Shearing during pregnancy — review of a policy to increase birthweight and survival of lambs in New Zealand pastoral farming systems

Development programming: Stress during gestation alters offspring development in sheep

Inappropriate management practices of domestic animals during pregnancy can be potential stressors, resulting in complex behavioural, physiological and neurological consequences in the developing offspring. Some of these consequences can last into adulthood or propagate to subsequent generations. We systematically summarized the results of different experimental patterns using artificially increased maternal glucocorticoid levels or prenatal maternal physiological stress paradigms, mediators between prenatal maternal stress (PMS) and programming effects in the offspring and the effects of PMS on offspring phenotypes in sheep. PMS can impair birthweight, regulate the development of the hypothalamic-pituitary-adrenal axis, modify behavioural patterns and cognitive abilities and alter gene expression and brain morphology in offspring. Further research should focus on the effects of programming on gene expression, immune function, gut microbiome, sex-specific effects and maternal behaviour of offspring, especially comparative studies of gestational periods when PMS is applied, continual studies of programming effects on offspring and treatment strategies that effectively reverse the detrimental programming effects of prenatal stress.

When to shear dairy ewes: before breeding, during pregnancy or let them unshorn?

Shearing dairy ewes may improve their fitness and heat tolerance during late pregnancy, with positive effects on lambs and lactation performances. To test this hypothesis, two Mediterranean breeds (MN, Manchega, n = 43; LC, Lacaune, n = 28), differing in lactation performances and fleece traits, were submitted to three shearing strategies: (i) shorn before breeding (SBB), (ii) shorn at day 100 of pregnancy (S100) and (iii) unshorn (CO). Ewes were bred in spring and gestated during summer. Fleece traits and respiratory rate were measured on pregnant (107-121 days) resting ewes at different barn temperatures. Blood and colostrum were sampled at lambing. Ewes suckled their lambs (28 days) and were machine-milked until 180 days of lactation. Lamb and ewe weights, and condition score of the ewes, were recorded throughout the experiment. Milk yield was assessed during suckling (fortnightly) and milking (daily), and milk was sampled for composition (fortnightly). Fleece extension and wool weight at S100 were 13 and 45% greater in MN than in LC ewes, respectively, but the ewe's respiration rate at late pregnancy, between 20 and 25 °C, did not vary among shearing treatments nor breeds. Nevertheless, S100 ewes had a 37% lower respiration rate than SBB and CO ewes at 28 °C. At lambing, SBB and S100 ewes had 86% higher glycaemia than CO in MN, but LC ewes did not vary. Shearing treatment had no effect in plasma insulin, β-hydroxybutyrate or non-esterified fatty acids (NEFAs) at lambing in both breeds. Lamb's birthweight and growth during suckling did not differ by shearing treatment in both breeds. Colostrum composition and milk yield during suckling were not modified by shearing treatment in either breed, although S100 suckling milk composition increased in MN (protein and casein, 6%) and LC ewes (total solids, 8%; fat, 18%), compared to CO and SBB. No effects were detected in milk yield or composition during milking, but S100 tended to yield 28% more milk than CO in the LC ewes. The S100 treatment improved the body reserves in late pregnancy, when compared to CO and SBB in both breeds. In conclusion, shearing dairy ewes in late pregnancy was a recommendable management practice under summer conditions, because alleviated heat stress and improved the weight of the ewes, without detrimental effects on lambing and lactational performances, the last tending to increase milk yield in high-yielding dairy ewes.

Breeding Ewe Lambs: An Australasian Perspective

A number of potential advantages have been identified for breeding ewe lambs at 7 to 9 months of age, including increased lifetime productivity and profitability. However, breeding at this young age in extensive pastoral systems, such as in Australasia, can be associated with a number of disadvantages resulting in uptake of this management procedure being relatively low. This review highlights the known differences between ewe lamb and mature ewe reproductive performance, thus differing in their management. The review then summaries the scientific literature of factors that affect ewe lamb reproductive success, with a focus on recent studies conducted under extensive pasture-based conditions in Australasia. In particular, this review outlines the importance of ewe lamb live weight and body condition score on their productivity. The potential long-term consequences of breeding a ewe lamb at a young age in terms of her future success and that of her offspring to weaning are briefly outlined. In addition, the potential impacts of selecting progeny born to ewe lambs as future replacement ewes are discussed. Throughout this review, optimal management guidelines from prior to breeding the ewe lambs until rebreeding at 2 years of age are provided. Lastly, areas requiring future research are identified and discussed.

