Layer Breeding-Concepts and Principles
B.L.N. Reddy, N.K. Praharaj, M.R. Reddy and S.V.R. Rao
In Indian sub continent layer breeding programmers are well established and it is essential for commercial poultry breeders to adopt breeding programmers as per the market need. Government organizations like Central Poultry Breeding Farms and Indian Council of Agricultural Research through its AICR.P Centers are engaged in layer breeding activities. Also poultry breeding companies from private sector are engaged in developing more viable and high producing crosses. Improvement in the genetic stocks was achieved through high intensity of selection and involving a large base population. The present emerging technologies in layer breeding are genetic engineering and biotechnological approaches to bring rapid improvement in the performance traits. Although, recent technologies have changed the approaches, still quantitative genetic methods are indispensable for conventional breeding and maintenance of the lines. Newer approaches in Quantitative Genetics are:
1. Best Linear Unbiased Prediction (BLUP).
2. Improved Selection Index
3. Infitesimal Model
4. Group Selection (Fairful, 1996) to improve the performance of the traits. This manuscript high lights some important concepts in layer breeding.
The trait egg production is polygenic in nature. It is a low heritable trait, while egg weight is moderately heritable. The components of egg production are as follows:
Age at Sexual Maturity: It is the age of the bird at which it lays the first egg. Early maturing hens lay more number of eggs, but smaller in size compared to late maturing ones. In order to prevent this problem lighting and feeding are monitored during growing period to delay sexual maturity. Age at sexual maturity is determined by both autosomal and sex linked genes. Sexual maturity of the pullets is highly correlated with the sexual maturity of dams. Males and females differ in transmitting the trait to their daughters. Hence males selected for breeding purposes should be the progeny from early maturing females.
Rate of Lay: Rate of lay is defined as the number of eggs produced in a given period of time. If the bird lays more eggs in a given time higher is its rate of lay. Clutch size determines the rate of lay. If the bird lays more number of eggs per clutch it is usually a better layer than the bird which lays fewer numbers of eggs. It is a lowly heritable trait. Family selection for this trait can bring about improvement.
Broodiness: Two dominant complementary autosomal genes A and C are responsible for broodiness. This behavior is seen in heavy breeds like Asiatic class. While Mediterranean breeds like Leghorn and Ancona are non broody. American class and English class are usually intermediate in broodiness. The most effective way in reducing broodiness in a strain is, to avoid breeding of daughters and/or sisters of brooding hen, even though they are the progeny of non broody dams. The sires whose dams or sisters show broodiness should also be avoided.
Persistency: It is the measure of the length of lying cycle. This factor is associated with egg production. The laying cycle of a hen is terminated by molting. The longer the laying cycle before the hen enters her molting period, the better she is for egg production. The laying cycle should be about 300 days.
Pauses: Pause is defined as the time between two clutches in which the hen is out of production. If the pause is more than 3 to 4 days, then the hen is considered as a low producer. Pause may be due to environmental and genetic factors. In winter months it is more common, and it is called as the winter pause.
Other than the components of egg production, the associated economic traits related to egg production are ;
Egg Weight: This is a highly heritable trait. It can be improved by selection. Birds which mature early tend to lay smaller eggs than those that mature late. The mature egg weight in layers should be approximately 55-58 gms. Egg weight is influenced by breed, nutrition, season and disease condition. It is also influenced by the type of housing.
Egg Quality: The quality of egg is measured by 1) Haugh Unit Score; 2) Albumin Index; 3) Yolk Index; 4) Shell Thickness, and 5) Presence or absence of blood and meat spots. These traits are low to moderately heritable. It can be improved by using various selection methods.
Shell Color: White and brown are most commonly preferred egg colors. Shell color is not having any nutritive value. It is characteristic of breed. Indigenous chickens lay tinted eggs and they are preferred in the market because of aesthetic value.
Shell Texture: The soundness of an egg is judged from its texture. This is important from a commercial point of view, because eggs are transported for marketing purpose. This trait is influenced by genetic, nutritional and environmental factors.
Body Size: Optimum body size of a hen at laying is an important factor to be considered for satisfactory egg size and persistency. Hence body weights at maturity. 20 weeks and 40 weeks are monitored to maintain good egg production and egg weight.
Feed Efficiency: It is measured as amount of feed consumed in Kg/dozen of eggs or as amount of feed consumed in Kg/ Kg of egg mass. Small sized layers with high production are more efficient than those heavy layers with high production. So this trait may be included in selection to develop feed efficient stocks, economical in production than normal sized ones in pure line stocks.
Fertility and Hatchability: Fertility and hatchability are low heritable traits, but can be improved by appropriate breeding methods. These traits can be improved by test mating the males before using them for breeding purpose. Fertility is defined as the percentage of eggs that are fertile out of total number of eggs set. Hatchability is defined in percentage that is total number of chicks obtained from the fertile eggs set and also expressed in percentage that is total number of chicks obtained from the total number of eggs set. Factors influencing fertility and hatchability are breed, strain, family and individuals within a family. Inbreeding decreases, while out breeding increases these traits. Age of the bird, nutrition, disease, management and environmental factors affect both fertility and hatchability.
Egg production Indexes: The various egg production Indexes are :
1. Hen housed egg production (HHEP): It is obtained by dividing the total number of egs produced by the number of birds housed in the laying pens. This does not take mortality into account.
