Shearing during late pregnancy increases size at birth but does not alter placental endocrine responses in sheep

Abstract

Shearing during the latter half of pregnancy is a common practice to improve flock health and productivity. Previous studies have demonstrated that shearing pregnant ewes at mid or late pregnancy is associated with an increase in lamb birth weight. In the present study, we used singleton Polypay × Dorset pregnant sheep, to investigate the potential roles of placental function and changes in maternal metabolism in underlying this increased birth weight response. Two groups were randomly established and blocked at enrollment by animal body weight, body condition score and subcutaneous adipose tissue depth. The groups were shorn (SH; n=18) or not (C; n=20) at gestational day (GD) 107 ± 1 (mean±SEM). Weekly maternal plasma samples were collected between shearing and birth; but only six samples were assayed for progesterone, pregnancy-associated glycoproteins (PAG1), glucose and non-esterified fatty acids (NEFA). At birth, sex, birth weight, lamb survival, and newborn body mass index (BMI) were recorded. Maternal body weight during mid- to late-pregnancy was similar between groups. Shearing resulted in increased lamb birth weight and BMI (P < 0.05) regardless of fetal sex but did not affect the maternal concentration of PAG1 or progesterone from GD 100 to 142. After shearing (GD100) and up to lambing, shorn females had higher circulating glucose concentrations (P < 0.05), but not NEFA, compared to the control group. Maternal circulating PAG1, progesterone, glucose or NEFA concentration across pregnancy did not differ according to lamb sex. Across pregnancy, birth weight was positively associated with PAG1 (P < 0.001), but not with progesterone concentrations. In conclusion, weight and BMI at birth was higher in both sexes upon shearing in singleton pregnancies. Despite PAG1 being associated with birth weight, late-pregnancy shearing did not alter the placental endocrine response. Whether other placental factors are altered upon shearing and may influence the increase in birth weight and BMI remain to be investigated.

Introduction

Shearing ewes during the latter half of pregnancy is a common management practice to improve flock health and productivity. Shearing during mid- to late-pregnancy increases voluntary feed intake (Parker et al., 1991) and milk yield without affecting milk composition (Sphor et al., 2011). In addition, shearing may enhance beneficial maternal and newborn behaviors that facilitate bonding and hence lamb survival at birth (Kenyon et al., 2006; Banchero et al., 2010). More importantly, mid- to late-pregnancy shearing increases birth weight of lambs, which has been associated in a few studies with higher feed intake rate (Vipond et al., 1987). The positive association between pregnancy shearing and increased birth weight has been reported in both singleton and twin pregnancies. This positive relationship however is not consistent, with some studies demonstrating that shearing increased birth weight in singletons, but not in twins and vice versa (Morris et al., 2000; Revell et al., 2002; Kenyon et al., 2006; Jenkinson et al., 2009). However, it remains unknown if mid-pregnancy shearing affects female and male lambs alike. Overall, the mechanisms underlying shearing-induced lamb birth weight and milk production increases remain unclear.

Previous studies have investigated the role of alterations in maternal metabolism mediating the shearing-induced increase in lamb birth weight. Pregnancy shearing increases maternal energy requirements which appear largely to be met by increased non-esterified fatty acids (NEFA) utilization as demonstrated using indirect, open-circuit calorimetry (Symonds et al., 1986). Shearing has been demonstrated to increase maternal glycemia (Symonds et al., 1986; Morris et al., 2000) and to decrease the response of insulin to a glucose challenge (Revell et al., 2000). Other studies have demonstrated that stress is not responsible for the increased birth weight upon shearing (Corner et al., 2007). Moreover, studies have observed conflicting results on the effect of mid-pregnancy shearing on maternal plasma insulin-like growth factor 1 (IGF-1), with either higher (Jenkinson et al., 2009) or lower (Revell et al., 2000) circulating IGF-1 in shorn ewes. Similarly, circulating concentration of NEFA was not altered (Symonds et al., 1986) or slightly increased after shearing (Symonds et al., 1988). Shearing ewes during pregnancy results in a transient increase in triiodothyronine (Symonds et al. 1988; Morris et al., 2000), although it does not seem to be the only contributing factor responsible for the shearing-induced birth weight effect. Despite findings suggesting the potential for a redirection of maternal glucose to the fetal-placental unit, there is a lack of direct evidence to confirm this.

