Control of oestrus in ewe lambs and yearling ewes with medroxyprogesterone acetate and fluorogestone acetate

Two experiments were conducted in a large flock of Berrichon purebred and Romanov × Berrichon F₁ crossbred ewes. In experiment 1, 157 ewe lambs were placed in six groups according to season of mating and maternal breed. Within each group equal numbers of females were treated with either medroxyprogesterone acetate (MAP) or fluorogestone acetate (FGA)-impregnated sponges for 14 days; dosage of PMSG at the time of sponge removal was 500 and 300 I.U. respectively for pure and crossbred. In experiment 2, 111 dry yearling ewes of the Berrichon breed were involved, of which 35, 51 and 25 had either a short lambing to treatment interval (approximately 3 months), a long lambing to treatment interval (approximately 6 months), or were primiparous or nulliparous. They were also treated either with MAP or FGA.The conception rates were extremely high (overall near 86%) in ewe lambs and not significantly affected by seasons or breeds or treatments. Prolificacy and number of lambs per ewe mated were higher in the F₁ crossbred group than in pure Berrichon females (respectively 1.91 vs. 1.34 lambs/mated ewe). No significant difference between the two progestagens could be observed for any of the productivity criteria. Fertility was equally satisfactory in yearlings (overall near 85%) irrespective of the interval between lambing and mating. However, the fertility of the nulliparous ewes (those that failed to conceive as ewe lambs) was significantly lower (72%). The number of lambs per ewe mated was 1.78 in yearlings with a 6-month interval from lambing to mating but was not significantly different from the numbers recorded in the other two groups (1.37 and 1.36). As in experiment 1, there was no difference in the results obtained after either of the two progestagen treatments.In conclusion, progestagen treatment has been shown in both ewe lambs and yearlings to be a very effective means of increasing lamb production in large flocks. This was achieved in ewe lambs regardless of the season and, in dry yearling ewes, regardless of the interval from lambing to mating in the anoestrous season.     

Ovarian response to hCG injections during the prepubertal period in three breeds of sheep with different ovulation rates and litter sizes

A positive correlation was reported between ovulation (OR) rate in response to hCG in prepubertal ewe lambs and adult ewes. The finding suggested that the response to hCG may be used to select prepubertal ewe lambs for prolificacy. We studied plasma FSH concentrations and ovarian response to hCG injection during the prepubertal period in breeds with high (Chios), medium (Florina) and low (Karagouniki) OR and litter sizes (LS). The results showed that Chios ewe lambs presented a higher profile of plasma FSH during the 3rd to 7th wk of age when compared to the breeds with medium (Florina) and low (Karagouniki) OR and LS. The ovarian response to hCG showed that the Chios ewe lambs responded earlier, by exhibiting ovulation at 6 wks and more effectively, by demonstrating a higher OR than the other two breeds. There was no correlation among plasma FSH concentrations during prepubertal age (3rd to 7th wk), ovulation rate after hCG injection, or litter size at first and second lambing for all breeds. Therefore we concluded that the plasma FSH concentrations and/or the ovarian response to hCG injection during the prepubertal period is not a suitable criterion for early selection for litter size in the adult ewe of these breeds.     

Ewe lambs with higher breeding values for growth achieve higher reproductive performance when mated at age 8 months

We studied the relationships among growth, body composition and reproductive performance in ewe lambs with known phenotypic values for depth of eye muscle (EMD) and fat (FAT) and Australian Sheep Breeding Values for post-weaning live weight (PWT) and depth of eye muscle (PEMD) and fat (PFAT). To detect estrus, vasectomized rams were placed with 190 Merino ewe lambs when on average they were 157 days old. The vasectomized rams were replaced with entire rams when the ewe lambs were, on average, 226 days old. Lambs were weighed every week and blood was sampled on four occasions for assay of ghrelin, leptin and ß-hydroxybutyrate. Almost 90% of the lambs attained puberty during the experiment, at an average live weight of 41.4 kg and average age of 197 days. Ewe lambs with higher values for EMD (P < 0.001), FAT (P < 0.01), PWT (P < 0.001), PEMD (P < 0.05) and PFAT (P < 0.05) were more likely to achieve puberty by 251 days of age. Thirty-six percent of the lambs conceived and, at the estimated date of conception, the average live weight was 46.9 ± 0.6 kg and average age was 273 days. Fertility, fecundity and reproductive rate were positively related to PWT (P < 0.05) and thus live weight at the start of mating (P < 0.001). Reproductive performance was not correlated with blood concentrations of ghrelin, leptin or ß-hydroxybutyrate. Many ewe lambs attained puberty, as detected by vasectomized rams, but then failed to become pregnant after mating with entire rams. Nevertheless, we can conclude that in ewe lambs mated at 8 months of age, higher breeding values for growth, muscle and fat are positively correlated with reproductive performance, although the effects of breeding values and responses to live weight are highly variable.     

