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.     

Effects of Melatonin,Progestagens, and the Ram on Out-of-Season Reproduction in Swedish Landrace Finewool Sheep

One hundred twenty-one Swedish Landrace Finewool ewes were treated with progestagen sponges (P), teaser ram stimulation (R), or melatonin implants plus teaser ram stimulation (M) in preparation for breeding with whole rams in August. Blood progesterone analyses from ewes in the R and M groups gave no evidence of luteal activity before the introduction of teaser rams. There were no significant differences between treatments for pregnancy rate (~90%). The P group had the most compact lambing season, while median breeding dates for M and R groups were delayed by one cycle. In those groups, the introduction of breeding rams was later found to have been too late. M and R differed significantly for probable conception date but not for lambing dates. Circa 30% of M ewes did not have a short 6 day ovulation cycle after the first ovulation, which resulted in a less concentrated lambing season than the other methods. Although no significant differences in litter size were seen among the 3 treatments, M had the highest group average, 2.25. The ewes in this study were not in very deep anestrous in the middle of August. This supports the conclusion that treatment with exogenous hormones is not necessary to breed Swedish Landrace Finewool ewes successfully in late August/early September.     

Effect of exogenous melatonin and plane of nutrition after weaning on estrous activity, endocrine status and ovulation rate in Salz ewes lambing in the seasonal anestrus

Forty-nine Spanish Salz ewes lambing in the second fortnight of March (20 March +/- 1.5 d) were used to determine the effects of exogenous melatonin and postweaning nutrition on endocrine status, date of first estrus and ovulation rate. Experimental design was a factorial defined by 2 postweaning planes of nutrition, 1.80 (high) and 1.35 (low) times the maintenance requirements, and treatment with a single 18-mg subcutaneous implant of melatonin (M) 32 d after lambing or no treatment control (C). Mean weaning to first estrus interval was shorter in treated than in control ewes (50.8 +/- 4.2 vs 87.6 +/- 6.3 d; P < 0.01). Considering both the treated and control animals together, the ratio between mean night and daytime plasma melatonin levels was significantly correlated with the implant insertion-first estrus interval on Day 5 (0.67; P < 0.01) and Day 35 (0.63; P < 0.05) after implantation. Melatonin implants induced a significant increase of mean LH concentrations at Days 14 and 33 after implantation (P < 0.01) without any significant influence of plane of nutrition. Ovulation rate was higher for treated than control ewes in the second estrus (P < 0.05). An interaction between plane of nutrition and exogenous melatonin on ovulation rate at the second cycle after weaning was detected (P < 0.01), being close to the significance in the first, fourth and fifth cycles (P < 0.1). These results suggest that exogenous melatonin in April may be an effective way of advancing the breeding season and enhancing ovulation rate associated with a low rather than a high plane of nutrition.     

Ovulation and lambing rates in Karaguniki ewes after treatment with follicular fluid

The aim of the present study was to examine the effect of steroid-free bovine follicular fluid (bFF) on both ovulation and lambing rates. For this purpose, 30 adult ewes of the Karaguniki breed were randomly allocated to three treatment groups (A,B and C; n=10 ewes each) during the breeding season of 1988. The ewes in Group A received bFF (6 ml iv) twice daily during their luteal phase, starting on Day 5 and lasting until Day 9. The ewes in Group B received a mixture of bFF/arachid oil (3 ml sc, 2:1) on Days 3, 4, 5, 10 and 12 of the estrous cycle. The ewes in Group C (Controls) were treated subcutanecusly with a mixture of steroid-free bovine plasma and arachid oil (2:1) on the same days as the ewes in Group B. Plasma concentrations of progesterone showed that the luteal function during the treatment cycle was normal in all treated and control ewes. The ovulation and lambing rates, however, were greater in Group A (2.5 +/- 0.2 and 1.9 +/- 0.3, respectively) and in Group B (2.1 +/- 0.2 and 1.6 +/- 0.1, respectively) than in Group C (1.5 +/- 0.2 and 1.2 +/- 0.3, respectively). Precipitating antibodies were detected in the plasma of Group B ewes only.     