Factors Associated with Mortality of Lambs Born to Ewe Hoggets

Simple Summary

Pre-weaning lamb deaths limit the production performance from ewes that are bred as hoggets (at 6–9 months of age). The present study aimed to investigate factors that are associated with increased likelihood of lamb death and the cause of that death, in lambs and in ewe hoggets over the lambing period. This was also compared to rates in mature-age ewes and their lambs. Lambs with low birthweights or that were born as twins were more likely to die, as were lambs whose dams had greater live weight changes during pregnancy. Ewes deaths during the lambing period accounted for approximately 11% of lamb deaths. The most common cause of hogget lamb deaths was stillbirth (lamb born dead) but this was an uncommon cause of death in lambs born to mature-age ewes. Management tools to increase lamb birthweights, and supervision of ewe hoggets at lambing time, are recommended.

Abstract

The reproductive performance of ewe hoggets is poorer than that of mature-age ewes due to production of fewer lambs with poorer survival. Scant data are available on the risk factors for, and causes of, the mortality for lambs born to ewe hoggets, the impact of ewe deaths on lamb loss, and the causes of death for lambs born to ewe hoggets vs. mature-age ewes lambing in the same circumstances. In this study, 297 lambs born to 1142 ewe hoggets were necropsied along with 273 lambs born to 1050 mature-age ewes. Low lamb birthweight, multiple litter size, and increasing ewe hogget average daily gain from breeding to late pregnancy were risk factors for lamb mortality. The most common cause of mortality for lambs born to ewe hoggets was stillbirth and the risk factors for stillbirth were similar to those for lamb mortality generally. Approximately 11% of ewe hoggets’ lamb deaths were due to the death of the dam. Causes of mortality differed between lambs born to ewe hoggets vs. those born to mature-age ewes. Management practices to increase ewe hogget lambs’ birthweights (particularly those from multiple litters) and supervision of ewe hoggets at lambing time are recommended.

Effect of early shearing during gestation on the productive and reproductive behavior of female sheep offspring in their first 18 months of age

The research has shown the interesting contributions of shearing in mid-gestation on the performance of lambs from birth to weaning. Other studies have reported that shearing at early pregnancy influences the development of the placenta and lamb live weight at birth. However, there was a lack of information on the effect of early-prepartum shearing on the behavior of the offspring from weaning onward. This study evaluated the effect of shearing ewes at 50 days of gestation on the growth, reproductive behavior and response to a gastrointestinal parasite challenge in the female offspring from weaning to 18 months old. Fifty-seven Polwarth female lambs were used, 22 being singles and 35 twins born to ewes either shorn at 50 days of pregnancy (PS, n = 23) or shorn at 62 days postpartum (U, control, n = 34) resulting in four subgroups: single lambs born to PS ewes (n = 8), born to U ewes (n = 14), twin lambs born to PS ewes (n = 15) or born to U ewes (n = 20). All progeny were managed together under improved pasture with a minimum forage allowance of 6% live weight on dry basis. Body weight, body condition score and fecal eggs count were recorded every 14 days from weaning to 18 months of age. Concentrations of progesterone were measured weekly (from 4 to 10 months of age and from 14 to 18 months of age) to establish the onset of puberty. Ovulation rate at an induced and a natural heat (545 ± 1.0 and 562 ± 1.0 day old) was recorded. Prepartum shearing did not affect the age at puberty or the ovulation rate of female offspring, but those born as singles were more precocious ( P = 0.03) and heavier ( P = 0.02) at puberty than twin born lambs. Both the average value of parasite egg count ( P = 0.0 7) and the Famacha index ( P = 0.02) for the entire study period were lower in lambs born to prepartum shorn ewes than those born to postpartum shorn ewes. In conclusion, shearing at 50 days of gestation did not affect the growth or the reproductive behavior of female offspring. However, female lambs born from ewe shorn during gestation showed a better response to the parasitic challenge, and further research is required to confirm this.

