J. Conington

Multi-trait selection indexes for sustainable UK hill sheep production

Three selection indexes for the UK hill sheep sector are derived to suit the extremes of hill production systems. These are: (i) intensive, where all surplus lambs not required for breeding are finished for slaughter, (ii) extensive, where all surplus ‘store’ lambs are sold to other farmers for finishing, and (iii) semi-intensive, which is intermediate between the two extremes, i.e. farms finish some lambs for slaughter and sell others as store lambs. Parameters for 12 breeding goal and index traits were estimated using a total of 3962 lamb records and 5944 ewe lambing records from Scottish Blackface sheep on two Scottish Agricultural College experimental hill farms. The breeding goal comprised carcass, maternal and survival traits. The evaluation of these indexes showed that improvements in maternal traits are possible, along with more modest improvements in carcass quality traits. Responses to selection are expected to be lower for the extensive farm in general, compared with the intensive farm. Evaluations of alternative indexes show that an index using measurements of fat and muscle on ewes rather than on lambs may be more cost-effective to implement in practice, compared with the original index, although this change results in a higher (i.e. undesirable) gain in mature size. Sensitivity analyses showed that in general, the indexes are robust to changes in economic values and to changes in heritability estimates.

A COMPARISON OF GROWTH AND CARCASS TRAITS IN SCOTTISH BLACKFACE LAMBS SIRED BY GENETICALLY LEAN OR FAT RAMS

An experimental programme was initiated in 1990 to investigate the consequences of selection for reduced fatness in hill sheep and to estimate genetic parameters for carcass traits in the same class of animal. Thirty-two progeny groups of lambs were born as a result of mating Scottish Blackface rams to ewes of the same breed on two Scottish Agricultural College hill farms in 1991 and 1992. Sires were from two divergent selection lines for subcutaneous back fat depth and were selected under ad libitum feeding conditions on an index combining live weight and ultrasonic fat depth at 20 weeks of age. The index was designed to alter body composition but not body weight. Weaned male progeny (reared extensively) were brought together from both farms in August of each year, reared on grass and finished for slaughter on swedes at a target condition score of 3. Measurements taken at weaning were: live weight, average ultrasonic measurements of average subcutaneous fat depth and muscle depth. At slaughter, traits measured were: pre-slaughter live weight, pre-slaughter condition score, age at slaughter, cold carcass weight, killing-out proportion, Meat and Livestock Commission (MLC) fat class and MLC conformation class. Side and shoulder dissections were made on proportionally 0·2 and 0·8 of lambs respectively and lean tissue, subcutaneous fat, intermuscular fat and bone were measured. Comparisons were made between the two genetic lines and genetic parameters were estimated from data adjusted to three different ‘end-points’: constant dissected subcutaneous fat weight, constant age at slaughter and constant cold carcass weight. Moderate to low heritability estimates were obtained for most traits: pre-slaughter live weight = 0·36, cold carcass weight = 0·39, fat class = 0·13, conformation class = 0·09, lean weight = 0·27, bone weight = 0·36 (constant subcutaneous fatness), intermuscular fat = 0·20, subcutaneous fat = 0·20 (constant cold carcass weight). Correlations between ultrasonic measurements at weaning, and slaughter and dissected carcass components were moderate to weak. However, there is sufficient genetic variation in the carcass traits to warrant inclusion in breeding goals for hill sheep. The results also show that genetic differences obtained by selection under intensive rearing conditions for divergent fatness are also seen in progeny reared under extensive (hill) conditions.

