Breeding programmes for smallholder sheep farming systems: II. Optimization of cooperative village breeding schemes

A simulation study was conducted to optimize a cooperative village‐based sheep breeding scheme for Menz sheep of Ethiopia. Genetic gains and profits were estimated under nine levels of farmers' participation and three scenarios of controlled breeding achieved in the breeding programme, as well as under three cooperative flock sizes, ewe to ram mating ratios and durations of ram use for breeding. Under fully controlled breeding, that is, when there is no gene flow between participating (P) and non‐participating (NP) flocks, profits ranged from Birr 36.9 at 90% of participation to Birr 21.3 at 10% of participation. However, genetic progress was not affected adversely. When there was gene flow from the NP to P flocks, profits declined from Birr 28.6 to Birr ?3.7 as participation declined from 90 to 10%. Under the two‐way gene flow model (i.e. when P and NP flocks are herded mixed in communal grazing areas), NP flocks benefited from the genetic gain achieved in the P flocks, but the benefits declined sharply when participation declined beyond 60%. Our results indicate that a cooperative breeding group can be established with as low as 600 breeding ewes mated at a ratio of 45 ewes to one ram, and the rams being used for breeding for a period of two years. This study showed that farmer cooperation is crucial to effect genetic improvement under smallholder low‐input sheep farming systems.     

Optimal village breeding schemes under smallholder sheep farming systems

Despite challenges in the implementation of livestock genetic improvement programs in developing regions, including centralized nucleus breeding schemes, these programs can contribute to the improvement of the livelihood of smallholder farmers. In this paper, we present a community- or village-based breeding scheme in which breeding activities are carried out by communities of smallholder farmers. We evaluated genetic responses and the rate of inbreeding from alternative village sheep breeding schemes that were based on a survey of existing flock structure and breeding management in a sheep-barley system in Ethiopia. This survey showed that individual flock sizes were small, and that the majority of farmers practiced mixed grazing and uncontrolled mating of their flocks in communal grazing lands within villages. Here we evaluated within-village schemes (selection across flocks within a village) and across-village schemes (selection across villages) at different intensities of ram selection (i.e. proportions of rams selected, P). Our results showed that under within-village schemes, intensity of selection could not be increased (i.e. P could not be decreased below 0.149) when the rate of inbreeding was constrained to an acceptable level of 0.01, resulting in low genetic gain. The most optimal scheme was found to be across-village selection with at least three villages cooperating and P = 0.05. Our results also indicated that genetic gain from village breeding schemes with mass selection and BLUP selection is comparable. Village breeding schemes can make a significant contribution to the genetic improvement of livestock in Ethiopia and other developing countries.     

Distribution and prevalence of footrot in Bhutan

The first cases of footrot in Bhutan were reported in sheep in 1990 at the National Sheep Breeding Centre (NSBC), which supplies breeding animals to village sheep flocks throughout Bhutan. Despite the presence of footrot at the Centre the distribution of apparently disease-free sheep continued. Cases of footrot were reported in village flocks soon after the disease was diagnosed at NSBC. A national survey was designed to establish the distribution and prevalence of footrot in Bhutan. This detected footrot in 19/94 village sheep flocks surveyed. The 19 affected flocks were distributed among nine different administrative districts whereas the villages selected were in 13 of a total of 16 sheep growing districts. The highest within-flock prevalences were among the seven flocks sampled in Bumthang district (mean 20.4%). The prevalence of the disease within flocks was generally much lower in other affected districts and in three districts a single affected animal was identified in the sample of 14 sheep examined in each village. Nationally, footrot prevalence was estimated to be 3.1% (95% CI 2.16-4.04%). There was a positive association between the receipt of animals from NSBC and the presence of footrot. The prevalence of the disease was higher in flocks with a migratory system of management than in those using a sedentary system. The relative risk of there being footrot in a migratory flock was nine-times higher than in a non-migratory flock. Only one strain of Dichelobacter nodosus (serogroup B) was identified among the 234 isolates obtained from the 19 affected flocks. Sheep with footrot healed quickly when treated with a vaccine made from this strain.     

