Reproductive potential and endocrinological responses of sheep kept under controlled lighting. II. Pituitary and gonadal responses of ewes and rams to a six-monthly light cycle

Forty entire ewes of mixed breeds were kept in environmentally-controlled rooms with a 6-monthly light cycle. Six mature spayed Border Leicester × Merino ewes and four mature intact Poll Dorset rams were kept under the same conditions.Over a period of 2 years (four light cycles) estimates were made of ovarian, testicular and pituitary activity in response to the artificial light regime. Non-pregnant ewes were bled twice weekly: peripheral plasma progesterone levels > 1 ng/ml were taken as indicative of ovarian activity. Testicular activity was estimated by weekly tests for peripheral plasma testosterone and scrotal sac volume. Pituitary activity was estimated monthly by the response to the injection (i.v.) of 75 μg gonadotrophin releasing hormone (GnRH) to rams and of 18.75, 37.5 or 75 μg to ewes, using peripheral plasma luteinising hormone (LH) estimations from the time of GnRH injection.Data for ovarian and testicular measurements were classed into categories according to week and for pituitary response to month of the light cycle.Cubic regression analyses were conducted on the percentage of ewes with progesterone levels > 1 ng/ml and on ram testosterone and scrotal measurements.Despite the irregularity of the curve for the light cycle, the ovarian and testicular responses of rams and ewes followed an alternating curve with peaks and troughs of activity separated by approximately 13 weeks in the 26-week cycle. The peak of ovarian activity occurred during the long daylength period which followed a 22-week period of decreasing daylength and was preceded by 1 month by peak ram testosterone and scrotal sac volume.The pattern of pituitary response was related to that of the actual light cycle and the data were subjected to time-lag regression. This econometrical technique revealed that there was a delay in pituitary response to daylength changes of 2 months for rams, and between 2 and 3 months for the spayed ewes. The peak pituitary response to the GnRH test occurred one month earlier for rams than for the spayed ewes, and coincided with the corresponding troughs of gonadal activity of each sex.The results showed that the breeding season of sheep can be compressed into 6 months and that the pattern of pituitary response follows the daylength pattern more closely than do measurements of gonadal activity. Peak reproductive activity in rams, as measured by pituitary and gonadal activity, precedes that of ewes by approximately 1 month.     

Effects of breed, ovarian steroids and season on the pulsatile secretion of LH in ovariectomized ewes

The effects of season and of oestradiol and progesterone on the tonic secretion of LH were studied in ovariectomized Merino and Suffolk ewes, two breeds which differ markedly in the seasonal pattern of their reproductive activity. In the absence of exogenous steroids, the frequency of LH pulses was lower and the amplitude of the pulses was higher in anoestrus than in the breeding season for Merino and Suffolk ewes 30 days after ovariectomy. In long-term (190 days) ovariectomized ewes, this seasonal change in LH secretion was observed in Suffolk ewes only. During seasonal anoestrus, treatment of ewes with subcutaneous oestradiol-17 beta implants (3, 6 or 12 mm in length) decreased the frequency of LH pulses in a dose-dependent manner, with Suffolk ewes being far more sensitive to the inhibitory effects of oestradiol than Merino ewes. The lowest dose of oestradiol (3 mm) had no effect on the secretion of LH in Merino ewes, but reduced secretion in Suffolk ewes. Treatment of ewes with the highest dose of oestradiol (12 mm) completely abolished LH pulses in Suffolk ewes, whereas infrequent pulses remained evident in Merino ewes. During the breeding season, oestradiol alone had no effect on the pulsatile release of LH in either breed, but in combination with progesterone there was a significant reduction in LH pulse frequency. Progesterone effectively decreased LH secretion in both breeds in both seasons. It was concluded that differences between breeds in the 'depth' of anoestrus could be related to differences in the sensitivity of the hypothalamus to both negative feedback by oestradiol and the direct effects of photoperiod.     

