A mail survey of factors affecting performance in 627 selected sheep flocks in south-eastern Norway

Abstract

Factors influencing rate of barren ewes, number of lambs born per pregnant ewe, and autumn live weight of the lambs were studied in 627 Norwegian sheep flocks based on herd level data from the Norwegian Sheep Recording System and a mailed questionnaire to the sheep farmers. The following factors were significantly associated (P<0.05) with rate of barren ewes: breed, herd average lambing date, type of production/age of farmer, routines for treatments for parasites, and housing type. Number of lambs born per pregnant ewe was significantly associated with rate of barren ewes, herd average lambing date, breed, percentage grass silage of total roughage, and no separation of gimmers during the housing period. Corrected autumn live weight of the lambs was significantly associated with breed, flock size, lambs born per pregnant ewe, no separation of gimmers during the housing period, and type of summer pasture.     

产羔期、补饲水平对考力代羊生产和繁殖性能影响的研究

为了研究草地一绵羊系统中草畜供求间的动态平衡，寻求在人工草地放牧条件下适宜的产羔时期及在此时期给产羔母羊的合理补饲水平，１９９０～１９９３年，在贵州省威宁县灼甫联户示范牧场进行了两期产羔期及补饲水平对绵羊生产性能影响的试验。产羔期的改变，导致了母羊及羔羊体重的不同变化趋势。结果显示出母羊体重对其生产、繁殖性能的重要性。春羔母羊配种前体重较之冬羔母羊提高９．７５ｋｇ，两者产羔率分别为１３９．２％和１１６．１％。春羔母羊较高的双羔率导致了春羔母羊较之冬羔母羊更低的繁殖成活率（分别为９１．２％和１０４．７％）。断奶时，春羔较之冬羔体重高５．２５ｋｇ，但断奶以后这种优势即消失了。补饲水平的差异对母羊及羔羊体重有一定的影响，但在牧草丰富的条件下表现得并不很强烈。     

Production responses and cost-benefit of long-acting pre-lambing anthelmintic treatment of yearling ewes in two commercial flocks in New Zealand

AIMS: To investigate the production responses and cost-benefit of administering a controlled-release anthelmintic capsule (CRC) to pregnant yearling ewes prior to lambing.

METHODS: Yearling ewes from two commercial sheep flocks (A, n=489; B, n=248) in the North Island of New Zealand were enrolled in the study. Prior to lambing, CRC containing albendazole and abamectin were administered to half the ewes while the other half remained untreated. Ewe liveweights and body condition scores were measured prior to lambing, at weaning and, for Flock B, prior to subsequent mating. Lambs were matched to dams shortly after birth and the weight and number of lamb weaned per ewe were determined. A cost-benefit analysis was undertaken for Flock B considering the increased weight of lamb weaned per ewe, and the weight of ewes at the next mating and the benefit in terms of lambs born.

RESULTS: The mean weight at weaning of treated ewes was greater for treated than untreated ewes by 2.76 (95% CI 0.64–4.88)?kg in Flock A (p<0.001) and 2.35 (95% CI ?0.41–5.12)?kg in Flock B (p=0.003); the weight of lamb weaned per ewe was greater for treated than untreated ewes by 1.43 (95% CI ?0.71 to –3.49)?kg in Flock A (p=0.041) and 3.97 (95% CI 1.59–6.37)?kg in Flock B (p<0.001), and ewe liveweight prior to subsequent mating was greater for treated than untreated ewes in Flock B by 4.60 (95% CI 3.6–5.6)?kg (p<0.001). There was no difference in the percentage of lambs reared to weaning between treated and untreated ewes in either flock (p>0.8). The overall cost-benefit of treatment for Flock B was NZ$9.44 per treated ewe.

CONCLUSIONS AND CLINICAL RELEVANCE: Pre-lambing CRC administration to yearling ewes resulted in increased ewe weaning weights and weight of lamb weaned in both the flocks studied. There was an economic benefit in the one flock where this was assessed.     

