Emmanuelle Haslin

Mating ewes to lamb at 12 months of age can enhance lifetime productivity but is associated with lower reproductive performance and lamb survival compared to mature ewes. Melatonin, an antioxidant, has been shown to improve placental function and lamb health when mature ewes were supplemented during pregnancy. This study investigated the effect of supplementing one-year-old ewes with melatonin during mid-pregnancy on their survival and the live weight and survival of their lambs. Single- (n = 2711) and twin-bearing (n = 2605) ewes were randomly assigned to Melatonin or Control groups at seven farms across southern Australia. The Melatonin group received an 18 mg implant of melatonin at 90–100 days after the introduction of the rams, which releases melatonin for 60–80 days, and the Control group were not supplemented with melatonin. Seven farms recorded paddock level information including fetal number, lambs present 200 days after the introduction of the rams, ewes present pre-lambing and 200 days after the introduction of the rams and ewe pre-lambing live weight. Lamb survival and ewe mortality were analysed with analysis of covariance. Three of the seven farms also collected individual lambs’ information (n = 4303 lambs) including weight at birth and weaning of lambs, date of birth and weaning, lamb sex and birth type. Individual lamb live weights and growth rate to weaning were analysed with linear mixed models. Supplementation with melatonin had no effect (P > 0.05) on ewe or lamb survival. Melatonin supplementation did not impact (P > 0.05) individual lamb birthweight, growth rate, or weaning weight, consistent with the observed results in lamb survival. These findings suggest that maternal supplementation with melatonin mid-pregnancy is not an effective strategy to improve the survival or early growth of lambs, or the survival of one-year-old ewes.

This study examined the long-term effects of increased growth rates between three and seven months of age on gene expression in the mammary gland of nondairy ewes during their second lactation. A total of 19 twin-bearing, two-year-old ewes that either had an increased growth rate between three and seven months of age (heavy; n = 9; 153 ± 2.2 g/d and 47.9 ± 0.38 kg at breeding) or did not have an increased growth rate (control; n = 10; 127 ± 1.9 g/d and 44.9 ± 0.49 kg at breeding) were selected. Mammary gland biopsies were collected at day 128 of pregnancy and day 30 of lactation to examine the expression of 37 genes involved in mammary cell development and milk fat metabolism using NanoString nCounter technology. Ewes in the heavy group tended to exhibit lower expression (P < 0.05) of acyl-CoA synthetase short-chain family member 1 (ACSS1), a gene critical for mitochondrial acetyl-CoA synthesis, energy production, and milk fat synthesis, and a trend toward (P = 0.11) lower expression of signal transducer and activator of transcription 5A (STAT5A), a regulator of mammary epithelial cell differentiation and survival. These lower expressions suggest potential carry-over effects of increased growth rate between three and seven months of age. However, no phenotypic differences were observed in lamb growth or live weight, and no differences were detected in the expression of downstream target genes or modulators of these pathways, suggesting limited functional impact on mammary gland development and lamb performance outcomes. Further investigations, including a functional assessment of lactation and use of comprehensive transcriptomic analyses, would be needed to understand the effects of increased growth rates between three and seven months of age on ewe mammary function and milk composition.

The lifetime productivity and efficiency of singleton- and twin-born ewe lambs born to one-year-old ewes were compared with twin-born ewe lambs born to mixed-age ewes. Romney ewe lambs either born as twins to mixed-age ewes (Heavier n = 135 and Control n = 135), or to one-year-old ewes as singletons (L1; n = 135) or twins (L2; n = 88) were included. The Heavier ewes weighed 47.9 ± 0.36 kg at ewe lamb breeding while Control ewes weighed 44.9 ± 0.38 kg. The lifetime number of lambs born and weaned, and the lifetime weight of lambs weaned were used to calculate both feed and ewe efficiency. Heavier ewes had greater lamb production as one-year-old ewes than L1, L2 and Control ewes, after which the yearly lamb production and efficiency did not differ between treatments. Heavier ewes had a greater lifetime number and weight of lambs weaned than L1 and L2 ewes but did not differ from Control ewes. The lower lifetime production of L1 and L2 ewes was driven by their low production as one-year-old ewes. Farmers, therefore, could select their replacements from ewes born to one-year-old ewes and have similar lamb production to ewes born to mixed-age ewes, providing they reach recommended pre-breeding live weights.