Stress and behavioural responses to winter shearing differ between pregnant and non-pregnant ewes

High progesterone concentrations reduce the stress responses in several mammals. Therefore, it may be expected that pregnant ewes have lower responses to stressors than anoestrous ewes. Our aims were to compare the stress response and the behavioural changes of pregnant and non-pregnant ewes sheared during winter. Two trials were with 20 were pregnant (group PR) and 17 non-pregnant ewes (group NP). In the first trial blood samples were collected immediately before and during the first 180 min after shearing, and cortisol, total protein, and albumin concentrations were measured, and globulin concentrations were calculated. In the second trial, the frequency in which each ewe was observed standing, lying down, walking and grazing were recorded from Day -3 to Day -1 (Day 0 = shearing), from Day 1 to Day 5, on Days 13 and 20-21 during 8 h/day. Scan observations were done every 10 min from 8:00 h to 12:00 h and from 14:00 h to 18:00 h (total = 50 recordings.day). Non-pregnant ewes had greater cortisol concentrations than PR ewes (P = .007). Non-pregnant ewes also had greater total protein concentrations than PR ewes (P = .029). Albumin concentration tended to be greater in NP than PR ewes (P = .064). Globulin concentration 30 min after shearing was greater in PR than in NP ewes (P = .047). Pregnant ewes were observed more frequently standing than NP ewes (P = .013). Non-pregnant ewes were observed more frequently lying down and walking than PR ewes (P = .039 and P = .009, respectively). Before shearing, on Day 2 and Day 4, NP ewes walked more frequently than P ewes (P = .007, P < .0001, P = .007 respectively). Pregnant ewes were observed grazing more frequently than NP ewes (P = .009). Pregnant ewes grazed more than NP ewes on Days 1 and 5 (P = .005 and P < .0001 respectively). Overall, we concluded that shearing was a strong stressor that provoked sustained changes in the behavioural pattern, physiological status and gregariousness intensity despite being or not pregnant. Some responses, as cortisol concentration, changes in the time walking and grazing and the decrease of the cohesiveness after shearing seem to be less pronounced in pregnant than in non-pregnant ewes.

Effects of late gestation shearing on BW, feed intake and plasma metabolite concentrations in Rambouillet ewes managed outdoors during winter

The majority of lambs in the United States are born from late winter to early spring and pregnant ewes are generally sheared in the last third of pregnancy. Although there are benefits to shearing before parturition, shorn animals may be more vulnerable to the cold, highly variable climatic conditions associated with these seasons. The objective of this study was to determine if late gestation shearing induces differences in individual BW, dry matter intake (DMI) and plasma metabolite concentration of finewool ewes managed outdoors during winter. Thirty-six mature, pregnant Rambouillet ewes (3.8±0.45 years; 76.8±11.4 kg) were managed in a drylot with ad libitum access to pelleted alfalfa in bunks capable of measuring individual daily DMI. The treatment group consisted of ewes sheared at ~5 weeks before the estimated parturition date (shorn; n=18). Unshorn ewes (n=18) remained in full fleece throughout the experiment and were shorn on the last day of the experiment ~2 weeks before the estimated parturition date. Blood was collected on days 0 (before shearing shorn group), 7, 14 and 21 (before shearing unshorn group) of the trial, and plasma was isolated and analyzed for non-esterified fatty acid (NEFA), β-hydroxybutyrate (BHB) and glucose (GLU) concentrations. There was no effect of shearing on ewe DMI or BW during the trial (P⩾0.35). Plasma NEFA and GLU concentrations were similar (P⩾0.36) between shearing groups, though plasma BHB concentration was 103.7 μmol/l greater (24.1%; P&lt;0.01) in unshorn ewes. Lamb BW at birth was not affected (P=0.30) by ewe shearing treatment. Under conditions of this study, no differences in economically important aspects of sheep production were observed between shorn and unshorn pregnant ewes.

