Lambs from ewes fed a poor-quality diet with realimentation exhibit increased incidence of hypothermia and divergent pre-weaning growth

Abstract

Depending on seasonal availability and quality of range forage, extensively managed ewes can experience nutritional challenge during any point of gestation. Neonatal lamb performance after severe experimental nutrient restriction during gestation has been well studied; however, there is a gap in understanding of how nutrient restriction experienced over the course of a production cycle impacts lamb success. Therefore, the objective of this study was to investigate early life performance of lambs from ewes fed a diet simulating winter forage in the Intermountain West. Timed-mated Rambouillet × Merino ewes were allocated to one of two dietary treatments, consisting of a diet formulated to meet nutritional requirements for the entirety of gestation based on National Research Council (NRC) recommendations (balanced diet) or a poor-quality diet fed from 30 to 125 d of gestation (dGA) and then realimented with the nutritionally balanced diet for the remainder of gestation. From 30 dGA to lambing, BW were obtained every 7 d and body condition scores (BCS) every 14 d. At the end of gestation ewes were group housed in a fully enclosed barn for lambing, to produce lambs born to balanced diet ewes (CONT; n = 33) and lambs born to poor-quality diet ewes (NC; n = 33). For the first week of life, rectal temperature and blood samples were collected daily from all lambs. Bodyweight and morphometrics (crown-rump length, CRL; abdominal circumference, AC; crown circumference, CC; and cannon bone length, CBL) were collected at birth, and then weekly thereafter. Treatment by time interactions were observed where poor-quality diet ewes weighed less (P < 0.05) from weeks 6 to 17 and had lower (P < 0.05). BCS from weeks 6 to 16 of the treatment period There were no differences in birthweight or birth morphometrics between treatments. There was an increased (P < 0.05) incidence of hypothermic events (rectal temperatures < 38 °C) in NC lambs in the first week of life, despite having normal blood glucose concentrations. Beginning at 4 wk of life through weaning (70 d), NC lambs weighed less (P < 0.05) compared to CONT. Nutrient challenged lambs also exhibited asymmetric growth, indicated by increased (P < 0.05) CRL/BW, CC/BL, CRL/BW, and AC/BW ratios compared to CONT. These data suggest that sustained moderate nutritional challenge during gestation, even when supplemented in late gestation, negatively impacts offspring pre-weaning performance.

Extensively managed pregnant ewes often fall short of their nutritional needs without supplementation. This study showed that a poor-quality diet during pregnancy can slow lamb growth and predispose newborns to hypothermia, potentially having lasting impacts on flock productivity.

Introduction

Maternal nutrition during gestation plays a critical role in determining offspring growth, development, and long-term health. In extensive ruminant livestock production systems, seasonal declines in forage quality and quantity pose a persistent challenge to meeting the nutritional demands of pregnant grazing animals. In the Intermountain West, ewes are often bred in the fall and graze native rangeland throughout the winter (Julian et al. 2024). This production cycle creates a misalignment between increasing maternal and fetal nutrient requirements and declining forage quality and availability as gestation progresses (Reynolds et al. 2019; Gauvin et al. 2020; Julian et al. 2020). As the season progresses from fall to winter, native forage becomes increasingly fibrous and lower in crude protein (CP) and total digestible nutrients (TDN), often making it inadequate to meet the nutritional requirements of pregnant ewes without supplementation. Thus, producers often provide supplemental feed during this time to support the nutrient requirements of gestating ewes (Bohnert and Stephenson 2016). Supplementation strategies typically target late gestation, when fetal growth and maternal nutrient demands peak; however, nutrient challenge can occur at any point of gestation and may have lasting consequences (Caton and Dhuyvetter 1997; McGuire et al. 2013; Campos et al. 2022).