Low survival of triplet-bearing ewes and their lambs represents lost production and a welfare issue. The effects of feed-on-offer (FOO; low: 1205 vs. high: 1980 kg DM/ha) and concentrate supplementation (low: 50 vs. high: 300+ g/ewe/day) levels during late pregnancy and lambing on the survival of triplet-bearing ewes and their lambs were investigated on 10 commercial farms using 1772 triplet-bearing Maternal ewes. Ewe and lamb survival were estimated at marking, and ewe body condition score (BCS) was recorded in late pregnancy and at marking. Although FOO treatment had no effect on triplet-bearing ewe mortality, receiving higher supplementation decreased mortality by 40% and increased BCS at marking by 0.14 compared with a lower supplementation (p < 0.05). Supplementation, FOO treatments, weather conditions during lambing and shelter availability had no effect on triplet-lamb survival. These findings suggest no additional benefit to triplet-bearing ewe survival when FOO levels exceed 1200 kg DM/ha during late pregnancy and lambing, but increased supplementation can reduce ewe mortality. Further research is required to determine the response to the supplementation level at lower FOO levels on triplet-bearing Merino ewes and their lambs and establish whether supplementation of triplet-bearing ewes during late pregnancy and lambing with higher levels of concentrates would be cost-effective.

This study estimated genetic parameters and identified candidate genes associated with live weight, and the occurrence of pregnancy in 1327 Romney ewe lambs using genome-wide association studies. Phenotypic traits considered were the occurrence of pregnancy in ewe lambs and live weight at eight months of age. Genetic parameters were estimated, and genomic variation was assessed using 13,500 single-nucleotide polymorphic markers (SNPs). Ewe lamb live weight had medium genomic heritability and was positively genetically correlated with occurrence of pregnancy. This suggests that selection for heavier ewe lambs is possible and would likely improve the occurrence of pregnancy in ewe lambs. No SNPs were associated with the occurrence of pregnancy; however, three candidate genes were associated with ewe lamb live weight. Tenascin C (TNC), TNF superfamily member 8 (TNFSF8) and Collagen type XXVIII alpha 1 chain (COL28A1) are involved in extracellular matrix organization and regulation of cell fate in the immune system. TNC may be involved in ewe lamb growth, and therefore, could be of interest for selection of ewe lamb replacements. The association between ewe lamb live weight and TNFSF8 and COL28A1 is unclear. Further research is needed using a larger population to determine whether the genes identified can be used for genomic selection of replacement ewe lambs.

This study evaluated the impacts of management of body condition score (BCS) between pregnancy scanning and lamb marking on the mortality of triplet-bearing ewes and their lambs at 19 research sites across Southern Australia. Triplet-bearing ewes of Maternal (crossbred or composite) or Merino breed were randomly allocated to treatment at pregnancy scanning at an average of 97 days from the start of joining: High or Low BCS. The BCS of individual ewes was assessed at pregnancy scanning, pre-lambing (average of 137 days from the start of joining) and marking (average of 165 days from the end of joining), and ewe and lamb mortality to marking, recorded for each mob. The average BCS at pregnancy scanning was 3.4 for Maternal ewes and 3.3 for Merino ewes. There were no breed by BCS treatment effects on the BCS of ewes at pregnancy scanning or lamb marking or on the change in BCS between pregnancy scanning and pre-lambing or between pre-lambing and marking. The change in BCS differed between the High and Low BCS treatments, between pregnancy scanning and pre-lambing (0.12 vs. −0.33; p < 0.001) and between pre-lambing and marking (−0.39 vs. 0.07; p < 0.001) but did not differ between breeds. The average BCS at marking for ewes managed at the High and Low BCS treatments was 3.1 and 3.0 for Maternals and 3.0 and 2.8 for Merinos. Survival of triplet-bearing Merino ewes (p < 0.01) and their lambs (p < 0.001) was greater when ewes were managed at the High BCS compared to the Low BCS. The BCS treatment did not impact the survival of Maternal ewes or their lambs. The survival of Merino but not Maternal lambs was higher when ewes were in greater BCS pre-lambing (p < 0.01) and when ewes gained BCS between pregnancy scanning and pre-lambing (p < 0.01). Ewe mortality was lower when ewes gained BCS between pregnancy scanning and pre-lambing (p < 0.05). Merino ewes were more likely to die than Maternal ewes for a given change in BCS between pregnancy scanning and pre-lambing (p = 0.065). Overall, our findings demonstrate that producers should manage the nutrition of triplet-bearing Merino ewes so that ewes are in greater BCS at lambing and/or to gain BCS between pregnancy scanning and lambing to improve ewe and lamb survival. Triplet-bearing Maternal ewes should be managed to gain BCS between pregnancy scanning and lambing to improve ewe survival.