Shearing ewes in the first third of gestation improves offspring performance

The effect of shearing ewes at 50 days of gestation on birth and weaning weight, vigour of the lambs in the first hour of life and the survival of single and twin lambs was investigated. Ultrasound pregnancy diagnosis was used to allocate 140 Polwarth ewes on Day 50 of gestation to one of four treatments, including two factors: shearing time (prepartum (PS) and postpartum (U)) and litter size (single (S) and twin (T)) resulting in the following treatments: single-bearing ewes shorn at 50 days of gestation (SPS, n = 39), single-bearing ewes unshorn (SU, n = 40), twin-bearing ewes shorn at 50 days of gestation (TPS, n = 28) and twin-bearing ewes unshorn (TU, n = 33). All ewes grazed together on improved pastures. Body condition score of the ewes was recorded once every 2 weeks from shearing until weaning, and plasma concentration of NEFA in ewes was measured once every 2 weeks from Day 30 postpartum until weaning (Day 104 ± 7). Gestation length, lamb behaviour during the first hour of life, lamb weight at birth and daily weight gain from birth to weaning and lamb survival were measured. Ewe body condition and plasma non-esterified fatty acids concentration was not affected by shearing time or litter size. Birthweight of the lambs was affected by shearing time (P = 0.0002), litter size (P < 0.0001) and sex of the lamb (P = 0.03). Lambs born to PS ewes had higher daily growth rate (P < 0.0001) than lambs born to U ewes (0.149 ± 0.07 vs 0.141 ± 0.07 kg, respectively). Birthweight affected the number of assisted lambs at birth (P = 0.01). Length of gestation was affected by shearing time (P < 0.0001). Sucking ability was improved by shearing time and litter size. Lambs born to PS ewes tried to suck (P = 0.01) and successfully sucked (P = 0.009) before lambs born to U ewes (23.3 ± 2 vs 28.7 ± 2 and 37.6 ± 1 vs 38.4 ± 1 min, respectively). Attempt and success to suck was affected by litter size (P = 0.004 and P = 0.05). TPS-born lambs tried to suck before (P = 0.004) and successfully sucked earlier (P = 0.0004) than TU lambs (26.1 ± 2.5 vs 33.7 ± 2 and 38 ± 1 vs 42.1 ± 2 min, respectively). Shearing time affected lamb survival at 20 days of age (P = 0.04). The improved survival in these lambs, expressed by a higher birthweight and vigour at birth, may be explained by better nutrition in utero in response to shearing during early pregnancy. In contrast to previous research on prepartum shearing, the present experiment shearing was done at 50 days of gestation. Early shearing in gestation might coincide with a period of high cell proliferation, intense vascular development and increased uterine and umbilical blood flows in the placenta which is in rapid development, and may result in improved outcomes when compared with shearing done on Day 70 or later of gestation.

EFFECT OF SHEARING DURING PREGNANCY ON PRODUCTIVE PERFORMANCE IN THE POST-PARTUM PERIOD OF EWES ON EXTENSIVE HUSBANDRY

The use of shearing during pregnancy has been described as a tool for improving productivity in sheep and for minimizing perinatal mortality in lambs through the increase of fetal development. This study assessed the effect of shearing around 74 days of gestation on the productive performance of ewes and lambs during the first month of life. Forty Corriedale ewes were inseminated in autumn in Southern Brazil. All ewes were kept together at the same pasture under extensive husbandry conditions. The ewes were randomly separated into two treatment groups: twenty animals were completely sheared at 74 ± 6 days of pregnancy, and twenty were kept without sheared during pregnancy, composing the control group. Ewes and their lambs were evaluated at three different times during the experiment: at birth, between 15 and 21 days post-partum and between 22 and 45 days post-partum. Ewes had their body condition score, body weight, placental weight, milk production and serum concentrations of beta-hydroxybutyrate measured, while lambs had hematocrit, hemoglobin, and plasma lactate and glucose, as well as body weight at birth and until wean determined. Values of hematocrit and hemoglobin were lower and body weight at birth and at wean was higher in the group of lambs born from sheared ewes. Placenta weight was higher in sheared ewes. Body condition score and beta-hydroxybutyrate showed no differences between groups. Milk production of sheared ewes (1.26 L/day) was higher than in control group (0.93 L/day). Shearing ewes at 74 days of pregnancy was efficient for the better development of lambs at post-birth, reducing perinatal mortality rates.

