Modern Approaches of Poultry Breeding
The domestication of jungle fowl marked the beginning of selective breeding of poultry. The breeds showed a range of traits, including production traits such as body size, musculature, egg production, and egg color. Industrial breeding started with the hybridization of selected pure breeding lines sampled from these base breeds and continued with a more and more intense further selection of the pure lines. The different breeding and selection technologies at different periods of time for the genetic improvement of poultry were introduced. Techniques such as Mass selection, Hybridization, Pedigree selection, Artificial insemination, Osborne index, Family feed conversion testing, Selection index, Individual feed conversion testing, BLUP breeding value estimation, DNA markers are used over decades.
Genetic Strategies for the Improvement of Broilers
Breeders set breeding goals as a reflection of their expectations of future market demands with the ongoing changes of production and consumption trends. Broiler growth (body weight) has consistently been the prime selection trait, Family selection for livability and eradication of egg-transmitted diseases at the pedigree level may have contributed to the reduction of mortality. Indirect carcass measurements (breast muscle thickness) were applied to the male selection. In direct measurements, the sib information was used in the index, there is a higher intensity of selection which in turn increases the rate of inbreeding. Whereas, indirect carcass measurements provide their own performance information for the selection of candidates, which increases the accuracy of selection and consequently improve the genetic gain. It also reduces the rate of inbreeding.
Genetic strategies for ascites in broilers
Ascites syndrome has been a source of concern to the poultry industry. The genetic selection is the best solution for eliminating ascites syndrome in near future. With the advent of molecular genetic research on the genetic basis of ascites, we can see the potential for the identification of genetic markers that can be used to eliminate ascites from modern commercial broilers.
Genetic strategies for heat stress in broilers
Heat stress is one of the most important environmental stressors challenging broiler production worldwide. With the rapid development of the poultry industry worldwide, importation of high-performance stocks to, hot regions are continuously increasing. The use of improper genotypes in these regions results in large economic losses due to decreased growth rate, reduced protein gain and high mortality. Three major genes associated with heat tolerance were identified in poultry and they were naked-neck gene (Na) (reducethe feather coverage), frizzle gene (F) (modify the shape of the feather), and dwarf gene (dw) (reduces body size). Among these three genes, most notable is the gene for naked neck (Na), which produces heat tolerance by reduced feather coverage thereby increasing the rate of heat dissipation. Hence, specialized breeding programs using specific indicators of adaptation to heat should be added to commercial selection programs for rapid growth to improve broiler performance in hot climates.
Genetic strategies for layer improvement
Egg production has always been important for the selection criteria applied to egg laying stock. Egg numbers have increased from less than 270 to 340 eggs due to advancements in genetics. Traditionally part record selection was advocated as a means of shortening generation interval to increase genetic progress. Recently apart from part-production, annual production is also taken into account for selection decision. However, use of the whole record will double the generation interval. In order to optimize genetic gain/unit of time multi-stage selection is followed as it reduces the cost and efforts (by discarding inferior birds at early age) and generation interval is minimized.
Selection strategy egg production and for feed consumption
In layers improved feed conversion throughout the 20th century by selection on increased egg mass production and smaller body size. They are the most important traits involved in the variation of feed consumption. 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. Commercial poultry geneticists have also been selecting on residual feed consumption for improving feed efficiency. From the majority of the selection experiments from large commercial populations, it is apparent that most laying hens have a remarkable ability to adjust their feed intake to requirement. However, a significant residual component of feed intake has been shown to exist hence in recent years, because of high heritability and absence of significant negative effect on production parameters, residual feed consumption is used as selection criteria to improve feed efficiency.
Egg quality traits and its improvement strategies
Eggshell quality traits, amount of cuticle, color of eggs, protein content of eggs, gene for vitelline membrane, and traits related to aesthetics in eggs all are under selectable genetic control. Whereas a completely separate breeding programme exists for the production of white and brown-shelled eggs. Practicing selection for egg quality will continue to be one of the most important aspects of the breeding strategy for egg-laying hens. Genetics and genomics have identified new strategies to address egg quality including the use of very high-density genotyping to allow genome-wide selection which has potential benefits for measurements that can be performed in one sex.