Use of progestagen-gonadotrophin treatments in estrus synchronization of sheep

The main objective of this study was to investigate the effectiveness of certain progestagen-gonadotrophin treatments on synchronization of estrus in sheep. In Experiment I, 30 Chios ewes were treated at the beginning of the breeding season with medroxyprogesterone acetate (MAP) intravaginal sponges for 12 days and a single i.m. treatment of either FSH (Group 1,10 IU, n = 8; Group 2, 5 IU, n = 8; Group 3, 2.5 IU, n = 8) or eCG (Group 4, 400 IU, n = 6) at the time of sponge removal. Ten days after sponge removal laparotomy was performed to record ovarian response. Clinical estrus was observed in more (though not at a significant level) FSH treated than eCG treated sheep (62.5% versus 33.3%). Administration of 400 IU eCG resulted in the highest mean number of CL perewe ovulating (2.8 +/- 0.2), with administration of 10 IU FSH producing the next best results (2.1 +/- 0.3). Statistically significant differences in the mean number of CL per ewe ovulating were found only between ewes in Group 3 (1.7 +/- 0.4) and Group 4 (2.8 +/- 0.2) (P < 0.05). In Experiment II, 53 Chios and 30 Berrichon ewes were treated during the mid-breeding season with MAP intravaginal sponges for 12 days and a single i.m. treatment of either 10 IU FSH (27 Chios and 16 Berrichon ewes) or 400 IU eCG (26 Chios and 14 Berrichon ewes), at the time of sponge removal. Ewes that were in estrus on Days 2-4 and 19-23 after sponge removal were mated to fertile rams. No significant differences were recorded between treatment or breed groups in the proportions of ewes observed in estrus after treatment. In the Berrichon breed, FSH administration resulted in higher lambing rates (93.8% versus 57.1%, P < 0.05) and higher mean number of lambs born per ewe exposed to rams (1.4 +/- 0.2 versus 0.8 +/- 0.2, P < 0.05) than that of eCG. After treatment with eCG, the mean number of lambs born per ewe exposed to rams was higher in the Chios than the Berrichon breed (1.4 +/- 0.2 versus 0.8 +/- 0.2, P < 0.05). After treatment with FSH, the lambing rate was higher in the Berrichon than the Chios breed (93.8% versus 63.0%, P < 0.05). In conclusion, a single FSH treatment (5 or 10 IU) at the end of progestagen treatment appears to be more effective than eCG for the induction of synchronized estrus in sheep at the beginning of the breeding season, with no cases of abnormal ovarian response observed. During the mid-breeding season FSH (10 IU) appears to be equally as effective as eCG (400 IU) in respect of lambing rate and mean number of lambs born per ewe.     

The effects of GnRH analogue (buserelin) or hCG (Chorulon) on Day 12 of pregnancy on ovarian function, plasma hormone concentrations, conceptus growth and placentation in ewes and ewe lambs

The objectives of this study were to determine the effect of GnRH analogue (buserelin) or human chorionic gonadotrophin (hCG, Chorulon) treatment on Day 12 of pregnancy on ovarian function, plasma hormone concentrations, conceptus growth and placentation in ewes and ewe lambs. After oestrus synchronization with progestagen sponges and eCG, all the animals were mated with fertile rams. Both ewes and ewe lambs (20 per treatment group) were given either normal saline or 4 microg GnRH or 200 IU hCG on Day 12 post-mating. Pre- and post-treatment plasma hormone concentrations were determined in seven pregnant animals per treatment group in samples collected 1h before and 0, 2, 4, 6, 8, 24, 48 and 72 h after treatment. Overall mean progesterone concentrations were higher (P<0.001) in ewes as compared with ewe lambs in saline-treated controls. GnRH or hCG treatment increased (P<0.001) mean plasma progesterone concentrations in both age groups, however, post-treatment concentrations were significantly (P<0.05) higher in ewes than in ewe lambs. Oestradiol concentrations were similar in the two control groups. In ewes, but not in ewe lambs, both GnRH and hCG treatments significantly (P<0.05) increased the mean oestradiol concentrations above pre-treatment levels. Moreover, post-treatment oestradiol concentrations in GnRH- and hCG-treated animals were significantly (P<0.05) higher than those in the saline-treated controls. LH release in response to GnRH treatment was greater (P<0.05) in ewes than in ewe lambs, whereas FSH release in ewes was less (P<0.05) than that of ewe lambs. The effects of GnRH or hCG on conceptus growth and placentation was determined at slaughter on Day 25. In ewes, GnRH treatment increased (P<0.05) luteal weight, amniotic sac width and length, and crown-rump length compared with controls, but had no effect on these parameters in ewe lambs. In ewes, hCG treatment also enhanced (P<0.05) luteal weight, amniotic sac width and length, crown-rump length, embryo weight and number of placentomes as compared with controls. In ewe lambs, there was no difference (P<0.05) between hCG and control groups in luteal weight, embryo weight and amniotic sac width but crown-rump length, amniotic sac length and the number of placentomes forming the placenta were greater (P<0.05). In conclusion, GnRH or hCG treatment on Day 12 of pregnancy can increase ovarian function, conceptus growth and placental attachment in ewes. However, these treatments were less effective in ewe lambs.     