Reduced cortisol and metabolic responses of thin ewes to an acute cold challenge in mid-pregnancy: implications for animal physiology and welfare

BACKGROUND

Low food availability leading to reductions in Body Condition Score (BCS; 0 indicates emaciation and 5 obesity) in sheep often coincides with low temperatures associated with the onset of winter in New Zealand. The ability to adapt to reductions in environmental temperature may be impaired in animals with low BCS, in particular during pregnancy when metabolic demand is higher. Here we assess whether BCS affects a pregnant animal's ability to cope with cold challenges.

METHODS

Eighteen pregnant ewes with a BCS of 2.7±0.1 were fed to attain low (LBC: BCS2.3±0.1), medium (MBC: BCS3.2±0.2) or high BCS (HBC: BCS3.6±0.2). Shorn ewes were exposed to a 6-h acute cold challenge in a climate-controlled room (wet and windy conditions, 4.4±0.1°C) in mid-pregnancy. Blood samples were collected during the BCS change phase, acute cold challenge and recovery phase.

RESULTS

During the BCS change phase, plasma glucose and leptin concentrations declined while free fatty acids (FFA) increased in LBC compared to MBC (P<0.01, P<0.01 and P<0.05, respectively) and HBC ewes (P<0.05, P<0.01 and P<0.01, respectively). During the cold challenge, plasma cortisol concentrations were lower in LBC than MBC (P<0.05) and HBC ewes (P<0.05), and FFA and insulin concentrations were lower in LBC than HBC ewes (P<0.05 and P<0.001, respectively). Leptin concentrations declined in MBC and HBC ewes while remaining unchanged in LBC ewes (P<0.01). Glucose concentrations and internal body temperature (T(core)) increased in all treatments, although peak T(core) tended to be higher in HBC ewes (P<0.1). During the recovery phase, T4 concentrations were lower in LBC ewes (P<0.05).

CONCLUSION

Even though all ewes were able to increase T(core) and mobilize glucose, low BCS animals had considerably reduced cortisol and metabolic responses to a cold challenge in mid-pregnancy, suggesting that their ability to adapt to cold challenges through some of the expected pathways was reduced.

Survival of twin lambs is increased with shrub belts

Abstract Perinatal lamb mortality is a major source of reproductive loss, particularly for twins. A study was conducted to determine whether provision of shelter in the form of shrub belts (a ‘maternity ward’) could increase survival of twin lambs compared with hedgerows, and whether hedgerows improve survival of single lambs compared with unsheltered paddocks. Measurements were recorded for Merino × Poll Dorset cross twin lambs born in hedgerows (phalaris or hessian) or shrub belts and single lambs born in hedgerows or unsheltered paddocks over the years 2007–2009. Records for 382 single and 726 twin lambs were used. The survival of single lambs was not increased (P = 0.06) by hedgerow shelter. The survival of twin lambs in shrubs was 10% higher (P < 0.05) than that in hessian hedgerow shelter in 2008–2009 (0.77 cf. 0.70), associated with a reduction in deaths from starvation and/or mismothering and/or exposure. The hessian shelter was associated with an increased (P < 0.05) growth rate to weaning of single lambs, but the growth rates of twin lambs were lower (P < 0.05) in shrub than in hessian shelter. In 2010, a second study of 178 twin Merino × Poll Dorset cross lambs found that survival of lambs born alive was not improved by shrubs compared with unsheltered paddocks (0.80 versus 0.77; P > 0.05). It is concluded that shrub belts which forced twin-bearing ewes to lamb in a sheltered environment reduced perinatal mortality in one of three datasets, but was not repeated. The shrubs take time to establish, and the benefit will be small if weather is mild during lambing.