Breeding practices,growth, and carcass potential of fat-tailed Washera sheep breed in Ethiopia

On-farm survey of farmers' breeding practices, breeding objectives, and selection criteria and on-station feedlot performance evaluation of Washera sheep were undertaken in Ethiopia. The survey revealed that most (79.8%) of the farmers had no breeding ram. The mating system was predominantly uncontrolled. A majority (75.5%) of the sheep owners reported that they herded their sheep flock by mixing with other livestock species mainly with cattle. During grazing, 44.6% of the farmers mix their sheep flock with neighboring sheep flocks. The major sheep production objective was to generate income from the sale of live sheep. Fast growth, appearance, coat color, and pedigree performance were important ram selection criteria, respectively. Ability to breed at early age, pedigree information, mothering ability, and lambing interval were important selection criteria for ewe, respectively. The on-station performance study involved evaluation of feedlot gains and carcass production under five levels of feeding regimes (300, 400, 500, 600, and 700 g day⁻¹ of concentrate feed) for a period of 93 days. The results indicated that the feedlot growth and carcass performance of Washera sheep were very high, with average daily weight gains of up to 126 g and carcass weight of 16 kg, with the optimal level of supplementation for Washera sheep being at 500 g of concentrate per day for a period of 93 days.     

PrP genotype frequencies in German breeding sheep and the potential to breed for resistance to scrapie

Genetic susceptibility to scrapie is associated with polymorphisms in three different codons of the ovine prion protein (PrP) gene (136, 154, 171). Studies of PrP genotypes linked to scrapie have revealed the resistance of homozygous PrPARR/PrPARR animals and the high risk of PrPVRQ/PrPVRQ and PrPvRQ/PrPARQ animals in scrapie-affected flocks. The selection of PrPARR/PrPARR genotypes may therefore provide a strategy for controlling clinical scrapie. The genotypes of 1361 German breeding sheep from 15 different breeds in northern Germany were determined. Apart from the wildtype allele PrPARQ, at least four mutually exclusive allelic variants were found. The greatest variability within the PrP gene was encountered in texel sheep, in which 14 PrP genotypes were found. In the important meat breeds, Suffolk, German whiteheaded mutton and German blackheaded mutton, the PrPARR allele was predominant, and in these breeds the breeding of scrapie-resistant pedigree flocks within four generations seems to be a feasible option. In the texel sheep, the German merino, the German milk and the German land sheep breeds, the frequency of the PrPARR allele was much lower, and in several breeds no homozygous rams were available for breeding purposes. In these breeds the breeding strategy would depend on the number of heterozygous rams available, but resistant pedigree flocks could be achieved within nine generations.     

Breeding programs for smallholder sheep farming systems: I. Evaluation of alternative designs of breeding schemes

Village‐ and central nucleus‐based schemes were simulated and evaluated for their relative bio‐economic efficiencies, using Ethiopia's Menz sheep as example. The schemes were: village‐based 2‐tier (Scheme‐1) and 1‐tier (Scheme‐2) cooperative village breeding schemes, dispersed village‐based nuclei scheme (Scheme‐3), conventional 2‐tier central nucleus‐based scheme (Scheme‐4), and schemes linking a central nucleus and village multiplier nuclei with selection in central nucleus (Scheme‐5) or in both central and village nuclei (Scheme‐6). Among village‐based schemes, Scheme‐1 gave the highest genetic progress, while Scheme‐2 was economically the most efficient with genetic gain in the breeding objective of Birr 5.6 and a profit of Birr 37.2/ewe/year. The central nucleus schemes were more efficient than the village schemes. Scheme‐4 was the most efficient with genetic gain in the breeding objective of Birr 13.5 and a profit of Birr 71.2, but is operationally more difficult as it requires a very large central nucleus. The choice between village and central nucleus‐based schemes would depend on local conditions (availability of infrastructure, logistics and technical knowhow and support). Linking central nucleus with village‐based nuclei (Scheme‐6) would be a feasible option to overcome the operational difficulties of the conventional central nucleus scheme. If a village‐based breeding program is envisaged as should be the 1st step in most low‐input systems, then Scheme‐2 is the most efficient. To scale out to an entire Menz breed level, Scheme‐3 would be recommended.     