Factors influencing the grazing behaviour of sheep in a Mediterranean climate

The diurnal pattern of grazing of sheep was studied in summer in 2 Mediterranean environments in 3 experiments. In the first experiment with Border Leicester × Merino sheep, it was found that in a given day the times of beginning and ending the major morning and afternoon grazing periods and the time spent grazing, were influenced by time of dawn or dusk, temperature, humidity and the time spent grazing the previous night. The temperature was found to influence the timing of events throughout the day but humidity mainly had an effect in the midday and afternoon.Six breeds of sheep were compared in a second experiment (Romney, Dorset Horn, Cheviot, Suffolk, Southdown and Border Leicester). There were significant differences between the breeds in the diurnal pattern of grazing, in grazing times and in responses to environmental conditions. The Cheviots and Suffolks differed from the other breeds because they began earlier in both the morning and afternoon. The Suffolks grazed for longer in all the grazing periods and differed significantly in grazing time from sheep of all other breeds.In a third experiment, sheep reared in a dry tropical environment (Abydos) behaved differently over 3 summers to sheep raised in a Mediterranean environment with dry summers (Baker's Hill). The main differences were in response to temperature and humidities in the morning and to length of day in the afternoon. The differences diminished with time.Generally, afternoon (pm) activity was more influenced by immediate grazing activity (night grazing time and morning (am) grazing time) than was the morning behaviour, but there were exceptions.     

Evidence that the onset of the breeding season in the ewe may be independent of decreasing plasma prolactin concentrations

Ten ewes of each of two breeds, Dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were given subcutaneous oestradiol-17 beta implants and then ovariectomized. Another 10 ewes of each breed were left intact. On 3 May 1982, all the ewes were housed in an artificial photoperiod of 16L:8D. After 4 weeks, half of the ewes of each breed and physiological state were abruptly exposed to a short-day (8L:16D) photoperiod while the others remained in long days (16L:8D). The time of onset of the breeding season was significantly (P less than 0.05) advanced in ewes switched to short days (12 August +/- 10 days) compared to those maintained in long days (4 September +/- 14 days). Dorset Horn ewes began to cycle (20 July +/- 7 days) significantly (P less than 0.001) earlier than Welsh Mountain ewes (19 September +/- 6 days). Disparities in the time of onset of cyclic activity in ewes of different breeds and daylength groups were echoed in disparities in the time at which plasma LH and FSH concentrations rose in oestrogen-implanted, ovariectomized ewes of the same light treatment group. Prolactin concentrations showed an immediate decrease in ewes switched to short days, but remained elevated in long-day ewes. Since the breeding season started in the presence of high prolactin concentrations in long-day ewes, it seems unlikely that prolactin is an important factor determining the timing of the onset of cyclic activity.     

The response of rams and ewes of three breeds to artificial photoperiod

Rams and ewes of the Romney Marsh (N = 6), Dorset Horn (N = 8) and Australian Merino (N = 8) breeds were subjected to 4 successive periods of alternating 6 h light/18 h dark ('short' days) and 18 h light/6 h dark ('long' days) preceded by 16 weeks of 12 h light/12 h dark. The initial period was of 32 weeks (16 weeks 'short' days; 16 weeks 'long' days) and the next 3 were of 24 weeks (12 weeks 'short' days; 12 weeks 'long' days). Rams of all breeds showed a cyclic pattern of growth and regression of testes associated with plasma testosterone concentration, influenced by the change in light regimen 15-19 weeks previously. Sexual behaviour was also cyclic but lagged by some 6-7 weeks. The changes were greatest in the Romneys and least in the Merinos in which a higher degree of sexual activity was evident even when the testes were regressed (P less than 0.001). This was the major breed difference. All ewes of the Romney and Dorset breeds showed marked seasonality related to the imposed light regimen, whereas only 1 of the 4 Merinos did so. The mean peak of ovarian activity in the former 2 breeds coincided with that of maximum sexual activity of rams housed with them; that is, some 6 weeks after maximum scrotal volume. The rams and ewes were subjected to serial blood sampling episodes for plasma LH and testosterone and tested for plasma LH release following GnRH administration. There was little variation between breeds in LH concentration. Testosterone concentration varied greatly in the ram, highest levels associated with the developed phase of the testes and with maximum LH pulse frequency. The LH response to GnRH changed with respect to the state of the gonads. Maximal responses were observed in the developing phase of testicular growth although this variation was greater in the Romney and Dorsets than in the Merinos (P less than 0.001). In the ewes, maximal responses were seen in the follicular phase (P less than 0.001), with no difference between the luteal and acyclic phases. There were no breed differences. Plasma pooled from the serial blood sampling episodes was assayed for prolactin.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of reproduction,genotype and anthelmintic treatment of ewes on Ostertagia spp. populations

