Does offering concentrate supplement during late pregnancy affect twin- and triplet-bearing ewe and lamb performance?

Abstract

This study investigated the effect of offering concentrate supplement during late pregnancy on twin- and triplet-bearing ewe and lamb performance. Twin- (n=40) and triplet-bearing (n=20) ewes were grazed on a 6 cm herbage height from day 70 of pregnancy until parturition. From pregnancy day 102, half of the ewes from each litter size were offered 400 g/ewe per day of concentrate sheep pellets. From day 102 until day 145 of pregnancy, ewes offered concentrate gained 60 g more liveweight per day than ewes offered pasture only (P<0.01). Ewes offered concentrate were also under less metabolic stress in late pregnancy, as indicated by lower (P<0.05) plasma beta-hydroxybutyrate (BOH) and non-esterified fatty acid (NEFA) concentrations. Offering concentrate increased lamb birth weight from 3.9 to 4.2 kg (P<0.05) and tended to increase lamb plasma gamma-glutamyl-transferase (GGT) concentrations at age 24–36 h (P = 0.08). It had no effect, however, on lamb plasma glucose or immuno-globulin G (IgG) concentrations within 24–36 h of age, lamb growth from birth until day 52 of lactation or lamb survival. Ewes offered concentrate reared a greater total weight of lamb to day 52 of lactation than ewes offered pasture only. The economic viability of offering a concentrate to these ewes grazing a 6 cm sward could not be established.

Keywords:

• sheep
• nutrition
• pregnancy
• concentrate supplementation
• twin- and triplet-bearing ewes
• ewe liveweight
• ewe body condition
• lamb birth weight

Introduction

Twin- and triplet-bearing ewe intake of herbage has been shown to be unrestricted when grazing ryegrass and white clover swards that are 4 cm or greater in height (Morris & Kenyon 2004). While herbage intake may be unrestricted at these sward heights, triplet-bearing ewes still need to mobilise their body reserves to maintain triplet-foetal growth in late pregnancy (Barry & Manley 1985; Morris & Kenyon 2004). These results suggest that ewes with large litters may be unable to meet their nutritional requirements when grazing an ad libitum diet of ryegrass and white clover. The high energy demand of triplet-bearing ewes during late pregnancy and their potential inability to consume enough nutrients in late pregnancy (Everts 1990) may explain why triplet-born lamb birth weights (Kenyon et al. 2005a; Dwyer & Morgan 2006), survival rates (Thomson et al. 2004; Kerslake et al. 2005) and growth rates to weaning (Morris & Kenyon 2004) are often disappointing.

Triplet-bearing ewes may find it difficult to consume enough pasture during late pregnancy because of limited space in the rumen (Everts 1990), and/or the bulk, high water content and greater fibre content of the pasture. The nutrient intake of ewes can be increased by offering concentrate supplements that are less bulky than pasture, have lower water content and less fibre (Dove 2002). This is supported by the finding of Orr & Treacher (1990) who showed that the total nutrient intake of twin- and triplet-bearing ewes is increased by offering concentrate supplement with a grass silage diet.

While offering concentrates to ewes grazing low-quality low herbage allowances in late pregnancy has been shown to have a positive effect on ewe and/or lamb performance (Hall et al. 1992; Stephenson & Bird 1992; Banchero et al. 2004), only a few studies have looked at offering concentrate on a high herbage allowance (Hinch et al. 1996; Kerslake et al. 2008 ,2009). The aim of this study was thus to investigate the effects of offering concentrate supplements during late pregnancy, to twin- and triplet-bearing ewes grazing a 6 cm sward height, on ewe and lamb performance.

Materials and methods

Experimental design

The experimental design was a replicated 2×2 factorial design, which included two litter sizes (twin- and triplet-bearing ewes) and two nutritional treatments: 6 cm herbage sward height with concentrate supplement (concentrate) or 6 cm herbage sward height only (non-concentrate).