Breeding for Diseases Resistance in Broilers and Layers
Salmonellosis, campylobacter, Marek’s disease, Newcastle disease avian influenza, and infectious bursal disease significantly affect the economy of the poultry sector. New opportunities have been arising in animal genomics and related technologies. With the availability of the draft chicken genome sequence, the genes that underlie the resistance loci can be identified and utilized. In recent years, advances in molecular genetics, the relationship between genes and their corresponding phenotypes, are beneficial for disease prevention and control. Most of recent strategies are now developed, combining structural, population, and functional genomics approaches.
SYSTEMS OF BREEDING
Whether a male or female bird can be kept for breeding purpose or not is determined largely by the kind of progeny (young ones) they produce. This is true regardless of the character involved. A male whose dam (mother) had a high record of egg production mated to a female with a high egg production record frequently produces daughters that lay prolifically (in large amounts). The results secured from a given mating are determined largely by the genetic contribution of the birds mated rather than by their physical appearance. The breeding systems can be classified depending on whether it is aimed to increase homozygosity or heterozygosity into random mating, inbreeding (breeding for increased homozygosity) and outbreeding (breeding for increased heterozygosity).
- Random Mating
Mating of individual without any selection. This is used in developing a control population which is required to compare and measure the effects of other breeding systems. Control population also helps to estimate the effects of the environment which in turn, helps to estimate the true genetic gain through any breeding method
- Inbreeding
It is defined as mating between individuals which are more closely related to each other than the average relationship between all individuals in a population. Inbreeding can be consistently carried out for several generations.
There are 3 distinct methods:
- a) Close inbreeding: Mating between sibs and parents and progeny. Full sib mating and back crossing of the progeny to the younger of the parents are often practiced.
- b) Strain Formation: Developing a small group of animals within a breed and variety with a special character in view. This is a mild form of inbreeding. For example Babcock strain of Single Comb White Leghorn developed to lay heavier eggs.
- c) Line breeding: This is inbreeding with an ancestral line and is the most intensive form of back-crossing. Line breeding is back crossing to the same parent for several generations in succession.
- c) Outbreeding This is the opposite of inbreeding in the sense that the relationship of the individuals which are mated is less close than the average relationship within the population. Mating between strains or inbred lines are the forms of outbreeding.
The methods of out breeding (cross breeding) are outlined below:
- a) Single or 2-way cross: Two different populations (inbred lines, strains or breeds) are crossed to produce a first filial (F1) generation which is purely for commercial purpose but not for breeding. F1 here usually exhibits hybrid vigour especially when inbred lines are involved. When two inbred lines of the same breed are crossed, the progeny is said to be incrossbred.
A X B AB
- b) Three-way cross: In this method, F1 crossbred females (AB) are mated to males of a third line (C), to obtain a F2 progeny (ABC). A X B AB X C ABC
- c) Four way cross: Two different single crosses (AB and CD) are crossed to obtain ABCD. A X B C X D AB X CD ABCD
This is usually practiced in poultry breeding for crosses between inbred lines of low viability since only a relatively small number of animals of the lines A, B, C, D need to be maintained. d) Crossing for production of a new breed: Different breed types have been crossed to produce the modern day breeds of farm animals so as to combine desirable traits from many sources. These foundation crosses have to be subjected to inbreeding combined with selection to consolidate them into true breeding populations (breeds). Example Cornish developed from Aseel, Malay and English game breeds.
METHODS OF MATING
Mating is defined as the pairing of a male and a female for the purpose of reproduction or production of young ones. The different methods of mating commonly practiced in poultry are as follows:
- Pen mating -A single male is segregated with a group of females in a pen during the breeding season. This requires marginally more labour. Number of females that can be allowed with a male (referred to as mating ratio) is 10 to 12 in case of Leghorn (Egg type birds) and 6 to 8 in case of meat type birds.
Pedigreeing is possible both on sire’s (father) as well as on dam’s (mother) side. However, if Pedigreeing is done only on dam’s side, multiple male mating can be employed in a larger pen.
- Stud mating
The males are usually confined at all times to small individual pens (stud) within the large laying pen. The female is held in the stud for a known period of time (till mated) after which it is removed and another is added. Two mating per week or at least once every 5 days is desirable for optimum fertility. This method requires more labour and is rarely practiced now-a-days.