Donald A. D., Morley F. H. W., Waller P. J., Axelsen A., Dobson R. J. and Donnelly J. R. 1982. Effects of reproduction, genotype and anthelmintic treatment of ewes on Ostertagia spp. populations. International Journal for Parasitology12: 403–411. Merino and Border Leicester × Merino (BL × M) ewes, nearly all of the same age and reared at the same site, were either unmated or mated to Border Leicester rams. Ewes of each genotype and reproductive status were untreated or were given a single pre-lambing drench with thiabendazole at 50 or 100 mg/kg a week before the start of lambing in spring on pastures at Canberra which had been contaminated during autumn and winter by adult sheep. The two genotypes grazed together within each combination of reproductive status and anthelmintic treatment which grazed separately. Thiabendazole was highly effective in removing both fourth stage larvae and adults of Ostertagia spp., the most abundant genus. Eight weeks after the pre-lambing drench lactating ewes carried larger Ostertagia spp. populations than did unmated ewes of both genotypes, but as a result of reinfection after treatment, differences between drenched and undrenched ewes were not significant. At this time lactating as well as unmated ewes harboured large populations of arrested early fourth stage larvae of Ostertagia spp. acquired during the last 8 weeks, showing that arrest of development is not prevented by lactation. There was strong evidence that some ingested larvae which became arrested in lactating ewes were rejected by unmated ewes. At all stages of the reproductive cycle studied, BL × M ewes were substantially more resistant to Ostertagia spp. infection than Merinos. No persistent benefits in parasite control or in animal production were detected from the pre-lambing drench.     

The effects of handling procedures,breed differences and treatment with lithium and dexamethasone on some blood parameters in sheep

Fifteen aged Merino and 15 aged Border Leicester ewes each divided into 3 groups of 5 for infusion with lithium chloride, lithium chloride plus dexamethasone and normal saline, and then subjected to 3 jugular venous blood samplings, 1 h apart, in a 3 × 3 × 2 experimental design involving times × treatments × breeds.The blood samples were examined for packed cell volumes, plasma and erythrocyte sodium and potassium concentrations, and plasma calcium concentrations.There were significant changes in packed cell volumes (PCV) 39 v. 30%; 39 v. 31%), plasma sodium concentrations (151 v. 149 mmol l⁻¹; 151 v. 148 mmol l⁻¹) and plasma potassium concentrations (5.3 v. 4.6 mmol l⁻¹; 5.3 v. 4.7 mmol l⁻¹) between Times 0 and 1 and between Times 0 and 2, respectively. There were no significant changes in plasma calcium or erythrocyte sodium or potassium concentrations associated with times. The evidence suggests that the times-effects were caused by different methods of handling the sheep immediately prior to each blood sampling, and this is discussed. The fall in PCV was greater than that recorded by other authors.There were highly significant (P < 0.01) breed differences in PCV (36 v. 31%), plasma calcium concentrations (2.0 v. 2.2 mmol l⁻¹) and erythrocyte potassium concentrations (10.1 v. 15.0 mmol l⁻¹) for Merino and Border Leicester ewes, respectively. There were no significant breed differences in plasma potassium or erythrocyte sodium concentrations.The mean plasma potassium concentration of 5.08 mmol l⁻¹ for the lithium-treated sheep was significantly higher than the means of 4.67 and 4.77 mmol l⁻¹ for lithium plus dexamethasone and saline-treated groups, respectively. There was no significant difference between the latter two means, and there were no significant treatment effects for any of the other blood constituents.     

Seasonal changes in LH secretion in normal ewes and ewes which grazed oestrogenic clover

Plasma luteinizing hormone (LH) concentrations were measured in normal (control) Corriedale X Merino (comeback) ewes and in clover-infertile comeback ewes which had grazed oestrogenic Yarloop clover (Trifolium subterraneum L. cv. Yarloop) for more than 4 years. Plasma LH concentrations were measured in samples taken at 20-min intervals for 6 h during the dioestrous stage of the oestrous cycle in the breeding season (BS) and during the anoestrous season (AS). In the control ewes during BS, transitory elevation in plasma LH concentration (pulses) occurred, reflecting secretory episodes, with a frequency of one per 5.2 h. This frequency fell to one per 16.5 h during the anoestrous season. In clover-infertile ewes, LH pulses occurred with a frequency of one per 4.5 h during BS and one per 4.9 h during AS (difference not significant). In the controls, plasma LH levels were higher (P less than 0.05) during BS (mean +/- s.d. = 1.2 +/- 0.4 ng/ml, n = 9) than in AS (0.7 +/- 0.3 ng/ml, n = 5). In the clover-infertile ewes, plasma LH levels in BS (1.3 +/- 0.6 ng/ml, n = 12) were similar to those of controls. During AS, plasma LH levels in the clover-infertile ewes (1.0 +/- 0.6 ng/ml, n = 10) remained similar to their BS levels, being significantly (P less than 0.05) higher than LH levels in the controls at this time. These studies indicate that the higher plasma concentrations of LH which have been reported in clover-infertile ewes arise from more frequent LH pulses. Furthermore, in contrast to normal ewes, average plasma LH, reflecting pulse frequency, is not reduced in AS. This supports the view that ingestion of phytooestrogens affects neural centres involved in regulating LH secretion.     

