Genetic progress in poultry species for meat production has contributed to the consistent growth in world production of poultry meat. The poultry species have a number of advantages over the larger species used for meat production. It is possible to maintain large pedigreed populations and use their high reproductive rates to transfer genetic progress to the production generations in less than five years. These populations continue to maintain high heritabilities despite, in some cases, prolonged selection. The history of selection progress in broiler chickens (Gallus gallus domesticus) is reviewed and compared with rates of progress in the duck (Anas platyrhyncos) and the turkey (Meleagris gallopavo).

The rates of genetic change for production traits such as growth, feed efficiency and yield have changed the physiology of the birds. Changes in selection criteria have been made to improve the robustness of the production stock. This allows them to perform well in a wider range of environments. These have been combined with improved definitions of the optimum environments for the birds to minimise any impact on welfare and health. This paper describes examples of selection in the broiler chicken aimed at improving skeletal quality and resistance to ascites. A number of the factors influencing future selection criteria are discussed. Breeding programmes have adapted to respond quickly to adverse genetic correlated responses. The need to combine selection for a large number of traits requires that the programmes are very efficient and use the best statistical techniques available for multivariate breeding value estimation.

Breeding programmes and improved knowledge of genetics have resulted in gradually increased genetic changes in pigs. In this paper the genetic changes in the European pig during the last century are analysed and compared with societal needs for the following decades. Dutch data are used to support the general trend in Europe and results of an EC project on breeding and society are used to identify societal needs.

Genetic changes in pigs started early this century, mainly in Northern Europe, by the gradual set up of local and regional breeding programmes. In the first 60 years the goal of these programmes was focussed only on leanness and feed efficiency. However, type, leg quality and breed characteristics were of utmost importance in these years for some breeding programmes. During these decades the genetic changes were negligible for reproduction traits. Progress was achieved especially for breed characteristics like type and colour and later also backfat thickness, daily gain and consequently improved feed efficiency.

With the introduction of crosses in the early 60's, specialisation in dam and sire lines but moreover in breeding programmes arose in the pig industry. This specialisation resulted rapidly in increased genetic changes, more breeds and/or lines and breeding goals that supported efficient lean meat production systems. Especially in the sire lines, this specialisation resulted in increased progress for daily gain, backfat thickness and feed efficiency. Improved knowledge of genetics increased genetic changes in leanness and feed efficiency during the last few decades, but at the same time attention could be and was paid to fertility and vitality. During the last decade most European pig breeding programmes realised anual genetic progress for daily gain of +20 g/day, lean meat % of +0.5% and litter size of +0.2 piglet/litter. Molecular genetics does not enable more genetic change for these traits, but does so especially for expensive or difficult to measure traits, e.g. meat quality, health and longevity. In commercial pig breeding programmes selection limits have not been reached yet. Nevertheless, there are experimental results that indicate unintended side effects of the present breeding goals.

Next to changes in breeding possibilities, the societal needs have also changed. During the first 80–90 years of the century, pig producers were aiming at production of lean meat at low(er) prices. In this period low prices were more important than quality and/or welfare of the pigs. Nowadays, diversification in consumers' needs is growing and quality of the product (including production circumstances) is important in Western Europe. The market for pork is changing from a market of producers into a market of (potential) buyers. Clearly, the success of any pig breeding programme next century will be judged by the consumer. Where a newly bred, or genetically modified, variety of animal fails to provide something people are prepared to buy, the breeding programme will be judged a failure. In this respect ethical issues are also becoming more important. Modern breeding techniques can threaten animal welfare; a very fast increase in one or a few traits can easily result in an insufficient increase or adaptation of the organs and skeleton. Animal integrety is another important issue, especially related to genetically modified animals. Clearly, the relationship between animal and human interests is central to any view.

From the analysis of genetic changes in pigs and the present and foreseen changes in market demands, it may be concluded that the pig breeding programmes were very successful; but they will have to change their goals. Less emphasis will be needed on low cost price and more emphasis on product quality, biodiversity and well balanced genetic changes for economically important traits.

In commercial egg type chicken breeding three and four way crosses are used to produce commercial layers. The primary breeders are using closed nucleus breeding programmes, with birds kept under maximum biosecurity. All grand parents and parents are produced from a closed nucleus for the world wide demand of commercial layers. The breeding goals have been focused for several decades on increasing number of eggs per hen housed. Additional traits have become more important during the last decade, i.e. feed efficiency, internal and external egg quality and general adaptability. Prior to each selection, weights for individual traits within the selection index are adjusted to meet market demands. Breeding stock and commercial layers have to be bred to perform adequately in a variety of systems ranging from large intensive cage units to free range management under different environmental conditions world-wide.