Severe nutrient restriction during gestation has been shown to induce adaptive fetal programming, a physiological response in which the fetus shunts the limited nutrients to vital organs at the expense of those deemed non-vital (Hales and Barker 2001; Barcellos et al. 2024). Although some effects of prenatal restriction may be partially mitigated with improved management, these developmental adaptations can persist in postnatal life. Lambs exposed to adverse in-utero environments may exhibit low birthweights, reduced skeletal muscle mass, poor body composition, decreased feed efficiency, and lower quality carcass traits (Vautier and Cadaret 2022). While the effects of severe undernutrition are well documented, the extent to which nutrient restriction reflective of winter range influences lamb performance remains less well defined. Low birthweight pathologies are estimated to cost the sheep industry 8–10% of annual product (APHIS 2015); however, this figure likely underrepresents animals affected by developmental programming in the absence of birthweight differences. Therefore, the objective of this study was to evaluate early-life performance (from birth through weaning) in lambs born to ewes fed a forage-based diet designed to create moderate nutrient restriction by quality. We hypothesized that this gestational insult would not result in overt birthweight differences, but that tissue specific adaptations underlying differences in animal performance would develop prior to weaning.

Materials and methods

Animals and experimental design

This study was approved by the Colorado State University Institutional Animal Care and Use Committee (Protocol no. 5286). Three-year-old Merino × Rambouillet ewes were synchronized using a 12-d CIDR with prostaglandin protocol. Briefly, Eazi-Breed CIDR’s (Zoetis, Parsippany, NJ) were placed on day 0, followed by prostaglandin injection (Lutalyse; Zoetis) on day 10. Upon CIDR removal on day 12, all ewes were placed with a mature ram fitted with a marking harness, and dorsal marking date was recorded. At ∼25 d of gestation (dGA) blood was collected from all ewes via jugular venipuncture and submitted to a commercial lab (The Dairy Authority, Greeley, CO) for pregnancy determination via pregnancy specific protein B assay (BioPRYN; Moscow, ID). After pregnancy confirmation, ewes were blocked by initial body weight (BW; 90.82 ± 3.26 kg) to ensure similar mean BW between treatment groups, randomly assigned one of two diets, and housed in individual pens for the treatment period. Ewes were let out daily into dry lot pens for exercise, socialization, and pen cleaning. At ∼60 dGA a flank ultrasound was performed to confirm maintenance of pregnancy and determine estimated progeny number.

Diet composition and treatment allocation

By 30 dGA ewes were housed in individual pens and transitioned to one of two diets with ad libitum access to water and individual mineral blocks (Redmond Agriculture, Redmond, UT). The balanced diet (n = 17 ewes) was formulated to meet or exceed the nutrient requirements for a mature medium-sized (90 kg) twin-bearing ewe, based on the Small Ruminant National Research Council (National Reserach Council 2007; ∼66% TDN and ∼11.5% CP) guidelines. The poor-quality diet (n = 20 ewes) was formulated to simulate the nutrient quality of winter forage based on previously reported data in the Intermountain West (Thomas and Kott 1995; Julian et al. 2024; ∼50% TDN and ∼6% CP). The balanced diet was composed of cracked corn (10.7%), corn silage (30%), grass hay (52%), and soybean meal (7.3%) on a dry matter (DM) basis (Table 1). The poor-quality diet was composed of corn silage (40%) and wheat straw (60%) on a DM basis. Both diets were sampled weekly and analyzed for nutrient composition by a commercial laboratory (SDK Laboratories, Hutchinson, KS). This study occurred over two continuous years to generate lambs from balanced diet ewes (CONT; n = 33; 3 singletons, 21 twins, 9 triplets; 14 males and 19 female) and from poor-quality diet ewes to generate nutritionally challenged offspring (NC; n = 33; 5 singletons, 19 twins, 9 triplets; 21 males and 12 females).

Table 1.

Composition of balanced and poor-quality diets.

	Diet
Ingredient	Balanced diet	Poor-quality diet
Corn silage, %	30	40
Grass hay, %	52	–
Cracked corn, %	10.7	–
Soybean meal, %	7.3	–
Wheat straw, %	–	60
Chemical composition
Dry matter, %	63.03	67.94
Crude protein, %	10.57	6.01
Acid detergent fiber, %	28.22	39.73
Neutral detergent fiber, %	36.15	59.37
Total digestible nutrients, %	62.33	57.81
Net energy for maintenance Mcal/kga	1.54	1.39
Net energy for gain, Mcal/kgb	0.81	0.66

^aNEm = {[1.37 × (TDN × 0.0361)] − [0.0138 × (TDN × 0.0361) × (TDN × 0.0361)] + [0.0105 × (TDN × 0.0361) × (TDN × 0.0361) − 1.12]} ÷ 2.205.

