Growth, endocrine profiles and reproductive responses of ewe lambs after medium and long-term treatment with ovine growth hormone

Two experiments were conducted to examine effects of exogenous ovine growth hormone (oGH) on growth and reproductive traits of ewe lambs. In the first trial, 30 Debouillet ewe lambs (4 months old) received either 0 or 2.5 mg, s.c. of oGH (Day 0 = first day of 98-day treatment). Ovarian cyclicity was determined by monitoring serum progesterone. Efficiency of gain (first 50 days of treatment) was more (P < 0.10) desirable in oGH-treated animals, but did not differ (P > 0.20) between groups during the last 48 days of treatment. Serum GH in alternate-day samples was elevated five-fold (P < 0.01). First estrus occurred 10 days earlier (P = 0.14) in oGH-treated ewe lambs. In a second trial, 45 ewe lambs were evenly divided into three groups receiving 0 mg of oGH (CON; 50 injections), 2.5 mg of oGH (GH98, 50 injections) or 25 injections containing 2.5 mg of oGH followed by 25 injections of 0 mg of oGH (GH48) on alternate days for 98 days before a breeding season. Ewe lambs receiving GH48 had higher (P < 0.05) gains the first 24 days than those receiving CON or GH98. Ewes receiving GH48 demonstrated first estrus (P = 0.06) 22 days before control ewes and 28 days before GH98 ewes. Other reproductive traits did not differ (P > 0.25). Serum GH was greatly elevated by injections of exogenous oGH, but neither serum insulin nor prolactin was affected. Short-term elevation of serum GH resulting from exogenous oGH injections marginally enhances reproductive and growth responses, but does not induce major changes in these traits in ewe lambs after 4 months of treatment.     

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.     

Induction of precocious puberty in ewe lambs by pulsatile administration of GnRH

Circhoral administration (250 ng/h, i.v.) of GnRH induced a preovulatory-like surge of LH and subsequent luteal function in 4 of 4 ewe lambs 1 month before expected date of puberty. Within 12h of the start of pulsatile delivery of GnRH, mean concentrations of immunoactive and bioactive LH increased significantly (P less than 0.05) and the LH surge occurred by 1.8 +/- 0.6 days of treatment. Mean concentrations of serum progesterone were elevated significantly (P less than 0.001) 3 days after the surge. The biopotency of LH (bioactive LH/immunoactive LH) before the GnRH-induced surge of LH did not differ from LH biopotency in ewe lambs receiving circhoral delivery of saline (0.41 +/- 0.05 and 0.46 +/- 0.04, respectively). Biopotency of LH declined markedly at the GnRH-induced LH surge (0.25 +/- 0.04), but biopotency of serum LH was significantly augmented (P less than 0.05) during the period of luteal activity (0.70 +/- 0.07). Regular oestrous cycles were observed in 3 of 4 ewe lambs after the 10-day GnRH treatment period. These results indicate that pulsatile delivery of GnRH is effective in inducing precocious puberty in ewe lambs. Increase in LH biopotency does not appear to be required in the pubertal transition to reproductive cyclicity in this species. Augmented LH biopotency may be important in support of luteal function after first ovulation.     

Study of LH response to GnRH in the young male as a criterion of genetic merit for female reproduction in sheep

A high and a low response line in sheep were selected on the basis of the mean concentration of LH in 10-week-old Finn-Dorset ram lambs after an i.v. injection of 5 micrograms GnRH. After 8 male generations the mean LH response of the high line was more than 5-fold that of the low line and the heritability of the selected trait was estimated at 0.44 +/- 0.015. Highly significant line differences in mean LH response to GnRH were also found in males at 20 weeks of age and females at 10 and 20 weeks of age and the genetic correlations between the four LH response traits appear to be close to unity. Large line differences in the mean FSH response to GnRH were also found in both males and females at 10 and 20 weeks of age. Selection had little effect on the physical characteristics of lambs. High-response line ewes entering their first breeding season at about 7 months of age showed oestrus earlier in the season and had higher ovulation rates and numbers of lambs born per ewe lambing than did low-response line ewes. In the second breeding season, at about 19 months of age, the only line difference was a higher ovulation rate early in the breeding season in high-line ewes. It is suggested that these changes may be mediated by a more rapid response in high-line ewes to increased GnRH stimulation at puberty or at the beginning of the breeding season.     

