Nutritional regulation of the genes encoding the acid-labile subunit and other components of the circulating insulin-like growth factor system in the sheep

Developmental Programming of Fertility in Cattle-Is It a Cause for Concern?

Cattle fertility remains sub-optimal despite recent improvements in genetic selection. The extent to which an individual heifer fulfils her genetic potential can be influenced by fetal programming during pregnancy. This paper reviews the evidence that a dam's age, milk yield, health, nutrition and environment during pregnancy may programme permanent structural and physiological modifications in the fetus. These can alter the morphology and body composition of the calf, postnatal growth rates, organ structure, metabolic function, endocrine function and immunity. Potentially important organs which can be affected include the ovaries, liver, pancreas, lungs, spleen and thymus. Insulin/glucose homeostasis, the somatotropic axis and the hypothalamo-pituitary-adrenal axis can all be permanently reprogrammed by the pre-natal environment. These changes may act directly at the level of the ovary to influence fertility, but most actions are indirect. For example, calf health, the timing of puberty, the age and body structure at first calving, and the ability to balance milk production with metabolic health and fertility after calving can all have an impact on reproductive potential. Definitive experiments to quantify the extent to which any of these effects do alter fertility are particularly challenging in cattle, as individual animals and their management are both very variable and lifetime fertility takes many years to assess. Nevertheless, the evidence is compelling that the fertility of some animals is compromised by events happening before they are born. Calf phenotype at birth and their conception data as a nulliparous heifer should therefore both be assessed to avoid such animals being used as herd replacements.

Impact of early weaning on small intestine, metabolic, immune and endocrine system development, growth and body composition in artificially reared lambs

AbstractThis study evaluated the effect of early weaning (EW) of artificially reared lambs using a restricted milk replacer (MR) feeding and step-down weaning system on the short- and long-term effects on growth, feed intake, selected blood metabolites and hormones, body composition, and small intestine development. Mixed-sex twin-born 2 to 5 d old lambs were randomly allocated to individual pens and fed MR at 20% of initial individual BW in week 1 and 15% in week 2 followed by weaning off MR by the end of week 4 (EW; n = 16) or week 6 (Control; Ctrl, n = 16) using a step-down procedure. Concentrate starter and fiber diets were offered ad libitum to week 9, then gradually removed over a 10-d period. All lambs were managed as a single group on pasture from weeks 6 to 16 of the trial. Feed intake was recorded daily in the first 6 wk, and BWs recorded weekly. At weeks 2, 4, 6, and 8, and pre- and postclostridial vaccination at week 8, blood samples were collected for analysis of selected blood metabolites, IGF-1, and immune function. Body composition was evaluated in eight animals per group at weeks 4 and 16 after euthanasia, and duodenal samples collected for histomorphometric evaluation. Early weaned lambs had lower DM, ME, CP, and NDF intake than Ctrl lambs at 21, 15, 21, and 36 d of rearing, respectively (P < 0.001), driven by lower intakes of MR from day 15 (P < 0.001) as per the experimental design, and lower total DMI of fiber (P = 0.001) from 21 to 42 d of rearing. Lamb BW tended (P = 0.097) to be lower in EW than Ctrl lambs from 5 to 10 wk of rearing, with lower ADG in EW lambs from weeks 3 to 6 (P = 0.041). Early weaning had negligible effects on duodenal morphology, organ, and carcass weights at weeks 4 and 16. Plasma metabolites (urea nitrogen, triglycerides, NEFA, glucose, and total protein) were similar between groups, while β-hydroxybutyrate was greater in EW than Ctrl lambs at weeks 4 and 6 (P = 0.018) but not week 8 indicative of early rumen development. Serum IGF-1 tended to be lower in EW than Ctrl lambs from weeks 2 to 6 only (P = 0.065). All lambs developed antibody responses postvaccination and there was no effect of treatment (P = 0.528). The results of this study illustrate that artificially reared lambs can be weaned off MR by 4 or 6 wk of rearing without compromising growth, small intestine morphology, major organ development, and body composition, nor immune function at either 4 (preweaning) or 16 (postweaning) wk of age.