Breeding with resistant rams leads to rapid control of classical scrapie in affected sheep flocks

ABSTRACT: Susceptibility to scrapie, a transmissible spongiform encephalopathy in sheep, is modulated by the genetic make-up of the sheep. Scrapie control policies, based on selecting animals of resistant genotype for breeding, have recently been adopted by the Netherlands and other European countries. Here we assess the effectiveness of a breeding programme based on selecting rams of resistant genotype to obtain outbreak control in classical scrapie-affected sheep flocks under field conditions. In six commercially-run flocks following this breeding strategy, we used genotyping to monitor the genotype distribution, and tonsil biopsies and post-mortem analyses to monitor the occurrence of scrapie infection. The farmers were not informed about the monitoring results until the end of the study period of six years. We used a mathematical model of scrapie transmission to analyze the monitoring data and found that where the breeding scheme was consistently applied, outbreak control was obtained after at most four years. Our results also show that classical scrapie control can be obtained before the frequency of non-resistant animals is reduced to zero in the flock. This suggests that control at the national scale can be obtained without a loss of genetic polymorphisms from any of the sheep breeds.     

Evaluation of the efficiency of alternative two-tier nucleus breeding systems designed to improve meat sheep in Kenya

A deterministic approach was used to genetically and economically evaluate the efficiency of five two‐tier nucleus breeding systems for meat sheep in Kenya. The nucleus breeding systems differed in terms of whether the system was closed or open, in the type of animals that were involved in the movement of genetic superiority and in the number of selection pathways in each system. These systems were compared under four alternative breeding objectives based on monetary genetic gain and profit per ewe. The first objective simulated a situation where the flock size cannot be increased due to non‐feed related constraints (FLOCK). The second specifically assumed that the flock size is restricted due to limited amount of feed resources (FEED). The third and fourth objectives assumed that sheep performed only tangible roles (TR) and both tangible and intangible roles (IR) in the production system respectively. Monetary genetic gains were highest for all objectives in an open nucleus system with a certain proportion of commercial‐born ewes being introduced in the nucleus while at the same time utilizing young rams from the nucleus to breed sires and dams for the nucleus and commercial sector (ONyre). Utilizing young rams in a closed nucleus system for the dissemination of superior genes resulted in higher annual monetary genetic gain than utilization of old rams. Profit per ewe was significantly higher for FLOCK and IR in ONyre. In a closed system that allowed for downward movement of dams from the nucleus to the commercial sector to breed sires and dams, profit per ewe was highest for FEED and TR. The success of a nucleus breeding system should also focus on the profitability and logistics of establishing it. The implication of these results on the choice of two‐tier nucleus breeding systems for the improvement of meat sheep is discussed.     

Effectiveness of photoperiod stimulation on reproductive performance of sheep housed continuously indoors on an accelerated breeding schedule.

Over five breeding periods, the reproductive performance (fertility, prolificacy and fecundity) of three strains of sheep housed continuously indoors was measured. The sheep were maintained as two separate flocks, A and B, with similar genetic background on an eight month accelerated breeding schedule under one of three lighting regimes, abrupt, contracted or constant, to induce ovarian activity. Overall fertility to one estrus was 59%. For flock A, fertility was greatest under the contracted lighting regime and lowest under the constant lighting regime; the opposite was true for flock B. Among the three strains, fertility was lowest for the Dam 2 strain in flock A. Prolificacy averaged 1.8 lambs per ewe lambing and was similar under each of the three lighting regimes. Fecundity (averaging 106%), the number of lambs born per 100 ewes exposed to rams, generally followed the pattern of fertility.     

Connectedness in Targhee and Suffolk flocks participating in the United States National Sheep Improvement Program

Connectedness among animals in separate flocks reduces the risk of biased comparisons when selecting across flocks on EBV. The objective in this study was to assess levels of connectedness in the genetic evaluation of weaning weight among Targhee and Suffolk flocks participating in the US National Sheep Improvement Program (NSIP). Among flocks currently participating in the NSIP, a total of 25,404 weaning weight and 35,794 pedigree records were available for 16 Targhee flocks, and 14,017 weaning weight and 18,311 pedigree records were available for 24 Suffolk flocks. Connectedness was measured by using 2 different methods. First, numbers of progeny with recorded weaning weights from linking sires (defined as sires with progeny in multiple flocks or sires born in one flock with progeny in another flock) were counted. Second, connectedness was measured by calculating the average prediction error correlation of mean flock EBV (flock r(ij)). Benchmarks for flock r(ij) were established, with 0.10 and 0.05 representing low and moderate risk of bias associated with comparing EBV among flocks, respectively. From 1995 through 2004, 44% of Targhee lambs with weaning weights were born to linking sires; in Suffolk lambs, that value was 23%. In 1990, 1995, and 2005, average flock r(ij) were 0.10, 0.19, and 0.28, respectively, among Targhee flocks, and 0.02, 0.02, and 0.04, respectively, among Suffolk flocks that participated in NSIP in all 3 yr. Among all active flocks in 2005, flock r(ij) averaged 0.13 in Targhees and 0.03 in Suffolks. Hierarchical clustering of flocks based on flock r(ij) revealed that all active Targhee flocks connected at a level near or above 0.10. In Suffolk flocks, 2 distinct clusters had formed, in which connectedness was relatively high within each cluster (flock r(ij) near 0.10) but was near zero between clusters. Risk of bias in comparing EBV among flocks in the Targhee was low; however, caution should be exercised when comparing EBV between Suffolk flocks from different clusters.     