Reproductive, hormonal and milk characteristics in ewe lambs treated with ovine growth hormone before breeding

One month before a fall breeding season, 30 6-mo-old fine-wool ewe lambs were allotted to one of three treatment groups consisting of either 0, 2.5 or 5 mg ovine growth hormone (oGH) daily for 10 d and then, on alternate days, for an additional 20 d. Five ewes from each treatment group were bled at hourly intervals for 8 h on Days 0 (first day of treatment), 9, 19 and 29. Milk yield and composition were examined on Day 21 post partum. Neither feed intake nor ewe weights differed (P > 0.20) among treatments. Serum insulin did not differ (P > 0.20) among treatments before or during the 5 h following treatment on Day 0 or 29; however, 6, 7 and 8 h after oGH administration, a linear, dose-dependent increase (P < 0.10) in serum insulin was noted. On Day 9, serum insulin differed linearly (P < 0.10) before and after treatment. On Day 19, serum insulin differed (P < 0.10) among the groups 2, 3, 6, 7 and 8 h following oGH treatment. One hour after treatment on Day 0, serum GH increased linearly (P < 0.01) among groups. Serum GH continued to rise 3 h after treatment and remained elevated through Hour 8 in ewes receiving oGH. Serum GH was increased (P < 0.01) before and after oGH administration on Days 9, 19 and 29 in GH-treated ewes. Percentage of ewes that cycled during treatment or breeding as determined by serum progesterone was similar (P > 0.20) among groups. Pregnancy rates did not differ (P > 0.20) in ewe lambs receiving 0 (70%), 2.5 (80%) and 5 (60%) mg oGH. No differences were detected (P > 0.40) in milk yield or composition among treatments. In fine-wool ewe lambs treated with 2.5 or 5 mg of exogenous oGH for 30 d before breeding, neither reproductive performance nor subsequent milk production were enhanced; however, these treatments increased serum GH and insulin.     

Exogenous growth hormone improves the number of transferable embryos in superovulated ewes.

The application of pGH (porcine Growth Hormone) to superovulated ewes was studied with the aim of improving the embryo yield. Thirty-seven ewes were superovulated with pFSH for 3 d and 18 of them were cotreated the third day with 0.50 mg of pGH. Embryos were surgically recovered on Day 7 after sponge withdrawal. Then, 102 morphologically healthy embryos were immediately transferred in pairs to 51 synchronized recipient ewes. The GH treatment did not significantly affect the percentage of ewes in estrus, the time of estrus onset or the ovulation rate. However, it improved synchronization by grouping estrus in a narrower range (12 h) in comparison to the control group (24 h); (16 to 28 h after sponge withdrawal vs 12 to 36 h; P < 0.05). The total amount of LH released during the preovulatory surge was lower in the GH than in the control group (P < 0.05). No differences were found between groups for other LH-related parameters such as basal levels, peak values or peak time from sponge removal. The proportions of unfertilized oocytes and degenerate embryos recovered were lower in the GH cotreated group (P < 0.05 and P < 0.01, respectively). This resulted in higher rates of transferable embryos and lambs born per donor ewe in the GH than in the untreated group (3.9 vs 1.7 and 2.28 vs 0.84, respectively; both, P < 0.05). These beneficial effects of GH would likely be due either to a direct action on oocyte maturation or to an indirect action on the oviductal environment.     

Serum thyroid hormones and reproductive characteristics of Rambouillet ewe lambs treated with propylthiouracil before puberty