The oestrus cycles of 509 (307 two-tooth and 282 mixed-aged) Romney ewes were synchronised using a progesterone-releasing device (Eazi-breed CIDR®, Pfizer, Auckland, New Zealand). The CIDR was inserted for 13 days and, on removal, ewes were injected with 400 IU of PSMG (Folligon®, Intervet, Boxmeer, Holland). At this time, ewe liveweight and body condition score (BCS) were recorded. BCS was measured on a scale of 1–5, 1 describing a ewe that was extremely emaciated and 5 a ewe that was very fat (Jefferies 1961). After weighing and body condition scoring, Romney ram hoggets were fitted with crayon mating harnesses and were introduced for a 5-day breeding period. At 49 days of pregnancy (P49), after the mid-point of the breeding period, crayon-marked ewes were weighed, body condition scored and pregnancy diagnosed using ultrasound technology. Ewes were marked as being non-pregnant or single-, twin- or triplet-bearing for future identification. All ewes were grazed as one group under commercial grazing conditions from the commencement of the breeding period until P70.

On P70, 20 twin- and 10 triplet-bearing ewes were allocated to a 6 cm herbage sward height (non-concentrate) and 20 twin- and 11 triplet-bearing ewes were allocated to a 6 cm herbage sward height with concentrate supplement (concentrate). A 6 cm sward height was chosen as previous research has shown that twin- and triplet-bearing ewe intake is unrestricted when grazing a sward height greater than 4 cm (Morris & Kenyon 2004). Each ewe nutritional treatment had two replicates and was balanced for ewe liveweight and body condition (pooled means of ewe liveweight and BCS for concentrate and non-concentrate treatment groups were 63.8 and 62.8 kg and 2.9 and 2.9, respectively). Throughout the remainder of the study, ewes were rotationally grazed weekly around four 2 ha paddocks at a stocking rate of 7 or 8 ewes/ha. Rotational grazing limited any potential paddock effects such as the potential sparing of pasture from pregnancy into lactation during supplementation.

From P102 until P145, concentrate-fed ewes were offered 400 g/day per ewe of sheep pellets (903 g/kg dry matter (DM), 141 g/kg DM crude protein (CP), 126 g/kg DM of lipid, calculated metabolisable energy (ME) content of 12.3 MJ/kg DM; Universal stock feed, Harvey Farms Ltd, Wanganui, New Zealand) in two 2.5×0.4 m troughs. The amount of 400 g/day was chosen because previous research has shown that offering 400 g/day to twin-bearing ewes grazing a restricted herbage sward height increases lamb birth weight (Kerslake et al. 2008), whereas offering 200 g/day did not (Kenyon et al. 2005b). During the concentrate feeding period, troughs were checked daily for residuals; however, there were none during the entire feeding period. Four weeks after the mid-point of lambing, all ewes were grazed as one group under commercial grazing conditions. Three triplet-bearing ewes and one twin-bearing ewe died during pregnancy.

The experiment was carried out at Kebble Farm, Massey University, Palmerston North, New Zealand (longitude 40.2°S, latitude 175.3°E, elevation 64 m). The experimental period was from 8 March to 10 October 2005 and the lambing period was from 12 August to 22 August 2005. During the lambing period, the average wind speed was 2.0 m/s, average rainfall 0.4 mm, average temperature 9.1°C (minimum 4.8°C, maximum 15.3°C), with an average cold stress index of 966.49 kJ/m² per h (Donnelly 1984).

Herbage management

Throughout the study, ewes and lambs were grazed on a ryegrass (Lolium perenne) and white clover (Trifolium repens) sward that was at least 4 years old. To ensure that ewes were grazed within a controlled range of herbage heights, herbage sward surface heights were measured at weekly intervals using a sward stick (Jenquip, New Zealand, 100 readings per paddock). If the herbage height was found to be below 5 cm, ewes were placed in an alternative paddock that had been prepared at the appropriate sward height. If above 7 cm, additional ewes were introduced to control the sward height. The aim was therefore to maintain the defined sward conditions irrespective of size of area grazed. Herbage mass was measured at two-weekly intervals using a rising plate meter (Ashgrove Pastoral Products, New Zealand, 50 readings per paddock) and was calculated from:

where MR is the meter reading (Hodgson et al. 1999).

