Genetic composition is a major factor which governs the productive performance in variousaspects of livestock. About 80 per cent performance of poultry is dependent on genecomponents and 20 per cent on environment component in exploiting genetic potential to itsmaximum possible extent. Basically if birds are of poor genetic make-up, whatever may bethe highly scientific management conditions prevail, they may not be desired economical andprofitable producers. The combination and fixing of high quality genes in specific breeds,varieties and strains involve consistent scientific and skillful breeding practices. It ishighly technical skilled job which requires persistent continuous efforts for long-time.Gregor Mendel is supposed to be the father of genetics who has formulated some offundamental laws in genetics on his observations on which most of today’s genetic skills depend.
BASIS OF VARIABILITY
No two individuals are similar. Each individual differs from another due to the genetic variation. This is because each parent transmits one gene or pair of genes to its offsprings and variability depends on the heterozygosity of number of gene pairs ofparents. A sample half of inheritance of each parent and sample half of environment isreceived by each offspring. The chicken has total 39 pairs of chromosomes and each carries many genes, some probably 100 also. The number of gametes produced are abundant resulting into enormous hereditary combinations. The expression of character is due to one or moregenes. The basic unit of inheritance is gene and not the character.
INHERITANCE AND ENVIRONMENT
As already stated earlier variation in any flock is not only due to genetic component but environment component is also responsible for it. The exploitation of genetic variation is rather slow because several genes are involved in expression of each character in addition to which environment, hormones, and management conditions are also responsible for producing response. The hatchability, fertility, viability (liveability), resistance to disease, egg production, flesh production are the characters of economic and fundamental importance, where environment frequently plays its role for full exploitation of these characters. For example, inspite of high hatchability genes, poor hatching can be obtained due to faulty incubation conditions or low egg production can be noted in highly genetic potential birds because of imbalanced diet and poor housing conditions.
SEX RATIO AND FERTILITY
The fertility in poultry to large extent depends on genetics and selection of bird as an inherent character, but it is equally affected by better management than inheritance. The most important management point for it is sex ratio formating. On an average one male for 15 to 16 females for replacement pullets and one male for 10 to 12 females for broiler breeders is optimum for good fertility, depending on light or heavy breeds.
The choice of allowing individuals to become parents for next generation is selection. In other words differential rate of repro-duction is also called as selection. It is necessary to maintain or increase variation in population and one of the important force for geneticimprovement.
Kinds of Selection:
There are two kinds of selection, viz. natural or artificial.
In case of random mating nature itself selects individuals for mating in flocks or population by preferential mating.
The selection applied by man with records and by adopting different methods is artificial selection. This involves intrapopulation and interpopulation selection.
Basis of Selection (Systems of Selection)
Depending on the base used for selection it is divided into the following systems:
It is also called as mass selection. Because the individual is selected from mass (flock or large number) on its own phenotypic value and put together in mass for mating, it is known as mass selection. The system is adopted for traits of high heritability and expressed in both sexes, where it results in faster genetic improvement.
The individuals are selected on the performance of their whole family. It is useful in case of low heritability characters like egg production and which is expressed in females only; similarly viability, where phenotype is not reliable indicator of genotype. The sire or dam family averages are compared to population mean and the whole family is either selected or rejected for higher or lower means, respectively. The system involves two types of selection, viz, progeny testing and sib selection:
The individual isselected on the basis of performance of it’s progeny, i.e. sons and daughters. But thebiggest problem with this system is the time consumed to obtain performance values of progeny. The part year production values, e.g. egg production upto 40 weeks of age can be efficiently used to overcome the difficulty of time consumption as it is positively correlated with full year production to make use of this selection system.
Sisters and brothers of individual are called as sibs. To avoid time consuming problem in progeny testing, individuals are selected on the basis of performance and appearance of their brothers and sisters. Similarly, to select birds for traits which are expressed in one sex only, for example, egg production which is expressed in pullets or hens, cockerels or cocks are selected on performance of their sisters.