Antral follicle growth and endocrine changes in prepubertal cattle,sheep and goats

In the growing heifer calve, there is an early post-natal, gonadotrophin driven increase in ovarian antral follicle growth. The endocrine regulation of and reason for this initial stimulation of ovarian follicular development are not fully understood. This initial endocrine activity appears to be later held in check by negative feedback suppression mechanisms until the heifer is of a sufficient body size to initiate oestrous cycles and to reproduce. There is increasing evidence from recent ultrasonographic studies, performed in the same groups of prepubertal heifer calves, that the development of ovarian antral follicles and tubular genitalia occur in parallel. There appear to be two distinct periods of enhanced development of the reproductive organs, from 2 to 14 weeks of age and again from 34 to 60 weeks of age, or just prior to puberty. First ovulation in heifers is preceded by a gradual increase in pulsed LH secretion, which results in enhanced antral follicle development and oestrogen production. It was demonstrated that prepubertal heifers produced recurrent antral follicular waves; maximum sizes and life span of the dominant follicles of waves, as well as periodicity and FSH dependency of wave emergence were similar to those in adult cattle. In does, no Graafian follicles are seen at birth and total follicle numbers increase to 2 months of age, and then decline to 5 months of age. In ewe lambs, studies using transrectal ovarian ultrasonography showed that antral follicle recruitment and growth increased after the first 2 months of age and just before puberty. This bi-phasic pattern of changes in ovarian follicle recruitment and growth is strikingly similar to that in heifer calves, but it contrasts with earlier post-mortem examinations of ovaries in ewe lambs. Unlike in cattle and adult ewes, the rhythmic pattern of follicular wave emergence was not established in pre- and peripubertal ewe lambs. The early increase in antral follicle numbers and size in ewe lambs may be, at least in part, due to changes in FSH release and potency, and enhanced follicle production prior to first ovulation is probably caused by an increase in the frequency of LH pulses.     

Bioactive and immunoreactive FSH concentrations in ewe and ram lambs over the first year of life

Several studies suggest that the concentration of immunoreactive (I) FSH measured in peripheral plasma by radioimmunoassay does not always reflect the level of bioactive (B) hormone capable of eliciting a biological response (e.g. oestradiol synthesis by Sertoli cells in vitro). The aim of this study was to measure both B-FSH and I-FSH concentrations in male and female sheep during the first year of life, and to relate this to pubertal development. The hypothesis being tested was that B-FSH is present in both male and female sheep during the prepubertal period and that discrete changes in B-FSH are associated with the onset of puberty. Eight ewe lambs and eight ram lambs were blood sampled fortnightly from 2 to 52 weeks of age. All samples were assayed for B-FSH and I-FSH content. Pubertal development was monitored in ewe lambs from behavioural oestrus and from plasma progesterone concentrations, and in ram lambs from penile and testicular development and from plasma testosterone concentrations. Mean I-FSH concentrations varied significantly with time after birth, in both females and males (P<0.01). In contrast, B-FSH was found to vary with time in females only (P<0.01). Around the expected time of puberty in ram lambs (i.e. at 30–40 weeks of age), and thereafter, I-FSH concentrations were undetectable (<0.2 ng ml⁻¹), whereas the B-FSH concentrations were measurable at concentrations up to twice the assay detection limit (0.8 ng ml⁻¹) until 38 weeks of age. In ewe lambs, but not ram lambs, there was a significant linear relationship between B-FSH and I-FSH values (R=0.595; P<0.005). When standardised about the time of puberty, B-FSH (P<0.05) but not I-FSH was significantly higher in ewe lambs that failed to reach puberty. No differences for either B-FSH or I-FSH between pubertal and non-pubertal ram lambs were noted. In summary, B-FSH was often measurable in plasma throughout prepubertal development in sheep and the concentrations often differed from those of I-FSH, especially in ram lambs. However, there appeared to be no discrete changes in B-FSH that could be directly related to specific pubertal events. It is concluded that although FSH may be a prerequisite for prepubertal testicular development and/or ovarian follicular growth, it is not a critical factor in determining whether puberty is attained during the first year of life in this seasonally breeding species.     

