Appropriate breeding strategies for associated/emerging traits in layers
Poultry improvement has been one of the great successes of classical selective breeding strategies. A large number of economic traits of egg type chickens have been/ or being subjected to selection of one kind or the other for their improvement. These traits include production parameters (sexual maturity, egg numbers, persistency in lay), external egg quality traits (egg weight, shell strength, shell color), internal egg quality traits (albumen quality, percentages of lipids and solids etc.), and bird traits (body weight, feed conversion, temperament, mortality). For many of these traits, selection is applied for enhanced performance during the early and late lay cycles. Attention is also directed to the traits that improve the performance of the parent stock that produce the commercial egg laying hens.
Many decades of selection for increased egg production and efficient feed conversion have resulted in stocks capable of remarkable levels of performance. In view of the decreasing rate of progress in egg production it seems that the emphasis of selection should be changed to meet specific market needs. Also, because of the changing structure of the worldwide egg industry, the commercial layer should be able to cope in a variety of field conditions and management systems such as high density cages, open houses or alternative systems i.e. adaptability to varying environments.
The production life of a commercial egg layer is considered to be about 80 weeks of age and may extended to 110 to 140 weeks of age if flock produces through a second or third laying cycle. Thus overall livability, including longevity, is an important component of productive flocks. There are many diverse factors that contribute to overall livability including behavior (temperament, reaction to stress, aggression to cage mates), physiological response (oviposition stress), and disease resistance. The commercial breeder must consider all of these factors to increase general livability. In the search for new ways to achieve further improvements in egg production efficiency, the associated traits and/ or some new emerging traits like residual feed intake, genetic resistance to heat and diseases, egg compositional traits and the bird’s welfare parameters merits the serious attention of the breeders. In this presentation, selection strategies for improving these traits in relation to overall economic efficiency of an egg breeding enterprise have been discussed.
Feed Conversion Efficiency
The most commonly used criteria for feed efficiency in laying hens are daily feed intake per hen, feed intake per egg, feed conversion (kg feed per kg egg mass) and egg income minus feed cost. Efficient egg production come from increased egg output and/ or reduced body weight. Individual feed consumption records enhance selection for efficiency of egg production but individual feed records are not easily measurable for a longer period of lay. So feed efficiency in commercial layers has been significantly improved by indirect and direct selection within and between lines. Findings have been reported that the ‘residual’ component of feed consumption, i.e. individual bird differences between predicted and actual feed intake, are partly heritable and respond to selection. Pirchner (1985) pointed out; 30-40% of the variation in individual feed consumption is unexplained by egg output and body weight. The analysis and interpretation of this ‘residual’ remains a continuing challenge for breeders. By definition, residual feed intake is not correlated with body weight or egg production. Maximum egg output per bird requires a stock-specific optimum body weight, which indicates that indirect selection for improved efficiency by lowering body weight is of questionable value. This leaves ‘residual’ feed consumption as the component of primary interest to the breeders in their efforts to improve feed efficiency (Flock, 1998). Breeding goal for commercial layer is maximum egg income over feed cost, not minimum RFC. Selection can be practiced directly on egg mass (maximum egg number, holding egg weight at a desirable level) and feed consumption using appropriate economic weights. Calculation of RFC has the advantage that total genetic progress in egg income over feed cost can be separated into three components: (1) higher feed consumption due to higher egg output, (2) more or less feed consumption due to change in body weight, and (3) lower feed consumption due to reduced RFC. Improvement in feed efficiency has been primarily due to increased egg mass production, but direct measurement of feed consumption may become of more value in the future (Arthur, 1986).
Shulman et al. (1994) reported from four generations of selection experiment results that RFC can be reduced by 1.5g/day per generation without changing body weight or egg output. The heritability of RFC from 16 to 42 weeks of age was 0.46. Genetic correlation between RFC and feed consumption was 0.50 but, as expected, correlation of RFC with body weight and egg production was not significant. Flock (1998) reviewed that a significant residual component of individual feed intake has been shown to exist, independent of body weight and egg mass output, and this can be subjected to selection.