^bNEg = {[1.42 × (TDN × 0.0361)] − [0.174 × (TDN × 0.0361) × (TDN × 0.0361)] + [0.0122 × (TDN × 0.0361) × (TDN × 0.0361) × (TDN × 0.0361) − 1.65]} ÷ 2.205.

Gestational management

From 30 to 125 dGA pair feeding was implemented. Briefly, ewes receiving the balanced diet were paired by BW with ewes receiving the poor-quality diet and initially fed at 3% of the balanced diet ewes BW. Feed delivery to each pair was then determined by the previous day’s intake for the ewes receiving the balanced diet and adjusted based on whether orts remained or feed buckets were slicked. This allowed for ewes fed the balanced diet to meet their nutrient requirements while ensuring the ewes receiving the poor-quality diet would not overcome intentional deficits in quality by increasing intake. At 125 dGA, all ewes remained in individual housing, and restricted diet ewes were realimented to the balanced diet ad libitum to simulate a late gestation supplementation period. For the entire study period, daily feed intake, weekly maternal BW, and 14 d body condition scores (BCS) were collected.

Lamb growth and development

Approximately 10 d from the expected lambing date, ewes were moved into a fully enclosed barn and group housed. Once in group housing, all ewes continued to be fed the balanced diet ad libitum. Ewes lambed independently in the group environment, unless intervention was needed due to dystocia, before being brought with their lambs to a jug in a separate temperature-controlled barn. Placental weight and cotyledon number were collected after passage. At birth, birth type (number of lambs born/ewe), rectal temperatures (AmerisourceBergen, Conshohocken, PA), BW, and birth morphometrics (crown-rump length, CRL; crown circumference, CC; cannon-bone length, CBL; and abdominal circumference, AC) were collected from lambs. Ewes and lambs were monitored for general health and lamb vigor based on clinical observations (including ability to stand, suckle, and overall responsiveness) for 24–48 h and then processed by ear tag and tail band placement before being released from the jug. Lambs and dams were maintained in the temperature-controlled barn, and weight and temperature collection continued daily beginning at 0700 for the first week of life. After the first week of life, BW and morphometrics were collected weekly until weaning at ∼70 d. Lambs remained with their dams throughout the preweaning period, with access to commercial creep feed (Ault Feedmill, Ault, CO) ad libitum beginning at 1 wk of age, to reflect natural production conditions.

Blood collection

Blood samples for glucose analysis were collected from lambs beginning at 0700 daily for the first week of life without prior withholding of nursing, reflecting post-prandial concentrations. Samples were collected via jugular venipuncture into syringes containing EDTA. Whole blood was immediately transferred to microcentrifuge tubes containing EDTA, and plasma was isolated via centrifugation (2,000 × g, 10 min) and stored at −20°C until glucose analysis could be performed. Plasma glucose concentrations from triplicate 5 µL aliquots were determined via the commercially available Infinity Glucose Hexokinase Reagent (Thermo Fisher Scientific, Waltham, MA). Intra-assay and interassay coefficients of variance were less than 10%.

Statistical analysis

Data were analyzed using the MIXED procedure of SAS 9.4 (SAS Institute; Cary, NC) and models were fit using restricted maximum likelihood (REML), and denominator degrees of freedom were estimated using the Kenward-Roger approximation. For maternal repeated measures variables, models included the fixed effects of treatment, time, birth type, year, and the interactions between treatment, time, and birth type. As ewes were assigned within a pair-feeding design, feeding pair was included as a random effect to account for the shared intake environment and to represent the experimental unit for maternal treatment. For maternal single time-point variables, the same fixed effects structure was used without the repeated statement. Repeated lamb outcomes were analyzed for the fixed effects of maternal treatment, time, birth type, year, sex, and the treatment, time, birth type, and sex interactions. Feeding pair was included as a random effect, and dam nested within pair was included as a random effect to account for non-independence among lambs born to the same ewe. Repeated measurements were modeled using lamb nested within dam and pair, as the subject. Single timepoint lamb variables were analyzed using the same fixed and random effects structure without the repeated statement. For all repeated measures, the covariance structure was selected for each outcome based on best-fit criteria using the lowest Akaike Information Criterion (AIC). Mean separation was performed using Fisher’s LSD test. Differences in hypothermic event frequency (core body temperatures below 38 °C) were assessed using a non-parametric Mann–Whitney U test due to non-normal distribution of count data. Differences were considered P ≤ 0.05, and tendencies were indicated when P > 0.05 and ≤ 0.10. All data are presented as least squares mean ± standard error of the mean.