Placental size and nutrient transfer capacity are known determinants of fetal growth and birth weight (Bell and Ehrhardt 2002). In the placenta, trophoblast binucleate cells secrete pregnancy-associated glycoproteins (PAGs; Green et al., 2000). PAGs can be used as a marker of placental function during gestation (Roberts et al., 2017) and are good indicators of feto-placental well-being (Gingrich et al., 2018). Thus, it is possible that the increased size at birth observed in shorn ewes may be explained by increased placental secretory capacity (PAGs and/or progesterone). To address this hypothesis, our aim was to determine if late-pregnancy shearing results in higher maternal circulating PAGs and/or progesterone leading to increased birth weight and size of the progeny. Given the sexual dimorphism in lamb weight (Gardner et al., 2007; Rosales Nieto et al., 2018), we also investigated if there was an interaction between birth weight effect in shorn sheep and sex of the lambs.

Material and Methods

All procedures in this study were approved by the Institutional Animal Care and Use Committee of Michigan State University (MSU) and are consistent with National Research Council’s Guide for the Care and Use of Laboratory Animals. The study was conducted at the MSU Sheep Research Facility (East Lansing, MI; 42.7°N, 84.4°W).

Animals

To investigate the effect of late-pregnancy shearing on placental function and pregnancy outcomes, 100 purebred Dorset and crossbred Polypay × Dorset were bred for 34 days (two full reproductive cycles) between August and September under natural conditions at the MSU Sheep Teaching and Research Center, East Lansing (MI) to purebred Polled Dorset rams (n=2, half-siblings). After the end of the breeding period, all ewes, were combined and moved into a 25-ha paddock further subdivided with electric fencing and with constant access to clean water to allow for 2–3 day grazing periods per subdivided paddock. After fetal number and gestational age were determined by ultrasonography (see details below), a total of 38 Polypay × Dorset ewes pregnant with singletons were enrolled in the study. From breeding to the day before ewes were moved indoors, the outdoor temperature ranged between 1 – 28 °C in October and −8 to - 18 °C in November. On average, when the pregnant females were 85 days of pregnancy (mean ± SEM; range 76 to 98 gestational days) they were moved indoors and housed in two 20 × 8 m pens until delivery (see treatment details below). Indoor temperature was never below freezing. All females were kept under the same environmental conditions and received the same diet througout the study (see details below).

Treatments and sample collection

Two groups were randomly established and blocked at enrollment by parity, animal body weight, body condition score and subcutaneous adipose tissue depth (Table 1). The groups were kept separately and were established to be shorn (Shearing (SH); n = 18) or not (Control (C); n = 20) at a gestational day (GD) 107 ± 1 (mean ± SEM; range: 98 to 120 gestational days). Control females were sham-shorn as follows to minimize differences in management and pregnancy stressors between study groups. Animals were handled for a similar duration to conventional shearing (approximately 3 min), but ensuring that no wool was removed. All females had approximately 8 months of wool growth prior to shearing. Females were machine shorn using 13 tooth, 95 mm wide combs (Heiniger ‘Quasar” combs, Heiniger, USA) leaving approximately 6 mm of stubble. All fleeces were weighed.

Table 1.

Maternal characteristics at gestational day (GD) 85 prior to shearing or sham-shorn (control) in Polypay × Dorset pregnant sheep.

	Control	Shearing	SEM1	P value
Body weight (kg)	72.2	73.8	6.4	0.64
Body score condition (points)	3.0	3.1	0.09	0.04
Fat depth (mm)	0.30	0.39	0.06	0.03

¹Standard error of the mean from the mixed model output.