Selection for superior growth advances the onset of puberty and increases reproductive performance in ewe lambs

The reproductive efficiency of the entire sheep flock could be improved if ewe lambs go through puberty early and produce their first lamb at 1 year of age. The onset of puberty is linked to the attainment of critical body mass, and therefore we tested whether it would be influenced by genetic selection for growth rate or for rate of accumulation of muscle or fat. We studied 136 Merino ewe lambs with phenotypic values for depth of eye muscle (EMD) and fat (FAT) and Australian Sheep Breeding Values at post-weaning age (200 days) for live weight (PWT), eye muscle depth (PEMD) and fat depth (PFAT). First oestrus was detected with testosterone-treated wethers and then entire rams as the ewes progressed from 6 to 10 months of age. Blood concentrations of leptin and IGF-I were measured to test whether they were related to production traits and reproductive performance (puberty, fertility and reproductive rate). In total, 97% of the lambs reached first oestrus at average weight 39.4 ± 0.4 kg (mean ± s.e.m.) and age 219 days (range 163 to 301). Age at first oestrus decreased with increases in values for PWT (P < 0.001), and concentrations of IGF-I (P < 0.05) and leptin (P < 0.01). The proportion of ewe lambs that achieved puberty was positively related with increases in values for EMD (P < 0.01), FAT (P < 0.05) or PWT (P < 0.01), and 75% of the ewe lambs were pregnant at average weight 44.7 ± 0.5 kg and age 263 days (range 219 to 307). Ewe lambs that were heavier at the start of mating were more fertile (P < 0.001) and had a higher reproductive rate (P < 0.001). Fertility and reproductive rate were positively correlated with increases in values for EMD (P < 0.01), FAT (P < 0.05), PWT (P < 0.01) and leptin concentration (P < 0.01). Fertility, but not reproductive rate, increased as values for PFAT increased (P < 0.05). Leptin concentration increased with increases in values for EMD (P < 0.001), FAT (P < 0.001), PWT (P < 0.001), PEMD (P < 0.05) and PFAT (P < 0.05). Many of these relationships became non-significant when PWT or live weight was added to the statistical model. We conclude that selection for genetic potential for growth can accelerate the onset of puberty and increase fertility and reproductive rate of Merino ewe lambs. The metabolic hormones, IGF-I and leptin, might act as a physiological link between the growing tissues and the reproductive axis.     

A note on the effect of vasectomised rams and short-term exposures to entire rams prior to the breeding period on the reproductive performance of ewe lambs

The aim of this study was to determine the effects of either a 17-day exposure to vasectomised rams or a short-term exposure to entire rams on ewe lamb reproductive performance. Seven hundred and twenty-one Romney ewe lambs, 7–9 months of age, were randomly assigned to one of four treatment groups. Treatments included exposure to vasectomised rams for 17 days prior to breeding, exposure to entire rams for 4 or 2 days prior to breeding or not being exposed to either vasectomised or entire rams. Unteased ewe lambs and those exposed to entire rams for either 2 and 4 days were less (P < 0.05) likely to be mated in the first 17 days of breeding only and pregnant in the first 17 days of breeding compared to ewe lambs exposed to vasectomised rams for a full 17 days pre-breeding. Ewe lambs exposed to entire rams for 2 days were more (P < 0.05) likely to be mated in the first 8 days of breeding, mated in the first 17 days of breeding only and pregnant in the first 17 days of breeding compared to unteased ewe lambs. Therefore, a short-term exposure to entire rams can be used to induce breeding activity in ewe lambs, although it is not as effective as a full 17-day exposure to vasectomised rams.     

Effect of post-mating GnRH analogue (buserelin) treatment on PGF2alpha release in ewes and ewe lambs