Effect of time of early weaning and time of lambing on accelerated lambing in Polypay sheep

Effects of early weaning, lactation, and day-of-year lambing on the ability of Polypay ewes to rebreed following winter and summer lambings were evaluated. Winter lambing ewes did not successfully rebreed while lactating. However, when winter-born lambs were weaned at 31 days postpartum, more ewes rebred and produced summer lambs (35.7%) than when lambs were weaned at 41 days postpartum (23.6%). Ewes that lambed during the early part of the winter lambing period had an advantage over later lambing ewes in the percentage which subsequently lambed the following summer. This was apparently the result of a difference in length of breeding exposure rather than a higher fertility rate during the early part of the breeding period. When summer lambing ewes were rebred during the early part of the summer breeding period (late summer to early fall), stress associated with lactation did not affect subsequent winter lambing performance. Summer lambing ewes belonging to a late weaning (80 days) treatment group did not differ (P>0.05) from those belonging to an early weaning (31 days) treatment group in winter fertility, prolificacy, day-of-year lambing or lambing interval.     

The effect of season and technique on synchronized and induced estrus and the induction of lambing in the ewe in a commercial setting

At 40 day intervals, groups of 87 to 142 commercial ewes of mixed breeding were subjected to a 5 day breeding period, following one of three estrus inducing or synchronizing treatments (intravaginal sponges containing 60 mg of medroxyprogesterone acetate) for 12 days with (progestin P.M.S.G. group) or without pregnant mares serum gonadotropin (P.M.S.G.) (Progestin group) at sponge removal, or a single injection of prostaglandin F(2alpha) (breeding season and early anestrus only) (PGF(2alpha) group)). Mean pregnancy rates (ewes lambing of those treated) and lambing percentages (lambs born per 100 ewes lambing) were 31 +/- 4%, 169 +/- 6%, 20 +/- 5%, 105 +/- 2% and 18 +/- 8%, 118 +/- 3%, respectively, for the three treatment groups above. Pregnancy rates for the progestin, P.M.S.G. group were 52% in late July, prior to the breeding season, 52% in September and declined to 14% in March (early anestrus). The time from the first of two daily injections of estradiol benzoate to lambing was 34.5 +/- 4.4 hours in ewes 142 to 146 days pregnant.     

Oxytocin-induced release of prostaglandin F2 alpha in postpartum beef cows: comparison of short versus normal luteal phases

The first postpartum ovulation after early weaning of calves (30 35 days of age) from cows is normally followed by a short luteal phase (6 10 days) unless the animals are pretreated with a progestogen (e.g. norgestomet). Reduced luteal lifespan in cattle is reportedly due to the premature release of a luteolysin (presumably prostaglandin F2 alpha [PGF2 alpha]). Therefore, the objective was to determine if oxytocin-induced release of PGF2 alpha (measured by the stable PGF2 alpha metabolite, 15-keto-13,14-dihydro PGF2 alpha [PGFM]) was greater for cows having a short compared to a normal luteal phase on Day 5 following the first postpartum estrus (Day 0). Thirty postpartum beef cows were randomly assigned into three groups (n = 10 per group) expected to have short (Short d 5) or normal (Norgestomet d 5 and Norgestomet d 16) luteal phases. Cows in Norgestomet d 5 and d 16 groups received Norgestomet (progestogen) implants for 9 days beginning 21 23 days postpartum. On Day 5 (Short d 5 and Norgestomet d 5) or Day 16 (Norgestomet d 16) following first postpartum estrus, each animal was injected (i.v.) with 100 IU oxytocin. In addition, cows in the Short d 5 group were subdivided into two groups following second estrus (normal luteal phase, n = 5 per group) to receive 100 IU oxytocin on Day 5 (Normal d 5) or 16 (Normal d 16), respectively. Estrous cycle length (means +/- SE) for cows in the Short d 5 group (8.7 +/- 0.4 days) was shorter (p less than 0.01) than for cows in all other groups (21.1 +/- 0.3 days).(ABSTRACT TRUNCATED AT 250 WORDS)     