Animal measurements

Unfasted liveweight and BCSs of ewes were measured at P70, P87, P102, P116 and P140. On P144, ewe back-fat depth on the left-hand side above the loin area (of the last rib) (Ultrasound, Auckland (Purchas & Beach 1981)) and ewe udder size were measured. Three measurements of udder size were taken from the posterior margin to the anterior margin of the udder. These included one along the midline of the udder and two parallel to the midline immediately medial to each teat (Mellor & Murray 1985). A 10 ml blood sample was collected by jugular venepuncture (Lithium heparin, Becton Dickson Vacutainer Systems, USA) at P102, P116, P130 and P140.

Within 12 h of birth, all lambs were identified to their dam, tagged and their sex and birth rank determined. In addition, lamb birth weights, crown–rump lengths and thoracic-girth circumferences were recorded. Surface area of the lamb was calculated using the mathematical formula for the total surface area of a cylinder.

Between 24 and 36 h old, a 5 ml blood sample (Lithium heparin, Becton Dickson Vacutainer Systems, USA) was collected by jugular venepuncture from all lambs. All ewes and lambs remained in their treatment groups and were rotationally grazed on a 6 cm sward height until weaning. All ewes and lambs were weighed 29 days (L29) and 52 days (L52) after the midpoint of lambing (15 August 2005).

Blood analysis

All blood samples were placed on ice until they were centrifuged at 1000g for 15 min. The plasma was removed and frozen at −20°C. Ewe plasma samples were analysed using diagnostic kits for glucose (Hexokinase method, Roche Diagnostics Ltd, Switzerland), beta-hydroxybutyrate (BOH) and non-esterified fatty acid (NEFA) (Sigma, Illinois, USA) concentrations. Lamb plasma samples were analysed for immuno-globulin G (IgG) (Standard direct ELISA assay), gamma-glutamyl-transferase (GGT) (Roche, diagnostics Ltd, Mannheim, Germany), glucose (hexokinase method, Roche Diagnostics Ltd, Switzerland), thyroxine (T4) and triiodothyronine (T3) (Radioimmunoassay kit, Coat-a-count Diagnostic Products Corporation, CA, USA) concentrations.

Statistical analysis

Herbage sward surface heights and mass were analysed using a general linear model (PROC GLM, SAS, SAS Institute, Cary, NC, USA) that contained the fixed effects of replicate (one, two) and concentrate supplement (non-concentrate, concentrate). Ewe liveweight gain, back-fat depth, ewe udder size, weight at L52 and total weight of lamb produced at L52 were analysed using a general linear model (PROC GLM). Fixed effects included replicate (one, two), concentrate supplement (non-concentrate, concentrate), litter size (twin, triplet) and the interactions between the main effects. Interactions were retained in the model if significant (P<0.05). If non-significant, interactions were removed and the model re-fitted.

Lamb birth weight, SA, crown–rump length, thoracic-girth circumference and lamb plasma metabolites (T3, T4, glucose, GGT and IgG) at age 24–36 h were analysed using a general linear model (PROC GLM). Fixed effects included replicate (one, two) concentrate supplement (non-concentrate, concentrate), birth rank (twin, triplet) and interactions between fixed effects. Lamb birth weight was added as a covariate for all dependent variables apart from lamb birth weight and surface area to body weight ratio. Liveweight at L29 and L52 and liveweight gain from birth until L29, L52 or from L29–L52 were analysed using the same general linear model (PROC GLM). The fixed effect of birth rank (twin, triplet), however, was removed and replaced with rearing rank (twin reared as a single or twin, triplet reared as a twin or triplet). No triplet-born lambs were reared as singles. Lamb birth weight was also not adjusted for in these models.

The T3, T4, GGT and IgG plasma concentrations were not normally distributed. To achieve a normal distribution, T3, T4, GGT and IgG were subjected to a log₁₀ transformation before being analysed using a general linear model. Back-transformed means are presented in the tables.