Twenty-four Rambouillet ewe lambs (average weight=43.7+/-1.2 kg, approximately 6 months of age) were used to examine the effect of thyroid suppression before the onset of puberty on serum thyroid hormones, body weights (BW), and reproductive performance. Beginning in early September, ewe lambs were randomly assigned to three treatments (n=8 lambs/treatment). All animals remained in a single pen (4 x 12 m) with access to salt, water, shade and alfalfa hay (2.5 kg per animal per day) throughout the experiment. Beginning on Day 0 (first day of treatment), all ewe lambs received daily treatments (gavage) for 15 days consisting of 0, 20, or 40 mg 6-N-propyl-2-thiouracil(PTU)/kg BW per day. Beginning on Day 15, the 20 and 40 mg treatments were lowered to 10 and 20 mg PTU/kg BW, respectively. All animals were treated for 28 days. Ovarian cyclicity was determined by twice weekly progesterone (P(4)) analysis. Thyroxine (T(4)) concentrations were similar on Day 0 (61.6, 54.8 and 56.9+/-2.5 ng/ml, P=0.17) in ewe lambs receiving 0, 20 and 40 mg PTU/kg BW, respectively. By Day 7, both PTU-treated groups had T(4) values less than 20 ng/ml (9.0 and 15.4+/-2.5 ng/ml) compared with 78.5 ng/ml in controls (P<0.01). By 7 days after termination of PTU treatment, serum T(4) had risen to 29.1 and 26.9 (+/-2.9)ng/ml in the 20/10 and 40/20 PTU groups, respectively. On Day 66, control ewes had 55.0 ng T(4)/ml compared with 43.1 and 39.0 (+/-2.6 ng/ml) for ewes in the 20/10 and 40/20 groups, respectively (linear, P<0.01). Serum triiodothyronine (T(3)) followed a similar pattern to that observed for T(4). Ewe lamb BW were similar (P>0.50) among groups throughout the treatment period. However, following the treatment, PTU-treated ewes tended (P<0.10) to weigh less than controls. Average Julian day of puberty was also similar (P>0.50) among treatments (286, 288 and 288+/-5 days; control, 20/10 and 40/20, respectively). Control ewes had a pregnancy rate of 75%, while both PTU-treated groups had pregnancy rates of 88% (P>0.20). The administration of PTU resulted in a rapid decline in serum T(4) and T(3) but neither time of puberty nor pregnancy rates were affected by lowered thyroid hormones.     

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

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

Pituitary response to LHRH, LH pulsatility and plasma melatonin and prolactin changes in ewe lambs treated with melatonin implants to delay puberty

Prepubertal ewe lambs were treated with empty or filled melatonin implants. The implants were placed s.c. at birth and pituitary responsiveness to various doses of LHRH, LH/FSH pulsatility and prolactin and melatonin secretion were examined at 10, 19, 28, 36 and 45 weeks of age. Control animals (N = 10) showed no consistent alteration in pituitary responsiveness to LHRH during development. Ewes treated with melatonin (N = 10) had puberty onset delayed by 4 weeks (P less than 0.03) but no effect of melatonin on LH or FSH response to LHRH injection was observed at any stage of development. In the control and melatonin-treated ewe lambs the responses to LHRH injection were lower during darkness than during the day at all stages of development. No consistent differences in LH or FSH pulsatility were observed between treatment groups or during development. Prolactin concentrations, however, failed to decrease at the time of puberty (autumn) in the melatonin-treated group. Melatonin-treated ewe lambs maintained normal rhythmic melatonin production which was superimposed on a higher basal concentration and showed the same increase in melatonin output with age as the control ewes. These results indicate that the delayed puberty caused by melatonin implants is not due to decreased pituitary responsiveness to LHRH or to dramatic changes in basal LH or FSH secretion.     

Plasma FSH and LH in prepubertal Booroola ewe lambs

Basal plasma concentrations (four 30-min samples) and GnRH-induced release of gonadotrophins were measured every 15 days between 30 and 90 days and at 110 days of age in Merino ewe lambs from the prolific Booroola ('B') flock (n = 18-23), the medium prolificacy ('T') flock (n = 14-20), and the 'O' flock (n = 4-8) of low prolificacy. At ages of 30 and 45 days B ewe lambs had mean basal plasma FSH concentrations of 145 and 122 ng/ml which were significantly higher (P less than 0.01) than those seen in T (45 and 53 ng/ml), and O (39 and 38 ng/ml) flock ewes. Between 60 and 110 days of age there were no significant differences between genotypes. The increment in FSH concentrations above basal levels induced by the subcutaneous injection of 100 micrograms synthetic GnRH was only significantly (P less than 0.05) greater in B than T and O genotype ewe lambs at 110 days of age but not at other ages. The basal plasma FSH differences between the B, T and O genotypes at 30 and 45 days of age were not consistently related to the size of litter in which lambs were born. At 30 days of age the mean plasma LH concentration of B, T, and O flock lambs were 2.6 +/- 0.5, 1.2 +/- 0.6 and 0.7 +/- 0.8 ng/ml respectively. These differences were not significant. At later ages there were also no significant differences between the genotypes with respect to basal LH, and the increase in LH induced by exogenous GnRH was always similar for the three genotypes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth hormone-releasing factor (GRF) induced growth hormone advances puberty in female buffaloes