Performance of Djallonké Sheep under an Extensive System of Production in Faranah,Guinea

A total of 147 ewes, 4 rams and 188 lambs of their progeny of the Djallonké breed of sheep were used to study the factors affecting reproductive and growth traits and the causes of lamb mortality. Data on ewes were collected during a 12-month period, while those on the lambs born to 123 of the ewes were collected until they were 12 months of age. The average fertility and abortion rates were 0.84 and 0.09. The fertility rate increased and the abortion rate decreased with increasing age of the ewes (p<0.05). The number of lambs born per ewe joined, litter weight at birth per ewe joined and litter weight at weaning per ewe joined were 1.28, 3.5 kg and 17 kg, respectively. The average numbers of lambs born per ewe, lambs born alive per ewe, lambs born dead per ewe and lambs per ewe that died between birth and weaning were 1.53, 1.43, 0.03 and 0.3, respectively. The age of the ewes significantly (p<0.05) affected all these traits except the number of dead lambs and the index of fertility (94%). The age of the ewes significantly (p<0.05) affected the birth weight and the weight at 6 and 12 months of age, whereas the lambing season significantly (p<0.05) affected all the growth traits studied. The type of birth was the most important source of variation in body weights of lambs. Sex had no significant (p>0.05) effect on the growth traits studied. The complex `starvation–bad management–light body weight at birth' caused 48% of the lamb mortality between birth and weaning, while diarrhoea, pneumonia and internal and external parasites caused approximately 52% of the lamb mortality over the same period. The seasonal raw mortality rate of the lambs before weaning was highest in the humid season.     

Comparative study of gastrointestinal helminthiasis in sheep on 'clean' grazing and permanent pasture under field conditions

An investigation of sources of helminth infection was carried out on three flocks: a traditional permanent pasture flock (A), one operating a 'clean' grazing system (B), and an East of Scotland College flock (C) which had operated a clean grazing system for nine years. Ewe and lamb worm egg output and pasture larval levels were recorded and tracer lambs were grazed during July and August on each farm. Considerable contamination was present on farm B fields compared with farms A and C, which resulted in higher worm burdens in late summer in farm B tracer lambs and lower weight gains. The main source of this infection was thought to be ewe periparturient egg output, as 21 per cent of ewes had positive worm egg counts over the lambing period. Differences in management practices between farms B and C, eg, earlier stocking of farm B fields, were also considered contributory factors.     

A matched case-control study of factors associated with retention of fetal membranes in dairy ewes in Southern Greece

We defined retention of fetal membranes (RFM) in dairy ewes as failure to expel the placenta within 6h after lambing the last lamb and designed a matched case-control study to identify factors that affect the risk of retention. For each ewe with RFM, the next ewe in the flock that lambed and expelled the placenta in <6h after lambing the last lamb was selected as control. Data analyzed included 92 pairs of ewes from 25 flocks comprising a total of 7275 ewes (median flocksize 270 ewes). Factors investigated for associations with RFM were induction of lambing, obstetrical assistance because of dystocia, parity, the number of liveborn lambs, the occurrence of stillbirth(s), the mean weight of the litter on the third day post-lambing and the occurrence of neonatal death in the litter. Conditional logistic regression indicated (1) that the risk of RFM increased linearly with increasing number of liveborn lambs and (2) that the risk of retention was 4-fold higher in ewes that received assistance at lambing than those that lambed normally.     

Risk factors for lamb mortality on UK sheep farms

Data on neonatal-lamb mortality and flock history were recorded by veterinary students working on 108 sheep farms in the UK in Spring 1997. The mean reported mortality incidence risk was 10.0% (median 9%; inter-quartile range 5.9–12.3%). The outcomes were incidence risks of stillbirth, perinatal (within 24 h of birth) mortality and postnatal (>24 h after birth) mortality. Exposures were screened for univariable associations with mortality risk. Logistic binomial multiple regressions adjusted for confounding, with farm as a random effect. This study has raised hypotheses for management factors associated with lamb mortality that warrant further study. Intensive rearing systems appear to be associated with increased perinatal and postnatal mortality, although housing ewes at lambing was associated with a decreased risk of stillbirth. High perinatal mortality also was associated with poor mothering-pen hygiene, flocks that foster more lambs, and failure to provide appropriate nursing for sick lambs. Larger flocks, poor ewe condition at breeding, and flocks with higher ewe-replacement rates were associated with higher postnatal mortality.     

The seroprevalence of maedi-visna in Ontario sheep flocks and its relationship to flock demographics and management practices.