The objectives of this study were to determine the effects of buserelin or saline treatment on ovarian function (Experiment 1), plasma PGFM concentrations and oxytocin stimulated prostaglandin F(2alpha) (PGF(2alpha)) release (Experiment 2) in ewe lambs and ewes. Welsh Halfbred ewes (n=26) and ewe lambs (n=24) were mated to vasectomised rams at synchronised oestrus and on Day 12 post-mating each animal was injected intramuscularly either normal saline or 4 microg buserelin. In Experiment 1, plasma progesterone and oestradiol concentrations were determined in samples collected by jugular venepuncture 1h before and at 0, 2, 4, 6, 8, 24, 48 and 72 h after treatment (n=7 per treatment group). Progesterone concentrations increased (P<0.05) from 2 to 8h after buserelin treatment and returned to basal levels after 72 h, whereas oestradiol concentrations were maximal at 2h post-treatment and returned to basal levels after 24h (P<0.05). Oestradiol concentrations were lower (P<0.05) in buserelin-treated animals than controls at 72 h post-treatment. Basal and post-treatment progesterone concentrations were greater (P<0.05) in ewes than in ewe lambs but oestradiol levels were similar for both age groups. Ovulation rate, determined by laparoscopy on Day 14, was similar for both age groups (ewes 1.1; ewe lambs 1.0). Buserelin treatment induced accessory corpora lutea in ewes (4/7; 57%) but not in ewe lambs (0/7; 0%). In the Experiment 2, plasma PGFM concentrations were determined in samples collected at 20-min intervals for 6h on Day 14 and at 20-min intervals for 1h before and at 10-min intervals for 1h and then at 20-min intervals for a further 3h period after an intravenous injection of oxytocin (1IU/kg body weight) on Day 15 post-oestrus. In this experiment there were five ewe lambs and six ewes per treatment group. There was no effect of buserelin treatment or age on basal PGFM concentrations on either Day 14 or 15. Although peak PGFM concentrations tended to be lower in buserelin-treated animals, the difference was not significant (P>0.05). However, peak duration following oxytocin challenge on Day 15 post-mating was shorter (P<0.05) in control ewes compared with control ewe lambs. In conclusion, buserelin treatment given on Day 12 post-oestrus enhances luteal function more in ewes than ewe lambs and after a transitory increase, reduces oestradiol concentrations in both ewes and ewe lambs. However, buserelin treatment does not significantly attenuate the luteolytic signal.     

A comparison between intravaginal progestagen and melatonin implant treatments on the reproductive efficiency of ewes

A total of 548 Manchega ewes were used to compare progestagen impregnated intravaginal sponges (12 days) plus eCG with melatonin implant (100 days) treatments, as possible stimulants of reproductive performance during the seasonal anoestrus and the lactation period. The use of both methods of oestrous manipulation induced a significant improvement in conception rate (78% for progestagen and eCG; 78% for melatonin versus 65% for the control) and fecundity (1.50 for progestagen and eCG; 1.55 for melatonin versus 1.26 for the control), when compared to the control group. Likewise, the lambing rate was also increased (1.17 for progestagen and eCG; 1.21 for melatonin versus 0.82 for control). A total of 35 and 39 extra lambs/100 ewes mated were produced, with an increase in productivity (percentage of extra lambs) of 43% and 48%, for ewes treated with progestagen sponges and melatonin, respectively, compared to the control group. The interval between ram introduction and parturition was significantly decreased, with matings in February–April (early seasonal anoestrus), compared to matings in March–May (seasonal anoestrus) in the group treated with progestagen sponges. An increase in the interval between parturition and the following mating was also recorded. As the body condition improved, the number of lambing ewes and fecundity were significantly improved in all groups. All the fertility results obtained with the progestagen sponges and melatonin treatments were similar. It could be concluded that progestagen sponges plus eCG and melatonin implants improved the reproductive performance of ewes, in seasonal anoestrus and during the lactation period, to a similar extent.     

Fertility in South Australian commercial Merino flocks: sources of reproductive wastage

To identify reasons for low fertility in the Merino in South Australia, we defined the extent and sources of reproductive wastage in flocks of maiden (n=14) and mature age (n=54) Merino ewes managed on 43 properties over 4 year. In a second study, reproductive wastage was examined in a very high ovulating flock of mature age South Australian Merino ewes heterozygous for the FecB gene, mated to lamb in either the autumn or spring. Losses in the latter flock were examined in less detail than the large-scale study, but allowed wastage to be partitioned between pre- and peri/post-natal sources. In the first experiment, reproductive wastage between mating and weaning was estimated at 59.7 potential lambs or ova lost per 100 ewes exposed to rams, representing 42.4% of ova shed. The main source of wastage occurred at lambing (55.6% of total loss), with death of twins a major contributor (35.3% of total loss). Other important sources of wastage were from partial failure of multiple ovulation (PFMO; 20.4% of total loss) and from ewes mating but not lambing (13.2% of total loss). Reproductive wastage did not vary with either age of ewe (maiden versus mature) or season of mating (October-December versus January-March). Mean ovulation rate, estimated as 141 per 100 ewes ovulating (range 100-200), varied with age of ewe (129 versus 144; maiden versus mature) but not with season of mating or year (1990-1991). Values for fertility, fecundity and lamb survival were 90, 127 and 73%, respectively. Fecundity of maiden ewes was lower than that of mature ewes (116 versus 130). Survival of single and twin lambs between birth and tail docking was 83 and 56%, respectively. Net reproductive efficiency, or lambs weaned per 100 ewes exposed to rams, averaged 81 (range 31-122). In the second experiment, mean ovulation rate of FecB Merino ewes was 316 per 100 ewes ovulating, with 242 ova (potential lambs) lost between ovulation and tail docking. This loss was equally shared between pre- and peri/post-natal sources (123 versus 119), and demonstrates severe limitation of the Merino to successfully bear and rear multiple litters. We concluded that future research in commercial Merino flocks be focused on lamb mortality, particularly of twins, and on PFMO, the major source of embryo loss.     