Genetic selection for extensive conditions

Abstract Humans have selected animals which are more suited for food production or other purposes, since the process of domestication of livestock began, whether this selection was done knowingly or unknowingly. The deliberate selection of improved breeds and strains has been a particularly important feature of agriculture in the last couple of centuries, especially in the industrialised countries. For most of this period, selection has been based on subjective assessment of the merits of animals, but objective tools for selection (e.g. performance recording and statistical methods for evaluation of genetic merit) have become widely used in the last few decades. These tools have been used to a large extent in pig, poultry and dairy cattle breeding, but to a much lesser extent in the beef cattle and sheep breeds common in extensive production systems. However, these extensive systems themselves have changed to a much lesser extent than those in which pigs and poultry are kept. Hence there have been opportunities for natural selection for traits conferring better adaptation to these environments. Additionally there has been some subjective selection for traits thought to confer better adaptation. There is considerable scope for wider uptake of existing objective methods of genetic improvement in the harsher areas of the UK, and elsewhere. However, these methods are likely to be more effective if the genetics of traits conferring adaptation to harsh environments are better understood, and if the most important of these traits are included in the breeding goal. Traits conferring better adaptation may include physical attributes such as litter size, fleece type and the ability to store body fat, some aspects of behaviour, especially maternal and grazing behaviour, and disease resistance. A better understanding of the relationships between production traits and these adaptation traits will also be critical for the development of appropriate, sustainable breeding programmes. This approach should reduce the risk of there being detrimental correlated effects of selection, and may provide opportunities to improve animal welfare.

Industry benefits from recent genetic progress in sheep and beef populations

An analytical model that evaluates the benefits from 10 years of genetic improvement over a 20-year time frame was specified. Estimates of recent genetic trends in recorded traits, industry statistics and published estimates of the economic values of trait changes were used to parameterise the model for the UK sheep and beef industries. Despite rates of genetic change in the relevant performance-recorded breeding populations being substantially less than theoretical predictions, the financial benefits of genetic change were substantial. Over 20 years, the benefits from 10 years of genetic progress at recently achieved rates in recorded hill sheep, sheep crossing sire and sheep terminal sire breeding programmes was estimated to be £5.3, £1.0 and £11.5 million, respectively. If dissemination of genetic material is such that these rates of change are also realised across the entire ram breeding industry, the combined benefits would be £110.8 million. For beef cattle, genetic evaluation systems have been operating within all the major breeds for some years with quite widespread use of performance recording, and so genetic trends within the beef breeds were used as predictors of industry genetic change. Benefits from 10 years of genetic progress at recent rates of change, considering a 20-year time frame, in terminal sire beef breeds are expected to be £4.9 million. Benefits from genetic progress for growth and carcass characters in dual-purpose beef breeds were £18.2 million after subtraction of costs associated with a deterioration in calving traits. These benefits may be further offset by unfavourable associated changes in maternal traits. Additional benefits from identification and use of the best animals available from the breeding sector for commercial matings through performance recording and genetic evaluation could not be quantified. When benefits of genetic improvement were expressed on an annual present value basis and compared with lagged annual investment costs to achieve it, the internal rate of return (IRR) on the combined investment in sheep and beef cattle was 32%. Despite a much higher rate of participation in performance recording, the present value of benefits and the IRR were lower for beef cattle than for sheep. The implications of these results for future national and industry investment in genetic improvement infrastructure were discussed.

Invited review: Improving neonatal survival in small ruminants: science into practice

