Friday, 10 May 2019

Problems and Issues Associated with Poultry Breeding

Problems and Issues Associated with Poultry Breeding

Consumption of poultry meat and eggs is increasing steadily. It has moved from a combined total of  5 million tonnes in 1992 to 117 million tonnes in 2000 (Executive Guide to World Poultry Trends,2001). Of the current total,8% is produced from turkeys,ducks or poultry species other than chickens. This chapter focuses on developments in chicken breeding. There are unique concerns for each of the other species,but developments in the breeding of the other species have in general paralleled those in the breeding of chickens.

Egg-type Chickens
Since the early 20th century,the breeding of egg-type chickens has seen significant changes. The genetic performance of the bird has improved substantially over this time. In order to be able to continue the improvement of the laying hen,further changes will need to be made. In the following discussion,consideration will be given to traits,methods of selection and industry structure.

Breeders today must select for,or at least monitor,the age at sexual maturity,rate of lay before and after molt,livability in the growing and laying house,egg weight,body weight,feed conversion,shell color,shell strength,albumen height,egg inclusions (blood and meat spots) and temperament,plus traits affecting the productivity of the parent. Since the early 1980s,the increasing proportion of eggs broken out for further processing has added additional traits, including percentage solids and lipids in
the egg.

Egg production per hen housed will continue to be the single most important trait under selection. However,the emphasis has been shifting from peak rate of lay to persistence of lay (Preminger and Flock, 1998). As flocks maintain high rates of lay for longer periods of time,they can be kept to advanced ages without being molted. While much is now known about the physiology of age-related changes,the elements that trigger these events remain elusive (Ottinger,1992). A better understanding of these processes is necessary for more effective selection,and may allow the identification of specific genes influencing ageing.

There is increasing use of induced molting to extend the laying life of the hen in much of the world,despite opposition to this practice by animal welfare activists in some developed countries. It can be expected that breeders will continue to work for improved post-moult performance for the foreseeable future. It has been stated that the economic impact of variability in disease resistance is relatively small and that it is not a high priority trait in most breeding schemes (Albers, 1993). However,the emphasis placed on disease resistance varies from one breeding firm to another. An epidemic of a specific disease can increase the importance of that disease in the minds of poultry producers and they may choose not to purchase stocks that are susceptible. For example,this has occurred in the USA (in the 1950s) and in Australia (in the 1990s) for the disease lymphoid silicosis. Relative Marek’s disease susceptibility affected buying decisions in the USA in the 1960s. Currently,feather pecking and resultant cannibalism is a problem in chickens housed in alternative systems in Europe and perceived differences among commercially available varieties are affecting sales.

Breeding for resistance to disease is difficult because of low instabilities and rapid evolution to more virulent forms among disease causing microorganisms. Instabilities are generally under 10% for total mortality, but somewhat higher for specific diseases (Gavora,1990). Some diseases thought to be
controlled by vaccination,such as bronchitis and Marek’s disease,keep reappearing due to the occurrence of variant viral strains. Breeding for resistance to specific diseases caused by microorganisms involves exposure of the birds to disease agents in a controlled manner,usually by inoculation of highly pathogenic variants of the organism.

This cannot normally be done in the pedigreed population under selection,due to the risk of killing excessive numbers of breeders and reducing effective selection for other traits. For this reason,the disease challenge is sometimes done in siblings or progeny of the birds under selection,at an isolated location,and the selection is done on a family basis. The deliberate exposure of birds to pathogenic agents raises questions from an animal welfare standpoint. There is need for improved methods of identification of genetically resistant birds. Marker-assisted selection,or better yet the identification and labelling of specific genes for resistance,will enhance progress. One such gene,influencing resistance to salmonella in chickens,has recently been reported (Hu et al., 1997).

An understanding of the genetics of the disease organisms themselves might make possible the use of pathogen-derived genes (Witter,1998),which,once inserted into the bird’s genome,could confer levels of resistance to the disease organisms not currently found in existing populations of poultry. To implement this theoretical strategy,these new constructs would have to be inserted through the use of transgenic technology. The trait with the most impact on profitability is feed conversion. The conversion of feed into eggs is primarily a function of egg numbers. It is also influenced by egg sizeand body weight. Breeders improved feed conversion throughout the 20th century, especially in brown egg stock,by selecting for increased egg mass and smaller body size. Since the mid-1980s,commercial poultry geneticists have also been selecting for improvement of that part of feed consumption not explained by egg mass and body weight. 