Maternal plasma samples were collected weekly beginning at the estimated GD50 and until postnatal day 1. All samples were collected at the same day of the week and at the same time (starting at 8:00 am). Only six of the collected samples; on gestational days (GD) 85, 100, 115, 137, 142 and the day after birth (postnatal day 1) were analyzed. In this study, we used a controlled mating period (34 days) and samples on gestational days were calculated based on the lambing date and pregnancy length of 145 days. Although synchronization protocols have been used in previous shearing studies (Revell et al., 2000; Kenyon et al., 2002; Corner et al., 2007) or not (De Barbieri et al. 2018), weekly sampling and later alignment of samples to the conception date provided an accurate assessment of gestational dates relative to shearing time.

Blood samples were collected by jugular venipuncture into 10 ml collection tubes containing sodium heparin (BD Vacutainer, Preanalytical Solutions, Franklin Lakes, USA) using 18 gauge 1 inch vacutainer needles. Blood samples were placed inmediately on ice and then centrifuged at 3000 rpm for 15 min. Plasma was separated into three aliquots and frozen at −20 °C until analysis for PAG1, progesterone, glucose, and NEFA. Analytical details are described below.

Diet

Females grazed following breeding to GD 85 on a pasture consisting of orchard grass (Dactylis glomerata), rye grass (Lolium perenne) and red clover (Trifolium pratense). During indoor housing, and until lambing, females received the same diet and were fed a total mixed ration (TMR) diet consisting of corn silage, mixed grass and legume silage and dry corn grain combined with a micronutrients fed at levels to meet the NRC (2007) requirements for energy according to stage in a singleton pregnancy. Females were group-fed and the TMR diet was offered daily between 3 to 4 pm. Nutritional composition of the TMR diets included dry matter, metabolizable energy and crude protein were estimated by wet chemistry methodologies (Dairy One, Ithaca, NY, USA) and are detailed in Table 2.

Table 2.

Nutrient composition (dry matter basis) of the total mixed ration (TMR) diet and energy and dry matter offered to Polypay × Dorset pregnant sheep during breeding (0–34 gestational days) mid-pregnancy (85–115 gestational days), and late pregnancy (116 gestational days to term).

	Breeding	Mid-Pregnancy	Late Pregnancy
ME (Mcal/kg)	2.35	2.35	2.42
CP (%)	12.3	13.5	14.1
ME intake (Mcal/ewe/day)	2.61	2.82	3.48
DM intake (kg/ewe/day)	1.11	1.20	1.44

CP: Crude protein; DM: Dry matter; ME: Metabolizable energy.

Ultrasonographic assessment

Pregnancy and number of fetuses were confirmed by transabdominal ultrasonography 75 days following the introduction of the rams and 44 days after the ram removal. Ultrasonography was performed with a 4 MHz transabdominal probe (GE LOGIQ Book XP Vet; Boston, MA, USA). Pregnancy was confirmed by detection of uterine fluid, placentomes, fetal membranes, or fetus. Gestational age was confirmed by assessing uterine depth (in early pregnancy), fetal length from crown to rump, biparietal diameter, and calcification of the fetal ribs and skull (Roberts et al., 2017).

Newborn outcomes

Within the first 3 hours after birth, sex, birth weight, lamb survival, and newborn morphological measures (body length, height, head length and width) were recorded. These morphological measures were used to determine body mass index (BMI), as an indicator of body growth and development, and calculated using following equation; BMI: (body weight (kg) / withers height (m) / body length (m)) × 10 as described by Tanaka et al. (2002).