Pedigree is the record of an individual’s ancestors including parents. This selection is also important because sample half of genes from each parent is transmitted to each of it’s offsprings. It is of immense importance in selection at earlier age, when traits in question might have not expressed themselves, i.e. it’s greatest applicability is in initial selection of sex-limited traits.
Methods of Selection on Phenotypic Values
The net value of an individual depends on many traits having varying economic importance.
The heritability and magnitude of genetic variance of traits differ from traits. The various traits may have phenotypic and genetic inter-relationships among themselves. Therefore, if too many traits are to be selected at a time, it may result in less improvement in specific traits. In this connection, Hazel and Lush have examinedsuperiority of the following selection methods:
This method involves selection of only one trait at a time for improvement till satisfactory level of improvement in it, selection efforts are then relaxed for this trait and concentrated on second trait, and so on. The improvement is simultaneous for traits which are positively correlated and vice versa for negatively correlated traits. But this is again time consuming method.
Independent Culling Levels
Selection for many traits can bedone at a time or simultaneously by setting up minimum standard level for each trait. Any
individual below of any one or more levels is culled irrespective of meritorious performance in other traits. Such method of selection will result in favouring of medium type of birds and may reject outstanding individuals in many economic important traits as they may not qualify themselves in traits of lesser economic importance.
Hazel and Lush (1943) have invented this method wherein selection for several traits at a time can be done by rectifying drawbacks in previous methods. It is also called as total score method as it includes estimation of the bird’s total breeding value (net merit) for two or more traits at a time. Each trait is given weightage depending on its economic importance, heritability and inter-relationships (genetic and phenotypic) among traits. The score of index of each trait is added to arrive at total score index. The method is more efficient in all circumstances, as it saves superior individuals in important economic traits even they are inferior in less important characters.
IDEAL BREEDING PROGRAMME
As seen earlier most of selection systems have some or the other limitations. Therefore, ideal breeding programme consists of combination of various selection systems, and method can be better used along with individual, family and pedigree selection systems. The traits like egg production, viability, fertility, etc. which have low heritability, can be
improved by adopting family selection. On the other hand successful improvement in characters like egg weight, shell quality, sexual maturity, growth rate, confirmation can be achieved by using individual selection. Therefore, a wise breeder fixes required characters in his bird by combining useful methods for his operation.
Selection of Birds for Breeding
Selection of Egg Type Lines
The suitable age for selecting egg type birds is 10 to 14 weeks where inferior birds are removed from the flocks. The body confirmation and development of structural body parts are given more weightage than body weight in selecting these breeders. Thebirds from each lines are selected separately and most of breeding procedures involve mating of males and females from various lines. The chicks from each line are sexed at day-old age and inaccuracies in sexing may lead to mixing of cockerels with pullets and vice versa. These birds of sexing error should be removed prior to mating. The unwanted males must be removed as early as possible to reduce the cost.
Basis for Selection of Individual Males and Females
1. The pedigree in past is assessed for several pedigree generations.
2. The appearance and performance of individual and its sibs in present is used for making final judgement.
3. The appearance and performance of individual in future is judged by appearance and performance of its progeny, generally of sons and daughters.
Selection of Meat Type Lines
As there is high correlation between weight of meat type parents and their broiler offsprings at eight weeks of age, their selection preferably is done at this age. The selection pressure required for males is more than that of for females. The correlation between weight of parents at sexual maturity and broiler offsprings is little due to which selection on weight basis of birds going into lay or after eight weeks is very difficult. Even though same percentage of birds are removed from each sex, more selection pressure
(culling) is applied for males than females. This does not result in any differences in the effects for next generation as males required are fewer than females.