Results

Maternal performance

Dry matter intake (DMI) did not differ between balanced diet and poor-quality diet ewes (1.65 ± 0.8 vs 1.63 ± 0.8 kg, respectively). A treatment by time interaction was observed for BW where poor-quality diet ewes weighed less (P < 0.0001) than balanced diet ewes from weeks 6 to 17 of the treatment period (Figure 1). Similarly, there was a treatment by time interaction for BCS where poor-quality diet ewes had lower (P < 0.0001) BCS than balanced diet ewes from weeks 6 to 16 of the trial period. Ewes in year one had greater (P = 0.002) DMI, weighed more (P = 0.008), and had greater (P = 0.005) BCS than ewes in year two. Dry matter intake, BW, and BCS all increased (P < 0.05) over time. Gestation length, placental weight, cotyledon number, and the ratio of cotyledons to placental weight did not differ between treatments. Placental weight was heaviest (P = 0.006) in triplet placentas, followed by twins, and then singletons. The ratio of cotyledons/placental weight was greatest (P = 0.05) in singleton placentas compared to triplets. The ratio of cotyledons/placental weight tended (P = 0.10) to be greater in twins compared to triplets, while singletons and twins did not differ. The ratio of cotyledons/placental weight tended (P = 0.08) to be greater in year two compared to year one.

Figure 1.

Weekly maternal bodyweights (A) and 14 d maternal body condition scores (B) for ewes fed a balanced diet ( n = 17) or poor-quality diet ( n = 20) diet from 30 to 125 dGA. Main effects of diet (Trt), week of trial (Time), and the interaction (Trt*Time) were considered. *Means differ by P ≤ 0.05, ^†Means differ by P ≤ 0.10.

Neonatal performance

There were no differences in birthweight or birth morphometric ratios between CONT and NC lambs or by year (Table 2). However, there was a birth type effect for birthweight with singletons being the heaviest (P < 0.0001) followed by twins and then triplets. Birthweight was not influenced sex. Ratios for CC/BW and CBL/BW were lowest (P = 0.0005 and P < 0.0001, respectively) in singletons, followed by twins, and then triplets. The ratio of CRL/BW was lower (P = 0.0005) in singletons, compared to twins and triplets, while CRL/BW tended (P = 0.09) to be lower in twins compared to triplets. The ratio of AC/BW was lower (P = 0.01) in singletons compared to twins and triplets, which did not differ from each other. Male lambs tended (P = 0.08) to have greater CBL/BW ratios compared to female lambs, but no other sex effects were observed in morphometric ratios.

Table 2.

Birth morphometrics between lambs born from dams fed a balanced diet (CONT; n = 33) vs a poor-quality diet (NC; n = 33) from 30 to 125 dGA.

	Experimental group		P-value
Measurement	CONT	NC
Birthweight, kg	4.42 ± 0.14	4.31 ± 0.16	0.59
CRL1/BW2, cm/kg	12.90 ± 0.64	13.74 ± 0.71	0.35
CC3/BW, cm/kg	6.83 ± 0.27	7.10 ± 0.30	0.47
CBL4/BW, cm/kg	3.84 ± 0.15	4.02 ± 0.15	0.34
AC5/BW, cm/kg	9.24 ± 0.40	9.43 ± 0.43	0.73

¹CRL = crown rump length.

²BW = bodyweight.

³CC = crown circumference.

⁴CBL = cannon bone length.

⁵AC = abdominal circumference.