Circulating hormonal, protein concentrations and metabolites

The Bovine Pregnancy Test Kit (IDEXX Laboratories) was used to semi-quantitatively determine PAG1 and has been validated for ovine species as previously described (Roberts et al., 2017). This kit detects the variants PAG-4, PAG-6, PAG-9, PAG-16, and PAG-19. Here, we use the term PAG1 to refer to the group of modern PAGs detected by this assay and secreted by the binucleate cells of the placenta (Sousa et al., 2006). In brief, samples were incubated in anti-PAG antibody coated plates followed by incubation with anti-PAG antibody and anti-IgG-horseradish peroxidase. The colorimetric reaction was read (450 nm) on a spectrophotometer (Elx808, BioTek, Winooski, VT, USA). Sample values were reported as serum sample minus negative controls after subtracting the mean absorbance value of the negative controls from the absorbance of each sample value. Values are expressed as optical density (OD). The intra-plate and inter-plate coefficient of variation (CV) for the positive controls were 2.7% and 6.3%, respectively.

Maternal plasma samples on GD 85, 115, 137, and postnatal day 1 were assayed for progesterone. Progesterone concentrations in plasma were determined using a commercially available direct, competitive ELISA assay (Ridgeway Science, Gloucestershire, United Kingdom) following manufacturer instructions as previously described (Roberts et al., 2017). Progesterone concentrations for each sample and plate control were calculated from the standard curve generated with a 4-parameter logistic regression model (Gen5 2.06.10, BioTek, Winooski, VT, USA). Intra- and inter-plate CV for the plate control were 3.6% and 8.1%, respectively.

Maternal plasma samples on GD 85, 100, 115, 137, and 142 were measured in duplicate and assayed for glucose by the enzymatic method using a commercially available kit (Sigma-Aldrich; Darmstadt, Germany) following manufacturer instructions as previously described (Karkalas, 1985). Concentrations of glucose were determined using an automated microplate spectrophotometer (SpectraMax 5, Molecular Devices, LLC, San Jose, CA, USA). Mean intra-assay CV based on two quality control pools measuring 77.6 ± 1 and 91.1 ± 1.9 mg/dl were 3.2 and 3.1%, respectively. The inter-assay CVs for the same quality control pools averaged 3.3 and 5.2%, respectively.

Maternal plasma samples on GD 85, 100, 115, 137, and 142 were measured in duplicate and assayed for NEFA using a commercially available kit (Wako Chemicals, Dallas, TX) a following manufacturer instructions as previously described (Matsubara et al., 1983). Concentrations of NEFA were determined using an automated microplate spectrophotometer (Elx808, BioTek, Winooski, VT, USA). Mean intra-assay CV based on three quality control pools measuring 122.1 ± 3.9, 180.9 ± 6.8 and 221.4 ± 7.1 mg/ml and were 5.3, 4.5 and 2.6, respectively. The inter-assay CVs for the same quality control pools, averaged 7.2, 7.1, and 9.1%, respectively.

Statistical analysis

Maternal body weight was analyzed using the linear mixed model procedures (PROC-MIXED). Fixed effect in the model was treatment and lamb sex. PAG1, progesterone, glucose and NEFA separated for gestation age were analyzed using the linear mixed model procedures (PROC-MIXED). Fixed effects in the model were treatment and lamb sex. Maternal body weight at sampling and progeny birth weight were included independently as covariates. In addition, circulating maternal concentration across pregnancy of PAG1, progesterone, glucose and NEFA were analyzed using the linear mixed model procedures (PROC-MIXED) allowing for repeated measures. The relationship between PAG1 across pregnancy and BWT was predicted using mixed models (PROC-MIXED) allowing for repeated measures. Birth weight and BMI were analyzed using the linear mixed model procedures (PROC-MIXED). Fixed effects in the model were treatment and lamb sex. The correlations among PAG1, progesterone, maternal body weight and birth weight were predicted using PROC GLM with MANOVA option which allows removal of major fixed effects. Fixed effects included in the model were treatment and lamb sex. All 2-way interactions among the fixed effect and covariates were included in each model and non-significant (P > 0.05) interactions were removed from the analysis. Significant differences between treatments for variables measured at different time points during pregnancy were analyzed using LSD of PROC GLM. Statistical significance was established at P < 0.05. All data analyses were performed using the SAS statistical package SAS version 9.3 (2010).