Exogenous bovine growth hormone-releasing factor (bGRF) at the dose rate of 10 microg/100 kg body weight was administered intravenously (i.v.) to six Murrah buffalo heifers as treatment group, while another six buffalo heifers served as control group which received the vehicle (0.9% NaCl solution) at an interval of 15 days for a period of 9 months to study the effect of bGRF on puberty onset associated with temporal hormonal changes in peri-pubertal buffalo heifers. Blood samples were collected at 3-day interval from all the animals during the experimental period and plasma harvested was assayed for growth hormonal (GH), luteinizing hormone (LH) and progesterone. The day that plasma progesterone was greater than 1.0 ng/ml for three consecutive sampling days was defined as the day of puberty. Exogenous bGRF administration increased (P = 0.02) plasma GH concentration in treatment group over control group during the treatment of bGRF as well as during the peri-pubertal period. Plasma progesterone concentrations increased transiently earlier (P = 0.05) by 58.5 days in bGRF-treated buffaloes than that in the control group. However, plasma LH concentrations were unaffected by the treatment of bGRF (P = 0.48). Both plasma GH and LH in the buffalo heifers increased (P < 0.01) over time preceding puberty and the higher hormonal concentrations were maintained during the onset of puberty, and thereafter, the concentrations of both the hormones declined (P < 0.05) after puberty. GH and LH were positively correlated both before puberty (r = +0.59 and +0.63; P < 0.05 for control and treatment group, respectively) and after puberty (r = +0.42 and +0.46; P < 0.05 for control and treatment group, respectively) indicating the interaction and/or close relationship of GH and LH in the mechanism of puberty in buffalo species.     

Effects of rapid weight gain to puberty on reproduction, mammary development and lactation in ewe lambs

The effects of rapid weight gain to puberty on reproduction, mammary development and milk production in ewes lambing at 13 mo of age were investigated on three trials. A total of 64 Dorset and 93 Suffolk ewe lambs were weaned at 42 d of age and their mean weight was 16 kg. These ewes were assigned, within breed groups, to either a finishing diet or a growing diet. Onset of puberty was determined by daily checks for estrus and ewes were bred beginning at 7 mo of age. In Trial 2, mammary gland development was determined in eight Suffolk ewes from each diet. Ewes on the finishing diet were younger at puberty than those on the growing diet (199 vs 206 d, P<0.05) but required more services per conception (1.3 vs 1.1, P<0.05). Dietary conception rate and lambing rate means were similar. Mean 4-h milk yield was lower (P<0.10) for ewes on the finishing diet (283 g) than for those on the growing diet (310 g). Mammary gland fat pad area was higher (P<0.05) for ewes on the finishing diet compared with those fed for growth. Gross and adjusted duct areas were higher in ewes on the growing diet, but differences were not significant. At puberty, negative correlation coefficients for milk yield with performance traits were as follows: daily weight gain, -0.184 (P<0.08); weight-to-height ratio -0.262 (P<0.01); body condition score, -0.189 (P<0.07); and body weight, -0.212 (P<0.05). Results of this study indicate that rapid weight gain to puberty impairs mammary gland development and milk production in ewe lambs.     