Induction of ovulation in the acyclic postpartum ewe following continuous, low-dose subcutaneous infusion of GnRH

Pituitary and ovarian responses to subcutaneous infusion of GnRH were investigated in acyclic, lactating Mule ewes during the breeding season. Thirty postpartum ewes were split into 3 equal groups; Group G received GnRH (250 ng/h) for 96 h; Group P + G was primed with progestagen for 10 d then received GnRH (250 ng/h) for 96 h; and Group P received progestagen priming and saline vehicle only. The infusions were delivered via osmotic minipumps inserted 26.6 +/- 0.45 d post partum (Day 0 of the study). Blood samples were collected for LH analysis every 15 min from 12 h before until 8 h after minipump insertion, then every 2 h for a further 112 h. Daily blood samples were collected for progesterone analysis on Days 1 to 10 following minipump insertion, then every third day for a further 25 d. In addition, the reproductive tract was examined by laparoscopy on Day -5 and Day +7 and estrous behavior was monitored between Day -4 and Day +7. Progestagen priming suppressed (P < 0.05) plasma LH levels (0.27 +/- 0.03 vs 0.46 +/- 0.06 ng/ml) during the preinfusion period, but the GnRH-induced LH release was similar for Group G and Group P + G. The LH surge began significantly (P < 0.05) earlier (32.0 +/- 3.0 vs 56.3 +/- 4.1 h) and was of greater magnitude (32.15 +/- 3.56 vs 18.84 +/- 4.13 ng/ml) in the unprimed than the primed ewes. None of the ewes infused with saline produced a preovulatory LH surge. The GnRH infusion induced ovulation in 10/10 unprimed and 7/9 progestagen-primed ewes, with no significant difference in ovulation rate (1.78 +/- 0.15 and 1.33 +/- 0.21, respectively). Ovulation was followed by normal luteal function in 4/10 Group-G ewes, while the remaining 6 ewes had short luteal phases. In contrast, each of the 7 Group-P + G ewes that ovulated secreted progesterone for at least 10 d, although elevated plasma progesterone levels were maintained in 3/7 unmated ewes for >35 d. Throughout the study only 2 ewes (both from Group P + G) displayed estrus. These data demonstrate that although a low dose, continuous infusion of GnRH can increase tonic LH concentrations sufficient to promote a preovulatory LH surge and induce ovulation, behavioral estrus and normal luteal function do not consistently follow ovulation in the progestagen-primed, postpartum ewe.     

Effect of ram exposure at the end of progestagen treatment on estrus synchronisation and fertility during the breeding season in ewes

Two experiments were conducted to examine the effects of ram exposure during the breeding season, in combination with progestagen treatment on estrus synchronization, fertility the LH surge and ovulation in ewes. Experiment 1 was subdivided into experiments 1a and 1b. In all experiments cross-bred ewes were treated with an intravaginal sponge for 12-14 days and three days before sponge withdrawal ewes were divided into control (no further treatment; n=191, 103 and 50 for experiments 1a, 1b and 2, respectively) or ram exposed (three mature rams per 50 ewes were introduced; +Ram; n=187, 99 and 49 for experiments 1a, 1b and 2, respectively). At sponge withdrawal ewes in Experiments 1a and 2 received 500 IU eCG and rams were removed from all the +Ram groups. In Experiments 1a and 1b, raddled, entire rams were introduced to ewes 48 h after sponge withdrawal. The timing of mating was recorded and ewes were maintained until lambing. In Experiment 2, estrus behavior was determined every 4 h and the time of the LH surge and ovulation were determined from a subset of 10 ewes per group. In Experiment 1a, less +Ram ewes were bred by 48 h after ram introduction (control 98% versus +Ram 89%, P<0.001) and in Experiments 1a and 1b 14% fewer (P<0.05) of the ewes bred in the first 3 h after ram introduction lambed to that service. In Experiment 1a, ram exposed ewes had a lower litter size than control ewes (1.93+/-0.06 versus 1.70+/-0.06 lambs per ewe; P<0.05). In Experiment 2, rams advanced (P<0.05) estrus, the LH surge and ovulation by 2-6 h compared with control ewes. We speculate that exposure of ewes to rams increased LH secretion and that this in turn increased follicle development and the production of oestradiol that led to a more rapid onset of estrus, the LH surge and ovulation compared to control ewes. Unexpectedly, ewes that were bred had lower fertility in the +Ram groups than control groups.     