Use of anthelmintics for nematode control in sheep in West Germany: results of a survey]

A survey by questionnaires was performed on 543 (0.9% of all) sheep flocks in western Germany in 1988/89 to obtain informations about deworming practices against gastrointestinal nematodes. The major informations are: Anthelmintic treatments were associated with the reproductive status of ewes and the breeding of lambs in most of the flocks. They were performed at an epidemiologically appropriate time in many cases but were not accompanied by essential management practices. (Pro)benzimidazole compounds were the anthelmintics most frequently used by more than 95% of the farmers; 22% and 13% of the farmers additionally or exclusively administered levamisole/pyrantel and ivermectin, respectively. On average, treatment of ewes and lambs against nematodes was performed 2.5 times and 2.8 times a year, respectively. The mean frequency of deworming differed regionally and was generally greater in pedigree breeding flocks than in commercial breeding flocks; it was significantly affected by the husbandry system, the number of ewes per flock, the size of available pasture and the class of anthelmintics employed. The results of this survey revealed that many German sheep farmers obviously lack an awareness of the rationale of preventive and economic control practices against gastrointestinal nematodes.     

The National Sheep Improvement Program: a review

A nationally organized sheep improvement program for sheep producers in the United States was implemented in 1987 under the name of the National Sheep Improvement Program (NSIP). This program completed a 3-yr Phase I project on February 16, 1990, that involved the definition of a uniform set of performance guidelines, development of an NSIP records processing center with associated performance recording materials and computer software, and the enrollment of both purebred and commercial flocks. Organizers of the NSIP have defined 12 traits of economic importance to the U.S. sheep industry for genetic evaluation: number of lambs born, total ewe productivity, six growth traits, and four wool traits. Genetic evaluations are currently being conducted on a within-flock basis and will move to an across-flock, within-breed basis when sufficient genetic ties between flocks are established. The genetic evaluations use BLUP procedures and provide genetic merit values in the form of expected progeny differences for every animal in a flock.     

Genetic evaluation of growth in farmers’ flocks of Madras Red sheep under long-term selection in a group breeding scheme

The Network Project on Sheep Improvement (NWPSI)–Madras Red field unit is a group breeding scheme involving 198 farmers’ flocks of Madras Red sheep in which selection for growth traits and rotation of rams have been practised for over two decades. Growth data collected from these flocks were used to evaluate the performance and understand the direct and expected responses to selection based on genetic parameters. The body weight at birth (BW), weaning weight (WW), 6-month weight (6W), 9-month weight (9W), 12-month weight (YW), pre-weaning average daily gain (ADG1, birth to 3 months), post-weaning ADG2 (3–6 months), ADG3 (6–9 months), ADG4 (9–12 months) and ADG5 (3–12 months) were 2.67, 10.05, 14.56, 18.36, 21.36, 80.13, 49.05, 43.00, 34.21 and 41.18 g, respectively. Univariate analyses were carried out using animal and sire models to estimate variance components. Heritability obtained from animal model for BW was 0.36 and the values for other body weight traits were almost unity. Heritability estimate for pre-weaning ADG1 was 0.31. Very high genetic variability was observed in spite of long-term selection and this sustenance of variability is one of the main advantages of a group breeding scheme, combining several flocks of smaller size. An increasing genetic and phenotypic trend was noticed for almost all the traits studied. The expected responses calculated based on genetic parameters also indicated scope for improvement.     