Genetic Resistance to Heat Stress
One of the primary objectives of a layer breeder is to develop a bird well adapted to more variable environmental conditions such as hot climates and to less intensive systems of feeding and housing. There are several genes that affect heat tolerance. Some, such as the dominant gene for naked neck (Na), affect the trait directly by reducing feather cover, while others, such as the sex-linked recessive gene for dwarfism (dw), reduce body size and thereby reduce metabolic heat output. The frizzle (F) gene may be useful in stocks that have to perform under hot humid conditions. This gene will reduce the insulating properties of the feather cover (reduce feather weight) and make it easier for the bird to radiate heat from the body. Horst (1989) suggested several other genes useful in making fowl tolerant of tropical conditions. The challenge for the poultry breeder is to introduce heat stress tolerance while retaining and improving the wide array of other economic traits needed in commercial chicken.
Immunity and Disease Resistance
Poultry production loses potential profit because of the negative impacts of disease and microbial contamination of improperly prepared poultry products can threaten human health. Consumers on the other hand desire less antibiotic use in food animal production along with safer food. Genetic approaches to health provide a natural and sustainable method to help ensure the wholesomeness of the food supply. Genetic differences in susceptibility and resistance to various diseases are well known in the chicken. The MHC has been shown to have a strong influence on disease resistance, particularly to the MDV (Bacon, 1987). Non MHC influences on disease resistance have also been documented. The NRAMP gene has been shown to influence susceptibility to Salmonella in poultry (Girard-Santosuosso et al., 2002). Better understanding of disease resistance mechanisms is needed before this information can be fully utilized by the poultry breeding industry. Candidate genes, micro arrays, and emerging field of proteomics all offer great potential in providing information on genetic control mechanisms for disease resistance.
Molecular genetics have recently enabled the search for molecular genetic markers for health. Although there are few notable examples of health traits controlled by single genes, the vast majority of health traits are controlled by multiple genes (Lamont, 1998, 2004). He further stated that the use of pure, recombinant proteins to enhance health in poultry is an emerging area that has resulted from research on the molecular genetics of the poultry immune response. The identification of genes encoding important communication molecules of the immune system, such as cytokines, has provided the means to produce large and pure quantities the natural proteins that the chicken uses to protect itself. These proteins can act enhancers for immune function, including vaccine response. Marker assisted selection (MAS) has considerable potential for poultry breeding companies. Pure-line pedigree stocks could be tested and then selected for disease resistance genes without exposure to any disease. The trait under selection in egg-layer stocks is primarily exposed only in females. MAS for disease resistance components could be done within males, resulting in enhanced efficiency of bird use. The current major limitation to this approach is the paucity of mapped chicken immune response genes and the limited number of DNA markers mapped on the chicken genome. These limitations should be eliminated once the chicken genome is sequenced.
Genetic resistance of poultry may be improved by selection for immune response to complex noninfectious antigens such as SRBC. Chicken selected for high antibody responses to SRBC showed higher resistance to some infectious diseases, e.g. Marek’s and New Castle disease virus (Pinard et al., 1992) and coccidiosis (Parmentier et al., 2001). The co selection between immune traits and production traits might be explained by a plieotropic effect of genes associated with immunoresponsiveness. The selection of poultry for fast growth rate may likely be accompanied by a reduction in specific immune response or, as a consequence, increased disease susceptibility (Bayyari et al., 1997).
Internal Egg Components
The trend in egg consumption is moving toward egg products at the expense of shell egg consumption. In USA and Europe, about 40% and 25%, respectively, of all eggs consumed are an industrially processed (Hartmann et al., 2000). Egg processing industries may require special characteristics of the eggs in order to improve profitability. Regardless of market demand, however, the biological reality that the primary role of an egg is to provide protection and nutrients for a developing embryo – not a food source for humans – can not be ignored. Although there is a continuous erosion of the shell market to increases in further processed product. The shell egg market consists of a perishable product remains large and is becoming increasingly automated.
Lower priority has been given to internal egg quality, such as yolk proportion, or to reproductive ability, such as fitness and survival of chick. The importance of yolk proportion is due to its large impact on yolk production and total egg dry matter, a trait of great importance for the egg processing industry. The egg processing industry is developing along two paths – food production and the production of non-food products that will add to the profit of egg industry (Ros, 1998). The yolk fat industry has noticed an increasing interest in egg yolk lipids for a wide range of technological and nutritional applications from a variety of industries (Schneider, 1999). Egg yolk contains several biologically active substances that have great potential in the field of medicine.