Regardless of treatment, singleton lambs weighed more (P < 0.0001) during the first week of life, followed by twins and then triplets. There was a time effect (P < 0.0001) in which lamb BW increased over time. Male lambs weighed more (P = 0.04) in the first week of life compared to females. Weights in the first week did not differ by year. Plasma glucose concentrations were similar between treatment groups, birth type, sex, years, and time. Lambs in the NC group tended (P = 0.08) to have lower rectal temperatures on days one, four, and five of life, but did not differ at any other timepoints or by birth type, sex, or year. When evaluated as hypothermic event frequency, NC lambs had an increased (P = 0.02) incidence of hypothermia compared to CONT, regardless of birth type, sex, time, or year (Figure 2).

Figure 2.

Hypothermic events per animal between lambs born from ewes fed a balanced diet (CONT, n = 33) and those born to ewes fed a poor-quality diet (NC, n = 33) from 30 to 125 dGA. Rectal temperatures were collected, and hypothermia was considered when rectal temperature was below 38 °C. *Means differ by P ≤ 0.05.

Pre-weaning growth

Beginning at 4 wk of age through weaning, there was a treatment by time interaction for BW, in which NC lambs weighed less (P < 0.0001) than CONT lambs (Figure 3). Bodyweight differences were also observed between birth types, with singleton lambs remaining the heaviest (P < 0.0001) followed by twins and triplets, which did not differ from each other. Lambs from year two weighed more (P = 0.003) than lambs from year one. CRL/BW, CC/BW, CBL/BW, and AC/BW were lower (P < 0.05) in CONT lambs compared to NC lambs. Birth type effects were also observed for CRL/BW, AC/BW CC/BW, and CBL/BW in which ratios were lowest (P < 0.05) in singletons followed by twins and then triplets, which did not differ from each other. Additionally, all morphometric ratios were greater (P < 0.05) in lambs from year one compared to year two and decreased (P < 0.0001) over time. Pre-weaning growth did not differ by sex.

Figure 3.

Growth performance from birth through weaning (70 d) between lambs born to ewes fed a balanced diet (CONT, n = 33) and those born to ewes feed a poor-quality diet (NC, n = 33) from 30 to 125 dGA. Data are presented as weekly bodyweight (A) and ratios of crown rump length to bodyweight (CRL/BW; B), crown circumference to bodyweight (CC/BW; C), cannon bone length to bodyweight (CBL/BW; D), and abdominal circumference to bodyweight (AC/BW; E). Effects of maternal diet (Trt), week of life (Time), and the interaction (Trt*Time) were considered and noted where significant. *Means differ P ≤ 0.05, ^†Means differ P ≤ 0.10.

Discussion

In this study, poor-quality diet ewes exhibited lower BW and BCS throughout gestation, well before traditional nutrient supplementation would begin. The effects of this plane of nutrition were also observed in the offspring, as lambs born to dams receiving the poor-quality diet had increased hypothermic event frequency in the first week of life, with normal growth and glucose concentrations. Additionally, NC lambs diverged in growth by BW beginning at 4 wk of life through weaning compared to CONT lambs. Morphometric ratios also differed in NC lambs, demonstrating asymmetric growth. Collectively, these results suggest that even moderate maternal nutritional challenge, common in extensive sheep production systems reliant on native forage, can impair lamb growth and resilience. While birthweight may remain unaffected, functional deficits in thermoregulation and postnatal growth have clear implications for lamb survival, health, and market value. These findings emphasize the need for strategic nutritional interventions during gestation, particularly in systems where supplementation is limited or delayed.