All ewe measurements that were taken over time (liveweight, BCS, and BOH and NEFA concentrations) from day P102 to P140 were analysed using a repeated measure (PROC GLM) analysis. This model contained the fixed effects of replicate (one, two), concentrate supplement (non-concentrate, concentrate), litter size (twin, triplet), time and the interactions of the fixed effects. If significant (P<0.05), interactions were retained in the model. If non-significant, interactions were removed and the model re-fitted.

The proportion of lambs surviving from birth to weaning was analysed as a binomial trait using the SAS procedure for categorical modelling (PROC GENMOD). Fixed effects included concentrate supplement (non-concentrate, concentrate), litter size (twin, triplet) and the interactions of the fixed effects. If not significant (P>0.05), interactions were removed from the model.

There were no significant differences found between replicates throughout this experiment. For the remainder of this paper, only the differences between ewe nutritional treatments and birth ranks will be presented.

Results

Herbage sward surface height and mass

For both ewe nutritional treatments, herbage measurements from P70 to P145 did not differ significantly in sward height (non-concentrate 6.6±0.21 cm, concentrate 6.1±0.22 cm) or mass (non-concentrate 1671±108 kg DM/ha, concentrate 1539±108 kg DM/ha).

Ewe liveweight and BCS

On P116 and P140, ewes that were offered concentrate were heavier (than P<0.05) ewes offered pasture only (Table 1). Litter size had no effect on ewe liveweight at P102, P116 or P140. From P102 to P145, concentrate-fed ewes gained 60 g more per day than ewes offered pasture only (P<0.01). Litter size and nutritional treatment had no effect on ewe liveweight on L29 or L52 (pooled means and standard deviations of ewe liveweight at L29 and L52 were 67.7±7.05 kg and 61.9±7.31 kg, respectively).

Table 1  The effect of offering concentrate (non-concentrate vs. concentrate) and litter size (twin- vs. triplet-bearing) on ewe liveweight at pregnancy day 102 (P102), 116 (P116) and 140 (P140) and on liveweight gain between P102 and P140 (mean±SE). Means within treatments with differing superscripts are significantly (P<0.05) different.

		Liveweight (kg)			Liveweight (kg)	Liveweight gain
	n	P102	P116	n	P140	P102 to P140 (kg/day)
Nutritional treatment
Non-concentrate	30	68.6±0.44	73.2±0.55^a	29	78.8±0.70^a	0.27±0.01^a
Concentrate	31	68.2±0.43	74.8±0.54^b	28	81.0±0.71^b	0.33±0.01^b
Litter size
Twin	40	67.9±0.37	73.5±0.47	39	79.4±0.58	0.30±0.01
Triplet	21	68.9±0.51	74.4±0.63	18	80.4±0.85	0.30±0.02

Ewe BCS at P102, P116 and P140 and ewe back-fat depth measured at P144 were not influenced by litter size or nutritional treatment (pooled means and standard deviations of ewe BCS at P102, P116 and P140 were 2.9±0.38, 2.9±0.43 and 2.8±0.42, respectively, and ewe back-fat depth measurements at P144 were 5.2±1.82 cm).

Ewe metabolic status

Ewes offered concentrate had greater (P<0.05) plasma NEFA concentrations at P102 and P116, but lower plasma NEFA concentrations at P130 and P140 (Table 2). Compared with the ewes offered pasture only, offering concentrate had no effect on ewe plasma BOH concentrations at P102, increased plasma BOH concentrations at P116 (P<0.01) and decreased plasma BOH concentrations at P130 and P140 (Table 3).