The tammar wallaby: a non-traditional animal model to study growth axis maturation

Maturation of the growth hormone (GH)/insulin-like growth factor 1 (IGF1) axis is a critical developmental event that becomes functional over the peripartum period in precocial eutherian mammals such as sheep. In mice and marsupials that give birth to altricial young, the GH/IGF1 axis matures well after birth, suggesting that functional maturation is associated with developmental stage, not parturition. Recent foster-forward studies in one marsupial, the tammar wallaby (Macropus eugenii), have corroborated this hypothesis. 'Fostering' tammar young not only markedly accelerates their development and growth rates, but also affects the timing of maturation of the growth axis compared with normal growing young, providing a novel non-traditional animal model for nutritional manipulation. This review discusses how nutrition affects the maturation of the growth axis in marsupials compared with traditional eutherian animal models.

Effects of nutritional manipulation on body composition in the developing marsupial, Macropus eugenii

When 60-day-old tammar wallaby pouch young (Macropus eugenii) are fostered to mothers at 120 days of lactation, their growth, developmental rate and maturation of their GH/IGF axes are markedly accelerated. To determine the effect of fostering on energy intake, body composition and fat accretion, we first measured total body fat and lean mass in these young. Next, we mimicked the triglyceride oleic and palmitic acid composition of 120-day milk by supplementing 60 day young with these fatty acids and comparing their growth with that of growth accelerated young. There was no difference in the weight or growth axis maturation of supplemented young but there was significantly more body fat in these and in the growth-accelerated fostered young than in controls. We conclude that the accelerated growth and GH/IGF axis maturation observed previously in fostered young is most likely due to increased milk consumption and earlier access to specific nutrients.

Growth axis maturation is linked to nutrition, growth and developmental rate

Maturation of the mammalian growth axis is thought to be linked to the transition from fetal to post-natal life at birth. However, in an altricial marsupial, the tammar wallaby (Macropus eugenii), this process occurs many months after birth but at a time when the young is at a similar developmental stage to that of neonatal eutherian mammals. Here we manipulate growth rates and demonstrate in slow, normal and fast growing tammar young that nutrition and growth rate affect the time of maturation of the growth axis. Maturation of GH/IGF-I axis components occurred earlier in fast growing young, which had significantly increased hepatic GHR, IGF1 and IGFALS expression, plasma IGF-I concentrations, and significantly decreased plasma GH concentrations compared to age-matched normal young. These data support the hypothesis that the time of maturation of the growth axis depends on the growth rate and maturity of the young, which can be accelerated by changing their nutritional status.

Effects of overfeeding naturally-mated adolescent ewes on maternal, fetal, and postnatal lamb growth