Effects of abnormal ovarian cycles during pre-service period postpartum on subsequent reproductive performance of high-producing Holstein cows

The primary objective of this study was to investigate the effects of abnormal ovarian cycles during the pre-service postpartum period on subsequent reproductive performance of high-producing Holstein cows. The study was conducted in a commercial dairy farm with approximately 150 lactating cows, in a subtropical region of Japan. Animals were kept in free-stall barn, and fed a total mixed ration. Cows that calved from June 2001 to July 2002 were included in the study. Milk samples were collected twice weekly from 2 to 11 weeks postpartum, and progesterone concentrations in skim milk were determined by ELISA. After a voluntary waiting period of 40 days, cows detected in estrus were bred by artificial insemination (AI). Pregnancy was confirmed by palpation per rectum 40-70 days after AI. Out of a total of 91 cases, 39 (42.9%) had normal ovarian cycles (ovulation within 45 days after calving, followed by normal ovarian cycles), 32 (35.2%) had prolonged luteal phase (i.e. luteal activity for >20 days), and 12 (13.2%) had anovulation or delayed first ovulation (i.e. first ovulation did not occur until >45 days after calving). The remaining (8.8%) had other types of abnormalities. When compared with cows with a normal ovarian cycle, prolonged luteal phase cows had a lower 100 days AI submission rate, conception rate and pregnancy rate (84.2% versus 56.3%; P<0.05, 50% versus 16.7%; P<0.05 and 42.1% versus 9.4%; P<0.01, respectively), and longer intervals to first AI and to conception ( 67+/-6 days versus 98+/-7 days and 95+/-9 days versus 136+/-11 days; P<0.01 for each). Similarly, when compared with cows with normal ovarian cycles, those with anovulation had lower 100 days conception rate and pregnancy rate (50% versus 0%; P<0.05 and 42.1% versus 0%; P<0.01, respectively), and longer intervals to first AI and to conception ( 67+/-6 days versus 93+/-12 days; P<0.05 and 95+/-9 days versus 155+/-14 days; P<0.01, respectively). Survival analysis of the data for calving to conception interval showed that cows with prolonged luteal phase or anovulation were getting pregnant at a slower rate, and took longer to get pregnant than the cows with normal resumption of ovarian cyclicity postpartum. In conclusion, abnormal ovarian cycles during the pre-service period postpartum adversely affected reproductive performance, including AI submission rate, pregnancy rate, interval to first AI, and calving to conception interval in high-producing Holstein cows.     

Sexual activity of Assaf ewes after October and February lambings

The first ovulation in 20 Assaf ewes and the first estrus in 54 out of 77 Assaf ewes after the October lambings occurred at mean intervals of 26 +/- 1.7 and 50 +/- 1.5 d postpartum, respectively. After the February lambings, nine out of 20 examined ewes had their first ovulation and estrus 35 +/- 2.5 and 71 +/- 7.7 d postpartum, respectively, and resumed normal cyclic activity in April. Five animals showed considerable ovarian activity but only occasional estrous activity during spring, while the remaining six ewes had a 6-to 7-mo-long anestrous season during which only occasional ovulations were detected in three of them.     