Table 2  The effect of offering concentrate and litter size on ewe plasma non-esterified fatty acid (NEFA) concentrations at different stages of pregnancy (mean±SE). Means within treatments with differing superscripts are significantly (P<0.05) different

		NEFA concentration (mmol/l)^1
	n	P102	P116	P130	P140
Nutritional treatment
Non-concentrate	30	−0.76±0.07 (0.47)^a	−1.18±0.12 (0.31)^a	−0.55±0.12 (0.55)^b	−0.71±0.13 (0.49)^b
Concentrate	31	−0.53±0.07 (0.59)^b	−0.84±0.12 (0.43)^b	−1.06±0.12 (0.34)^a	−1.23±0.13 (0.29)^a
Litter size
Twin	40	−0.70±0.06 (0.50)	−1.08±0.11 (0.34)	−0.92±0.10 (0.44)	−1.02±0.11 (0.37)
Triplet	21	−0.59±0.08 (0.55)	−0.95±0.15 (0.39)	−0.68±0.14 (0.43)	−0.91±0.16 (0.38)
^1Data were log-transformed; transformed data with SE are presented with back-transformed means in parentheses.

Table 3  The effect of offering concentrate and litter size on ewe plasma beta-hydroxybutyrate (BOH) concentrations at different stages of pregnancy (mean±SE). Means within treatments with differing superscripts are significantly (P<0.05) different.

		BOH concentration (mmol/L)^1
	n	P102	P116	P130	P140
Nutritional treatment
Non-concentrate	30	−1.01±0.05 (0.36)	−1.07±0.04 (0.34)^a	−0.96±0.05 (0.38)^b	−0.56±0.05 (0.57)^b
Concentrate	31	−0.90±0.05 (0.41)	−0.90±0.04 (0.40)^b	−1.15±0.05 (0.33)^a	−0.94±0.05 (0.39)^a
Litter size
Twin	40	−0.97±0.04 (0.38)	−0.97±0.04 (0.38)	−1.03±0.04 (0.36)	−0.75±0.04 (0.47)
Triplet	21	−0.94±0.06 (0.39)	−1.00±0.05 (0.37)	−1.07±0.06 (0.34)	−0.74±0.06 (0.47)
^1Data were log-transformed; transformed data with SE are presented with back-transformed means in parentheses.

Ewe nutritional treatment had no effect on plasma glucose concentrations at P102, P116, P130 or P140 (pooled means and standard deviations of plasma glucose concentrations at P102, 116, 130 and 140 were 3.7±0.46, 3.9±0.64, 3.8±0.44 and 3.9±0.47 mmol/l, respectively). All ewe plasma metabolites measured were unaffected by litter size at P102 and 116, 130 and 140.

Ewe udder size

At P144, ewe nutritional treatment tended (P=0.08) to have a significant effect on ewe udder size (concentrate ewes 56.8±1.49 cm, non-concentrate ewes 53.2±1.47 cm). Triplet-bearing ewes had (P<0.05) larger udder sizes than twin-bearing ewes (57.2±2.05 cm and 51.2±1.03 cm, respectively).

Lamb liveweight and dimensions

Lambs born to ewes offered concentrate had heavier (P<0.05) birth weights than lambs born to ewes offered pasture only. In addition, twin-born lambs were heavier (P<0.001) than triplet-born lambs. Twin-born lambs had longer crown–rump lengths (P<0.001) and greater thoracic-girth circumference (P<0.001) than triplet-born lambs. This birth rank effect on lamb body dimensions, however, was not apparent after adjustment for lamb birth weight (Table 4).

Table 4  The effect of offering concentrate and lamb birth rank (twin- vs. triplet-born) on lamb birth weight, crown–rump length and thoracic-girth circumference (mean±SE). Means within treatments with differing superscripts are significantly (P<0.05) different.

	n	Birth weight (kg)	SA:Bw^1	Crown–rump length (cm)	Thoracic-girth circumference (cm)
Nutritional treatment
Non-concentrate	66	3.9±0.09^a	179.8±7.08	51.6±0.47	36.3±0.37
Concentrate	63	4.2±0.10^b	174.1±6.82	52.6±0.48	37.0±0.38
Lamb birth rank
Twin	78	4.5±0.09^a	165.8±7.04^a	53.4±0.43^a	38.0±0.34^a
Triplet	51	3.6±0.11^b	188.1±6.88^b	50.8±0.53^b	35.3±0.41^b
^1SA:BW, surface-area-to-birth-weight ratio.