Pre-partum shearing of ewes under pastoral conditions improves the early vigour of both single and twin lambs

This study tested the hypothesis that ewes bearing single or twin lambs shorn at 70 or 120 days of pregnancy would give birth to lambs more vigorous than those born to unshorn ewes, increasing the chances of lamb survival. This was tested in two experiments. In the first, 57 adult Corriedale ewes bearing single lambs were allocated to three treatments; shearing at Day 70 of gestation (S70, n = 18), shearing at Day 120 of gestation (S120, n = 19) or unshorn (US, n = 20). In the second, 57 Corriedale ewes bearing twins were allocated to the same treatments: S70, n = 18; S120, n = 20; or US, n = 19. All ewes grazed native pasture until penned and fed individually to requirements during the last 10 days of pregnancy. Gestation length, lamb birthweight, lamb behaviour during their first hour of life, blood glucose and colostrum amount and composition were measured. In the first experiment, with single lambs, birthweights were similar (4.8, 4.7 and 4.7 kg for S70, S120 and US ewes, respectively; s.e. = 0.17, P = 0.47) and the ewes had lengths of gestation of 149 ± 0.6, 148 ± 0.5 and 147 ± 0.6 days respectively. Seventy-eight percent of S70 and 61% of S120 lambs sucked compared with 21% of US lambs (P < 0.001). In the second experiment, with twin lambs, S70 lambs were heavier than S120, which were heavier than US lambs (3.9, 3.4 and 3.1 kg; s.e. = 0.1, P < 0.0001). Ninety-seven percent of S70 and 92% of S120 lambs stood in their first hour of life compared with 68% of US lambs (P < 0.05). Sixty-seven percent of S70 and 63% of S120 lambs sucked compared with 22% of US lambs (P < 0.001). The length of gestation was 148 ± 0.4, 147 ± 0.5 and 144 ± 0.5 days for S70, S120 and US ewes. In both experiments there was no difference in the amount or quality of the colostrum produced by the ewes of the different treatments. We conclude that pre-partum shearing improves lambs’ behaviour independently of their birthweight and suggest that this could be because the lambs are more physiologically mature at birth.

Mid-pregnancy shearing can increase birthweight and survival to weaning of multiple-born lambs under commercial conditions

In several field studies, mid-pregnancy shearing has increased lamb birthweight, which may be associated with an increase in lamb survival. However, animal numbers have been too low in these studies to accurately examine any effects on survival rates to weaning. This study was designed to investigate the effect of mid-pregnancy shearing on the birthweight of multiple-born lambs and their survival rates to weaning under commercial farming conditions at 2 different sites. At Riverside, 657 twin- and 47 triplet-bearing ewes were either shorn at mid-pregnancy (n = 347) or left unshorn (n = 357). Similarly, at Tuapaka, 532 twin-, and 60 triplet-bearing ewes were either shorn at mid-pregnancy (n = 291) or left unshorn (n = 301). Mid-pregnancy shearing significantly (P<0.05) increased the birthweights of lambs at each site (by 0.44 and 0.13 kg at Riverside and Tuapaka, respectively). Mid-pregnancy shearing significantly (P<0.05) increased lamb weaning weights (by 1.07 kg) and survival rates to weaning (by 5.5%) at Riverside, but not at Tuapaka. These findings show that mid-pregnancy shearing under commercial farming conditions can increase the birthweight of multiple-born lambs and, when the birthweight response is large enough to reduce the numbers of otherwise lightweight lambs, it is associated with an increase in survival rates to weaning.