During gestation there is competition for nutrients between the ewe, placenta, and fetus, with these demands being greater with multiple fetuses and towards the end of gestation when mammary development and colostrum synthesis are also occurring (Gootwine et al. 2007; Reynolds et al. 2010; Hughes 2021). Reduced BW and BCS in ewes fed the poor-quality diet, suggests that the nutrient density of the diet was insufficient to meet the increasing metabolic demands of pregnancy, leading to mobilization of maternal reserves to support fetal development (Reed et al. 2007; Steinhauser et al. 2021; Tillquist et al. 2023). This has been well described in the literature, and for this reason, producers often choose to supplement dams in late gestation when nutrient demands are at their highest. The supplementation strategy employed in this study did rescue maternal BW at the end of gestation, yet it is likely the earlier nutrient deficits influenced fetal development. Importantly, gestation length, placental weight, and cotyledon number were unaffected by maternal diet, indicating that placental development may be preserved under moderate nutritional challenge. This differs from previous studies with various nutrient restrictions, predominantly by intake, where alterations in placental development were reported (Dandrea et al. 2001; McMullen et al. 2005; Vonnahme et al. 2007; Lekatz et al. 2010; Ma et al. 2011; Vonnahme 2018; Edwards et al. 2020). Throughout literature, it has been established that outcomes are dependent upon timing, duration, and severity of insult and this may explain the lack of obvious placental changes in this study. The observed birth type effect on placental weight and cotyledon to placental weight ratios is consistent with previous reports linking increased fetal load and increased nutrient demand and does not appear to be due to aberrant placental development (Vonnahme et al. 2003; Dwyer et al. 2005; Ocak et al. 2009; Zywicki et al. 2016). Thus, reduced maternal reserves paired with normal placental morphometrics suggest that impacts on the fetus are due to a direct reduction in nutrient delivery due to maternal plane of nutrition rather than placental insufficiency. Decreased nutrient exchange due to reduced blood flow could be occurring without differences in placental weight or cotyledon number, as placental angiogenesis is dependent on maternal nutrition (Redmer et al. 2004). With a more severe restriction in mid-gestation, umbilical blood flow is reduced and not recovered by realimentation despite normal placental blood flow, weight, and cotyledon size near term (Lemley et al. 2012; Vasquez-Hidalgo et al. 2023). Although not directly measured in this study, similar effects may be occurring, but further research is needed.

Neonatal outcomes revealed no differences in birthweight or morphometric ratios between treatment groups, supporting the hypothesis that ewe nutritional challenge in this study was not severe enough to induce overt fetal growth restriction. It is possible that late gestation nutrient supplementation was adequate to recover fetal size but not to reverse thrifty adaptations in tissue development and functionality that occurred earlier in gestation (Pillai et al. 2017). Other studies utilizing nutrient restriction as a maternal insult have found variable birthweight differences in both sheep and cattle (Ford et al. 2007; Vonnahme et al. 2007; Long et al. 2009). When evaluated as separate groups, the nutrient restricted but normal size fetuses exhibited characteristics of both control and nutrient restricted groups, depending on the variable of interest, indicating that tissue specific differences may be occurring in the normally sized fetuses whose dams were nutrient restricted (Sandoval et al. 2020). The most common losses in the first week of life are due to lamb hypoglycemia and hypothermia and are often concurrent and compounding (Santos et al. 2023). The increased frequency of hypothermic events in NC lambs suggest compromised thermoregulatory capacity. Neonatal thermoregulation is accomplished by non-shivering thermogenesis, shivering thermogenesis, colostrum intake, and metabolism (Dwyer et al. 2016; Plush et al. 2016). While low birthweight is associated with increased hypothermia, the present study demonstrated that NC lambs have difficulty thermoregulating despite normal birthweights and euglycemia. This may reflect subtle developmental programming impairments, particularly in brown adipose tissue function or metabolic rate, which are not captured by birthweight alone. Maternal nutrient restriction has been associated with decreased brown adipose tissue (Ojha et al. 2013; Satterfield et al. 2013; Yang et al. 2021; Ma et al. 2025) and in the current study, ewes received the restricted diet during the normal window of brown adipose development (70–120 dGA; Gao et al. 2021) which may have impaired deposition. After the first 24 h of life in lambs, shivering thermogenesis and metabolism are primarily responsible for heat production and rely on normal skeletal muscle development and metabolism. Multiple models of fetal growth restriction have demonstrated altered skeletal muscle fiber size, number, and metabolic activity (Funston et al. 2010; Yates et al. 2016; Posont et al. 2019) and it is possible those impacts were occurring in these animals; however, this was not investigated. Such impairments in neonatal thermoregulation can increase susceptibility to environmental stressors and mortality, especially in harsh winter conditions and may help explain losses in seemingly healthy lamb crops within the first week of life.