The objective of this study was to evaluate the effects of overfeeding naturally-mated adolescent ewes (Ovis aries) on maternal, fetal, and postnatal lamb growth, hormone concentrations, and lamb carcass characteristics. Two experiments were conducted in which singleton-bearing adolescent ewes were fed a diet containing 2.72 Mcal/kg ME at a rate which met NRC gestational age requirements (MN; n = 10 in Exp. 1, n = 7 in Exp. 2) or were fed the same diet ad libitum (15% refusal rate) throughout gestation (HN; n = 7 in Exp. 1, n = 6 in Exp. 2). Ewe BW was greater (P < 0.05) for HN than MN ewes beginning on 75 d and 52 d of gestation for Exp. 1 and 2, respectively. Final BCS was greater (P ≤ 0.05) for HN than MN ewes in both experiments; 3.5 vs. 3.0, respectively, for Exp. 1, and 4.8 vs. 2.9, respectively, for Exp. 2. Fasting maternal blood insulin concentrations were greater (P ≤ 0.05) in HN ewes near term (135 d of gestation), whereas fasting maternal glucose concentrations were greater (P ≤ 0.05) during most of the second half of gestation in HN ewes, for both experiments. Gestation length did not differ (P = 0.69) between treatments in Exp. 1, but in Exp. 2, HN ewes had shorter (P = 0.01) gestation lengths (144 vs. 149 d) and had increased (P = 0.002) dystocia scores. Fetal abdominal circumference was greater (P < 0.05) in lambs from MN than HN ewes at 97 d of gestation in Exp. 1 (20.8 vs. 17.4 cm) but did not differ (P = 0.94) between treatments at 95 d of gestation in Exp. 2 (averaging 20.5 cm). There were no differences (P ≥ 0.15) in lamb BW, abdominal circumference, crown-rump length, and biparietal distance at birth; or in postnatal BW and plasma concentrations of glucose, insulin, and lactate in either experiment. There were no differences (P ≥ 0.18) in HCW, dressing percentage, LM area, fat thickness, or KPH between treatments in Exp. 2. Although there was no difference (P ≥ 0.31) between treatments in concentrations of IGF1 or IGF2 mRNA in liver samples collected at harvest, lambs from MN ewes had greater (P ≤ 0.05) concentrations of IGF1R and INSR mRNA, suggesting long-term effects of maternal diet on postnatal hepatic function. In conclusion, excess nutrition during gestation in naturally-mated adolescent ewes did not affect birth weight or postnatal performance of offspring.

Effect of dietary n-3 polyunsaturated fatty acid supplementation on bovine uterine endometrial and hepatic gene expression of the insulin-like growth factor system

Supplementation of cattle diets with n-3 polyunsaturated fatty acids (n-3 PUFA) has been suggested to have positive effects on fertility. In addition, the actions of the insulin-like growth factor (IGF) system both systemically and locally have been shown to influence reproductive processes. The objective of this study was to evaluate the effect of dietary n-3 PUFA supplementation on hepatic and endometrial expression of IGF signalling genes in cattle. Beef heifers were supplemented with a rumen protected source of either a saturated fatty acid (palmitic acid; CON) or high n-3 PUFA diet (n-3 PUFA) for 45 days before slaughter and tissue recovery. Transcription level of candidate IGF signalling genes was measured by reverse transcription quantitative real-time PCR (RT-qPCR) in total RNA isolated from uterine endometrial and liver tissue from seven CON and seven n-3 PUFA supplemented animals. Compared to controls, mRNA abundance in n-3 PUFA liver tissues was higher for IGF-2R, IGFBP-1 and IGFBP-5 (P < 0.05); lower for GHR-1A (P < 0.05); and unchanged for IGF-1, IGF-2, IGF-1R, IGFBP-2, IGFBP-3, IGFBP-4, IGFBP-6, ALS and GHR(total) (P > 0.05). Compared to controls, mRNA abundance in n-3 PUFA endometrial tissues was higher for IGF-2, IGF-1R, IGF-2R and IGFBP-2 (P < 0.05); lower for IGF-1, IGFBP-3 and IGFBP-6 (P < 0.05); and unchanged for IGFBP-1, IGFBP-4, IGFBP-5 and GHR(total) (P > 0.05). Thus, dietary supplementation of cattle with n-3 PUFA affects transcription of genes involved in IGF signalling, in a tissue dependent fashion.