Yolk proportion is composed of the traits, yolk weight and egg weight and thus by selecting on yolk proportion, a simultaneous selection on yolk weight and egg weight takes place. Furthermore, the higher dry matter content of these eggs would make them desirable to the food processing industry. The effect of feeding and management on yolk proportion are more limited to a particular period of time (Hartmann and Wilhelmson, 2001). If breeding is to be used to increase yolk yield, the optimal design of the breeding programme must be used to increase each specific case and, if possible, breeding should be combined with a management or feeding system which allows the full genetic potential to be expressed.
Kreuzer et al. (1995) presented repeatability of 0.4-0.6 for emulsion stability suggesting genetic variation in the chemical composition of the yolk. Ambrosen and Rotenberg (1981) estimated heritability of the phospholipids concentration ranging between 0.05 and 0.14. Hartmann et al. (2000) found that one round of divergent selection was enough to produce a response in yolk proportion. Furthermore, a sustained effect of selection was observed over the entire laying period. They further observed a decrease in egg weight when selecting on the basis of high yolk proportion and vice versa when selecting on the basis of low yolk proportion.
In general, egg production traits appear to be genetically negatively correlated with yolk proportion. Hartmann and Wilhelmson, (2001) reviewed heritability estimates for various yolk traits and observed intermediate sized heritabilities of dry matter concentration in the yolk which is an essential trait in the context of extraction of biologically active substances from the yolk. The yolk dry matter constitutes about 60% of the total egg dry matter and could therefore be expected that there is a relationship between feed intake and traits relating to yolk deposition. Mennicken et al. (1996) estimated the genetic correlation between feed intake and the proportion of yolk to be 0.16 and the correlation between the proportion of the yolk and body weight to be 0.21.
Selection for high feed efficiency resulted in hens producing more polyunsaturated fatty acids (PUFA) and thereby the ratio between the PUFA and the saturated fatty acids increased (Zaky et al., 1996). With the exception of blood and meat spots, all of the egg quality traits have moderate to high heritability (Siegel, 2004). This consistency across traits suggests considerable additive genetic variation, and hence they should be amenable to modification through individual phenotypic selection. The internal components of an egg are not in balance and they change with age. It is likely that selection for an increased yolk yield would affect egg production which needs to be considered.
However, lack of selection response may also be observed because of physiological limits. In terms of egg composition this may be expressed as reduced hen fertility. Eggs with an extreme composition will probably not be readily hatch able, with the result that the hens producing these eggs will not reproduce and a selection response is therefore unattainable.
In recent years there has been an increasing awareness of bird welfare implications and, associated with decreasing demand for old laying hens, attention has turned to negative impact of osteoporosis on the condition of the skeleton of hens sent for processing at the conclusion of their laying cycle (Belyavin, 1995). The decreased skeletal strength is associated with the reduction in meat quality of the carcasses from birds that have suffered breakages. The main skeletal problems in laying hens are associated with loss of bone mineral during the laying period. Genetic selection for greater productivity in birds is one of the crucial factors in the decline in bone strength of hens over time. In egg laying strains, intensive selection for egg output has resulted in birds of low body weight that lay a large number of eggs on a low food intake. Over a laying year, the amount of calcium these birds can deposit in their shells can be up to 20 times their total body content. If methods cannot be found to alleviate this problem, end-of-lay hens may become a liability to the producer. Possible approaches to this challenge include altering genetic selection criteria and the use of other means to modify those factors which might influence bone strength, such as body size and the nutritional management, including levels of vitamin supplementation (Whitehead and Wilson, 1992).
Eggshell quality, including eggshell strength typically deteriorates in commercial flocks after 70 weeks of age (Boorman et al., 1985) and the incidence of osteoporosis has likewise been shown to increase as the bird ages (Fleming et al., 1994). Origin of osteoporosis is not well defined. It has been suggested that the problem is quite genetic in origin resulting from the breeding of light weight, energetically efficient birds that maintain a high rate of lay over a prolonged period on a low food intake (Whitehead and Wilson, 1992; Cransberg et al., 2001). Bishop et al. (2000) reported that morphometric traits involving cancellous and medullary bone volumes were found to be poorly heritable. In contrast, higher heritability of other characteristics like tibia strength, humerus strength, and keel radiographic density were 0.45, 0.30 and 0.39 respectively. There was also a positive correlation between body weight and bone strength. Thus genetic selection seems to offer the best prospects for improving bone quality and resistance to osteoporosis in hens. This response can also be made to be independent of body size though faster progress could be made if body weight were to be allowed to increase.