Results

Offspring outcomes

Late-pregnancy shearing resulted in increased birth weight and BMI in singleton pregnancies (P < 0.05; Table 3). Male lambs were heavier and larger than female lambs (P < 0.05; Table 3). The interactions between birth weight and lamb sex and between BMI and lamb sex were not significant (P > 0.05; Table 3).

Table 3.

Effect of mid-pregnancy shearing or sham shearing (control) in Polypay × Dorset pregnant sheep on progeny weight and BMI at birth (mean ± SEM).

Treatment	Control	Shearing	SEM1	P value
Birth Weight (kg)	5.0	5.4	0.34	0.03
BMI	0.40	0.44	0.04	0.04
Sex	Female	Male
Birth Weight (kg)	4.9	5.5	0.34	0.005
BMI	0.39	0.44	0.04	0.01
Birth Weight Treatment * Sex				0.6
BMI Treatment * Sex				0.4

¹Standard error of the mean from the mixed model output.

BMI: Body Mass Index.

For treatment, data are combined for males and females. For sex, data are combined for control and shearing treatment.

Maternal outcomes

At enrollment (GD85), maternal body weight in control and shearing groups averaged 71.7 ± 3.1 and 73.8 ± 3.1 kg, respectively (P > 0.05; Table 1; Figure 1). At GD147, body weights were similar in the control and shearing groups (P > 0.05; C: 84.9 ± 3.7 kg vs. SH: 86.3 ± 3.2 kg; Figure 1). On average, females from the shearing treatment lost 2.4 kg of body weight of which ~75% was fleece weight (average: 1.8 ± 0.1 kg). From GD85 until lambing, body weight change in control and shearing groups averaged 228 ± 16 g/day and 218 ± 10 g/day, respectively and did not differ according to treatment (P > 0.05).

Figure 1.

Mean (± SEM) body weight through pregnancy in Polypay × Dorset pregnant sheep shorn (black line) or not (gray line; control group) on gestational day 100. Dotted line denotes shearing time. PND1 denotes postnatal day 1.

Circulating PAG1 increased from GD85 until GD142 in both the control and the shearing group (P < 0.05). As expected, both groups showed a decline in maternal PAGs between the last pregnancy sample (GD142) and the first lambing sample (postnatal day 1; Figure 2). Late-pregnancy shearing did not affect the concentration of PAG1 by gestational age or across pregnancy and was similar to that of the control group (P > 0.05; Figure 2). PAG1 across pregnancy did not differ between fetal sex or the interaction between treatment and fetal sex (P > 0.05). Circulating progesterone increased from GD85 until GD137 in both control and shorn groups (P < 0.05). Similar to that observed for PAG1, progesterone concentrations were reduced at lambing. Late-pregnancy shearing did not influence the concentration of progesterone by gestational age or across pregnancy (P > 0.05; Figure 2). Maternal progesterone concentration across pregnancy did not differ according to the fetal sex and there was no interaction between treatment and fetal sex (P > 0.05).

Figure 2.

Mean (± SEM) maternal circulating pregnancy associated glycoprotein (top panel; PAG1; Optical Density [OD]) and progesterone (middle panel) in in Polypay × Dorset pregnant sheep that were shorn (black line) or not (gray line; control group) on gestational day 100. Dotted line denotes shearing time. PND1 denotes postnatal day 1. Bottom panel: Relationship between PAG1 concentrations and birth weight across pregnancy (P < 0.01). Dotted lines represent upper and lower 95% confidence limits.

Maternal circulating glucose concentration was similar between treatments before shearing at GD85 and GD100 (P > 0.05; Figure 3). After shearing (GD100) and up through lambing, shorn females had higher circulating glucose concentrations compared to the control group (P < 0.05; Figure 3). Maternal circulating glucose concentration across pregnancy did not differ according to fetal sex (P > 0.05). No interaction was observed in glucose concentrations between treatment and fetal sex (P > 0.05). Maternal circulating glucose concentration across pregnancy was not related to birth weight or BMI (P > 0.05). Late-pregnancy shearing did not influence maternal concentration of NEFA by gestational age or across pregnancy and was similar to control group (P > 0.05; Figure 3); except on GD115 when maternal concentration of NEFA tended to differ (P = 0.06; Figure 2). NEFA across pregnancy did not differ according to fetal sex or in the interaction between treatment and fetal sex (P > 0.05).