Placental restriction of fetal growth decreases IGF1 and leptin mRNA expression in the perirenal adipose tissue of late gestation fetal sheep

Placental restriction (PR) of fetal growth results in a low birth weight and an increased visceral fat mass in postnatal life. We investigated whether PR alters expression of genes that regulate adipogenesis [IGF1, IGF1 receptor (IGF1R), IGF2, IGF2R, proliferator-activated receptor-γ, retinoid-X-receptor-α], adipocyte metabolism (lipoprotein lipase, G3PDH, GAPDH) and adipokine signaling (leptin, adiponectin) in visceral adipose tissue before birth. PR was induced by removal of the majority of endometrial caruncles in nonpregnant ewes before mating. Fetal blood samples were collected from 116 days gestation, and perirenal visceral adipose tissue (PAT) was collected from PR and control fetuses at 145 days. PAT gene expression was measured by quantitative RT-PCR. PR fetuses had a lower weight (PR 2.90 ± 0.32 kg; control, 5.12 ± 0.24 kg; P < 0.0001), mean gestational arterial Po2 ( P < 0.0001), plasma glucose ( P < 0.01), and insulin concentrations ( P < 0.02), than controls. The expression of IGF1 mRNA in PAT was lower in the PR fetuses (PR, 0.332 ± 0.063; control, 0.741 ± 0.083; P < 0.01). Leptin mRNA expression in PAT was also lower in PR fetuses (PR, 0.077 ± 0.009; control, 0.115 ± 0.013; P < 0.05), although there was no difference in the expression of other adipokine or adipogenic genes in PAT between PR and control fetuses. Thus, restriction of placental and hence, fetal substrate supply results in decreased IGF1 and leptin expression in fetal visceral adipose tissue, which may alter the functional development of the perirenal fat depot and contribute to altered leptin signaling in the growth-restricted newborn and the subsequent emergence of an increased visceral adiposity.

Effects of maternal cold exposure and nutrient restriction on the ghrelin receptor, the GH-IGF axis, and metabolic regulation in the postnatal ovine liver

Maternal cold exposure of pregnant sheep promotes fetal growth, whereas nutrient restriction (NR) can reverse this effect. The present study was designed to establish whether cold exposure induced by winter shearing of the mother at 70 days gestation (term=147 days), with or without NR (induced by a 50% reduction in maternal food intake from 110 days gestation), has specific effects on mRNA abundance of hepatic genes related to growth and liver energy metabolism that could regulate postnatal body and liver growth. Measurements of hepatic gene expression for the GH secretagog receptor-1a (GHSR-1A), peroxisome proliferator-activated receptor (PPAR)alpha, phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase activity together with glycogen content were made in the livers of offspring at 1 and 30 days of age. Maternal NR reduced liver mass at day 1, whereas offspring of cold-exposed mothers had larger livers at day 30 irrespective of maternal diet. Cold exposure resulted in the up-regulation of GHSR-1A mRNA abundance and reduced glucose-6-phosphatase activity at 1, but not 30 days of age, whereas IGF-II mRNA was decreased at 1 and 30 days. PPARalpha mRNA abundance was enhanced, while PEPCK was reduced in 30-day old offspring of cold-exposed mothers. NR caused reductions in IGF-I mRNA and, at 1-day postnatal age, down-regulated GHR, while, at 30 days, reduced GHSR-1A gene expression and hepatic glycogen content. In conclusion, we have shown that maternal cold exposure and NR have different effects on the hepatic GH-IGF and metabolic axis that may contribute to changes in liver growth over the first month of life.

Uterine and hepatic gene expression in relation to days postpartum, estrus, and pregnancy in postpartum dairy cows

The somatotropic axis consisting of growth hormone, the growth hormone receptor (GHR) insulin-like growth factor (IGF)-I, and IGF binding proteins changes with the stage of lactation and nutrition of the cow and may be 1 mechanism through which lactation and nutrition affect the establishment of pregnancy. The objective of this study was to quantify GHR, IGF-I, and IGF binding protein-2 (IGFBP-2) mRNA in liver and uterine endometrial tissue at 4 stages of lactation (40, 80, 120, and 160 days in milk) and around the time of artificial insemination. Estrus was synchronized with GnRH and PGF2alpha, and cows were inseminated 12 h after estrus. Uterine biopsies were collected immediately before the second injection of PGF2alpha (before estrus), at the initiation of standing estrus, and 4 d after estrus. Liver biopsies were collected once on 4 d after estrus. The abundance of GHR, IGF-I, and IGFBP-2 mRNA in liver and uterus was determined by real-time quantitative PCR. The amount of liver IGF-I mRNA was positively correlated with plasma IGF-I concentrations. Cows that became pregnant after AI had more GHR and IGFBP-2 mRNA in their liver than cows that did not become pregnant. There was no effect of DIM or pregnancy status on abundance of uterine mRNA; however, uterine GHR and IGF-I mRNA was most abundant at estrus. In summary, cows at different stages of lactation or with different pregnancy statuses had similar quantities of uterine mRNA. In contrast, liver quantities of mRNA differed relative to pregnancy status. These data provide evidence that liver indices of metabolic state may be indicative of pregnancy success.