Effect of plane of protein after weaning on resumption of reproductive activity in Rasa Aragonesa ewes lambing in late spring

The effect of protein supplementation after weaning on the resumption of reproductive activity in a Spanish breed of ewes (Rasa Aragonesa) with a reduced seasonality was studied. Two equal groups of ewes were weaned in the first 2 weeks of July. From weaning to the end of the experiment both groups were fed identical energy-content rations but with different protein levels (high, Group H; low, Group L). The mean time between weaning and first detected estrus was 64 days. No differences between groups were found either in relation to this interval or in the ovulation rate in the first estrus. Ewes showing a multiple ovulation rate in the first cycle anticipated the onset of sexual activity after weaning. Significant differences in the ovulation rate were detected between the 2 groups from September to the end of the experiment (1.55 vs 1.05 corpora lutea for Groups H and L, respectively; P<0.001). In conclusion, the effect of a protein supplement after weaning in Rasa Aragonesa ewes lambing in late spring is reflected at mid-term in the early breeding season, with an evident rise of ovulation rate in the nutrient supplemented group.     

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.     

Estrus, ovulation and conception following timed insemination in Romney ewes treated with progestagen and gonadotropins.

The estrus — ovulation time relationships was examined in Romney ewes treated with progestogen (intravaginal sponge) and gonadotropins (PMSG + HCG or PMSG alone) prior to (January) and during (April) the breeding season. The conception rate of ewes inseminated at predetermined times after treatment was also investigated.Ewes exhibited estrus sooner after sponge removal in April than in January (34.9 v 38.9 hrs, P < 0.001). The interval from sponge removal to ovulation was also shorter in April than in January (56.3 – 62.1 hrs, P < 0.01). There were no significant differences between treatments or season on the mean interval from estrus to ovulation. Types of gonadotropin treatment had no effect on the estrus — ovulation time relationships. There were no significant effects of season, hormone treatment or time of insemination on lambing rate.     

Repeatability of the duration of oestrus and breed differences in the relationship between druation of oestrus and ovulation rate of sheep.

The duration of oestrus and the time interval from removal of progestagen-impregnated pessaries to the onset and end of oestrus were examined in Texel, Finnish Landrace, Galway and Fingalway (Finnish Landrace X Galway) ewes. The differences among the breeds in the relationship between these variables and ovulation rate at the controlled oestrus were also investigated. Breed differences were significant for all traits except the interval from pessary withdrawal to the onset of oestrus. The relationship between ovulation rate and both the interval from pessary withdrawal to the onset of oestrus and the duration of oestrus differed significantly among the breeds. The repeatability of the duration of oestrus was significant for Texel and Rambouillet ewes (mean = 0.5) and for pooled data from ewe lambs of various breeds. It was concluded that, in view of the breed differences in the relationship between ovulation rate and duration of oestrus and other traits, generalizations should not be made from among-breed to within-breed relationships. The high repeatability for the duration of oestrus may mean substantial heritabilities for the physiological determinants of oestrus duration.     

Comparison of two progestogens during out-of-season breeding in a commercial ewe flock

A comparison was made of the relative effectiveness of subcutaneous ear implants containing 2 mg Norgestomet or vagnial pessaries containing 60 mg medroxyprogesterone acetate (MAP) to induce estrus and conception in dry anestrous ewes. Groups of ewes were treated with one of the two progestogens for 14 d, and 500 IU pregnant mare serum gonadotropin (PMSG) was administered intramuscularly at the time of progestogen withdrawal. No significant differences in estrus induction, pregnancy rate or number of lambs born per ewe lambing were observed. Ewes treated with Norgestomet had 96% estrus, 60% pregnancy rate and 1.4 lambs per ewe lambing. Comparably, ewes treated with MAP had 94% estrus, 65% pregnancy rate and 1.7 lambs per ewe lambing. Norgestomet implants compared favorably with MAP pessaries for estrus induction and breeding of commercial, dry anestrous ewes.     