Despite intensive selection for egg production the decrease in genetic variation observed in closed commercial lines is not yet critical. Peak production is approaching the biological limit of one egg a day. During this period genetic and phenotypic variation have been significantly reduced. But for early production (sexual maturity) and late production (persistency) genetic variation is still high. In a mating scheme avoiding full and half sib matings no serious inbreeding depression is observed. To achieve continued future genetic progress, selection pressure will shift to other traits like internal and external egg quality and perhaps behaviour traits which still respond to selection.

Primary breeders are responding to this challenge by testing pedigreed cross-line hens in a wide range of environments and housing systems while the pure-line elite stock is kept under conditions of maximum biosecurity. Marker assisted selection is already part of commercial breeding programmes. In the past, blood typing has been used to improve Marek's resistance, whereas today anonymous microsatellites which are linked to traits of economic interest are used for selection. In particular, selection between full sib males can give a major improvement.

The whole industry is getting more specialised. While the genetic potential of the birds is improved management and nutrition have also to be adapted to changing demands. The general goal for the future is to breed chickens with the ability to function well within a wider range of production conditions and do not respond to the slightest stress.

The major ruminant species, dairy and beef cattle and sheep, represent a rather heterogeneous group as regards genetic improvement, which to a large extent reflects their respective breeding structures. In the UK, the beef cattle and sheep industries still span many different breeds, have small herds/flocks, and have been relatively unaffected by agribusiness investment, and are assumed to have a traditional pyramid breeding structure, in which progress is determined by a small number of breeders. Recording of production information, which to date has focussed on terminal sire characteristics, is relatively recent, and until the use of across herd evaluations, genetic progress was probably limited. However in recent years there have been gains in both growth and muscling. There is little evidence or concern for undesired consequences in commercial flocks, partly because of the extensive use of crossbreeding in these industries, which exploits both breed complementarity and heterosis.

By contrast, the dairy industry is now dominated by purebred Holsteins. Increasingly breeding activities are both global in scope and dominated by a small number of large breeding companies. Because most traits of interest are only expressed in the female, improvement programmes have continued to focus on progeny testing, with test daughters in many herds. Most recording schemes and promotional activities emphasise production and type traits. The dairy industry is also notable for the publication of bull progeny test results, so that top bulls can then be used as sires of the next generation of by all companies. These bull evaluations now extend to international rankings.

Data from the US indicates continuing genetic progress for production traits in the Holstein, particularly since the 1960s, by when progeny testing had been established, frozen semen widely used, and adequate statistical procedures in place for evaluating bulls. Genetic progress is also evident for type traits. There is now growing concern and evidence of undesirable genetic changes in fertility, disease incidence and overall stress, despite improved nutrition and general management. Altering this situation will require both the recording of such traits and the use of that information by breeding companies, especially in sire selection.

Breeders of poultry and pigs have selected for some combination of increased growth rate, decreased fatness and increased muscularity. Increasingly various fitness traits are included in the index used. The consequences of such selection include complex effects on nutritional and environmental requirements, at least some of which are reliably predictable using suitable models. Appropriate changes to the environment and to nutrition as selection proceeds will help to avoid unwanted effects occurring. Among the predictable effects are that higher ratios of nutrients to energy, and lower temperatures, will be needed by the improved genotypes. Selection for growth rate must eventually exhaust the capacity of the support systems – digestive, respiratory, circulatory and excretory – to cope with the increased metabolic rate. Selection for increased yield of valuable parts will cause these problems to occur earlier. While it is possible to predict that these problems will occur it cannot be predicted when they will. Breeders need to be aware of these problems, and use all possible routes to help them in reducing their severity. Where the appropriate actions for fitness selection, and nutritional and environmental modifications, are taken the occurrence of the problems will be delayed.

In nutritional terms productivity in dairy cows is best expressed in multiples of their energy requirements for maintenance, which is directly related to Metabolic Body Weight (MBW). Energy requirements can further be specified as those for free energy (fME), ketogenic energy (kME), glucogenic energy (gME) and aminogenic energy (aME). For maintenance, energy is required in an approximate ratio fME:gME:aME of 85:5:10.For a production level of 6x maintenance the required ratio of fME:kME:gME:aME is approximately 18:31:29:22, hence 70% of the required ME has to be delivered as specific nutrients.