Figure 3.

Mean (± SEM) maternal circulating non-esterified fatty acids (NEFA, mEq/ml; top panel) and glucose (ng/ml; bottom panel) in in Polypay × Dorset pregnant sheep that were shorn (black line) or not (gray line; control group) on gestational day 100. Dotted line denotes shearing time. * P < 0.05, ** P < 0.01, NS: No significant.

Association among variables

A significant positive correlation was observed between circulating PAG1 and progesterone (r: 0.48; P < 0.001) and between circulating PAG1 and birth weight (r: 0.19; P < 0.05). The correlation between maternal body weight and birth weight was also positive (r: 0.36; P < 0.001). There were no significant correlations between PAG1 and maternal body weight (r: 0.06; P > 0.05), circulating progesterone and maternal body (r: 0.15; P > 0.05) or between circulating progesterone and birth weight (r: 0.016; P > 0.05). Across pregnancy, circulating PAG1 concentration, but not progesterone, was positively associated with birth weight (P < 0.001; Figure 2).

Discussion

Previous work aiming to understand the underlying mechanism by which lamb birth weight increases after shearing during late pregnancy has investigated maternal feed intake, maternal glycemia and maternal circulating growth factors (Symonds et al., 1986; Parker et al., 1991; Morris et al., 2000; Jenkinson et al., 2009). However, the potential role of the placenta has remained unexplored thus far, which was the aim of this study. Similar to previous work (Kenyon et al., 2002; Corner et al., 2007), we observed that late-pregnancy shearing increased birth weight in singleton pregnancies irrespective of offspring sex. However, our findings demonstrate that neither the placental endocrine nor the maternal metabolic function evaluated in this experiment were associated with the lamb birth weight increase.

Effect of late-pregnancy shearing on weight and size at birth of progeny

The observed increase in lamb birth weight upon late-pregnancy shearing is consistent with that observed in Border Leicester × Romney and Romney sheep (Kenyon et al. (2002; 2006). In our study, ewes were fed a diet to meet the requirements for conceptus growth (NRC 2007), which appears to be a major consideration necessary for a shearing-induced increase in size at birth. This is particularly important because restriction of the maternal diet in experimental conditions or via natural limitations of some pastoral environments that fail to meet nutrient requirements, do not result increased size at birth associated to shearing (Kenyon et al. 2002; 2006; Banchero et al, 2010).

As previously reported (Gardner et al., 2007; Rosales Nieto et al., 2018) male offspring were also larger in the present study, but no interaction between fetal sex and shearing on size at birth was found. Differences in birth weight were paralleled by changes in body morphometry as measured by BMI. Body morphometry was also previously observed to be altered by late-pregnancy shearing (Corner et al., 2006; deNicolo et al., 2008) with lambs from shorn ewes exhibiting larger girth and crown to rump length. This is consistent with our findings where lambs born to shorn ewes had a larger BMI to that of control ewes.

Effect of shearing on maternal body weight

Late-pregnancy shearing resulted in maternal body weight change with a setback in body weight immediately after shearing due to wool loss. This weight loss was followed by a compensatory weight gain a week later. These findings align with studies where females animals lost weight immediately after shearing, but recovered weight post-shearing (Symonds et al., 1988; Morris et al., 2000; deNicolo et al., 2008). The observed post-shearing weight loss is likely the combination of fleece removal along with some adipose tissue mobilization.