This is referred to as residual feed consumption. Incorporation of selection on residual feed consumption will improve feed efficiency at a faster rate than selection on egg mass and body weight alone (Nordskog et al.,1991). To accomplish this, consumption is measured for individual hens. Expected feed consumption for each hen is calculated from the bird’s egg mass and body size using a linear model. Residual feed consumption is calculated by subtracting expected intake from the measured intake. Hens with high levels of residual intake are culled.

Feed conversion in the USA and Canada has improved from 2.95 g feed g−1 egg in 1960 (Agricultural Research Service,1960) to 2.01 g g−1 in 2001 (R.L. Nicholson,California,2001,in CMC Strain Performance Reports). Further continued improvement will be aided by a better understanding of the factors influencing feed conversion,including feather cover,activity and feed wastage. Some aspects of egg quality continue to improve,while others remain unchanged. Little change is occurring in overall egg weight,as most commercial varieties have already been selected to fit the needs of the markets in which they are sold. However, there is selection for attainment of the desirable egg size at an earlier age. This requires concurrent selection against increased egg size at a later age because of the strong genetic correlation between early and late egg size.

Inclusions (so-called blood and meat spots) have been selected to low levels of occurrence in white egg stock,so that little additional response can be achieved. In brown egg populations,there continues
to be genetic variability for inclusions,and effective selection is practiced to reduce the incidence of blood and meat spots. In brown egg varieties,effective selection also continues for eggshells with a darker brown color. Some selection is practiced for albumen height,so that Hugh units will remain at acceptable levels in markets where this measure is incorporated into egg grading standards. Shell strength improvement also continues and should improve for the foreseeable future,allowing the hens to produce a lower number of cracked eggs and eggs to be kept for longer periods of time.

New challenges are arising in relation to the use of eggs for further processing. Buyers of liquid egg are setting standards for the percentage of solids or lipids in the liquid egg product. In the USA,buyers of mixed white and yolk require that the mix contain at least 24.2% solids. If the level of solids is below this,processors must add yolk to increase the level. This reduces profits for the processor since yolk generally receives a higher price than albumen. In Italy,where yolks are in high demand for the production of pasta,buyers have established a standard of 10.5% lipid in the yolk. Both solids and lipids vary from one commercial cross to another (Ahn et al.,1997). Breeders can influence these traits by switching parent lines used to produce their commercial cross. However,it is extremely difficult to select within populations,since measurement of solids and lipids for individual birds is time-consuming and expensive. Quicker, cheaper methods are needed for the measurement of solids and lipids. 

Another issue affecting further processed eggs is the strength of the vitelline membrane. Egg whites must whip into foam with a good height. Contamination of the white with yolk will reduce the foam height. If the yolk ruptures during separation of the white,the contaminated product must be removed,reducing the speed of the breaking process and reducing the value of the product. Therefore,breeders must maintain intelligent membrane strength.

As more attention is focused on animal welfare,several traits increase in importance. Foremost among these is bird-to-bird aggression,which can lead to cannibalism and which also impacts feather cover. Craig and Muir (1996) have shown that selection in group cages can be used to reduce aggression. At least one commercial breeder has used this practice for over 30 years. Layers from this firm have relatively low levels of cannibalism when their beaks are left untrimmed (Craig and Lee, 1989).

Another welfare-related issue is the increasing demand for eggs produced by floor-housed birds (free-range,organic,etc.). The EU has issued a directive requiring the elimination of conventional cages by 2012. Cannibalism is an important issue for floorhoused birds. Nesting behaviour is another important issue,as birds must search out the nest. Eggs laid on the floor are more likely to be soiled and require special labor for collection.

The EU currently mandates 550 cm2 per bird in cages. Guidelines of the US United Egg Producers call for 432 cm2 per bird for all birds placed by the year 2012. Currently most commercial white egg layers in the world are housed in cages at 310–350 cm2 per bird. Breeders will need to take the changing cage densities into account in their breeding plans.

Other welfare-related factors that are likely to become of increasing concern to breeders include the killing of unneeded cockerels and maintenance of skeletal integrity as the bird ages. If induced molting is banned,this will also alter optimal breeding strategies.