The active immunization of the cyclic ewe against an estrone protein conjugate

Four mature, cyclic ewes were given injections (I.M.) of a conjugate of 1,3,5 (10)-estratrien-3-ol-6,17-dione, 6 carboxyoxime bovine serum albumin (immunized ewes) on day 3 after estrus, and at days 10, 20, 40, 58, 91 and 134 after this initial treatment. Six control ewes treated with carrier emulsion alone continued to cycle normally. Three of the immunized ewes failed to exhibit estrus, an associated preovulatory surge of LH and ovulation. One ewe showed 1 abnormally short estrous period and then became anestrus. Injection of an estrone-protein-conjugate at days 3 and 13 after estrus did not appear to interfere with the rate of structural luteolysis of the corpus luteum present, but plasma concentrations of progesterone reached abnormally high luteal phase levels and in 2 ewes failed, subsequently to decline to normal follicular phase levels. Estrone binding capacity rose as early as day 9 after first treatment, and concentrations of LH rose as early as day 14. Subsequently, plasma levels of LH, estrone and progesterone and antisera titer rose; the only significant cross reaction of the antisera was with estradiol 17beta (11.32 +/- 2.80%).     

Effect of diet and GnRH administration on post-partum ovarian cyclicity in autumn-lambing ewes

Using autumn-lambing ewes, this study investigated (i) the effects of diet on gonadotrophin secretion and responsiveness of the hypothalamic-pituitary-ovarian axis to exogenous GnRH during the early post-partum period; and (ii) whether ovulation prior to completion of uterine involution results in an increased incidence of aberrant ovarian cycles. Thirty-two ewes rearing 1.9+/-0.12 lambs were equally allocated to two dietary treatments at lambing (22 October +/-0.2 day). Diets comprised ad libitum hay and 1.5 kg per ewe per day of one of two concentrates (11.5 MJ ME, 195 g CP per kg) containing 300 g kg(-1) cracked maize grain (M) or 300 g kg(-1) sugar beet pellets (S). Half of the ewes on each diet (G) received 25 i.v. injections of 250 ng GnRH in 2 ml 0.9% saline at 2 h intervals from days 12-14 post-partum while remaining ewes (N) were monitored for the resumption of spontaneous ovarian cyclicity. Blood samples were obtained from all ewes throughout the study (lambing to 18 December) for measurement of circulating hormone concentrations and the uteri and ovaries of all ewes were examined via laparoscopy on day 21 post-partum. There were no effects of dietary treatment on ewe daily live weight loss, lamb daily live weight gain or the immediate post-partum increase in circulating FSH concentrations. Diet did not affect insulin concentrations or LH pulse frequency on day 12 post-partum but LH pulse amplitude was lower in ewes fed concentrate M compared to concentrate S (1.4+/-0.10 versus 1.7+/-0.12 ng ml(-1), respectively, P<0.05) and this was associated with an increased interval to the resumption of spontaneous ovarian cycles (35+/-3.1 versus 26+/-2.1 day, respectively, P<0.05). Administration of exogenous GnRH increased (P<0.05) the proportion of ewes on both diets that ovulated within 20 days of parturition and advanced the onset of ovarian cyclicity in ewes fed concentrate M by 9.5 days (significance of interaction, P<0.05). Four ewes, all of which ovulated before day 22 post-partum, had extended luteal activity while in remaining ewes, duration of the first luteal phase was inversely related to the time of first ovulation (r(2)=0.16, P<0.05). Results demonstrate that (i) the onset of ovarian cyclicity is influenced by diet and can be advanced by administration of exogenous GnRH; and (ii) ovulation during the early post-partum period is associated with an increased incidence of extended luteal activity.     