Neonatal mortality in small ruminant livestock has remained stubbornly unchanging over the past 40 years, and represents a significant loss of farm income, contributes to wastage and affects animal welfare. Scientific knowledge about the biology of neonatal adaptation after birth has been accumulating but does not appear to have had an impact in improving survival. In this paper, we ask what might be the reasons for the lack of impact of the scientific studies of lamb and kid mortality, and suggest strategies to move forward. Biologically, it is clear that achieving a good intake of colostrum, as soon as possible after birth, is crucial for neonatal survival. This provides fuel for thermoregulation, passive immunological protection and is involved in the development of attachment between the ewe and lamb. The behaviour of the lamb in finding the udder and sucking rapidly after birth is a key component in ensuring sufficient colostrum is ingested. In experimental studies, the main risk factors for lamb mortality are low birthweight, particularly owing to poor maternal nutrition during gestation, birth difficulty, litter size and genetics, which can all be partly attributed to their effect on the speed with which the lamb reaches the udder and sucks. Similarly, on commercial farms, low birthweight and issues with sucking were identified as important contributors to mortality. In epidemiological studies, management factors such as providing assistance with difficult births, were found to be more important than risk factors associated with housing. Social science studies suggest that farmers generally have a positive attitude to improving neonatal mortality but may differ in beliefs about how this can be achieved, with some farmers believing they had no control over early lamb mortality. Facilitative approaches, where farmers and advisors work together to develop neonatal survival strategies, have been shown to be effective in achieving management goals, such as optimising ewe nutrition, that lead to reductions in lamb mortality. We conclude that scientific research is providing useful information on the biology underpinning neonatal survival, such as optimal birthweights, lamb vigour and understanding the importance of sufficient colostrum intake, but the transfer of that knowledge would benefit from an improved understanding of the psychology of management change on farm. Developing tailored solutions, on the basis of adequate farm records, that make use of the now substantial body of scientific literature on neonatal mortality will help to achieve lower neonatal mortality.

Prediction of total body tissue weights in Scottish blackface ewes using computed tomography scanning

Thirty cull Scottish Blackface ewes were scanned three times over a period of 1 week using X-ray computed tomography (CT). Cross-sectional CT reference scans were taken at seven anatomical sites per ewe: ischium (ISC), femur (FEM), hip (HIP), 5th lumbar vertebra (LV5), 2nd lumbar vertebra (LV2), 8th thoracic vertebra (TV8) and 6th thoracic vertebra (TV6). Ewes were then slaughtered and dissection measurements collected. Results of multiple regression analyses suggested that five reference scans allow accurate prediction of total weights of bone, muscle and fat (carcass and internal). The most informative cross-sectional scans were ISC, HIP, LV5, LV2 and TV8, from which prediction equations were derived. Fat and muscle weights were predicted accurately (R 2 = 80 to 99%) but bone weight was predicted less accurately (R 2 = 56%). Repeatabilities were high for the CT measurements used to predict fat and muscle (0•82 to 0•99) but lower for those used to predict bone (0•19 to 0• 86).

Testing selection indices for sustainable hill sheep production – lamb growth and carcass traits

Two selection indexes, one intended for lamb producers and finishers and one for store lamb producers, were derived using genetic parameters for carcass and maternal characteristics from Conington et al. (2001) and economic values from Conington et al. (2004). This paper summarizes responses to selection for lamb traits only, after 5 years of selection (1998 to 2003) on two farms using these selection indexes. The index for lamb producers and finishers, evaluated on farm 1, with a flock size of 680 ewes, includes economic weightings for maternal traits as well as carcass weight, fat and conformation grades, whereas the index for store lamb producers, evaluated on farm 2, with a flock size of 580 ewes, only includes economic values for maternal traits and lamb growth to weaning. Three selection lines of Scottish Blackface sheep per farm were created with the first lambs born to each line in 1999. These lines were selection (S), control (C) and industry (I); they were of equal size on each farm. Five top- and five average-performing ram lambs were selected each year for the S and C lines respectively using a multi-trait best linear unbiased prediction (BLUP) implementation of the indexes. The I-line used four mature rams bought from industry, selected on appearance only, i.e. adherence to breed ‘type’. Results showed that 5 years after the implementation of the index, the S line had significantly higher index scores than the C or I lines on both farms. The means (s.d.) for the average index scores in 2003 are 114 (328), 119 (371) and 451 (328) (farm 1), and −8 (146), −11 (130), and 250 (129) (farm 2) for the C, I and S lines, respectively, giving predicted net differences (S-C) of £3·38 (farm 1) and £2·58 (farm 2) per ewe. Phenotypic responses showed significant S v. C differences in weaning weight on both farms. As predicted from previous analyses, no changes in carcass quality traits were seen at farm 1 although S-line carcass weights tended to be heavier than those from the C or I lines. The results show that genetic improvement using multi-trait selection indices has been successful and it is a viable, long-term strategy to improve levels of production for hill sheep in extensive environments.