Prenatal androgen exposure and growth and secretion of growth hormone and prolactin in ewes postweaning

Growth and secretion of growth hormone (GH) and prolactin (PRL) in ewe lambs exposed to androgen during fetal development were investigated. Testosterone cypionate was administered to the pregnant dams from approximately Days 28 to 84 of gestation. Ewe lambs from dams that received androgen exhibited masculinized external genitalia and some masculine behavioral characteristics. Intact androgenized ewe lambs grew faster (P less than 0.05) and were more efficient in conversion of food to body gain (P less than 0.05) than ewe lambs born to untreated dams over the period from 70 to 224 days of age. One-half of the ewe lambs in each group was ovariectomized at 58 days of age. Ovariectomy had no effect on subsequent growth or efficiency of growth in the control ewe lambs. However, ovariectomy of androgenized ewe lambs abolished the observed stimulated rate of growth and decreased the improvement in efficiency of food conversion. Blood samples were collected from the lambs at 85 and 136 days of age at 15-min intervals for 8 hr to determine parameters of GH and PRL secretion. Prenatal androgen exposure had no effect on any parameter of GH or PRL secretion. These data indicate that prenatal androgen exposure altered differentiation of growth potential in ewe lambs, but the growth response was not mediated through dramatic changes in secretion of adenohypophysial somatotropic hormones, GH and PRL.     

The effects of pasture availability for twin- and triplet-bearing ewes in mid and late pregnancy on ewe and lamb behaviour 12 to 24 h after birth

This experiment was conducted to evaluate the effect of offering ewes two different feeding levels, during mid and late pregnancy, on ewe and lamb behaviour 12 to 24 h after birth. Romney ewes, bearing twin (n = 80) or triplet foetuses (n = 56), were allocated to a pasture sward height of 2 or 4 cm between 70 and 107 days of pregnancy. In late pregnancy (day 107 to 147), half of the ewes were reallocated the alternate sward height, which produced four treatments: 2-2, 2-4, 4-2 and 4-4. Ewes were weighed on days 65, 92, 107 and 130 of pregnancy and lamb live weights were recorded 12 to 24 h after birth. Twelve to 24 h after birth the maternal behaviour score (MBS) of the ewes were determined, whilst their lambs were tagged. After the lambs were released, the behaviour of each ewe and her lambs was observed for 5 min. Ewe treatment and litter size had no effect on ewe MBS. However, as MBS increased (ewes stayed closer to lambs during tagging), ewes bleated less in a high-pitch and were quicker to make contact with their lamb. During the observation period, ewes in the 4-4 treatment had a greater percentage of their bleats in a low pitch (P < 0.05) than ewes in the 2-2 and 4-2 treatment (61.3% v. 41.3% and 38.8% low bleats, respectively) and more lambs born to 4-4 ewes (95%) bleated than lambs born to 2-2 ewes (84%; P < 0.05). However, lambs born to ewes in the 2-2 treatment bleated earlier than lambs in all other treatments (P < 0.05). Lambs born to 4-4 ewes were less likely (P < 0.05) to move towards their dam in order to make contact than lambs born to 2-2 or 4-2 ewes (3.1% v. 16.9% and 16.7%, respectively). These findings suggest that under the conditions of the present study, ewe nutrition had little effect on maternal behaviour. However, lambs born to ewes offered 2 cm pasture sward heights during mid and/or late pregnancy (2-2, 2-4 and 4-2 treatments) displayed behaviour that demonstrated greater 'need' whereas lambs born to ewes offered 4 cm during mid and late pregnancy sought less attention from their dam.     

Insulin-like growth factor-I feedback regulation of growth hormone and luteinizing hormone secretion in the pig: evidence for a pituitary site of action