Prenatal exposure to excess testosterone modifies the developmental trajectory of the insulin-like growth factor system in female sheep

Experimental elevation of maternal testosterone (T) from 30 to 90 days of gestation leads to intrauterine growth retardation (IUGR) and increased prepubertal growth rate in female lambs. This study tested the hypothesis that prenatal T treatment during mid-gestation alters the trajectory of the fetal insulin-like growth factor (IGF)-insulin-like growth factor binding protein (IGFBP) system to promote IUGR and subsequent postnatal catch-up growth in female lambs. Plasma IGF-I and IGFBPs were measured by radioimmunoassay and Western ligand blot, respectively, on 65, 90 and 140 days (d) of gestation, at birth, approximately 5 months (prepubertal, the catch-up growth period), and approximately 9.5 months (postpubertal). Northern blot analysis was used to measure hepatic mRNA content of IGF system components during fetal stages. At fetal 65 d, plasma protein and hepatic mRNA content of IGFBP-1, an inhibitor of IGF bioactivity, was elevated in prenatal T-treated fetuses although body weight did not differ. There was a transient increase in plasma IGF-I and IGFBP-3 concentrations at fetal 90 d in prenatal T-treated fetuses. Hepatic IGF-I mRNA and plasma IGFBP-3 content were reduced by 140 d when body weight was reduced in prenatal T-treated fetuses. Plasma IGFBP-2 content was significantly reduced in prenatal T-treated newborns, but by 4 months these females had significantly higher circulating IGF-I and IGFBP-3 concentrations and faster growth rates than control females. After puberty, plasma IGF-I remained elevated in prenatal T-treated females. These findings provide evidence that prenatal T excess programmes the developmental trajectory of the IGF/IGFBP system in female sheep to reduce IGF bioavailability during IUGR and increase IGF bioavailability during prepubertal catch-up growth.

Prenatal programming of postnatal productivity and health of livestock: a brief review

Human epidemiological evidence has suggested that metabolic perturbations during fetal life may increase predisposition to cardiovascular disease, type 2 diabetes and obesity in later life. A growing number of controlled experiments on sheep and other large animal species are adding to the already large body of experimental evidence from rat studies in supporting the ‘fetal origins’ hypothesis. Of particular practical relevance are findings that maternal undernutrition in late pregnancy can predispose lambs to glucose intolerance and increased adiposity in early adulthood. This effect may be exacerbated by high energy intakes and limited capacity for muscle growth in undernourished or growth-retarded lambs during early postnatal life. Recent Australian studies have demonstrated the effects of prenatal nutrition on postnatal growth and meat production in beef cattle, and on quantity and quality of wool production in sheep.