The objectives of this study were to describe the serological prevalence of maedi-visna in a sample of Ontario sheep flocks, and to identify management and demographic variables that were associated with seroprevalence for maedi-visna. A sample of 103 sheep flocks in Ontario was randomly selected from those flocks participating in the Red Meat Plan. The owners of these flocks were surveyed regarding management procedures on their farms, and blood samples were taken from a random sample of ewes in each flock. At least one ewe tested serologically positive, based on the agar gel immuno-diffusion test, in 69.9% of the farms. Positive serological reactions occurred in 20.9% of the 3880 sheep tested. Flock demographics and farm management variables were considered in a multiple regression model, and several factors were positively associated with higher maedi-visna seroprevalence rates. These included the average age of the flock, the number of years the owner had been sheep farming, the practice of using foster ewes, the practice of allowing lambs to have contact with other ewes that are lambing, and the average pasture acreage per ewe.     

Epidemiologic factors involved in perinatal lamb mortality on four range sheep operations.

Four shed-lambing operations in western Colorado were monitored during the 1984 spring lambing season to determine the causes and rates of perinatal lamb mortality. The number of lambing ewes per flock ranged from 513 to 1,712, and lambing percentages ranged from 131 to 180%. Overall perinatal lamb mortality ranged from 8.2 to 12.2%. Most lamb deaths occurred during parturition or within 24 hours after parturition. More than 85% of all lamb deaths were in lambs born to ewes having 2 or more lambs. The leading causes of lamb death were starvation, dystocia, stillbirth (unknown cause), and infectious diseases. A wheel model was used to categorize factors causing lamb deaths into 4 groups: physical, social, host, and biological, and to present data on perinatal lamb mortality in a simple visual model. In all flocks, social and biological factors resulted in most of the lamb deaths. On the basis of our findings, we suggest that interventions designed to improve ewe-lamb bonding and to reduce infectious agents and the incidence of prolonged parturition may reduce lamb mortality.     

Effects of alternative lamb production systems, terminal sire breed, and maternal line on ewe productivity and its components.

The 4 yr productivity of 25% (QF; n = 533) and 50% (HF; n = 531) Finnsheep ewes exposed to either Suffolk or Columbia rams in one of three production systems was monitored to test the effects of system, terminal sire breed, maternal line, and their interactions on annual market lamb production. Ewe lambs and yearlings were randomly assigned to either a high-input accelerated lambing system (HIGH), a high-input annual system (MED), or a low-input annual system (LOW). Nursery facilities were available for weak lambs or those born in triplet or more births for the HIGH and MED but not for the LOW systems. Accelerated lambing protocol required early weaning. Sex-adjusted lamb weaning weights were corrected to within-system mean ages of 42 d for HIGH and 70 d for MED and LOW. The HIGH ewes weaned 1.55 lambs per year compared with 1.46 for MED and 1.18 for LOW (P less than .01). However, because of early weaning, HIGH yielded the lowest weight of weaned lamb per year. The MED ewes weaned 5.9 and 11.1 more kilograms of lamb per year than the LOW and HIGH ewes, respectively (P less than .01). The HIGH system may be economically feasible if young lambs could be inexpensively grown to feeder or market lamb weight. The HIGH ewes did not, however, increase lamb numbers in proportion to increased exposures compared with the annual systems. Breed-group effects for ewe productivity (kilogram of lamb weaned per ewe per year) were consistent across management systems, although some interactions among breed group and system were present for components of productivity. Suffolk rams yielded an advantage of 1.6 kg of weaned lamb per exposure over Columbia rams (P less than .05) due to a 3% better lamb survival (P less than .01) and heavier weaning weight, especially in the LOW system. The HF ewes weaned .1 more lambs per exposure than QF ewes (P less than .01); .06 of the lambs were nursery-reared. However, lambs from HF ewes had a 2% lower survival rate (P less than .05) and were 1.4 kg lighter at weaning (P less than .01), so overall productivity among HF and QF ewes was similar.     

Evaluation of Columbia, USMARC-Composite, Suffolk, and Texel rams as terminal sires in an extensive rangeland production system: I. Ewe productivity and crossbred lamb survival and preweaning growth