Effects of melatonin treatment on the onset of ovarian activity,reproductive parameters and PRL plasma levels of immature ewes

The aims of this study were to investigate the effect of chronic treatment with melatonin on prolactin plasma profiles, the onset of ovarian activity and the fertility of immature ewes. Beginning in late June, 30 maiden ewes were administered 2.5 mg melatonin (i.m.) daily until mid-September. Progesterone and prolactin plasma concentrations were determined by validated radioimmunoassays on samples collected every 5 days from 17 June until 31 October. Prolactin (PRL) plasma concentrations in the control animals were highest at the beginning of the experiment and lowest towards its conclusion. In melatonin-treated ewes, PRL levels dropped just after treatment began and were similar to those observed in the control animals at the end of the experimental period. The ovarian activity was advanced by approximately one month by the administration of melatonin, while the mean date of lambings was advanced by about two weeks compared with the controls. Fertility in the treated animals was very similar to that of the controls; the prolificacy (lambs/pregnant ewe) was 2 in the treated and 1.62 in the control ewes.     

Effects of melengestrol acetate and P.G. 600 on fertility in Rambouillet ewes outside the natural breeding season

The effects of melengestrol acetate (MGA) and P.G. 600 on ewe fertility outside the natural breeding season were evaluated. Rambouillet ewes were assigned to one of four groups: (1) control (C; n=92); (2) PG600 (n=86); (3) MGA (n=99); and (4) MGA+PG600 (n=92). A pellet with or without MGA (0.3mg/ewe/d) was fed at 0.15kg/ewe/d for 7d. On the last day of pellet feeding, ewes were given either saline or 5mL of P.G. 600 i.m. (400IU equine chorionic gonadotropin (eCG) and 200IU human chorionic gonadotropin (hCG)). Ultrasonography was performed between Days 20 and 25 of gestation for ewes that were mated during the first 6 d of the breeding period from the MGA (n=15) and MGA+PG600 (n=8) groups, and the number of luteal structures and embryos were counted. During the first 6d of the breeding period, MGA increased (P<0.05) the percentage of ewes that mated and conceived when compared to C and PG600 (24.2% vs. 3.3% and 10.5%, respectively). Relative to MGA, the mean (+/-S.E.M.) number of luteal structures per ewe was enhanced (P<0.03) in MGA+PG600 (1.53+/-0.13 vs. 2.38+/-0.42, respectively), however as pregnancy progressed, the number of embryos (1.5+/-0.13 vs. 1.8+/-0.16, respectively) and lambs born (1.3+/-0.15 vs. 1.5+/-0.27, respectively) did not differ. Treatment with MGA reduced (P<0.01) the interval from ram introduction to lambing relative to groups that did not receive MGA (168+/-0.8d vs. 171+/-0.6d, respectively). In conclusion, treatment with MGA increased the percentage of ewes conceiving early in the breeding period. Although P.G. 600 increased the number of luteal structures present per ewe, it did not significantly enhance ewe prolificacy.     

Plasma hormone profliles and fertility in ewe lambs given progestagen supplementation after mating

Clun Forest ewe lambs (n = 124) were used to investigate the effects of post-mating progestagen supplementation on fertility. The animals were assigned to 1 of 3 three treatments: Group A (n = 41) served as the controls, Group B (n = 42) received 3 weekly injections of 6 mg of medroxyprogesterone acetate (MAP), while Group C (n = 41) was treated with intravaginal sponge containing 60 mg of MAP; all treatments were administered from Day 5 to Day 26 post mating. Supplementation did not increase the percentage of animals pregnant or those lambing: Group A, 72.2 and 66.6%; Group B, 57.5 and 50.0%; and Group C, 67.5 and 60.0%, respectively. Furthermore, there was no effect of supplementation on plasma progesterone, prolactin, cortisol, growth hormone, insulin, or glucose concentrations (P>0.05). However, pre- and post- mating hormone profiles differed significantly between the animals that lambed or aborted and the animals which were found to be barren at lambing. In the barren animals, progesterone concentrations were lower 4 days before and 9 to 33 days after mating (P<0.01), while overall prolactin concentrations were higher throughout the trial (P<0.01). But there was no difference between barren and fertile lambs in cortisol, growth hormone, insulin or glucose concentrations (P>0.05). These results indicate that progestagen supplementation does not increase the reproductive performance of ewe lambs. However, infertility is associated with reduced luteal function and increased prolactin concentration before and after mating.     