- Artificial Insemination
Artificial Insemination (AI) is the technique by which seminal fluid (semen) of male is introduced or deposited into the female reproductive tract by a pipette. One cock will yield about 0.5 to 1.0 ml of semen depending upon the body weight. About 0.05 to 0.10 ml of semen is enough to inseminate one hen. This technique of mating is having many advantages and few disadvantages.
The advantages of artificial insemination are as follows:
- Allows unlimited number of single male mating without requiring extensive breeding equipment; · Preferential mating avoided; · Accurate Pedigreeing possible; · If, due to some reason, a male of superior qualities cannot mate, it can still contribute to the next generation; · If the males are too heavy (as in the case of Broad Breasted White Turkey) or too old for natural mating, AI is the method of choice; · Hybridization between two different species is possible (eg. chicken-quail hybrid); · In caged-layers also, fertile eggs can be obtained only by the AI; · Problems of trap-nesting are avoided; and · The incidence of sexually transmitted diseases is reduced or avoided Insemination of chicken must be done during the afternoon hours by which time most of the birds are expected to have laid eggs rendering their oviduct empty. Frequency of insemination is twice a week or at least once in every five days.
The disadvantages of AI are:
- It require more labour; · Chances of cross contamination of birds through the inseminating equipment, especially of paratyphoid infection is possible; and · Involves handling of birds which may cause stress.
Shift System of Mating
It is desirable to obtain about 100-125 chicks to evaluate a male. This requires a total time which is inversely proportional to the mating ratio. If a male has 6, 12, and 16 dams, the approximate time required to obtain the desired number of progeny is 33, 17 and 13 days, respectively. To reduce this time, the males can be shifted from one pen to another so that it will have more mates (female birds) through which it can be evaluated. Similarly, the dams also have more mates making the comparison between dams more critical and accurate. The major drawback of the system is the overlapping of the paternity when the males are changed since the viability of spermatozoa ranges from 7-21 days. It has been found that this problem of overlapping of pedigree can be avoided greatly by using AI, because, the sperms held in the oviduct cannot compete with those in the fresh semen. Therefore, the eggs laid on the second day after AI is attributable to the new sire. Similar results is possible with natural mating, but the chances of preferential mating or time taken before the new male mates with the female concerned restricts the accuracy under natural mating. The other problems include the difference in vigour, virility and fertility of the successive cocks, difficulty in record keeping and more labour involvement etc.
The shift normally followed is as follows:
- 1 st day First shift males in breeding pens. · 2 nd day Start of collection of fertile eggs. · 15th day Removal of first shift males. · 20th day AI using the semen of second shift males and the males are allowed into the pen. · 22nd day Eggs are laid from 8th to 21st day (both days inclusive) are pooled and designated to have been fertilized by first shift males. Start of collection of fertile eggs on 22nd day and onwards. · 29th day Second shift males removed. · 35th day AI using the semen from third shift males and the males allowed into the pen. Eggs collected from 22nd to 34th day are pooled and designated to have been fertilized by second shift males; and so on. However, pen mating or AI is the commonly practiced methods of mating in most of the poultry breeding farms.
- Flock Mating
In this type of mating, large numbers of hens in a flock are kept with cocks in the ratio of 10 hens per cock. Method is good to reduce operating cost because of large number of fowls per unit. This method is preferred where pedigree records are not maintained.
This system of mating has the following disadvantages:
- Males develop tendency to fight each other (When one male become aggressive prevents others from mating) · Dominating tendency of male causes low fertility · Pedigree cannot be maintained
Conclusion
Genomics in poultry breeding has seen notable progress. The search for new measurements that more reliably reflect the traits which are the ultimate target of selection, whether it is a reduction in bacterial contamination, the resistance of the shell to damage or processing and nutritional qualities will be a major focus. In the future genomics could well play an important role in supporting breeders in selection programs. This will affect the structure of breeding programs and also impact the integration of breeding in the poultry production system. The new knowledge of the molecular basis of poultry phenotypes that are generated along the way will be used to engineer and redesign the poultry genome with novel technologies, and genetically engineered poultry breeds