A 3-yr study was conducted to comprehensively evaluate columbia, suffolk, usmarc-composite (composite), and texel breeds as terminal sires in an extensive rangeland production system. The objective was to estimate breed-of-ram effects on ewe fertility, prolificacy, and dystocia, and sire breed effects on lamb survival and growth until weaning at approximately 132 d of age. Data were from 22 columbia, 22 composite, 21 suffolk, and 17 texel rams with 957 exposures to 574 adult rambouillet ewes (3- to 7-yr-old at lambing), 908 lambings, and 1,834 lambs. Ram breed did not affect ewe fertility (mean = 94.9%; p = 0.73), total number born per ewe lambing (mean = 2.02 lambs; p = 0.20), number born alive per ewe lambing (mean = 1.90 lambs; p = 0.24), or number weaned per ewe lambing (mean = 1.45 lambs, p = 0.94). Dystocia rates were different (p = 0.01) for ewes mated to columbia (12.2%), composite (13.5%), suffolk (25.7%), and texel rams (31.9%) during 1 yr of the study, but differences among ram breeds were not repeatable (p ≥ 0.38) during the other 2 yr. Suffolk-sired lambs were heavier (p ≥ 0.02) at birth (5.5 kg) and weaning (40.3 kg) than lambs sired by the other breeds, which did not differ (p ≥ 0.34) for birth weight (mean = 5.3 kg). Texel-sired lambs (37.4 kg) were lighter (p ≥ 0.02) at weaning than columbia- (38.8 kg) and composite-sired (38.4 kg) lambs, which did not differ (p = 0.40) for weaning weight. Sire breed effect approached significance (p = 0.06) for lamb survival to weaning; estimated survival probabilities were 0.87 (columbia), 0.89 (composite), 0.93 (suffolk), and 0.86 (texel) for lambs reared by their birth dam. Interaction between sire breeds and birth weight affected (p < 0.001) lamb survival and revealed that lightweight columbia- and suffolk-sired lambs had a greater risk of death than lightweight lambs sired by composite and texel rams, but risk of death did not increase substantially for heavyweight lambs from any of the breeds. When mated to adult rambouillet ewes in an extensive rangeland production system, the use of suffolk rams is warranted to improve preweaning growth of market lambs and is not predicted to affect ewe fertility, ewe prolificacy, dystocia, or lamb survival compared with the other sire breeds we tested.     

A survey of sheep diseases in Canada.

A mail survey of disease occurrence in Canadian sheep flocks was conducted. The survey, which covered the period from September 1982 to August 1983, utilized flocks on the Record of Performance (ROP) sheep program and relatively complete data were available from 116 flocks. Data about lambing rates, incidence of a variety of lamb and ewe diseases and reasons for culling were obtained. At the same time a retrospective evaluation of records of diagnoses of sheep diseases recorded at diagnostic laboratories across the country was performed. Data from the years 1978 to 1982 were obtained and summarized. A lambing percentage of 153% (1.53 lambs live born per ewe lambing) was observed and an additional 0.05 lambs were stillborn. The major identified causes of mortality amongst lambs were starvation, pneumonia, scours and accidents. Pasteurella spp. were the etiological agents most commonly associated with pneumonia in lambs and Escherichia coli had the same predominant position with regards to nonparasitic scours. A large discrepancy existed between the proportional mortality rates for internal parasites and coccidiosis as determined from the farm survey data compared to diagnostic laboratory data. This suggests that clinical parasitism may not be adequately recognized at the farm level. Abortions in ewes occurred in approximately half the flocks, but generally at a low level and no severe abortion storms occurred. Pneumonia was the most commonly identified cause of mortality in ewes and although Pasteurella spp. appear to be the most important etiological agents, regional differences were apparent.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of sarcoptic mange on the reproductive performance of ewes and transmission of Sarcoptes scabiei to newborn lambs

In each of two experiments, the reproductive performance of ewes naturally infested with sarcoptic mange and of uninfested ewes was assessed in in-season natural breeding conditions (experiment I) or in out-of-season controlled breeding conditions (experiment II). A statistically significant difference was observed in the total lambs born per ewe (infested ewes, 1.2; controls, 1.6), in the liveborn lambs per ewe (infested ewes, 1.2; controls, 1.5) and in the lamb body weight per ewe (infested ewes, 3.33kg; controls, 4.53kg) under natural breeding conditions, as well as in the lamb body weight per ewe (infested ewes, 3.37kg; controls, 4.14kg) among ewes under controlled breeding conditions. No adverse effects were observed in the other reproductive indices, cycling rate, mating rate, return-to-oestrus rate, abortion rate, lambing rate and stillbirth rate. Sarcoptic mange was observed in the offspring of infested ewes after 30 days and in the offspring of uninfested controls 60 days after their birth.     