Implications of Host Genetic Variation on the Risk and Prevalence of Infectious Diseases Transmitted Through the Environment

Previous studies have shown that host genetic heterogeneity in the response to infectious challenge can affect the emergence risk and the severity of diseases transmitted through direct contact between individuals. However, there is substantial uncertainty about the degree and direction of influence owing to different definitions of genetic variation, most of which are not in line with the current understanding of the genetic architecture of disease traits. Also, the relevance of previous results for diseases transmitted through environmental sources is unclear. In this article a compartmental genetic–epidemiological model was developed to quantify the impact of host genetic diversity on epidemiological characteristics of diseases transmitted through a contaminated environment. The model was parameterized for footrot in sheep. Genetic variation was defined through continuous distributions with varying shape and degree of dispersion for different disease traits. The model predicts a strong impact of genetic heterogeneity on the disease risk and its progression and severity, as well as on observable host phenotypes, when dispersion in key epidemiological parameters is high. The impact of host variation depends on the disease trait for which variation occurs and on environmental conditions affecting pathogen survival. In particular, compared to homogeneous populations with the same average susceptibility, disease risk and severity are substantially higher in populations containing a large proportion of highly susceptible individuals, and the differences are strongest when environmental contamination is low. The implications of our results for the recording and analysis of disease data and for predicting response to selection are discussed.

Breeding for resistance to mastitis in United Kingdom sheep, a review and economic appraisal

Mastitis is a problem in the sheep industry, and its incidence varies widely with how it is recorded, the breed of sheep and the farm. Virtually all the published information about the genetics of mastitis refers to dairy breeds of cattle and sheep, and there is little information for meat sheep breeds. Many dairy breeding programmes worldwide use the somatic cell count (scc) in milk as an indicator of resistance to clinical and subclinical mastitis, but it is difficult to measure in meat sheep breeds. Molecular genetic technologies may therefore be a more practical way to assess susceptibility to mastitis. This paper reviews the genetics of mastitis and considers the opportunities for breeding for resistance to mastitis, with particular reference to sheep. In addition, to investigate the potential economic effects of mastitis in a purebred sheep population, a computer model of flock dynamics was developed. By making a modest set of assumptions about the key farm parameters that influence lowland sheep production, the model showed that breeding for resistance (or other control methods), if it could reduce the risk of contracting mastitis by 10 per cent, would be worth £8.40 per ewe, equivalent annually to £2·7 million for the purebred sector of the Texel breed alone.

A genetic epidemiological model to describe resistance to an endemic bacterial disease in livestock: application to footrot in sheep

Selection for resistance to an infectious disease not only improves resistance of animals, but also has the potential to reduce the pathogen challenge to contemporaries, especially when the population under selection is the main reservoir of pathogens. A model was developed to describe the epidemiological cycle that animals in affected populations typically go through; viz. susceptible, latently infected, diseased and infectious, recovered and reverting back to susceptible through loss of immunity, and the rates at which animals move from one state to the next, along with effects on the pathogen population. The equilibrium prevalence was estimated as a function of these rates. The likely response to selection for increased resistance was predicted using a quantitative genetic threshold model and also by using epidemiological models with and without reduced pathogen burden. Models were standardised to achieve the same genetic response to one round of selection. The model was then applied to footrot in sheep. The only epidemiological parameters with major impacts for prediction of genetic progress were the rate at which animals recover from infection and the notional reproductive rate of the pathogen. There are few published estimates for these parameters, but plausible values for the rate of recovery would result in a response to selection, in terms of changes in the observed prevalence, double that predicted by purely genetic models in the medium term (e.g. 2–5 generations).