The effect of the infusion of insulin during the luteal phase of the estrous cycle on the ovulation rate and on plasma concentrations of LH, FSH and glucose in ewes

The role of insulin in mediating pituitary responses to nutrition was investigated in 30 mature Border Leicester X Merino ewes. The ewes were infused with saline (n = 15) or bovine insulin at 0.4 IU/kg/d (n = 15) for 72 h during the luteal phase of the estrous cycle The ewes were housed in individual pens and were fed, ad libitum, a diet of low quality straw. Their estrous cycles were synchronized with prostaglandin (PG), with infusions given over Days 9 to 11 of the estrous cycle. A further injection of PG was given at the end of the infusion, and the subsequent ovulation rate was determined by endoscopy 12 d later. Blood samples were collected every 4 h from Day 8 until 52 h after the final PG injection for the determination of plasma FSH, insulin and glucose concentrations. On Day 11 blood samples were also taken every 20 min for 24 h for the determination of LH pulse characteristics. During the infusion of insulin, its concentration rose 4-fold and remained elevated until the end of infusion, when it fell to pretreatment concentrations. Glucose concentrations were significantly reduced during the insulin infusion and rose to pretreatment concentrations after infusion. In control ewes glucose and insulin concentrations did not change. Ovulation rate of treated ewes was not affected by the insulin (1.9 +/- 0.07) compared with that of control ewes (2.0 +/- 0.10). Neither were FSH concentrations affected by treatment with insulin, although a significant interaction of treatment with time was observed in the 36 h after infusion. The pre-ovulatory decline in FSH concentrations was delayed by about 8 h in the insulin treated ewes. The mean (+/- SEM) LH pulse frequency (4.3 +/- 0.4 vs 1.8 +/- 0.3 pulses per 24 h) and the mean (+/- SEM) concentration of LH (0.48 +/- 0.04 vs 0.32 +/- 0.03 ng/ml) were both significantly reduced by insulin. These results indicate that insulin-induced hypoglycaemia inhibits LH secretion in cyclic ewes and implicates insulin as a mediator of normal hypothalamo-pituitary function.     

Accuracy of pedigree and genomic predictions of carcass and novel meat quality traits in multi-breed sheep data assessed by cross-validation

Background

Genomic predictions can be applied early in life without impacting selection candidates. This is especially useful for meat quality traits in sheep. Carcass and novel meat quality traits were predicted in a multi-breed sheep population that included Merino, Border Leicester, Polled Dorset and White Suffolk sheep and their crosses.

Methods

Prediction of breeding values by best linear unbiased prediction (BLUP) based on pedigree information was compared to prediction based on genomic BLUP (GBLUP) and a Bayesian prediction method (BayesR). Cross-validation of predictions across sire families was used to evaluate the accuracy of predictions based on the correlation of predicted and observed values and the regression of observed on predicted values was used to evaluate bias of methods. Accuracies and regression coefficients were calculated using either phenotypes or adjusted phenotypes as observed variables.

Results and conclusions

Genomic methods increased the accuracy of predicted breeding values to on average 0.2 across traits (range 0.07 to 0.31), compared to an average accuracy of 0.09 for pedigree-based BLUP. However, for some traits with smaller reference population size, there was no increase in accuracy or it was small. No clear differences in accuracy were observed between GBLUP and BayesR. The regression of phenotypes on breeding values was close to 1 for all methods, indicating little bias, except for GBLUP and adjusted phenotypes (regression = 0.78). Accuracies calculated with adjusted (for fixed effects) phenotypes were less variable than accuracies based on unadjusted phenotypes, indicating that fixed effects influence the latter. Increasing the reference population size increased accuracy, indicating that adding more records will be beneficial. For the Merino, Polled Dorset and White Suffolk breeds, accuracies were greater than for the Border Leicester breed due to the smaller sample size and limited across-breed prediction. BayesR detected only a few large marker effects but one region on chromosome 6 was associated with large effects for several traits. Cross-validation produced very similar variability of accuracy and regression coefficients for BLUP, GBLUP and BayesR, showing that this variability is not a property of genomic methods alone. Our results show that genomic selection for novel difficult-to-measure traits is a feasible strategy to achieve increased genetic gain.     