Effects of supplementary feeding around lambing time on the productivity of Javanese thin-tail ewes

The extent to which ewe reproduction could be improved by supplementary feeding around lambing time was investigated with four groups each of 16 ewes which were run continuously with fertile rams until they had lambed three times. One group was fed at a maintenance level throughout and the other three groups were given supplementary feed during the last four week of each pregnancy only, or during the last four weeks of pregnancy plus the first six weeks after each lambing, or during the last four weeks of pregnancy plus the first 12 weeks after each lambing. The average body weight of lambs weaned at 13 weeks of age increased (P less than 0.001) progressively from 6.0 to 9.1 kg with increasing duration of supplementary feeding. There were however no significant differences between treatment groups in mean interval between successive lambings (202 days), ovulation rate (2.26), litter size (1.58), lamb birth weight (1.8 kg), preweaning lamb mortality (37%) or number of lambs weaned per lambing (0.99).     

A field study of reproductive performance and productivity of Pelibuey ewes in Southeastern Mexico

The objective was to evaluate the reproductive performance and productivity of Pelibuey ewes in two flocks in Southeastern Mexico. Animals grazed on irrigated star grass (Cynodon plectostachyus). In flock 1, lambs grazed with their mothers from 7 to 70 days of age (weaning age), whereas in flock 2, they were confined and offered chopped grass, a commercial concentrated and managed under restricted suckling until weaning at 60 days of age.. The statistical model included the fixed effects of flock, year and season of lambing, parity number, litter size at weaning, first order interactions and the random effects of ewe and the error term. The means ± SD for age at first lambing, prolificacy, lambing interval (LI), weaning weight, and productivity were 477?±?85.5 days, 1.41?±?0.51 lambs, 255.9?±?51.8 days, 16.5?±?4 kg, and 18.1?±?4.42 kg of lamb weaned, respectively. Repeatability estimates for the latter four traits were 0.09, 0.07, 0.11, and 0.13, respectively. All main effects in the models were significant (P?<?0.001), except for year on LI. Prolificacy and productivity increased with parity number. The shortest LI was in the rainy season. Ewes with two or three lambs produced 8 and 14 kg of lamb more at weaning than single lamb ewes. In conclusion, reproductive performance and productivity could be improved, and the low repeatability estimates suggest that management should be used in a short term rather than genetic approaches.     

Performance of Polypay, Coopworth, and crossbred ewes: II. Survival and cumulative lamb and wool production over 4 years.

Six ewe genotypes, generated by mating Coopworth (C), Polypay (P), and Suffolk (S) rams to P and Coopworth-type (Ct) ewes, were exposed to Hampshire rams for spring lambing from 1987 through 1990. Data from 1,013 exposures and 973 resultant lambings were used to analyze reproductive traits and cumulative ewe productivity over 4 yr. Ewe body and fleece weights were also analyzed. Ewes from S sires weaned the heaviest lambs and ewes from P sires weaned the largest number of lambs, resulting in similar total weight of lamb weaned per ewe mated. Coopworth-sired ewes weaned the least total weight of lamb per ewe mated. Ewes weaning twins produced 54% more total litter weight per ewe than those weaning singles. Annual ewe survival averaged 95%, ranging from 93% for S x P and C x P ewes to 97% for P x Ct and C x Ct ewes. When cumulative number and weight of lamb produced was assessed on the basis of all ewes starting the trial, P-sired ewes were highest, followed in order by daughters of S and C sires. Suffolk-sired ewes (67 kg) were 13% heavier than daughters of P rams and 19% heavier than daughters of C rams. Adjustment of lamb production for ewe metabolic body size resulted in C-sired ewes being more efficient than the heavier S-sired ewes. Coopworth-sired ewes produced 32% more wool than ewes sired by the other two breeds. Differences in wool production between ewes weaning one or two lambs were small.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of lasalocid and supplement level on productivity of gestating ewes grazing winter range

Two winter feeding trials (1985-86; 1986-87) were conducted to evaluate the productivity of gestating ewes fed lasalocid (L) and two supplement levels while grazing Montana winter range. Five hundred range ewes were randomized within age and breed each year and allotted to .15 or .23 kg hd-1.d-1 of a 20% CP supplement and either no L or L at 70 mg hd-1.d-1. Feed treatments began on 18 Dec. approximately 100 d before the first expected lambing date and continued for 84 d. Ewes fed .23 kg of supplement per day gained more (P less than .01) total weight (4.9 vs 4.0 kg) during the 84-d experiment and had higher (P less than .05) grease fleece weights (4.2 vs 4.0 kg) than those fed .15 kg of supplement. Lasalocid had no effect (P greater than .05) on ewe weight change or grease fleece weights. Supplement level had no effect (P greater than .05) on reproduction, lamb mortality and lamb performance. Ewes fed L had a greater (P less than .05) percentage of lambs born per ewe than those not fed L (120.7 vs 112.1%); lamb performance was similar (P greater than .05) between treatments. However, because a greater percentage of lambs were born per ewe starting the experiment, ewes fed L produced more (P less than .05) kilograms of weaned lamb than those not fed L (25.9 vs 23.4 kg). In conclusion, ewes fed L and grazing winter range weaned more kg of lamb than did controls because of an improved lambing percentage.(ABSTRACT TRUNCATED AT 250 WORDS)     