Although low birthweight and asymmetric growth at birth are hallmarks of developmental programing due to brain sparing (Galan et al. 1999; Hales and Barker 2013; Macko et al. 2013), neither were observed in this study. Reduced growth and asymmetry emerged in the preweaning period, supporting the idea that late gestation supplementation recovered size, but previous fetal adaptations induced by poor maternal nutrition were not fully corrected. Previous studies with birthweight differences have demonstrated compensatory gain in the postnatal animal, favoring rapid adipose deposition (Yates et al. 2011; Long et al. 2013; Tillquist et al. 2023) which does not appear to be occurring in this study. Fetal adaptations to in-utero nutrient restriction results in reduced postnatal performance even when offspring are exposed to a nutrient rich diet, as mechanisms of metabolic thrift persist into postnatal life. Skeletal muscle is particularly sensitive to nutrient repartitioning due to its high metabolic activity. Numerous studies have reported effects such as altered muscle mass, fiber number, fiber type proportions, glucose transporter expression, aberrant metabolism, and more (Zhu et al. 2006; Du et al. 2010; Cadaret et al. 2019; Ithurralde et al. 2021, 2023). While muscle dynamics were not directly evaluated in this study, the combination of reduced growth and altered thermoregulation in NC lambs, despite normal glucose concentrations, suggest that skeletal muscle and possibly endocrine adaptations may be occurring. Nevertheless, animals on this reduced growth trajectory would take longer to reach target weights for market or reproduction and have negative implications for production efficiency and producer profitability.

Birth type effects were seen throughout this study, with singletons outperforming twins and triplets consistently. This outcome reflects well-established principles of intrauterine competition, where increasing fetal number reduces uterine capacity and nutrient allocation in ewes carrying multiples (Pérez-Clariget et al. 2023). As a result, multiple born lambs exhibited lower birth weights, more pronounced asymmetry, and slower postnatal growth trajectories compared to singletons. These findings highlight the compounded vulnerability of multiple fetuses in a nutrient deficient environment. Interestingly, sex effects were minimal in this study. Male lambs only differed from female in body weight during the first week of life and otherwise did not differ. While this pattern was not expected, it is not unprecedented as a recent study also reported sex effects were lost in multiples when maternal restriction occurred during gestation (Sales et al. 2024). Collectively, these results demonstrate that both birth type and intrauterine environment interact to shape postnatal growth, with multiples disproportionately affected, which has important implications for flock management, feed allocation, and productivity.

Conclusion

Poor-quality nutrition during gestation, representative of winter forage conditions in extensive sheep production systems in the Intermountain West, adversely affects maternal body condition and compromises offspring performance. Although birthweights were similar, lambs born to ewes receiving a poor-quality gestational diet exhibited impaired thermoregulation and reduced growth through weaning, traits that may not be immediately apparent at birth but carry significant economic consequences. These findings underscore the importance of maternal nutrition beyond late gestation and challenge the assumption that only severe restriction warrants intervention. Strategic supplementation during mid-gestation may mitigate developmental programming effects and improve lamb viability and productivity. Future research should explore the mechanistic basis of these adaptations and evaluate long-term outcomes in growth, carcass quality, and reproductive performance to fully understand the impact of gestational nutrition. This knowledge could be utilized to evaluate the economic tradeoff between increased/improved supplementation strategies in extensive operations and lamb performance throughout life to inform management decisions.

Glossary

Abbreviations

AC

abdominal circumference

BCS

body condition score

BW

bodyweight

CBL

cannon-bone length

CC

crown circumference

CP

crude protein

CRL

crown-rump length

DM

dry matter

dGA

days of gestation

TDN

total digestible nutrients

Author contributions

Rachael M. Stucke (Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing—original draft, Writing—review & editing), Alison M. Kuderka (Investigation, Methodology, Writing—review & editing), Ellen A. Roberts (Investigation, Methodology, Writing—review & editing), Ryne D. Haggard (Investigation, Methodology, Writing—review & editing), Alejandro Quevedo (Investigation, Methodology, Writing—review & editing), Terry E. Engle (Conceptualization, Data curation, Investigation, Methodology, Validation, Writing—review & editing), and Caitlin N. Cadaret (Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing—original draft, Writing—review & editing)

Conflict of interest statement. The authors declare no real or perceived conflicts of interest.

Data availability statement

The datasets generated and analyzed during this study are available in the USDA Ag Data Commons.