Effect of late-pregnancy shearing on placental function

Consistent with previous reports, maternal circulating PAGs, a marker of placental function during gestation (Roberts et al., 2017), increased across pregnancy in singleton pregnancies and began to decline a day after lambing in control females (Roberts et al., 2017). Shearing however did not affect maternal circulating PAGs or progesterone, the two indices of placental endocrine function evaluated. This adds to the understanding of the role of the placenta in shearing-induced fetal weight gain as placental growth is not affected by shearing in mid-pregnancy (Revell et al., 2002).

We also evaluated whether or not circulating PAG1 were different in ewes according to the sex of the fetus. Despite studies reporting higher PAG concentrations in male-carrying pregnancies (Ranilla et al., 1994), we did not observe a difference in circulating PAG1 between female and male pregnancies irrespective of shearing during late pregnancy. Because the IDEXX PAG1 assay used only measures 5 out of 15 ovine PAGs (Sousa et al., 2006), and the role of PAGs in ruminant pregnancies remains obscure, whether other PAGs may be involved in the shearing related increase in fetal weight remains to be determined.

Similar to previous reports (Ranilla et al., 1994), maternal progesterone increased across pregnancy only to decline a day after lambing in all females. Similar to PAG1, circulating progesterone was not altered in shorn females. Despite circulating progesterone being positively associated with fetal growth (Kleemann et al., 1994), maternal progesterone did not vary according to fetal sex and was not affected by late-pregnancy shearing. These findings extend our previous work demonstrating a linear increased in progesterone across pregnancy and a positive relationship between PAG1 and progesterone (Roberts et al., 2017). Furthermore, we observed a positive relationship between circulating PAG1 and birth weight, as previously reported when both, singleton and twin pregnancies were combined (Vandaele et al., 2005). The strong association between PAG1, but not progesterone, and birth weight supports the concept that PAG1 may play a role in fetal growth.

Effect of late-pregnancy shearing on maternal metabolic factors

In females, maternal glucose increased immediately post-shearing and remained elevated to term similar to previous work (Symonds et al., 1986; Morris et al., 2000). The increase in glucose is likely a metabolic adaptation to the cold exposure after fleece removal (Symonds et al., 1988; Revell et al., 2000) and, it hypothesized to be related to the birth weight response. Additional work is needed to document that maternal metabolism and fetal-maternal glucose partitioning may be altered upon shearing in favor of increased fetal growth.

Late pregnancy is generally a lipolytic state, even in well-fed ewes, as maternal tissues become more dependent on NEFA so glucose is spared for the growing conceptus (Petterson et al., 1994). Maternal NEFA, an indicator of adipose tissue mobilization, was not consistently affected by shearing throughout pregnancy similar to those reported in Leicester × Swaledale ewes (Symonds et al., 1986). However, in a different experiment by the same authors, circulating NEFA differed between groups (control vs. shorn) only 3 days after shearing, but not thereafter (Symonds e t al., 1988). Similar to Symonds et al. (1988), our data reflect a transient elevation of NEFA following shearing that was not sustained. Despite a lack of sustained NEFA response in shorn ewes, it is possible that shearing in combination with exposure to a cold environment creates an increased reliance of maternal tissues on NEFA as a primary source of energy.

In conclusion, late-pregnancy shearing increased weight and BMI at birth in singleton pregnancies. The transient elevation of maternal glucose circulating concentration was observed upon late-pregnancy shearing was not associated with birth weight. The placental endocrine response (progesterone and PAG1) and maternal NEFA circulating concentration was not altered by late-pregnancy. Whether other placental factors are altered upon shearing and are responsible for the increase in birth weight remains to be investigated.

Implications.

Shearing during mid-pregnancy has been demonstrated to have several positive outcomes for sheep farm profitability including increased birth weight. Our results confirm that shearing during mid-pregnancy increases birth weight and indicate that increased fetal size is not associated with indices of placental endocrine function. Increased size of lambs at birth improves survival in early life with these benefits extending into early postnatal life manifest as improvements in early postnatal growth performance. Therefore, mid-pregnancy shearing is advised for sheep housed in late pregnancy as a means of improving neonatal survival and growth performance in early life.