The psychoendocrine evaluation of lamb development has demonstrated that maternal deprivation and milk replacement alters health, behavior, and endocrine profiles. While lambs are able to discriminate familiar and non‐familiar conspecifics (mother or lamb), only lambs reared with their mother develop such clear social discrimination or preference. Lambs reared without mother display no preference for a specific lamb from its own group. Differences in exploratory and emotional behaviors between mother‐reared and mother‐deprived lambs have also been reported. As these behavioural abilities are supported by the brain, we hypothesize that rearing with maternal deprivation and milk replacement leads to altered brain development and maturation. To test this hypothesis, we examined brain morphometric and microstructural variables extracted from in vivo T1‐weighted and diffusion‐weighted magnetic resonance images acquired longitudinally (1 week, 1.5 months, and 4.5 months of age) in mother‐reared and mother‐deprived lambs. From the morphometric variables the caudate nuclei volume was found to be smaller for mother‐deprived than for mother‐reared lambs. T1‐weighted signal intensity and radial diffusivity were higher for mother‐deprived than for mother‐reared lambs in both the white and gray matters. The fractional anisotropy of the white matter was lower for mother‐deprived than for mother‐reared lambs. Based on these morphometric and microstructural characteristics we conclude that maternal deprivation delays and affects lamb brain growth and maturation.

Ewe lamb live weight at first mating (7 months of age) is positively associated with reproductive performance and led to the recommendation of a minimum mating live weight of 40 kg. This experiment examined the effects of heavier mating live weight of ewe lambs on their live weight, fertility, lambing percentage and their progeny performance to weaning. Ewe lambs (n = 270) were randomly allocated to one of two treatments. The Heavy group (n = 135) was preferentially fed until mating, achieving 47.9 ± 0.36 kg, while the Control group (n = 135) had an average mating live weight of 44.9 ± 0.49 kg. Fertility, lambing percentage, ewe live weights, the survival and live weights of their progeny were recorded. There was a 28% increase in fertility and a 59% increase in lambing percentage in the Heavy group compared to the Control group. Progeny survival and live weights did not differ (P > .05) by treatment. A curvilinear relationship between fertility and ewe live weight at mating was identified, suggesting that ewe lambs should be mated at a live weight of 50-55 kg to maximise fertility and lambing percentage without reducing ewe live weight nor the progeny performance.

The current study investigated the effects of growing ewe lambs to a heavier live weight at breeding on their subsequent reproductive performance and live weight at two and three years of age, and the growth of their progeny to weaning. Two groups of ewe lambs were bred at seven months of age at an average pre-breeding live weight of either 47.9 ± 0.36 kg (Heavy; n = 135) or 44.9 ± 0.49 kg (Control; n = 135). Breeding performance, fertility, litter size, lambing percentage, ewe wither height, live weight, progeny survival, and live weight were recorded at two and three years of age. None of the variables measured differed between the Heavy and Control ewes at either two or three years of age (P > 0.10). This suggests that farmers can breed their Romney-type ewe lambs at an average live weight of 48 kg without any negative impacts on reproductive performance at two or three years of age nor on their progeny's live weight or growth to weaning. Although breeding heavier ewe lambs improved reproductive performance during their first year, further investigations are needed to assess their overall lamb production and efficiency over multiple breeding seasons.

Currently, 30–43% of New Zealand sheep farmers breed their ewe lambs, but few retain the offspring as replacements for their flock. No difference in lamb production as a yearling among singletons and twins born to ewe lambs and twins born to mature ewes has been reported, provided the ewe lambs had reached the 60–65% of their likely mature weight prior to breeding at seven to eight months of age. The aim of this experiment was to determine the lamb production from singletons and twins born to ewe lambs and twins born to mature ewes during their first two years of lambing. The experiment included 8-month-old ewes born as twins to mature ewes (M2, n = 135), singletons born to ewe lambs (L1, n = 135), and twins born to ewe lambs (L2, n = 88), bred during the same period to the same rams, over two years. The efficiency of lamb production (total litter weight at weaning divided by the pre-breeding weight of the ewe, for all ewes presented for breeding) after two years of production was not significantly different (p > 0.05) among the groups (0.40 ± 0.02, 0.39 ± 0.02, and 0.39 ± 0.03, for M2, L1, and L2, respectively).