Three experiments (EXP) were conducted to determine the role of insulin-like growth factor-I (IGF-I) in the control of growth hormone (GH) and LH secretion. In EXP I, prepuberal gilts, 65 ± 6 kg body weight and 140 days of age received intracerebroventricular (ICV) injections of saline (n = 4), 25 μg (n = 4) or 75 μg (n = 4) IGF-I and jugular blood samples were collected. In EXP II, anterior pituitary cells in culture collected from 150-day-old prepuberal gilts (n = 6) were challenged with 0.1, 10 or 1000 nM [Ala15]-h growth hormone-releasing hormone-(1-29)NH2 (GHRH), or 0.01, 0.1, 1, 10, 30 nM IGF-I individually or in combinations with 1000 nM GHRH. Secreted GH was measured at 4 and 24 h after treatment. In EXP III, anterior pituitary cells in culture collected from 150-day-old barrows (n = 5) were challenged with 10, 100 or 1000 nM gonadotropin-releasing hormone (GnRH) or 0.01, 0.1, 1, 10, 30 nM IGF-I individually or in combinations with 100 nM GnRH. Secreted LH was measured at 4 h after treatment. In EXP I, serum GH and LH concentrations were unaffected by ICV IGF-I treatment. In EXP II, relative to control all doses of GHRH increased (P < 0.01) GH secretion. Only 1, 10, 30 nM IGF-I enhanced (P < 0.02) basal GH secretion at 4 h, whereas by 24 h all doses except for 30 nM IGF-I suppressed (P < 0.02) basal GH secretion compared to control wells. All doses of IGF-I in combination with 1000 nM GHRH increased (P < 0.04) the GH response to GHRH compared to GHRH alone at 4 h, whereas by 24 h all doses of IGF-I suppressed (P < 0.04) the GH response to GHRH. In EXP III, all doses of IGF-I increased (P < 0.01) basal LH levels while the LH response to GnRH was unaffected by IGF-I (P > 0.1). In conclusion, under these experimental conditions the results suggest that the pituitary is the putative site for IGF-I modulation of GH and LH secretion. Further examination of the role of IGF-I on GH and LH secretion is needed to understand the inhibitory and stimulatory action of IGF-I on GH and LH secretion.     

Repeatability of LH responses by lambs to monthly challenge with synthetic gonadotrophin releasing hormone (GnRH)

The aim of this experiment with ram and ewe lambs was to test the hypothesis that there are consistent individual differences in Luteinising Hormone (LH) response to Gonadotrophin Releasing Hormone (GnRH).Pre-puberal Border-Leicester × Merino lambs (15 of each sex) aged 9 weeks were challenged with either 0, 30 or 60 μg synthetic GnRH each month for 7 months (December to June). The lambs were then rested from this monthly routine until they were challenged an eighth time in September at 48 weeks of age. Luteinising Hormone response (area under LH release curve) was measured each month and the repeatability of individual LH responses calculated.There was a significant interaction (P < 0.01) between treatment month and sex reflecting a fall in LH response by ram lambs after a peak in February, while at the same time responses by ewe lambs increased to peak again in May. LH response also increased with GnRH dose (30 vs. 60 μg; P < 0.05).Responses by individual lambs were ranked 1 to 5 each month within sex and GnRH doses (30 and 60 μg only). Highest ranked lambs had LH responses 1.4 to 7.0 times larger than lowest ranked lambs. Repeatability of rank between months was poor in all groups except ewe lambs given 60 μg GnRH, where three of the five lambs repeated a particular rank at 5 of the 8 sample months. However, the repeatability of response in this group was not considered to be sufficient to reject the null hypothesis. It was concluded that if consistent individual differences do exist they may be subtle and easily masked by factors such as GnRH dose, sex, age and season.     

Reduced growth hormone secretion prolongs puberty but does not delay the developmental increase in luteinizing hormone in the absence of gonadal negative feedback

Previous studies have shown that the growth hormone (GH) axis is important for timing the later stages of puberty in female monkeys. However, it is not clear whether these growth-related signals are important for the initiation of puberty and early pubertal events. The present study, using female rhesus monkeys, used two approaches to answer this question. Experiment 1 tested the hypothesis that reduced GH secretion would blunt the rise in nocturnal LH secretion in young (17 mo; n = 7) but not older adolescent ovariectomized females (29 mo; n = 6). Reduced GH secretion was induced by treating females with the sustained release somatostatin analogue formulation, Sandostatin LAR (625 microg/kg). Morning (0900-0930 h) and evening (2200-2230 h) concentrations of bioactive LH were higher in older adolescent compared to young adolescent females. However, diurnal concentrations were not affected by the inhibition of GH secretion in either age group when compared to the placebo-treated, control condition. Experiment 2 tested the hypothesis that reduced GH secretion induced in young juvenile females would delay the initial increase in nocturnal LH secretion and subsequent early signs of puberty. In order to examine this hypothesis, puberty in control females (n = 7) was compared to those in which puberty had been experimentally arrested until a late adolescent age (29 mo) by the use of a depot GnRH analogue, Lupron (750 microg kg(-1) mo(-1); n = 7). Once the analogue treatment was discontinued, the progression of puberty was compared to a group treated in a similar fashion but made GH deficient by continuous treatment with Sandostatin LAR (n = 6). Puberty occurred as expected in control females with the initial rise in evening LH at 21 mo, menarche at 22 mo, and first ovulation at 30 mo. As expected, Lupron arrested reproductive maturation, but elevations in morning and evening LH and menarche occurred within 2 mo of the cessation of Lupron in both Lupron and Lupron-GH-suppressed females. In contrast, first ovulation was delayed significantly in the Lupron-GH-suppressed females (41 mo) compared to the Lupron-only females (36 mo). These data indicate that within this experimental model, reduced GH secretion does not perturb the early stages of puberty but supports previous observations that the GH axis is important for timing the later stages of puberty and attainment of fertility. Taken together, the data indicate that factors that reduce GH secretion may have a deleterious effect on the completion of puberty.     