No. of eggs laid
HHEP%= ———————— X100
No. of birds housed
2. Hen day egg production
No. of eggs laid
HHEP% = ———————
No. of hen days
3. Survivor egg production
No. of eggs laid
SEP%=------------------------ --- X100
No. of surviving birds
Selection Methods in Layers
Main objective of the layer breeding programme is to bring improvement in layer traits such as egg number, egg weight, fertility, hatchability, egg quality, feed efficiency and viability. There are number of selection methods to improve the traits.
Individual Selection: This is practiced for highly heritable traits like body weight and egg weight. Here selection is practiced on the basis of phenotypic performance, which gives satisfactory results.
Family Selection: This is practiced for lowly heritable traits like egg production. Phenotype of an individual is not a reliable indicator of the genotype for such traits. Family selection may be sire family selection or dam family selection. Sire family selection, in which average is of the sire family, is the sole basis of selection. Since family averages form the basis of selection, either the whole family is selected or the whole family is rejected. Individuals in the family although differing, are treated at par as far as selection is concerned. Large family size is better for the estimation of breeding values and for getting better response.
Combined Selection: This is mostly used in improving the lowly heritable traits. It uses information on the individual phenotypic values and its sire and dam family averages and other relatives with appropriate weights given to each of the components to increase the accuracy of selection. It is superior to individual and family selection as it combines the advantages of both individual and family selection. Egg production trait can be improved by this method of selection.
Sib-Selection: If particular trait is expressed in only one sex like egg production, which is not expressed in opposite sex, selection will be based on the performance of the relatives like full or half sibs of other sex. This method of selection is called sib selection.
Progeny Testing: The selection of parents is based on the average performance of the progeny called progeny testing; this results in greater response than other selection methods. This was used for the development of high producing layer stocks. Now a days it is not very popular as it increases the generation interval and decreases the progress per unit of time. This is important for the improvement of slaughter traits, since individuals measured are not available for selection.
Muti-trait Selection Methods:
The selection practiced for several traits simultaneously to improve the performance of individuals is called multitrait selection. These are as follows:
1. Tandem Selection .-This is the selection for different traits at different times and it is least efficient than other methods of selection.
2. Independent culling level selection: In this method of selection culling levels are fixed for each trait. This is easier to practice and requires less right assumptions. This permits selection at different stages for different characters and helps in the reduction of breeding costs.
3. Index Selection : This method of selection is superior to the other two methods of selection. This permits simultaneous selection of several traits to optimize genetic gains. This is a more balanced approach, since it combines information on various traits on the basis of their economic importance. It requires unbiased estimates of genetic and phenotypic parameters of the component traits of the index and their relative economic values. The efficiency of index selection increases with increase in the number of traits to be selected, but the response in the individual traits becomes less. Hence only those traits, considered more important should be included in the index.
Intra vs. Inter Population Selection Methods
Selection of individuals within a population to reproduce the next generation using one or combination of methods presented above is known as intra population or closed flock selection. This method increases the frequency of desirable genes which act additively. The response to be realized from this method depends upon the magnitude of additive genetic variance available in the population. Selection in pure strains which results in improvement in crosses is the principle in closed flock selection.
Inter population selection or inter line selection uses the performance of the crosses to develop the pure lines. There are two methods of selection:
1. Recurrent Selection
2. Reciprocal Recurrent Selection.
The primary objective of inter line selection is the improvement of cross performance, irrespective of perform ace of the two lines involved.
1. Recurrent Selection: In this type of selection only one of the two populations is pure bred and it is improved in reference to the other population. This reference population is called tester line, which is either inbred or cross of two inbred lines. This type of selection utilizes the non additive genetic variance.
2. Reciprocal Recurrent Selection:
This method of selection increases the frequency of both additive and non additive genes, hence improves both pure line as well as cross line performance. Both populations should be open bred with a history of nicking. This method of selection is indicated for the population, which has ceased to respond to conventional closed flock selection methods.
FEMALES A Males X B Females
* Rank the sires of A and dams of B on the cross progeny performance and select best sires of A
MALES FEMALES B Males X A Female
* Rank the sires of B and dams of A on the cross progeny performance and select best sires of B best dams of A.
* Reproduce the strain A separately by mating best B males X B females
Genotype-Environmental interactions (G X Es):
In layers, housing, temperature, humidity, diets, vaccines and disease exposure can result in G X Es (Sheridan, 1990 Fairful and Gowe, 1990). Usually the optimal selection environment is the one in which the birds are expected to perform better (Yamada and Bell, 1980). Selection for the highest individual performers, in a group environment will have lower productivity due to competitive effects (Muir, 1996). In commercial layer selection hens are evaluated and selected in single cages but reared in multiple cage environments. Commercial breeders are located in temperate climates. Layers selected in these climates must perform in various climates of the world. This is especially problematic in hot climates (Gowe and Fairful, 1995).
Hence, breeders have to set up joint ventures with the companies in various parts of the world under which sub populations are selected for local conditions. To the best of present knowledge, best dams of B.
* Reproduce the strain A separately by mating best A males X A females optimal selection environment is the environment under which the bird is expected to have excellent performance,Marker Assisted Selection: Marker assisted selection depends on linkage disequilibrium (Muir, 1994), which is disrupted by recombination. Crosses of inbred lines can result in the desired equilibrium. Adequate family sizes to establish a QTL marker association are possible from large half sib populations. The models used for quantitative trait loci are additive. However non additive genetic effects are important for commercial traits in layers. G X Es affects the genome as a whole. Genetic background affects the value of QTL and estimates of relationship between markers and QTL (Dunnington, 1993). Markers have been used in selection for disease resistance and could potentially be of value in selection for sex limited traits.