Effects of phyto-estrogenic alfalfa consumption on plasma LH levels in cycling ewes

Plasma luteinizing hormone (LH) concentrations were determined in five Dorset ewes fed orchard grass hay (Dactylus glomerata) and five ewes fed alfalfa (Medicago sativa). Total phyto-estrogen content ($\text{X}\text{±}\text{SEM}$ genistein equivalents) of the orchard grass hay and alfalfa was 16.9 ± 2.9 and 118 ± 12.3 ppm respectively. LH was determined at regular intervals during the estrous cycles synchronized with progesterone impregnated pessaries and characterized by marker ram and vaginal cytology.Peak LH levels in control ewes (40.1 ± 5.5 ng/ml) were lower (P<0.05) than in ewes fed phyto-estrogenic alfalfa (66.0 ± 16.8 ng/ml). Results also indicate that the LH peak may occur later (P<0.05) in the estrus period of ewes fed phyto-estrogenic alfalfa (15.4 ±4.5 h). These experiments may suggest that peak LH concentrations are elevated and delayed further into the estrus period in ewes fed phyto-estrogenic alfalfa.     

Duration of the breeding season and response to reproductive manipulation in five breeds of sheep under northern prairie conditions

The breeding season was 157, 154, <126, 210 and 217 days for Rambouillet, Columbia, Suffolk, Rambouillet x Finnish Landrace and Columbia x Finnish Landrace ewes respectively. Treatment of cyclic ewes with pregnant mare serum gonadotropin (PMSG) (500 IU), following a 12-day treatment with progestin-containing intravaginal sponges, did not affect fertility, but did decrease the time from sponge removal to estrus, (control 48.0 +/- 3.1 hr; PMSG 39.4 +/- 1.8 hr) to the preovulatory surge of LH (control 52.7 +/- 2.8 hr; PMSG 39.0 +/- 1.7 hr) and FSH (control 52.3 +/- 2.9 hr; PMSG 42.8 +/- 1.6 hr) and caused an elevation of serum LH levels prior to the preovulatory surge (control 1.25 +/- 0.18 ng/ml; PMSG 2.31 +/- 0.22 ng/ml). Exposure of the purebred ewes to 18 hours of daylight in January, decreasing by 30 minutes a week subsequently, counteracted the seasonal reduction in the number of ewes lambing following induced breeding under natural daylight in May. Prolificacy was greatest in crossbred ewes and their fertility was not affected by season. Gestation period was longer for fall-bred ewes and varied with breed.     

Plasma luteinizing hormone and prolactin in normothermic and hyperthermic ovariectomized ewes

The effect of bromocriptine on concentrations of luteinizing hormone (LH) and prolactin (PRL) as well as the rhythmicity of episodic profiles of plasma LH were investigated in twelve ovariectomized ewes exposed to 3-day trials during which ambient temperature/humidity conditions maintained either normothermia or induced an average of 1.4°C increase of rectal temperature (hyperthermia). In 24 of 48 trials, ewes received twice daily subcutaneous injections of 1 mg bromocriptine beginning at 1900 hr on day 1. Plasma PRL and LH were measured at 10-min intervals for 4 hr on days 2 and 3. Bromocriptine significantly decreased plasma PRL (65 ± 6 vs 5 ± 1 ng/ml), mean plasma LH (11.0 ± 0.2 vs 6.5 ± 0.2 ng/ml) and tended (P < 0.1) to decrease LH rhythmicity. In hyperthermic placebo-treated ewes, plasma PRL was increased (65 ± 6 vs 212 ± 20 ng/ml) and mean LH was decreased (11.0 ± 0.2 vs 8.2 ± 0.2 vg/ml) compared to normothermic, placebo-treated ewes, but there was no effect of hyperthermia on LH rhythmicity. Bromocriptine treatment of hyperthermic ewes decreased mean PRL (212 ± 20 vs 32 ± 9 ng/ml) on both days of sampling although mean levels were significantly higher on day 2 than on day 3(54 ± 14 vs 10 ± 6 ng/ml). Perhaps because mean LH was already inhibited in hyperthermic ewes, bromocriptine did not further decrease mean LH (8.2 ± 0.2 vs 6.6 ± 0.2 ng/ml), but LH rhythmicity was decreased (P < 0.01). There was no significant difference in mean LH between normothermic ewes receiving bromocriptine and hyperthermic ewes receiving bromocriptine (6.5 ± 0.2 vs 6.6 ± 0.2 ng/ml). These results indicate that bromocriptine inhibits PRL and LH secretion in normothermic ewes. In hyperthermic ewes, the inhibitory effect of bromoriptine on PRL was even more pronounced, but the effect on LH release was minimal perhaps because LH was already inhibited by hyperthermia.     