Modelling of a recording scheme for market-oriented smallholder pig producers in Northwest Vietnam

Village breeding programmes are being developed by an integrated long-term project for smallholders in Northwest Vietnam to improve pig production in different production systems. In total, 120 smallholders with 169 sows in 5 of the 9 project villages were approached in single person interviews, using a structured questionnaire. Frequency analysis of data on breeding management and ranking of smallholders' trait preferences and selection criteria used for pigs were performed with SAS 9.1, using the FREQ procedure. Survey results indicate that both improved local breeds and exotic genotypes should be incorporated in future village breeding programmes for market-oriented smallholder pig production, improving the reproductive and growth performance as well as the carcass quality. In a next step, a model reflecting the status quo at farms with market-oriented pig production was developed integrating data from farmers' survey and information from the project's current recording scheme. A deterministic approach was used to assess the profitability and genetic merit of the current recording scheme. Modelling results show that the current recording scheme is unprofitable (− 33.90 € sow^− 1). As continued success of village breeding programmes depends on the profitability of breeding measures, the long-term sustainability of the current recording scheme seems unlikely. Genetic gains are achieved in production and carcass quality traits, while a small reduction in reproduction traits can be observed. In a last step, possible effects of increased pig performances on the profitability and genetic merit of the current recording scheme were evaluated. Effects of increased pig performances on the genetic and economic success of the recording scheme are generally limited. Further model calculations are necessary for finding possibilities to improve smallholder pig breeding in a profitable way.     

中系马里努阿犬兴奋性选育模式研究

统计中系马里努阿犬3个世代系谱资料和兴奋性状测试记录,对中系马里努阿犬兴奋性状不同的选育方法进行模拟研究。利用系谱资料,F3和F4世代兴奋性状数据,估算个体育种值,依据表型和育种值数据,对F4犬群按照个体表型、家系表型、个体育种值、家系育种值进行排序,各选择35%个体,筛选其在F5代中的个体,统计分析各种选择方法后代表型和遗传进展,评测不同选择方法对中系马里努阿犬兴奋性状选育最优方法。研究结果:选择中系马里努阿犬个体育种值好于选择家系育种值好于选择家系好于选择个体表型,选择中系马里努阿犬公犬效果好于选择母犬的效果。     

Analysis of litter size and days to lambing in the Ripollesa ewe. II. Estimation of variance components and response to phenotypic selection on litter size

A performance data set from 376 Ripollesa purebred ewes of the experimental flock of the Universitat Autònoma of Barcelona was analyzed using a bivariate Bayesian threshold-linear model. The data set contained 1,598 litter size records and 1,699 days-to-lambing records. The model included the additive genetic effect of each animal and 3 nongenetic sources of variation: ewe age, year of lambing, and the permanent environmental effect characterized by the ewe. The flock was phenotypically selected for litter size since 1986, and replacement ewes and rams were selected from the progeny of the more prolific ewes, which had at least 3 deliveries recorded. The phenotypic trend for litter size was positive, whereas days to lambing followed an unclear pattern. Both traits had low heritabilities; 0.13 for litter size and 0.11 for days to lambing. Response to selection was evaluated through (a) the average breeding value of the ewe lambs chosen annually, and (b) the average breeding value of the overall flock. The first measurement suggested a positive trend for litter size, although it showed important oscillations. On the other hand, the average breeding value for the overall flock showed a stable positive tendency after yr 4 of selection, with estimates clearly different from zero after yr 11 of selection. A significant increase in the incidence of multiple births was observed, with a mode of approximately 10%. The correlated response in days to lambing did not show a significant trend. The effect of year of lambing also positively influenced both litter size and days to lambing, although important oscillations were observed between years. Results indicated that litter size in sheep can be effectively improved through phenotypic selection, even in small flocks; moreover, days to lambing could also be genetically improved, given the estimate obtained for its heritability.     

Results of a postal survey of scrapie in the Shetland Islands in 2003

In February 2003, a postal survey of 1279 sheep farmers in the Shetland Islands yielded 586 responses (46 per cent response rate). The principal aim of the survey was to gather information on the history and control of scrapie. Overall, 28.5 per cent of the respondents thought they had had a case of scrapie in their flock at some time. There was a slow increase in the proportion of affected flocks during the 1970s, followed by a more rapid increase during the 1980s and early 1990s, and a decline from the mid-1990s onwards. The peak proportion of affected flocks was approximately 6 per cent in 1994. Of the farmers who had ever had scrapie in their flock, 97.1 per cent had attempted to control the disease. The most common method of control was breeding from non-susceptible tups, used by 90.6 per cent of the affected flocks and 75.1 per cent of the flocks that had never been affected. A comparison of the characteristics of the affected and unaffected flocks indicated that an increased risk of scrapie was associated with the larger flocks, the open flocks and the flocks that bought in lambs. The basic reproduction ratio for the spread of scrapie between flocks was estimated to be 1.47, and the mean duration of an outbreak within a flock was estimated to be approximately two years.     