Selection of males
First of all we should know percentage of males to be retained at eight weeks of age which is more than the requirement of sexual maturity. Usually 60 per cent males are retained at day-old age. Forty per cent at eight weeks and 12-15 per cent at sexual maturity. The percentage is more at initial and middle stages because of depletion due to mortality and culling in later period. For selection at least weigh 15 per cent birds individually in a flock or in pen as representative of all birds at eight weeks of age.Record these weights on a separate sheet starting from heaviest and ending with lightest.According to percentage required count the number of males to be kept out of these 15 per cent, starting from heaviest with counting towards lightest. The figure of weight at which percentage of retaining is reached becomes the minimum weight for selection. Now discard the males weighing below minimum standard weight fixed by individual weighing. The sample weight must be taken in every pen of house to nullify effect of variation.
Selection of females
The selection for females is done with lesser selection pressure than males. About 80 per cent females are retained at eight weeks of age. Body weight is not much exerted for selection of females. The selection some times may be done with same procedure adopted for males but usually it is done by following the normal method of culling, i.e. birds are selected on the basis of general appearance, body condition, confirmation, moulting pattern, etc.
Methods of Mating
The methods of mating play major role in obtaining fertility of eggs from breeder birds.There are five commonly used methods of mating, out of which pen mating and flock mating are of commercial importance. While stud mating, shift mating and artificial insemination (AI) are important from research point of view.
Generally followed for pedigree hatching where parents of offsprings can be determined with trap nesting of females. More number of females are allowed to mate with single male in small flock in separate pens for each male. But fertility may not be as good as of flock mating due to likeness in mating, i.e. the male heading the pen may not like to mate with particular female and vice-versa.
This is the common method of mating used in most of the breeding practises. About 20-30 males are run with 250-300 females in flocks in a section of house. This reduces the chances of likeness or social order in mating and very good fertility is obtained, but parentage of offspring cannot be known.
The male is housed in a pen or coop and females are individually kept one by one with male for mating time only and removed. The method is excellent for increasing the utility of outstanding males to increase the offsprings mating, and therefore, it is more expensive.
In this method males are shifted from one pen to another after certain period of time, which helps in thorough testing of females as they are exposed to several males for mating.But to maintain accuracy of parentage this method is little difficult because fertile eggs can be produced for one to two weeks even after removing male from that pen. The recommendation for overcoming this problem is to discard eggs for one week after shifting of old male and housing of new male in particular pen. By adopting shifting after short span of time, large number of males can be tested with adequate assessment of females also for breeding.
The method is not commonly used in chicken but it is quite common in turkey breeding due to lower fertility problems. The reasons for not becoming it common may be the non- availability of trained personnel, more labour involved and handling stress to birds. But if practised, it is excellent method to increase the efficiency of breeding programme. Because it increases the utility of outstanding males, eliminates completely social order in mating, minimises risk of disease spread and increases accuracy in parentage determination, it is more advantageous.
In fact poultry birds are prolific breeders and they bred in all seasons. Therefore, there is no specific breeding season for poultry unlike that of other livestock but fertility percentage may vary in different seasons. For that also only season may not be the sole cause for reduced fertility. The season may act as minor contributing factor to lower down fertility. For example in summer season fertility is little bit reduced than mansoon and winter. That is excessive heat in summer may reduce breeding instinct of birds adding to causes of lowering down of fertility. Therefore, it may be said that rainy and winter seasons are comparatively better seasons for poultry breeding than summer.
BREEDING SYSTEMS IN POULTRY
The object of any breeding programme is to improve genetic make-up of progeny by maintaining genetic variance, for increasing the productive performance along with descent appearance. For keeping the variation, which is effect of heterozygosity, homozygosity is also essential to induce variation. At time of reaching of selection limit or selection plateau, status and condition is maintained to obtain highest production for some period and selection with breeding system in opposite direction can be adopted to avoid drastic deterioration of genetic composition which may result in sudden drop in productive performance. Therefore, the breeding systems used in any breeding programme should have the following objectives:
i. Increase in homozygosity which constitutes inbreeding,
ii. Increase in heterozygosity which involves outbreeding.
iii. Maintain the status quo position which is done by random mating. Ultimate result of breeding in term of genetic language is alteration of gene and genotype frequency, either in forward or backward direction as per desire and requirement.