Ewe nutritional treatment had no effect on lamb liveweight at L29, L52 or liveweight gain from birth until L29 or L52 (Table 5). Offering concentrate did, however, have a positive effect on liveweight gain from L29 to L52 (P<0.05).

Table 5  The effect of offering concentrate and lamb rearing rank (twin-born reared as single or twin vs. triplet-born reared as a twin or triplet) on weight at lactation day 29 (L29) and 52 (L52) and on weight gain from birth to L29 and L52, and from L29–52 (mean±SE). Means within treatments with differing superscripts are significantly (P<0.05) different

					Liveweight gain (kg/day)
	n	Liveweight at L29 (kg)	n	Liveweight at L52 (kg)	Birth to L29	Birth to L52	L29 to L52
Nutritional treatment
Non-concentrate	50	12.1±0.35	48	15.8±0.41	0.27±0.013	0.22±0.008	0.16±0.017^a
Concentrate	51	11.5±0.32	50	16.2±0.40	0.25±0.013	0.22±0.008	0.21±0.018^b
Lamb rearing rank
Twin reared as single	5	12.4±0.74^a	5	18.3±0.99^a	0.30±0.032^c	0.29±0.021^b	0.28±0.043^c
Twin reared as twin	62	12.2±0.27^a	61	16.0±0.30^bc	0.27±0.009^b	0.23±0.006^b	0.17±0.012^b
Triplet reared as twin	22	12.0±0.55^a	20	15.1±0.56^cd	0.24±0.016^b	0.21±0.011^ab	0.17±0.022^ab
Triplet reared as triplet	12	10.6±0.53^b	12	14.5±0.71^d	0.22±0.021^a	0.17±0.014^a	0.11±0.028^a

Triplet-born lambs reared as triplets were lighter (P<0.05) than all other rearing ranks on L29. On L52, triplet-born lambs reared as triplets were lighter (P<0.05) than all twin-born lambs. Twin-born lambs reared as singles were heavier (P<0.05) at L52 than all other rearing ranks. From birth until L29, twin-born lambs reared as singles gained a greater (P<0.05) amount of weight than all other rearing ranks. In addition, twin- and triplet-born lambs reared as twins gained a greater (P<0.05) amount of weight than triplet-born lambs reared as triplets. From birth to L52 and from L29 to L52, triplet-born lambs reared as triplets gained less (P<0.05) weight than twin-born lambs reared as either singles or twins.

Lamb plasma thyroid hormone concentrations

There was a significant interaction between ewe nutritional treatment and birth rank for both plasma T3 and T4 concentrations (P<0.05 and P<0.001, respectively) (Table 6). Triplet lambs born to ewes offered pasture only had lower plasma T4 and T3 concentrations than twin lambs born to ewes offered pasture only. There were no differences, however, between twin and triplet lambs born to ewes offered concentrate. Twin lambs born to concentrate-fed ewes had lower plasma T4 and T3 concentrations than twin-lambs born to ewes offered pasture only. There were no differences in plasma T4 and T3 concentrations between triplet lambs born to ewes offered concentrate or ewes offered pasture only. Lamb birth weight had a positive effect (P<0.001) on plasma T4 concentrations, where a 1 kg increase in lamb birth weight increased plasma T4 concentrations by 0.09 (±0.03 (SE)) nmol/l.

Table 6  The effect of offering concentrate and lamb birth rank (twin- vs. triplet-born) on lamb plasma thyroid hormone concentrations at 24–36 hours after birth (mean±SE). Means within treatments and interactions with differing superscripts are significantly (P<0.05) different.