Fertility in South Australian commercial Merino flocks: aspects of management

Wide variation in reproductive performance of commercial Merino flocks in south central Australia is the result of genetic and environmental influences that are both amenable to change through decisions of management. Relationships of reproductive traits (estrus, ovulation, fertility, fecundity, lamb survival, and lambs weaned) with variables that graziers can change or modify (strain of Merino, day or month of exposure of ewes to rams, ram effect or teasing, length of the mating period, ram percentage, days between weaning and next mating, stocking density and flock size at lambing, ewe liveweight, and condition) are reported in this paper, the third in a series. Small differences were observed between medium and strong-wool South Australian Merino strains for reproductive traits. Choosing the time of year that ewes are exposed to rams, between late spring to autumn, may result in reduced ovulation rate during early summer (December) giving a potentially smaller net reproductive efficiency (lambs weaned). The ram effect or teasing, used by about 50% of graziers to synchronise lambing, could be effectively employed to the end of January. The technique was not reproductively advantageous when compared with flocks that were not teased. The percentage of rams mated to ewes varied widely (approximately 1-3%) and did not alter flock fertility, suggesting that a substantial proportion of graziers could safely reduce the number of rams purchased. A positive relationship between incidence of estrus during the first 14 d of the cycle and the number of days from weaning to next mating and a negative relationship of returns to service with the same variable indicates that managers should consider increasing the time allowed for recovery of liveweight and body condition by adjusting age at weaning, length of the mating period, or both. Lamb survival was curvilinearly related to flock size and not stocking intensity, with the optimum size at about 400 ewes. The number of lambs weaned per 100 ewes exposed to rams increased by 1.0 kg(-1) increase in liveweight at mating. We concluded that the major factor controlling net reproductive efficiency is nutritional in origin through its effects on ewe liveweight and condition, and is a factor that can be largely manipulated through management.     

Effects of sire breed on ewe dystocia, lamb survival and weaned lamb output in hill sheep systems

A study was carried out on six hill farms, located in the main hill regions of Northern Ireland, over three breeding seasons to investigate the effect of sire breed on ewe dystocia, lamb survival and weaned lamb output. On each farm, groups of 26 to 40 purebred Scottish Blackface ewes (n = 3174) were crossed with Blackface, Swaledale, North Country Cheviot, Lleyn and Texel sires by natural service (year 1) and artificial insemination (years 2 and 3). Each of the mating groups was initially balanced for ewe age, live weight and condition score, and a total of 15 sires of each breed were used over the 3 years. In total, 2272 ewes were recorded at lambing, producing 3451 lambs, over the 3 years. Sire breed had a significant effect (P < 0.001) on lamb birth weight with Blackface and Swaledale the lowest and Texel the highest. The proportion of ewes with dystocia was higher with Texel sires compared with Lleyn, Swaledale and Blackface sires (P < 0.05). Ewes mated to Cheviot sires had more dystocia than those mated to Blackface and Swaledale sires (P < 0.05), while ewes mated to Lleyn sire breed had more dystocia (P < 0.05) than those mated to Blackface sire breed. Most of the incidence of dystocia was attributable to increased lamb birth weight (P < 0.001), and some to litter size (P < 0.05). Swaledale, Cheviot, Lleyn and Texel sire breeds increased the incidence of malpresentations (P < 0.05). Lamb live weight at weaning was higher with Texel- and Cheviot- and Lleyn-sired lambs compared with Blackface- and Swaledale-sired lambs (P < 0.01). The level of lamb mortality at birth was not affected by sire breed. However, lamb mortality at weaning was lower for Lleyn-sired lambs compared with the other lamb genotypes (P < 0.05). These results indicate that the use of sires from larger breeds, such as Lleyn and Texel, within the Blackface ewe flocks can increase lamb output, with no apparent detrimental effects on lamb mortality or ewe survival, but increased assistance at lambing will be required.     