Hormonal relationships during the periovulatory period among ewe breeds known to differ in fertility after cervical artificial insemination with frozen thawed semen

Our previous work indicates that ewe breed differences in fertility following cervical AI with frozen-thawed semen are due to failure of normal sperm transport and/or early embryo development. Here we examined differences in hormone concentrations about the time of ovulation among more (Finnish Landrace and Belclare) and less (Suffolk and Texel) fertile ewes after AI with frozen thawed semen. In Experiment 1, oestradiol concentrations were measured in samples collected frequently from 12h before to 18h after the LH surge and progesterone was measured in samples collected from 9 to 27h after the LH surge in Suffolk (n=24), Texel (n=20) and Finnish Landrace (n=27) ewes. In Experiment 2, oestradiol concentrations were measured in samples collected frequently from 24h before to 6h after the LH surge and progesterone was measured in samples collected from 6h to 6 days after the LH surge in Suffolk (n=35) and Belclare (n=30) ewes. In Experiment 1, there was an effect of breed, time and their interaction (P<0.001) on oestradiol concentrations between -12 and +6h but only breed differences at +12 and +18h (P<0.01). Progesterone concentrations increased over time (P<0.001) and the rate of increase was significantly greater in Finnish Landrace than in the other two breeds. In Experiment 2, oestradiol concentrations were unaffected by breed. There was an interaction between breed and time with the rate of increase of progesterone being greater in Belclare than Suffolk ewes (P<0.001). In conclusion, differences in hormone concentrations in the periovulatory period are not consistent with ewe breed differences in fertility; however, we have showed that progesterone concentrations rise earlier in the more prolific breeds and suggest that this may explain reported ewe breed differences in embryo development.     

Effects of fasting ewes before mating on their reproductive performance

Border Leicester x Merino ewes were fasted for the first six days or the second six days of the oestrous cycle preceding mating in the fall season. When not fasting, ewes were grazed with a control group on irrigated pasture. Compared with control ewes, fasting had no effect on oestrous occurrence or fertilization but did reduce the ovulation rate, especially where fasting occurred early in the oestrous cycle.     

Moderate increases in peripheral blood estradiol concentration in the adult ram do not directly inhibit testosterone secretion.

Two experiments were conducted in July with adult Dorset x Leicester x Suffolk rams to determine whether increases of 150 or 300% in estradiol (E2) concentration in peripheral blood (from 6.3 +/- 0.8 pg/mL in control rams) would affect testosterone secretion directly as well as indirectly via the hypothalamic-pituitary axis. After 4 days of estradiol treatment (experiment 1) provided with subcutaneous polydimethylsiloxane implants filled with crystalline estradiol, luteinizing hormone (LH) and testosterone secretions were reduced by 50% (p < 0.05) in both groups of rams because of subtle decreases in pulse frequencies and amplitudes. Estradiol treatments were also associated with decreases in mean follicle-stimulating hormone (FSH) concentration (30-50% in both groups, p < 0.05) and increases in mean prolactin concentration (35% in low-E2 group; 105% in high-E2 group, p < 0.05), but testicular responsiveness to an LH challenge (single intravenous dose, 10 micrograms NIH-LH-S25) remained normal. When along with estradiol treatment, 10-micrograms doses LH were given every 80 min (experiment 2), testosterone secretion increased by 265% (p < 0.05) in both treated and control rams. Relative to day -1, secretion on day 4 was characterized by higher (p < 0.05) pulse frequencies and baseline concentrations and lower (p < 0.05) pulse amplitudes; values for all characteristics were similar to those for Dorset x Leicester x Suffolk rams in the breeding season. Interestingly, the decreases in mean FSH concentration brought about by estradiol and (or) LH treatments were not any greater than in experiment 1, and estradiol's ability to elevate mean prolactin concentration was blocked completely.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of progesterone on the GnRH-induced secretion of oestradiol and androstenedione from the autotransplanted ovary of the anoestrous ewe.