		Thyroid hormone concentration (nmol/l)^1
	n	T4	T3
Nutritional treatment
Non-concentrate	55	4.71±0.03 (111.1)	1.57±0.04 (4.8)
Concentrate	53	4.70±0.03 (109.9)	1.53±0.04 (4.6)
Lamb birth rank
Twin	68	4.76±0.03 (116.4)^b	1.67±0.04 (5.3)^b
Triplet	40	4.70±0.03 (109.9)^a	1.44±0.05 (4.2)^a
Interactions
Twin
Non-concentrate	38	4.81±0.04 (122.7)^b	1.75±0.05 (5.7)^c
Concentrate	30	4.70±0.04 (109.9)^a	1.59±0.05 (4.9)^b
Triplet
Non-concentrate	17	4.61±0.06 (100.5)^a	1.39±0.07 (4.0)^a
Concentrate	23	4.69±0.04 (108.9)^a	1.47±0.06 (4.3)^ab
^1Data were log-transformed; transformed data with SE are presented with back-transformed means in parenthesis.

Lamb plasma glucose GGT and IgG concentrations

Ewe nutritional treatment had no effect on plasma glucose and IgG concentrations of lambs at age 24–36 h. There was a tendency (P=0.08) for plasma GGT concentrations to be greater in lambs born to concentrate-fed ewes than in lambs born to ewes fed pasture only (concentrate 7.43±0.10 IU/L (1685.8), non-concentrate 7.38±0.13 IU/L (1603.6) (transformed data with SE are presented with back-transformed means in parentheses). Plasma glucose concentrations at age 24–36 h were greater in twin-born lambs than triplet-born lambs at 24–36 h of age (twin-born 6.7±0.18 mmol/l, triplet-born 5.6±0.23 mmol/l). Birth rank had no effect on plasma GGT and IgG concentrations at 24–36 h. Lamb plasma GGT concentrations were positively correlated (R = 0.29) with plasma IgG concentration at age 24–36 h (P<0.002). No correlation was found between plasma GGT and glucose or IgG and glucose.

Lamb survival was not affected by ewe nutritional treatment (Table 7). Triplet-born lambs had lower survival rates than twin-born lambs. Ewes offered concentrate reared a heavier (P<0.05) total weight of lamb to L52 than ewes offered pasture only.

Table 7  The effect of offering concentrate and lamb birth rank (twin- vs. triplet-born) on lamb survival from birth until lactation day 52 (L52) and total weight of lamb produced per ewe at L52 (mean±SE). Means within treatments with differing superscripts are significantly (P<0.05) different.

	Lamb survival (%)		Total weight of lamb (kg/ewe)
	n	Birth to L52	n	L52
Nutritional treatment
Non-concentrate	48	0.9±0.29^1 (70.1^2)	29	28.7±1.32^a
Concentrate	50	1.4±0.33 (80.0)	28	32.0±1.35^b
Lamb birth rank
Twin	66	1.8±0.32 (85.2)^b	39	30.0±0.92
Triplet	32	0.5±0.29 (62.0)^a	18	30.7±1.87
^1Log^10 transformed
^2Back-transformed

Discussion

The aim of this study was to investigate the effects of offering concentrate supplement during late pregnancy, to twin- and triplet-bearing ewes grazing a 6 cm sward height, on ewe and lamb performance. Supplementing a ryegrass and white clover sward with concentrate led to a moderate improvement in ewe liveweight, a reduction in ewe plasma NEFA and BOH concentrations in late pregnancy, and a 300 g increase in lamb birth weight. While ewe herbage and concentrate intakes were not measured in this trial, these observed improvements suggest that offering concentrate increased the total nutrient intake of twin- and triplet-bearing ewes, and that substitution of herbage was minimal. Improvements in ewe liveweight, body condition and lamb birth weights of concentrate-fed ewes, when compared with ewes fed silage only, have previously been attributed to the greater nutrient intake of concentrate-fed ewes (Orr & Treacher 1990). In a similar trial, the current authors have also shown that the herbage intakes of concentrate-fed ewes and ewes fed pasture only are the same when grazing a 6 cm sward of ryegrass and white clover, suggesting that substitution of pasture for concentrate on a 6 cm sward did not occur (Kerslake et al. 2009).