Ovulation response to pregnant mares' serum gonadotrophin in prepubertal ewe lambs of different Booroola genotypes

Ovulation rates following treatment with PMSG were measured in prepubertal ewe lambs of known Booroola genotype to evaluate the technique as a method of early identification of Booroola genotypes. Two experiments were conducted with homozygous (FF), heterozygous (F+) and non-carrier (++) interbred $\text{1}\text{2}$ Merino $\text{1}\text{2}$ Romney breed type ewe lambs and a third experiment compared first cross F+ $\text{1}\text{2}$ Merino $\text{1}\text{2}$ Coopworth breed type with contemporary Coopworths (++). The ewe lambs aged 5–6 months were injected with 400 or 600 i.u. PMSG. The percentage of ewe lambs ovulating following treatment with PMSG was similar in all experiments (75–77%) but in Experiment 1 the FF and F+ lambs had a significantly higher proportion ovulating than the ++ lambs (P<0.05). The mean ovulation rate of FF ewe lambs treated with PMSG was 1.03 (Expt 1) and 1.69 (Expt 2) higher than ++ lambs (P<0.05). In Experiment 1 the mean ovulation rate of F+ ewe lambs was 0.33 higher than ++ ewe lambs but this difference was not significant. However, in Experiments 2 and 3 the mean ovulation rate of F+ ewe lambs treated with 600 i.u. PMSG was 1.06 and 0.45 higher than ++ ewe lambs respectively (P< 0.05). These results show that PMSG treatment is a promising technique for identifying the Booroola genotype of ewe lambs at an early age but further experiments with different dose rates of PMSG and different ages and liveweights of lambs are required to determine optimum dose rates and time of treatment.     

Disappearance of opioidergic tone on LH secretion in underfed prepubertal sheep

A study was conducted to determine whether an opioid tonus inhibitory of LH secretion is present in underfed prepubertal sheep. Ten Suffolk ewe lambs were subjected to food restriction during 60 days. During this period they were allowed to pasture only 2 hours per day while control ewe lambs were allowed for 10 hours. Body weight and plasma blood levels of glucose, urea and total proteins were measured weekly. At the end of this period, an intravenous injection of Naloxone (NAL, 1.5 mg/kg BW) was given to control and underfed animals followed 60 min later by an intravenous injection of LHRH to test the pituitary responsiveness. Underfed animals did not show an increase in plasma LH while control animals presented a rise from 0.28 +/- 0.08 to 2.02 +/- 0.6 ng/ml after the NAL stimulus (P less than 0.05). The response to LHRH was similar in both group of animals. Basal plasma levels of insulin were lower in underfed ewe lambs than in control animals (P less than 0.05). Underfed animals were placed on plain feeding with a schedule similar to control lambs for 30 days and the same experiment was repeated. During this occasion, NAL increased plasma LH concentration in both group of lambs. Levels of plasma insulin were not different in both groups. The lack of effect of NAL on LH secretion in food restricted ewe lambs suggests that the opioid modulation of LH secretion is absent by underfeeding in female prepubertal sheep.(ABSTRACT TRUNCATED AT 250 WORDS)     

Oocyte pick-up in juvenile lambs affects neither onset of puberty nor their future fertility

A study was designed to evaluate whether the time of onset of puberty and fertility of young ewe lambs would be affected by oocyte pick-up conducted in single or repeated sessions during the first months of lambs' live. Five groups of lambs from the Karagouniko breed were used (A-E each n=12). In group A no treatments were applied (control group) while, laparoscopical follicular aspiration (OPU) was performed early in the third, fourth and fifth month of lambs age (groups C-E, respectively). From the second to fifth month of their age, group B lambs were aspirated four times in monthly intervals. All lambs were weighed at birth, weaning, at second month and monthly thereafter until the eighth month of age. Progesterone priming and ovarian stimulation by serial FSH administrations proceeded each OPU session. To determine onset of puberty blood progesterone concentration was assayed in samples collected initially every week and after the seventh month of age twice weekly. From the seventh month a fertile ram was introduced in each group and oestrous behavior/mating was daily monitored and recorded. Pregnancy diagnosis was carried out by transabdominal ultrasound scanning 55 days after rams' removal. At the fourth and fifth month of age group B lambs were lighter (p<0.05) than controls, but this difference was later equalized. The time of onset of puberty did not differ between groups (p=0.069) and ranged between 224 and 270 days. Some animals (n=15) entered puberty with a full-length luteal phase having progesterone concentration greater than 1ng/ml, while others (n=32) exhibited one or two short luteal phases before luteal length restoration. During the first breeding season 41 animals were fertilized and maintained pregnancy to term, without noticeable differences between groups (p=0.555). During the second breeding season, all ewes were naturally served and lambed at the expected time. It is concluded that OPU in young dairy lambs does not affect the time of onset of puberty, the endocrine profile of the lambs and it does not compromise their future fertility even if it is applied at four successive months.