A practical approach to managing the risks of iodine deficiency in flocks using thyroid - weight: birthweight ratios of lambs

AIM: To establish a protocol for determining the iodine status of a flock and managing the risks of iodine deficiency, using thyroid-weight:birthweight ratios of newborn lambs and serum iodine concentrations of ewes. METHODS: Data were collected from iodine supplementation studies in Southland and Rangitikei, of sheep fed exclusively pasture or pasture plus brassica during the latter half of gestation (n=350, or 89 per treatment group). The ewes were supplemented pre-mating or at the time of pregnancy scanning with an injection of long-acting iodised oil. Serum iodine concentrations were measured in ewes prepartum in Rangitikei and postpartum at both locations. The thyroid-weight:birthweight ratios (as g/ kg) in 229 newborn lambs were determined at post-mortem examination and compared between iodine supplemented vs unsupplemented flocks using probit analysis. Samples of pasture and kale were analysed monthly for determination of iodine and selenium. RESULTS: Initial mean serum iodine concentration of all ewes was 41 mug iodine/L. Supplementation increased serum iodine concentrations regardless of forage fed, and concentrations remained high for between 127 and 206 days. The range of thyroid-weight:birthweight ratios in lambs from supplemented ewes was 0.09-0.70 (mean 0.35, standard deviation (SD) 0.147) g/kg. Among lambs from unsupplemented ewes, the range was much wider (0.21-8.5; mean 1.61, SD 1.95 g/kg). About half of those ratios were >0.8 g/kg and clearly indicative of goitre, including 62% from the groups on brassica and 18% from the Rangitikei group fed exclusively pasture. Probit calculations showed that a ratio of 0.40 (95% confidence interval (CI)=0.29-0.47) g/kg predicted with 35% probability, and a ratio of 0.80 (95% CI=0.70-0.99) g/kg predicted with 90% probability that a lamb came from an unsupplemented flock. CONCLUSIONS: Compared to iodine concentrations in forages, thyroid-weight:birthweight ratios more accurately reflected the iodine status of the flock and could be used to identify which flocks to supplement the following year. Serum iodine concentrations of ewes measured before or after lambing did not reflect forage fed, but values near 40 mug/L were associated with goitrous lambs. CLINICAL RELEVANCE: Thyroid-weight:birthweight ratios >0.8 g/kg were indicative of iodine deficiency, and ewes should be supplemented pre-mating or during pregnancy to prevent goitre the following year. Ratios <0.4 g/kg rarely occurred among deficient flocks, so the probability of benefit from supplementation was low. Intermediate ratios were ambiguous, and the iodine status of the flock could not be determined from biomarkers. In such cases individual-farm supplementation trials might be required to detect and manage the risks of marginal deficiency.     

Selective breeding for rearing ability as an aid to improving lamb survival

Lamb mortality to weaning age was compared in Merino flocks selectively bred from maternal ancestors with high or low lifetime rearing ability. The High Efficiency (HE) flock comprised the descendants of ewes that had reared lambs on 3 or all occasions from 4 rearing opportunities. The Low Efficiency (LE) flock comprised descendants of ewes that had failed to rear any progeny on 2, 3 or all occasions from 4 rearing opportunities. During the first 3 years of testing the response to selection the mean mortality to weaning among HE single lambs was 19.5% of lambs born compared with 32.3% in the LE flock (p less than 0.001). Mean mortality of twin lambs in the HE flock was 21.5% compared to 36.3% in the LE flock (p less than 0.01). In the HE flock significantly fewer single lambs died during and within 3h of birth and significantly fewer twins died during birth and shortly thereafter, and between 3h and 2 days of age. The viability of LE singles and twins may have been lower overall as the mortality rate in most periods was higher than that of their HE counterparts. The results highlighted the major impact of intrinsic maternal and, or foetal defects on lamb survival. It was concluded that rearing ability was heritable, and that selection for rearing ability offers excellent prospects for alleviating the hitherto intractable problem of high perinatal lamb mortality.