Two experiments were conducted during the anoestrous period in Border Leicester x Merino ewes with ovarian autotransplants to study the effects of a single injection of 20 mg progesterone on follicular steroid secretion. The aim of these experiments was to determine whether pretreatment with a 20 mg intramuscular injection of progesterone could reduce GnRH-induced ovarian steroid secretion in anoestrous ewes. In both experiments, an injection of 150 ng GnRH induced an LH pulse in all ewes with a maximum concentration 10 min (the first post-injection sample) after injection. Oestradiol and androstenedione secretion increased progressively after the GnRH-induced LH pulse and reached maximum rates of secretion between 60 and 90 min before decreasing slowly to pre-injection rates at 150 min. There were no differences in the pattern of secretion of oestradiol (measured in both experiments) or androstenedione (measured only in Expt 2). In Expt 1, the injection of progesterone 72 h before the challenge with GnRH had no effect on the maximum rate of oestradiol secretion from the autotransplanted ovary. However, in Expt 2, when progesterone was given either 36 or 60 h before GnRH, there was a significant suppression in the maximum rate of secretion of both oestradiol and androstenedione between 60 and 90 min after GnRH injection. These data show that pretreatment of anoestrous sheep with progesterone can suppress LH-stimulated steroid secretion from the ovary and indicate that progesterone may have a direct effect on oestrogenic follicles in sheep.     

Influence of breed and season on ovarian and pituitary response in progestogen-PMSG-treated ewes

Breed and seasonal effects on LH release, ovarian steroid secretion, and ovulation were evaluated in mature Finnish Landrace (Finn) and Hampshire ewes that received either a progestogen-PMSG treatment in May, July and November (experiment 1) or estradiol-17beta (50 mug) in May and July (experiment 2). The progestogen-PMSG treatment increased plasma estradiol within 12 hr after the PMSG injection at all three treatment periods and resulted in plasma LH and estradiol profiles similar to those during proestrus in cyclic ewes. Season, but not breed, affected the time from PMSG injection to preovulatory LH surge (56.5+/-1.4 hr in November vs 77.1+/-3.4 hr in July). Ovulation rate was higher in Finn than Hampshire ewes except in July when it decreased in Finn ewes. Magnitude of the estradiol-mediated LH release was decreased in July in Finn but not Hampshire ewes. Seasonal effects on reproduction in progestogen-PMSG treated ewes appear to be mediated through pituitary gonadotropin secretion with breed differences as to time and/or intensity of the seasonal effect(s).     

Plasma gonadotropin and progesterone concentrations during the estrous cycle of Finn, Suffolk and Targhee ewes

Hormonal profiles during the estrous cycle of Finn, Suffolk and Targhee ewes were compared in six ewes of each breed. Blood samples were drawn by venipuncture at 8-h intervals from onset to onset of consecutive estrous periods. Number of corpora lutea (CL) and ovarian follicles >/=3 mm in diameter on Day 10 (estrus = Day 0) were observed using endoscopy. Estrous cycle length was 14.9, 15.6 and 16.4 d (P<0.01) in Finn, Suffolk and Targhee ewes, respectively. Finns had more (P<0.001) CL (3.5) than Suffolks (2.0) and Targhees (1.8), but luteal phase progesterone concentrations were similar among breeds in peak level and area under the curve. In Finn ewes, the amplitude of the preovulatory LH surge was lower (P<0.01) and tended to occur later in estrus; otherwise LH levels and patterns were similar among breeds. A coincident follicle stimulating hormone (FSH) preovulatory surge occurred in most ewes, the amplitude of which was related to that of luteinizing hormone (LH); r = 0.67, P<0.01. Plasma FSH levels and patterns were similar in Finn, Suffolk and Targhee ewes and most ewes had three to four secretory episodes. Follicles >/=3 mm averaged 1.8, 1.0 and 1.2 (P>0.1) in Finn, Suffolk and Targhee ewes, respectively. Results indicate that the higher ovulation rate of the Finn ewe is not elicited by increased FSH levels at any stage of the estrous cycle.