Effects of suckling mode on endocrine control of reproductive activity resumption in Texel ewes lambing in July or November

To study the effects of the overlapping of seasonal and lactation anestrus and the influence of the suckling mode on the resumption of reproductive activity in Texel ewes, two experiments were carried out after the July and November lambings. The frequency and the duration of suckling decreased with the age of the lambs, and the suckling intensity in the lambs that were allowed to suck unrestrictedly was three times higher than in lambs restricted to sucking three times a day. The overlapping of seasonal anestrus and lactation delayed the resumption of ovarian and estrus activity, but no difference was observed between dams suckling single lambs and twins. The resumption of ovarian and estrus activity was much earlier after the November lambing than after the July lambing. Weaning after the November lambing shortened the interval between parturition to first estrus but not to the first luteal function. The reduction of suckling intensity by suckling management had no effect on the resumption of ovarian and estrus activity. In early postpartum, suckling inhibited the luteinizing hormone (LH) pulsatile secretion and consequently the first LH discharge. However, the earlier restoration of gonadotropin stimulation in dry ewes was not immediately followed by ovarian activity. The suckling inhibition may be due to a temporary disturbance in hormonal balance, the rise in prolactin (PRL) and cortisol secretions. Plasma estradiol 17beta (E2) concentrations were higher in dry than in suckling ewes in early postpartum. Follicle stimulating hormone (FSH) secretion was not involved in the process of delaying the resumption of reproductive activity after lambing.     

Ovulation, fertilization and lambing rates, and peripheral progesterone concentrations, in ewes inseminated at a natural oestrus during November or February.

The objective of this study was to determine the relative importance of seasonal changes in ovulation rate, fertilization rate and embryo survival as the cause of reduced lambing rates in ewes mated in February compared with those mated in November. The study was conducted at 57 degrees N using mature Mule ewes and Suffolk rams. Sixty ewes were allocated equally to five groups: unbred (UB) or mated at a natural oestrus during November (N) or February (F) by natural (N) or cervical artificial (A) insemination. Groups were maintained separately at pasture supplemented with hay. A raddled vasectomized or non-vasectomized ram was present with UB, NN and NA groups from 26 October 1995 to 1 January 1996 and with UB, FN and FA groups from 25 January 1996 to 31 March 1996. Ewes marked by the ram were recorded twice a day, and those in groups NN, NA, FN and FA were inseminated at their second behavioural oestrus. For all ewes, blood samples were obtained once a day from introduction of the vasectomized rams until 30 days after mating (groups NN, NA, FN and FA) or 20 days after the first oestrus (group UB), and ovulation rate was measured by laparoscopy 7 days after the first oestrus. For ewes in groups NN, NA, FN and FA, ovulation rate was measured again after the second oestrus and ova were recovered from six ewes per group for assessment of fertilization before autotransfer. Pregnancy and lambing rates were recorded at term. Mean (+/- SE) dates of the first recorded oestrus for ewes in groups NN, NA and UB, and FN, FA and UB were 4 +/- 1.1 November and 4 +/- 0.9 February, respectively, and intervals between the first and second oestrus were 16 +/- 0.2 and 17 +/- 0.3 days (P < 0.01), respectively. Ovulation rates were 2.6 +/- 0.08 and 2.0 +/- 0.05 (P < 0.001), and peripheral progesterone concentrations during the luteal phase were 8.5 +/- 0.25 and 7.6 +/- 0.31 ng ml-1 (P < 0.05), for November and February, respectively. The difference in peripheral progesterone concentration was not solely attributable to the difference in ovulation rate. There was no significant effect of month or method of insemination, or of embryo recovery and autotransfer procedures on pregnancy rates and the proportion of ewes that became pregnant were NN 0.92, NA 0.83, FN 0.67 and FA 0.75. For ewes undergoing embryo recovery and autotransfer, ova recovered per corpus luteum were 1.00, 0.93, 1.00 and 0.92, fertilized ova per ovum recovered were 0.69, 0.92, 1.00 and 0.83, and lambs born per corpus luteum were 0.62, 0.79, 0.78 and 0.58 for NN, NA, FN and FA groups, respectively. There were no significant seasonal effects on fertilization rate or embryo survival. It is concluded that a seasonal decline in ovulation rate is the primary cause of reduced lambing rates in ewes mated in February compared with those mated in November. Pregnancy rates were high after mating in both periods and were not enhanced by the use of cervical insemination.