In future, birds might be selected not only for traits to improve the economics of egg production, but also for skeletal traits to improve both bird welfare and value of the processed carcass. Bone breakages are an important welfare problem in laying hens (Gregory and Wilkins, 1989; Gregory et al., 1994) and there is strong evidence that genetic variation exists for osteoporosis, the physiological disease that underlies predisposition to bone fracture (Bishop et al., 2000). The prevalence of broken bones may be related to the high requirements for calcium deposition in eggshells and there is likely to be genetic variation for the propensity to remove material from the skeleton to furnish the increased requirements for eggshell formation in birds with higher rates of lay. The extent of between-breed genetic variation for bone and eggshell breaking strengths and between-breed genetic correlation (Taylor and Hnizdo, 1987) for bone and eggshell strength were determined to address these questions.
The large variation observed in the bone characteristics of hens at the end of lay, phenotypically unrelated to egg production in a flock of highly productive hens (Rennie et al., 1997), suggest that the problem of osteoporosis may be alleviated by genetic selection, perhaps without serious consequence for egg productivity. As suggested above, it is likely that laying hen osteoporosis is polygenic in origin. Identification of the specific genes associated with osteoporosis and their association with quantitative trait loci for bone quality could lead to the development of markers that could form the basis for commercial MAS for resistance to osteoporosis. Most important will be to establish a selection index to overcome the antagonistic genetic correlation between shell quality and bone strength.
The selection of chickens in the intensive production system has resulted in remarkable increases in production efficiency, but some production practices may subject animals to unintended stress, such as crowded social environment for laying hens housed in battery cages. This situation may affect animal welfare as well as increase stress-related diseases. One solution to these problems is to improve the animal’s ability to cope with the intensive production environment through genetic adaptation. Because of inherent differences in behavioral and physiological homeostasis in response to diseases and stressful stimuli, selective breeding of chickens has become a major tool to combat these problems and improve animal’s well being (Buchenuer, 1990; Siegel and Dunnington, 1997). It is likely that genetic variability of behavioral traits will receive more attention in the future.
Use of Marker Genes
Availability of molecular tools such as genetic maps (Groenen et al., 2001) makes it possible to unravel the genetic bases of complex traits. Studies dealing with QTL for body weight using different crosses of chickens have revealed a number of QTL controlling growth. Knowledge about genetic markers, linked to genes affecting QTL may be of great use in selection breeding programs (Dekkers and Hospital, 2002).
Discovering a DNA marker close to a trait-gene will enable powerful marker assisted selection (MAS) techniques to be used. This method of selection depends upon the identification of genetic markers for the desired trait and can be implemented very early in the life of the young chick. The markers can be found after relatively small-scale experiments in a laboratory and, with disease resistance, after a measured challenge under controlled conditions. Whether DNA markers can be found for trait genes where the heritability is low is an unknown, but intriguing possibility (Bulfield, 2004). He further stated that despite the utility of QTL and MAS, such breeding procedures are operating ‘blind’ in not knowing the exact role of the gene(s) in the QTL. The ultimate scientific and commercial value is in identifying the trait-gene (s)’ themselves within the QTL. This is a challenging task requiring a wider range of molecular tools including: cDNA libraries, ESTs, DNA micro arrays, and BAC libraries of the target genome, established conserved synteny to information-rich species and ultimately sequencing the target genome itself directly.
With this new advancement in identifying the major genes (marker gene), the evaluation of the genetic value of an individual consists of two elements: the effect of a marker gene (treated as fixed) and the polygenic effect of the trait of interest (treated as random). So, it seems that molecular genetics may not replace the conventional poultry breeding. However, it could be a source of additional genetic variability widening the scope for future genetic progress. It is more likely that over time these techniques will be incorporated into current poultry breeding practices to solve specific problems and to create new products.
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