During late pregnancy, nutrients are partitioned not only towards foetal growth but also towards mammary development and colostrum synthesis (Robinson 1983). While offering concentrate had no effect on plasma IgG concentrations, offering concentrate tended to increase ewe udder size and lamb plasma GGT concentrations. Ewe udder size and plasma GGT concentrations have been previously used as an indirect measure of colostrum production (Mellor & Murray 1985) and intake (Parker & Nicol 1990; Kenyon et al. 2005a). In addition to this, in the present study, plasma GGT concentrations were positively correlated with plasma IgG concentrations. These results combined could indicate that offering concentrate to ewes grazing a 6 cm sward height may have a positive effect on colostrum let down or production and, consequently, colostrum intake of the lamb. Offering concentrate supplement during late pregnancy to ewes grazing a low (Stephenson & Bird 1992) or high (Murphy et al. 1996) herbage allowance has previously been shown to increase colostrum and milk production.

Ewe nutrition during pregnancy can influence the thyroid status of the foetus (Symonds 1995). Thyroid hormones T3 and T4 are important for the maturation and functional development of brown adipose tissue and heat production directly after birth (Polk 1988; Symonds 1995). In the present study, triplet lambs born to pasture-fed ewes had lower plasma thyroid hormone concentrations than twin lambs born to pasture-fed ewes. While this may indicate that triplet-born lambs have a lower potential for heat production at age 24–36 h (Symonds 1995), previous studies have reported no differences between birth ranks in plasma thyroid hormone concentrations at age 24–36 h (Kerslake et al. 2010) and during the first three days of life (Dwyer & Morgan 2006). Interestingly, offering concentrate to twin-bearing ewes decreased plasma T3 concentrations in their offspring. This relationship was not observed in triplet-born lambs and the reason for this decrease in twin-born lambs is unknown. These results may suggest that offering concentrate during pregnancy could decrease heat production in twin-born lambs, where previous research has shown that plasma T3 concentrations of twin-born lambs at age 24–36 h are associated with maximum heat production at this age (Kerslake et al. 2010).

Offering concentrate during late pregnancy to ewes grazing high herbage allowances has been shown to improve lamb survival to weaning (Hinch et al. 1996). In the present study, offering concentrate had no effect on lamb survival to weaning. This finding should be treated with caution, however, as the numbers of lambs in the present study were insufficient to accurately determine survival differences between nutritional treatments. Ewes offered concentrate, however, reared a greater total weight of lamb at L52 than ewes fed on pasture only. Total weight of lamb weaned takes into account both lamb growth and survival. To be economical, concentrate supplementation must result in a production difference that covers the expense of the concentrate and the associated labour and equipment required. While it is probable that the production gains observed in this trial (approximately 3.3 kg of lamb weaned per ewe) are not large enough to justify the costs associated with offering concentrate, the degree, if any, of substitution is not known. Therefore the economic viability of offering concentrate to twin- and triplet-bearing ewes grazing a 6 cm sward can only be hypothesised.

Conclusion

Offering concentrates to twin- and triplet-bearing ewes grazing a 6 cm high sward moderately improved both ewe and lamb performance. Offering concentrate supplements during late pregnancy resulted in an increase in ewe liveweight gain, a decrease in metabolic stress and an increase in birth weight of offspring. These positive effects suggest that offering concentrate increased the total nutrient intake of twin- and triplet-bearing ewes and that substitution of the 6 cm high herbage was minimal. In addition to increasing lamb birth weight, offering concentrate tended to increase colostrum production and, potentially, colostrum uptake by the lambs. Offering concentrate also increased total lamb weight at L52. Further studies are required to investigate the substitution rate and the economic viability of offering twin- and triplet-bearing ewes concentrate when grazing a 6 cm sward height.

Acknowledgements

The authors wish to express their thanks to Beef + Lamb New Zealand, Massey University for funding this research and to the National Centre for Growth and Development for personal financial assistance to the first author. The authors also wish to acknowledge GA Poole and DL Burnham for their contributions in the field.