Hepatic gene and protein expression of primary components of the IGF-I axis in long lived Snell dwarf mice

Recent evidence indicates that the GH/IGF-I axis plays a key role in the control of aging and longevity. To better understand this biological relationship we examined the mRNA and corresponding protein levels of primary IGF-I axis genes in the livers of young and aged long-lived Snell dwarf mice relative to their age-matched controls. We demonstrated that the level of IGF-I and ALS mRNAs is dramatically decreased in both young and aged dwarf livers, transcripts encoding IGF-IR and IGFBP-I are elevated in young dwarfs, but normalize to control levels in aged dwarf livers while transcripts encoding IGFBP-3 are elevated only in aged controls. Interestingly, regulation at the protein level of several IGF-I axis components in the Snell dwarf appears to involve both altered gene expression and post-translational regulation. In this study, we reveal both concordant and discordant relationships between mRNA and protein levels for particular components of the IGF-I axis, illustrating that some of these gene products are not solely regulated by transcriptional mechanisms. These results are consistent with a delay in the molecular maturation of the IGF-I axis in dwarf livers, suggesting the preservation of some neonatal characteristics in young adult and aged dwarf livers. Our studies provide gene expression and protein abundance profiles for components of IGF-I axis that are distinguishing characteristics of both young and aged dwarf mice, and suggest that delayed development of the IGF-I axis in the young adult Pit1(dw/dwJ) dwarf liver may play an important role in the endocrine regulation of mammalian longevity.

Relationships between the plasma concentrations of insulin-like growth factor-I in dairy cows and their fertility and milk yield

The relationships between insulin-like growth factor-I (IGF-I) and the fertility and milk yield of Holstein-Friesian dairy cows were investigated. The concentration of IGF-I in blood was measured weekly from one week before to 12 weeks after calving in 177 multiparous cows and at four times during this period in 142 primiparous cows; the concentration of IGF-I in milk was measured in 50 of the multiparous cows. The plasma concentrations of IGF-I were higher in the primiparous than in the multiparous animals. In the primiparous cows, high concentrations of IGF-I before calving were associated with longer calving to conception intervals. Conversely, in the multiparous cows low concentrations of IGF-I before and after calving were associated with a failure to conceive, despite repeated services. Multiparous cows with IGF-I concentrations of greater than 25 ng/ml in the week after calving were 11 times more likely to conceive to first service than those with lower concentrations. Concentrations of IGF-I greater than 50 ng/ml at first service increased the likelihood of conception five-fold. Cows with higher peak milk yields had lower plasma concentrations of IGF-I and took longer to return to ovarian cyclicity. The negative relationship between milk yield and return to cyclicity was stronger in the multiparous cows (P < 0.002) than in the primiparous cows (P < 0.04). The concentrations of IGF-I in milk followed a different pattern and were not associated with the changes in plasma IGF-I or fertility.

Plasma Leptin Is Regulated Predominantly by Nutrition in Preruminant Lambs

In juvenile and mature animals, the plasma concentration of leptin is regulated by adiposity and nutrition. However, the timing of these influences on plasma leptin, and their relative importance in early postnatal life, are unknown. We investigated these plasma leptin influences in sheep, a species characterized during fetal life by leanness and insensitivity of leptin to variation in maternal nutrition. Small and large neonatal lambs were randomly assigned to either a diet sustaining an average daily weight gain (ADG) of 148 g/d (Low plane) or ate ad libitum a diet sustaining an ADG of 337 g/d (High plane). A subset of animals were slaughtered at 7.5, 10, 15 and 20 kg of body weight. Birth size had no effect on plasma leptin concentrations and adiposity at birth or at later times. Plasma leptin concentrations increased within 6 d of birth in the High plane lambs (P < 0.01) and continued to rise over time. In contrast, plasma leptin concentrations never changed in the Low plane lambs despite increasing adiposity. The positive association between plasma leptin concentration and adiposity was greater in the High plane than in the Low plane lambs, suggesting an independent effect of nutrition. Consistent with this finding, lipid accretion rates, a variable that is mostly independent of adiposity, was a strong predictor of plasma leptin concentrations only in the High plane lambs (R(2) = 0.77, P < 0.01). A positive association between plasma insulin and leptin developed over time in the High plane lambs (R(2) = 0.75, P < 0.01 on d 40), but was not seen in the Low plane lambs. These data indicate that both nutrition and adiposity regulate plasma leptin synthesis in early postnatal life, but in contrast to adulthood, the effects of nutrition appear to be predominant.