Intensive animal agriculture has raised concerns among the general public about the effect on the environment of high concentrations of livestock. Manure output has become a concern and producers are asked to control not only the total amount of manure spread on the land,but also the amount of moisture in the manure (for fly control) and the phosphorus content. Birds that consume less feed will excrete less, so selection for reduced residual feed consumption should also result in less manure to spread on the land. There are genetic differences between varieties in the amount of moisture in the farces. Varieties with dry droppings are more prone to the development of urologists (Lent and Wide man, 1993). It is possible to select for drier droppings on an individual bird basis (Preminger et al.,1994) but care should be taken not to increase the incidence of urologists.

The greatest potential for increased egg consumption is in the tropics,where per capita egg consumption levels are low and are increasing. Much research has already been done on resistance to heat stress but,to date,no bird bred specifically for resistance to heat stress has captured much market
share in the tropics in general. As a result of depressed feed consumption and poorquality feed ingredients in tropical areas, there may be a benefit in having a bird for the tropics with a large appetite (Ansah,2000).

However,feed prices are very high in most of the tropical countries,so the ultimate solution for the production of eggs at economical prices in these areas may be the construction of controlled-environment houses to utilize the efficiencies of the modern layer.

Beginning about 1970,the advent of highspeed computers and the development of sophisticated statistical estimates of genetic value have permitted improved rates of within-line improvement. Questions remain concerning the effect of statistical tools such as best linear unbiased prediction (BLUP) that incorporate family information on the rate of exhaustion of genetic variability (Muir,1997). Accelerated loss of genetic variation due to the use of BLUP may necessitate early outcrossing of the pure lines that make up these crosses, to reintroduce genetic variability. 

Alternatively, selection rules could be considered that would allow for more conservation of genetic variance and optimal balance of long-term vs. short-term response. The use of marker-assisted selection (MAS) is expected to increase the accuracy of breeding value information and to be especially useful for traits that have low heritabilities or are difficult to measure. MAS will also allow the improved utilization of available ‘selection space’ (Soller and Medjugorac,1999). This underutilized ‘selection space’ is provided by the surplus males that are available in chicken breeding
stock. Far more males can be produced than are needed since,at the time of selection, full brothers without progeny tests all have identical predictions of breeding values for traits that can only be measured in females. Vallejo et al. (1998) found several markers for genes controlling resistance to Marek’s disease. Lamont et al. (1996) reported on markers for egg production,and Van Kaam et al. (1999) reported on markers for feed efficiency.

The use of transgenesis plays a major role in the breeding of plants. Several companies are now striving to develop transgenic strains of chickens that can be used to produce pharmaceuticals or other valuable proteins in eggs,but this tool has yet to be applied to the breeding of commercial poultry stocks. Since the single-celled chicken zygote is difficult to manipulate and then reintroduce into the egg for further development, transgenic poultry are more difficult to produce than are transgenic plants or mammals. With the developing concern about genetically modified organisms (GMOs),commercial breeding firms are now being required by some consumers to state that they are not using GMOs. This has a chilling effect on the interest of breeders in using the transgenic tool.

Eventually transgenesis will prove too valuable to ignore and commercial hens will become available that have enhanced performance due to the introduction of DNA that has been synthesized in the laboratory or that originates from other species.

Industry structure
much fewer in number and much larger in size. Three holding companies now control the majority of the breeding work on the commercially available breeding stock for egg-type chickens,though their products are marketed under nine different brand names. The reduction in the number of breeding firms has been due to international competition and to the high cost of maintaining modern breeding,marketing and distribution programmes in comparison with potential income.

The reduced number of breeding firms has raised concerns about reduced competition and an associated reduction in the potential for innovative research and development (Sheldon,2000). From an insider’s perspective,competition is still intense, some of it among companies within the same groups. However,there has been a dramatic reduction in the number of geneticists working for breeding firms and in the total number of chicken populations under selection.

There is also concern about the narrowness of the base of the genetic stock now being marketed. There is danger in this situation due to the potential susceptibility of ‘monocultures’ to new diseases that could destroy or damage a genetically uniform population,as happened with maize in the southern corn leaf blight epidemic in the USA in 1970 (Duvick, 1978).

There has also been increasing planned and unplanned loss of stock used as resource populations in the public sector (Pisenti et al.,2001). Some of the lost stock was developed over a period of many years,and their loss reduces the scope of future research. From the standpoint of genetic variability for long-term improvement in commercial stock,the important factor is not the preservation of unique research populations or of the degree of heterozygosity within populations,but the maintenance of allelic diversity across the species (Notter, 1999). The combined losses of research and commercial populations formerly held by now defunct breeders can limit the future  genetic potential of the chicken.

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