Systems to Enhance Homozygosity
Any breeding system that increases homozygosity within breed or variety is known as inbreeding or rather to increase homozygosity inbreeding is adopted. Inbreeding is mating of closely related individuals wherein relationship is more close than the average of population. In inbreeding there will be one or more common ancestors from which part of gene samples (gametes) are drawn. Depending on closeness of relationship among the individuals, inbreeding can be grouped into three types, viz, close inbreeding, line breeding, and foi !nation of strain.
The mating of parent and progeny of mating between sibs is known as close inbreeding. This is followed for formation of inbreed lines and if relationship is less closer than that of cousins then inbreeding effect is very mild which cannot be counted also. As stated earlier with breeding of close relative the heterozygosity can be brought down to 10-12 per cent or even below it in 10-12 generations.
It is inbreeding within the highly admired sire or dam ancestoral line to increase number of individuals of outstanding male or female in population. The daughters of different generations are mated back to outstanding sire or sons of various generations are mated back to outstanding sire or dam, so that resultant progeny has more and more percentage of genes of these outstanding parents. An inbred line should have at least 50 per cent inbreeding co-efficient. Mating of full sister-brother for three generations or of half, brother-sister for six generations can produce inbred line with 50 per cent inbreeding co-efficient.
Formation of Strain
This involves mating of double cousins, hence it is called as milder form of inbreeding.This is done to establish high productive traits in population to use it for production of commercials. It is adopted within the breed, so that selection can be carried out within and between strains to evolve better commercials. It also reduces heterozygosity but a slower rate than close or line breeding.
IMPORTANT ASPECTS OF BREEDING
the capacity of quantitative character to be transfered from parents to offspring is called as heritability.In terms of real genetic language the ratio of additive genetic variance to total phenotypic variance is heritability, i.e. h2 = va/vp. The improvement in progeny is fast, when heritability of that character is high and vice-versa. But inspite low heritability, a significant improvement in specific character can be achieved by using suitable selection procedures.
As stated earlier any deviation from expected mean performance is called as heterosis, and positive heterosis is generally known as “hybrid-vigour” or “nicking”. Heterosis can arise from out-crossing in between various lines of same breed or strain crossing or cross-breeding.
The rigidity or intensity of selection is known as selection pressure. It is designated in terms of percentage of flock retained for breeding purpose. Higher the percentage of flock retension, lower will be the selection pressure and vice-versa. For example, selection pressure will be much more, when superior 15 per cent birds in flock are retained for breeding as compared to 50 per cent use of birds as parents.
The age of laying first egg or the distance between day of hatch and first egg laid, is termed as sexual maturity. Earlier is the sexual maturity, more productive the bird is because it gets additional period to lay in its laying cycle of year. But some times in specific breeds pullets are presented from early sexual maturity to avoid production of smaller size eggs, egg bounds and prolapse before proper development of body.
Intensity of lay:
It is capacity of bird to lay in defined period of time. It can also be termed as rate of lay and what breeder is interested is high intensity of lay, which is the ability to lay at rapid rate. The better is intensity of lay, the more will be financial returns. In this connection the size of clutch is important. The number of eggs laid on consecutive days without gap, is known as clutch. The longer is the clutch size, the higher will be theintensity of lay. One way of measuring intensity of lay is to calculate simple percentage of production while other way is the size of clutch. It is important genetic trait but management also governs intensity of lay.
It can be defined as early sexual maturity.
The ability of hen to continue laying for longer period in her first laying cycle. In other words it is measure of length of Laying year of hen. The laying is generally terminated by moult in late summer. The longer is the length of laying cycle, more persistent the hen is. Once again it is important genetic trait associated with egg production. Persistency is highly (+ve, 0.75) correlated with annual egg production and hence important contributing factor to hen-housed egg production of bird