Age-dependent regulation of the acid-labile subunit in response to fasting-refeeding in rats

The GH-dependent, hepatocyte-derived acid-labile subunit (ALS) regulates IGF release from the serum by forming ternary complexes containing IGF binding protein (IGFBP)-3 or IGFBP-5. Malnutrition suppresses ALS and IGF-I expression in a development-dependent manner. Our aim was to investigate whether the effect of feeding following fasting was similarly age dependent. We fasted juvenile and adult rats for 48 h and then refed them, collecting serum and liver tissue at 8, 24, and 48 h. These were compared with rats before fasting (0 h controls) and animals fed throughout the study (free-fed controls). During fasting, serum ALS fell to 25 +/- 5.3% of 0 h controls in juveniles but only 56 +/- 6% in adults. Within 24 h of refeeding, ALS in juveniles had returned to 0 h control levels, and by 48 h to free-fed levels, whereas there was no significant refeeding response in adults during this period. Circulating IGF-I and IGFBP-5 showed similar age-dependent responses to refeeding, rising significantly faster in juveniles. IGFBP-3 did not show this response. Furthermore, hepatic ALS and IGF-I mRNA showed no age-differential response to fasting and refeeding, suggesting posttranscriptional regulation. Neither regulation of hepatic GH receptor nor ALS clearance rates could explain the age-dependent effect. We hypothesize that development-dependent regulation of ALS and IGF-I during refeeding may involve a posttranscriptional hepatic response that is not GH dependent.

Effects of birth weight and postnatal nutrition on neonatal sheep: III. Regulation of energy metabolism

This study investigated effects of birth weight and postnatal nutrition on regulation of energy metabolism in the neonatal lamb. Low (mean +/- SD 2.289 +/- 0.341 kg, n = 28) and high (4.840 +/- 0.446 kg, n = 20) birth weight male Suffolk x (Finnsheep x Dorset) lambs were individually reared on a liquid diet to grow rapidly (ad libitum fed, ADG = 337 g, n = 20) or slowly (ADG = 150 g, n = 20) from birth to live weights (LW) up to approximately 20 kg. At birth, small newborns had higher plasma concentrations of urea nitrogen (mean +/- SEM 8.31 +/- 0.25 vs 6.39 +/- 0.32 mM, P = 0.002) and somatotropin (ST, 49.1 +/- 17.0 vs 10.8 +/- 4.3 ng/mL, P = .045) and lower IGF-I (36.1 +/- 6.8 vs 157.7 +/- 21.8 ng/mL, P < 0.001) than large newborns. Plasma glucose (1.42 +/- 0.23 vs 2.63 +/- 0.95 mM, P = 0.147) and insulin (0.09 +/- 0.02 vs 0.13 +/- 0.06 ng/mL, P = 0.264) concentrations did not differ. Urea nitrogen concentration in plasma peaked and then declined rapidly in all lambs during the first week postpartum, and plasma ST declined on a body-weight-related basis from birth. During rearing to 20 kg LW, plasma insulin was higher in low- vs high-birth-weight lambs. Lambs fed ad libitum had greater plasma concentrations of glucose, urea nitrogen, insulin, and IGF-I compared to those fed a restricted diet (ADG = 150 g). The results suggest that during the early postpartum period, newborn lambs exhibit the fetal characteristic of high rates of amino acid oxidation. The results also support the notion that, at birth, low-birth-weight lambs are less mature than high-birth-weight lambs in aspects of metabolic and endocrine development, which may enhance their capacity to utilize amino acids for energy production and to support gluconeogenesis during the immediate postpartum period. Being small at birth also resulted in elevated plasma insulin concentrations when adequate nutriment to support moderate or rapid growth was provided postpartum, although it remains to be elucidated whether this more chronic effect persists in the longer term.