Sexual maturation of ewes raised without ram exposure in a controlled lighting environment

Finn x Dorset ewe lambs (n = 70) born in the spring (March 28 to April 6) from two successive lambing seasons were evaluated for age at first ovulation in the absence of mature rams. Ewe lambs were born in a controlled, short light (8L:16D) photoperiod or in ambient light (13L:11D). At about 10 to 11 wk of age, ewe lambs were allocated to a short (8L:16D) or long (16L:8D) light environment. Plasma progesterone (P(4)) concentrations were measured as an index of first ovulation. First exposure of ewes to sexually mature rams was in November. Most ewe lambs (77%) ovulated before ram exposure. More lambs (P < 0.025) born in ambient light and raised in short light reached puberty with typical cycles of plasma progesterone compared to other treatments. Long days tended to retard the onset of puberty. Although pregnancy rate did not differ across light treatments, more ewes became pregnant from the ambient-light born and short-light raised treatment. Photoperiod is an important factor affecting the onset of sexual maturation and genesis of normal luteal progesterone secretion in the ewe lamb.     

Dietary protein or energy restriction influences age and weight at puberty in ewe lambs

The working hypothesis was that restricted protein content in diets of prepubertal ewe lambs would delay time of onset of puberty to a greater extent than restricted energy content. Restriction of dietary protein (metabolizable protein, MP) or energy (ME) intake on age and weight at puberty was studied in ewe lambs in two experiments. In Experiment 1, 85 crossbred ewe lambs were assigned by weight to one of five dietary treatment groups: control (CTL; 18 mg ME day⁻¹ and 173 g MP day⁻¹); energy restricted (ER; 10.2 mg ME day⁻¹ and 96 g MP day⁻¹); protein restricted (PR; 11 mg ME day⁻¹ and 66 g MP day⁻¹); ER for 9 weeks then switched to CTL (ER/CTL); PR for 9 weeks then switched to CTL (PR/CTL). Ewe lambs were weighed biweekly, bled weekly, and serum was assayed for progesterone to determine time of puberty. Average age at puberty differed among lambs fed the various diets (P < 0.01). Ewe lambs in the CTL group were younger and weighed more at puberty. Lambs fed diets restricted in protein or energy content were similar in age at puberty. Dietary switching, ER/CTL and PR/CTL, affected body weight and gain (P < 0.05), but did not result in changed age at puberty compared with lambs fed ER or PR diets. In Experiment 2, two breed types (maternal type and meat type) were used to compare responses of dietary protein restriction to dietary energy restriction. In Experiment 2, lambs (n = 60) were assigned to either the CTL, PR or ER diets. Lambs in the CTL group gained more weight than lambs fed the PR diet and lambs fed the PR diet gained more weight than lambs fed the ER diet (P < 0.05). Diet affected age (P < 0.001), but not weight at puberty (P = 0.13). Dietary energy and protein restriction influenced age at puberty in ewe lambs with energy restriction having a greater influence on delaying onset of puberty than protein restriction. Realimentation did not overcome effects of previous feed restriction by inducing an earlier age at puberty than ewe lambs fed the ER or PR diet throughout the experimental period. Body weight gain prepuberty is more important than type of dietary restriction (protein or energy) with regard to impacting age at puberty. We, therefore, reject the working hypothesis that dietary protein restriction prepuberty would have a greater impact in delaying onset of puberty than dietary energy restriction.     

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.