During their productive life, dairy cattle alternate periods with high and periods with low risk of an inadequate nutrient supply. A period of high risk is between 10 days pre-partum and 90 days postpartum. High risk in this period means a high susceptibility for metabolic and reproductive disorders, many of which are interrelated. Negative Energy Balance (NEB) in early lactation is a metabolic status that is almost inevitable in high yielding dairy cows. In severe cases this is an important predisposing factor for metabolic disorders (paturient paresis, fatty liver, ketosis, displaced abomasum) and reproductive disorders (undetected heat, delayed ovarian cycle, reduced fertility, increased between calving interval).

Adequate nutrient supply is required from 2 weeks pre-partum onwards. Critical issues are energy density, distribution between structural and non-structural carbohydrates and distribution between fME, kME, gME and aME. Practical solutions, which can help alleviate the metabolic and reproductive problems, could be a nutrient based feed evaluation system, a lower peak yield combined with an improved persistence and maybe a deliberately increased calving interval.

The poultry meat industry, in continual pursuit of improved efficiency, has demanded rapid growth rate. Primary breeding companies have responded to industry pressure and growth rate has increased in an almost linear fashion. Despite the obvious advantages to industry profitability, it can be argued that increased growth has placed more emphasis on the demand tissues of growth than those systems or organs that supply the substrates for rapid growth and/or are essential to support the increase in body mass. One of the consequences is that modern broilers are not as adaptable to their environments as their predecessors were. Rapid growth has also produced problems not seen in slower growing birds. Skeletal and cardiovascular disease (sudden death syndrome (SDS), ascites) are examples of growth related problems. Although it can not be said that rapid growth automatically will result in these problems, there is no doubt that the!re is a relationship.

The relations between genetic change in domestic livestock and infectious disease (including both its epidemiology and the animal's reaction to it) are examined. The overall picture is confusing because there are different, and seemingly unrelated, ways of considering the issue. An attempt is made to put these together into a more general framework. Four processes of particular interest are distinguished and discussed in more detail: (i) the way a population's genetic potential for immunocompetence can be changed by breeding, (ii) the way an animal's immunocompetence is influenced by that animal's production potential, in combination with the environmental resources that are available to it at a given time, (iii) the way the disease status of an animal (and a population of animals) is influenced by its immunocompetence, and (iv) the way the production level of an animal is influenced by activation of its immune system. Ultimately, all four processes influence the realised level of production.

This comes down to four questions that need to be addressed: (i) can we use genetic variation in immunocompetence in animal breeding? (ii) does a higher production potential (today's direction of breeding) have a negative impact on immunocompetence? (iii) does improved immunocompetence result in improved health? and (iv) how large is the negative impact of disease on production?

Selective breeding has been an important component in the increased output and efficiency of animal production since the 1950's. At the same time there has been increasing moral concern over the welfare of modern farmed animals with much of the focus on the environment and management of farm animals and relatively little consideration of the impact of genetic change on welfare. This is now changing, partly because of some well-publicised examples where selection has led to ‘undesirable’ side effects, and because of the perceived welfare risk of emerging biotechnologies. This paper will address whether and how moral concerns over animal welfare should place limitations on genetic change in animal production.

A combination of better management and genetic selection for good health and fertility would provide a more effective long term solution for economic loss due to diseases and poor fertility. This would also help to address public concerns about the use of medical treatment in milk production. A balance in the genetic improvement of health and fertility together with milk production could be achieved through their inclusion in national genetic selection indices, for which genetic parameters are needed. One of the main objectives of this study was to estimate genetic parameters for various disease and fertility traits in the UK dairy cattle population, using records from a national recording scheme run by Livestock Services UK Ltd. Genetic analysis of traits recorded as present or absent (binary traits; e.g. diseases) requires the use of non-linear threshold models, because linear models require normality assumptions (e.g., Gianola 1982). The other objective of this study was to estimate genetic parameters for binary disease and fertility traits based on threshold animal models and to compare results with those from linear animal models.

Broiler breeders conduct their breeding programs only in optimal environments, claiming that because farmers are instructed to provide the broilers with optimal management, genotype by environment interactions (GxE) are not important for the broiler industry. However, with the rapid development of the poultry industry worldwide, high-performance broiler stocks are now being imported to developing countries where environmental control, mainly the mitigation of hot climates, is not feasible. Moreover, results from several studies suggest that due to the increase in genetic potential for rapid growth rate, resulting from successful breeding programs, broilers are becoming more sensitive to rather small environmental deviations from optimal conditions (Leenstra and Cahaner, 1991; Cahaner and Leenstra, 1992; Cahaner et al., 1993; Settar et al., 1999; Yunis et al, 1999). Hence, also in developed countries, many broilers will be, or are already being reared under suboptimal hot environments.