The effect of mid- and late-pregnancy shearing of hoggets on lamb birthweight, weaning weight, survival rate, and wool follicle and fibre characteristics

Pregnancy shearing of mature ewes has been shown to influence lamb birthweight and sometimes lamb survival to weaning and wool follicle characteristics. However, the effects of shearing hoggets in mid- or late-pregnancy under New Zealand’s pastoral conditions have not been examined. This study was designed to determine the effects of mid- and late-pregnancy shearing on lambs born to Romney hoggets. Single-bearing hoggets were either shorn at average day 79 of pregnancy (n = 74), at average day 119 (n = 80), or left unshorn (n = 76) while their twin-bearing counterparts were either shorn at average day 79 of pregnancy (n = 18) or left unshorn (n = 12). Mid-pregnancy shearing was found to increase the birthweights of singles (by 0.32 kg, P < 0.05) in comparison with unshorn ewes; however, this relationship was not observed in twins. Neither mid- nor late-pregnancy shearing affected lamb survival to weaning. Singles born to late-pregnancy shorn ewes had whiter wool (P < 0.05) in comparison with lambs born to unshorn dams. Dam treatment had no other effects on follicle or wool characteristics. The results of this study indicate that mid-pregnancy shearing of hoggets can be used as a technique to increase birthweight in single lambs but not twin-born lambs. Pregnancy shearing had no effect on lamb survival rates to weaning, whereas shearing in late-pregnancy had minor effects on wool colour that would be unlikely to affect wool value in Romneys. Shearing in mid-pregnancy had no effect on either progeny follicle or wool characteristics.

The effect of progesterone supplementation post mating and shearing of ewes in early pregnancy on the reproductive performance of ewes and birthweight of lambs

AIM

To determine whether short-term progesterone supplementation post mating or shearing of ewes in early pregnancy affected either the proportion of ewes that lambed or that had multiple lambs, or the birthweight of lambs.

METHODS

Romney ewes (n=457) were synchronised in oestrus using controlled internal drug-releasing (CIDR) devices containing progesterone, and mated to Romney rams over a 5-day period. The mid-point of mating (Day 0) occurred 2 days after the withdrawal of CIDR devices. Ewes mated (n=397) were randomly allocated to one of four treatment groups: shearing at Day 5, shearing at Day 30, no shearing, and no shearing plus progesterone supplementation using a CIDR device inserted on Day 3 for 6 days. During the period from Day 5 to Day 27, six harnessed Suffolk rams were placed with the ewes and matings recorded. At Day 48, all ewes that did not return to the Suffolk rams were scanned for pregnancy using ultrasound. At Day 140, single- and multiple-bearing ewes were set-stocked at 15.1 and 12.2 ewes/ha, respectively, and equivalent numbers of ewes from each treatment group were placed in each paddock. Blood samples from 10 unshorn and 10 progesterone-supplemented ewes were collected on Days 3, 6 and 9, and analysed for plasma progesterone concentrations. Lambs were identified to their dam and weighed within 12 h of birth, and again at 27 and 93 days after the mid-point of lambing. The ewes were weighed at regular intervals throughout the trial.

RESULTS

Plasma progesterone concentrations of supplemented ewes were higher than those of unsupplemented ewes (3.28 vs 1.75 ng/ml) on Day 6 (p=0.02) but not on Day 9 (4.58 vs 4.63 ng/ml). Treatment of ewes had no effect on either the proportion of ewes which lambed to the synchronised mating period or that had multiple lambs. Lambs born to ewes shorn at Day 30 tended (p=0.09) to be heavier at birth (by 0.28 kg) than those born to unshorn ewes but this effect was not evident when data were corrected for length of gestation. Neither shearing at Day 5 nor progesterone supplementation had any effect on the birthweight of lambs, and the treatment of ewes had no effect on the survival rate of lambs to weaning.

CONCLUSIONS

Progesterone supplementation for 6 days beginning 3 days post mating did not increase either the proportion of ewes that lambed or that had multiple lambs, or the birthweight of lambs. Shearing 5 days after mating had no significant effect on the reproductive performance of ewes and need not be avoided, but is unlikely to result in an increase in lamb birthweight. Shearing ewes at Day 30 may result in an increase in the birthweight of lambs but, ideally, ewes should be further advanced in pregnancy before shearing is undertaken.