A case-control study on scrapie in Norwegian sheep flocks

Scrapie first was detected in indigenous sheep in Norway in 1981, and from 1995 to 1997 an increase in the number of flocks with scrapie cases was recorded. These flocks were mainly in one geographical region. A study to identify risk factors for scrapie was conducted. The study had three frequency-matched controls selected for every case within the same Veterinary District. A questionnaire was submitted to 176 sheep flocks (42 had been scrapie flocks). The data obtained by the questionnaire were linked to data collected from governmental and industry registers. After imputing missing data using single random imputation, the statistical analysis was performed using multivariable conditional logistic regression.

Purchase of female sheep from scrapie flocks, sharing of rams, or sharing of pastures between different flocks were the risk factors associated with the occurrence of scrapie. Of factors potentially sustaining and promoting the infection in the flock, number of winter-fed sheep, number of buildings for housing sheep, rams and ewes shared room during mating period and increase in the flock size were associated with scrapie. We interpret these findings to show that factors involving transfer of sheep between flocks or direct contact between sheep of different flocks are important for the spread of scrapie. Management factors are important for the development of scrapie. However, it was not possible to discriminate between the different management factors in this study at the flock level. Also, factors indicating awareness and interest of the farmer (as well as willingness to contact a veterinarian for diseased sheep) were related to the detection of scrapie in the flock.     

Prospects and strategies for genetic improvement of the dairy potential of tropical cattle by selection

The import of genetic material and the use of crossbreeding to improve the dairy merit of tropical cattle has been criticized for eroding the livestock genetic resources of the tropics. An alternative is genetic improvement of the indigenous cattle through selection. The objective of the present paper is to examine the feasibility of this alternative.Constraints to genetic improvement of tropical cattle through selection are discussed. Low reproductive rates and high calf mortality reduce the intensity of selection. The generation interval, which is long in cattle, is further prolonged by the late sexual maturity and the long calving intervals in most tropical breeds. The most serious constraint is, however, that the extensive milk recording schemes which support dairy cattle breeding programmes in many temperate countries are almost non-existent in the tropics. In this situation, the most realistic approach to improvement through selection is to start with a single nucleus herd (or a group of cooperating herds).Two alternative selection programmes (with and without progeny testing) for a closed herd of 500 cows are outlined. The alternative which assumed no progeny testing, i.e. selection of bulls on pedigree information only, gave the fastest genetic improvement (predicted at 36 kg/year).By distributing breeding bulls from the herd the genetic progress can be disseminated to the outside population with a time lag of about two generations. A nucleus herd can supply about 15 selected bulls per 100 cows in the herd per year, enough for a population of several thousand cows.     

An assessment of genetical methods in the control of scrapieUne contribution de méthodes génétiques dans la lutte contre la tremblanteEinschätzung genetischer Methoden bei der Bekämpfung der Traberkrankheit der Schafe

Scrapie is caused by a virus-like agent which is transmitted to sheep by both maternal and horizontal routes of infection. The disease can be controlled by selective culling of bloodline relatives of scrapie cases. However, this depends on accurate breeding records which are often not available. Two major studies, with Cheviot and with Herdwick sheep, have shown that ‘susceptibility’ to experimental infection with one source of agent is controlled mainly by a single gene and that the dominant allele confers ‘susceptibility’. There is evidence that scrapie agent may still replicate in some ‘resistant’ sheep, but only after a long delay. For this reason, ‘resistance’ is best regarded in terms of an extended incubation period. Also, ‘resistance’ to one strain of agent does not mean ‘resistance’ to all strains. A ‘resistant’ flock of Swaledale sheep is being developed by experimentally infecting all animals and breeding from the survivors. There have been extensive outbreaks of natural scrapie in the ‘susceptible’ flocks of Cheviot and Herdwick sheep. Investigations of these outbreaks might be able to show whether or not sheep selected for ‘resistance’ could be used to limit the spread of infection in flocks with a serious scrapie problem.