the adrenal gland influences luteal activity in sheep. Twelve Finnish Landrace x Southdown ewes were either bilaterally adrenalectomized (n = 6) or sham adrenalectomized (n = 6) during the breeding season. At approximately 37 and 47 d after surgery, all ewes received intramuscular injections of cloprostenol to synchronize estrus. Blood samples were taken via jugular venipuncture at 48-h intervals between 1 and 19 d after the last cloprostenol treatment. Serum concentrations of progesterone were determined in each of these samples. Analysis of variance showed that concentrations of progesterone during the luteal phase were lower (P<0.05) in adrenalectomized ewes than in sham-operated controls, but that patterns of progesterone were similar for both groups of sheep. Based on these results, we conclude that the adrenal gland does not appear to be necessary for initiation of luteal regression in ewes.     

Ovarian function in ewes during the transition from breeding season to anoestrus

Transrectal ovarian ultrasonography was conducted in six Western white-faced ewes for 35 days from the last oestrus of the breeding season, to record the number and size of all ovarian follicles > or = 3 mm in diameter and luteal structures. Blood samples were collected once a day for estimation of serum concentrations of follicle-stimulating hormone (FSH), oestradiol and progesterone. Each ewe had five follicular waves (follicles growing from 3 to > or = 5 mm in diameter) over the scanning period. The duration of the growth phase of the largest ovarian follicles did not differ (P > 0.05) between waves, but follicular static and regressing phases decreased significantly (P < 0.05) after the decline in serum progesterone concentrations at the end of the last luteal phase of the breeding season. The intervals between the five follicular waves were: 9.2+/-0.4, 5.2+/-0.7, 8.3+/-0.8 and 5.8+/-0.7 days; the two shorter intervals differed (P < 0.05) from the two longer intervals. Using the cycle-detection program, rhythmic increases in serum FSH concentrations were detected in all ewes; the amplitude, duration and periodicity of FSH fluctuations did not vary (P > 0.05) throughout the period of study. The number of identified FSH peaks (7.8+/-0.5 peaks per ewe, per scanning period) was greater (P < 0.05) than the number of emerging follicular waves. Serum concentrations of oestradiol remained low (< or = 1 pg/ml) on most days, in five out of the six ewes studied, and sporadic elevations in oestradiol secretion above the non-detectable level were not associated with the emergence of follicular waves. The ovulation rate was lower than that seen during the middle portion of the breeding season (November-December) in white-faced ewes but the transitional ewes had larger corpora lutea (CL). Maximal serum concentrations of progesterone appeared to be lower and the plateau phase of progesterone secretion appeared to be shorter during the last luteal phase of the ovulatory season in comparison to the mid-breeding season of Western white-faced ewes. During the transition into anoestrus in ewes, the endogenous rhythm of FSH release is remarkably robust but the pattern of emergence of sequential follicular waves is dissociated from FSH and oestradiol secretion. Luteal progesterone secretion is suppressed because of fewer ovulations and diminished total luteal volume, but it may also result from diminished gonadotropic support. These season-related alterations in the normal pattern of ovine ovarian cycles appear to be due to reduction in ovarian responsiveness to gonadotropins and/or attenuation in secretion of luteinizing hormone (LH) occurring at the onset of the anovulatory season in ewes.     

Post-insemination milk progesterone concentration and embryo survival in dairy cows

Logistic regression analysis was used to evaluate the relationship between post-insemination milk progesterone concentration and embryo survival, and between milk yield and milk progesterone concentration. Milk samples were collected on Days 1, 4, 5, 6, and 7 (insemination=Day 0) following 871 inseminations in spring-calving dairy cows. Milk progesterone concentrations were measured by enzyme-immunoassay and pregnancy diagnosis was conducted with transrectal ultrasonography at approximately Day 30. There was a negative linear relationship (P<0.01) between milk progesterone concentration on Day 4 and embryo survival while, in contrast, there was a positive linear and quadratic relationship between milk progesterone concentration on Days 5, 6 and 7 (P<0.05) and also between the rate of change in progesterone concentrations between Days 4 and 7 inclusive and embryo survival (P<0.05). There was a weak negative linear relationship between average daily milk yield at the time of insemination and milk progesterone concentrations (P<0.001). There was no association between many production parameters, including liveweight and body condition score measured at various stages between calving and insemination, and milk progesterone concentration between Days 4 and 7 inclusive (P>0.05). In conclusion, low progesterone during Days 5-7 (after insemination) was associated with low fertility in dairy cows and there were indications of a range of progesterone concentrations within which embryo survival was maximal.     

Seasonal changes of gonadotropin-releasing hormone secretion in the ewe.

A sustained volley of high-frequency pulses of GnRH secretion is a fundamental step in the sequence of neuroendocrine events leading to ovulation during the breeding season of sheep. In the present study, the pattern of GnRH secretion into pituitary portal blood was examined in ewes during both the breeding and anestrous seasons, with a focus on determining whether the absence of ovulation during the nonbreeding season is associated with the lack of a sustained increase in pulsatile GnRH release. During the breeding season, separate groups (n = 5) of ovary-intact ewes were sampled during the midluteal phase of the estrous cycle and following the withdrawal of progesterone (removal of progesterone implants) to synchronize onset of the follicular phase. During the nonbreeding season, another two groups (n = 5) were sampled either in the absence of hormonal treatments or following withdrawal of progesterone. Pituitary portal and jugular blood for measurement of GnRH and LH, respectively, were sampled every 10 min for 6 h during the breeding season or for 12 h in anestrus. During the breeding season, mean frequency of episodic GnRH release was 1.4 pulses/6 h in luteal-phase ewes; frequency increased to 7.8 pulses/6 h during the follicular phase (following progesterone withdrawal). In marked contrast, GnRH pulse frequency was low (mean less than 1 pulse/6 h) in both groups of anestrous ewes (untreated and following progesterone withdrawal), but GnRH pulse amplitude exceeded that in both luteal and follicular phases of the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of method of administration on ovarian responses of seasonally anestrous ewes administered gonadotrophin releasing hormone

Mature ewes were treated during the anestrous season with saline (I) or GnRH either intramuscularly in saline (II), subcutaneously in carboxymethylcellulose (CMC) (III) or subcutaneously in gelatin capsules (IV). Fifty μg of GnRH or 1 ml of saline were administered to 22 ewes in experiment 1. In experiments 2 and 3, forty-seven and 10 ewes received 250 μg GnRH or 1 ml of saline. Ewes were bled for progesterone determination prior to treatment and up to 12 or 13 days after treatment. In experiment 3, ovaries were observed via mid-ventral laparotomy 4 days after treatment and ovarian structures recorded. Ewes were classified into one of four progesterone response categories: cyclic, transient, prolonged or no response. The only treatment that changed the progesterone response from the saline-treated controls was GnRH in gelatin capsules. More ewes in this group were classified with a prolonged progesterone response (40%) than in the saline control group (0%). GnRH (in gelatin capsules)-treated ewes in the prolonged progesterone response category had higher concentrations of plasma progesterone than GnRH (in saline or CMC)-treated ewes with a prolonged progesterone response. For the GnRH (in gelatin capsule)-treated ewes, the prolonged progesterone response was similar to progesterone in ewes during the estrous cycle and all ewes in the prolonged progesterone category had corpora lutea (experiment 3). In summary, implanting the GnRH in gelatin capsules subcutaneously in seasonally anestrous ewes increased the ovulation response and enhanced corpus luteum function over ewes administered GnRH in saline intramuscularly.