Inheritance and variability of economically useful traits in poultry

The data presented in Table 14 were obtained by different authors under unequal conditions and in different years. Therefore, the indicators are approximate, yet they still allow for noting a significant differentiation in the degree of heritability of various traits.

Many of them, related to meat productivity (live weight, feathering, etc.) and egg quality (egg weight), possess high heritability, which facilitates selection for their improvement. However, such crucial traits as egg production, fertility, and poultry viability, which determine economically significant qualities in the production of eggs and meat, are weakly heritable.

The magnitude of the heritability coefficient depends on the genetic diversity of the population. In the initial period of breeding work with a population, the variability of heritable traits is usually high, but under the influence of selection and mating for selected traits, it decreases, and the breed, line, or group of poultry becomes increasingly uniform in terms of heritable qualities.

The repeatability of the trait level in adjacent years increases, and what is known in practice as "calibration" for desirable economically useful qualities occurs, resulting in economically significant uniformity of such parameters as:

• broiler carcasses;
• egg weight;
• egg shape;
• shell color;
• other commercial product qualities.

Variability represents the differences that arise between poultry of the same species, breed, or line under the influence of the hereditary material itself, as well as environmental conditions. Variability, inherent in individual birds with at least small differences between them, provides material for natural selection and, along with the correlation of traits, is used in the creative work of the breeder. Hereditary variability is of great importance in this regard, the regularities of which form the basis for work on improving existing and creating new breeds and lines of poultry.

A direct positive correlation between traits facilitates selection; a negative one, conversely, necessitates the use of more complex methods for improving heritable but closely unrelated economically useful qualities.

Examples of the influence of the nature of correlation on selection:

• The correlation between growth rate and pectoral muscle development is most often positive, which allows for the simultaneous improvement of these valuable qualities in meat poultry farming through selection and mating methods.
• The tendency toward a negative relationship between egg production and egg weight requires more complex selection methods to improve these traits.

Correlations between economically useful traits in poultry are almost always curvilinear in nature. Egg weight increases with an increase in the live weight of hens, but only within limits specific to each population. For instance, hens of the Russian White breed, whose live weight exceeds 2.5 kg, in certain populations lay smaller eggs than layers with a slightly lower live weight; the egg production of hens with the highest and lowest live weight, as well as the hatchability of eggs obtained from them, is lower compared to the average indicators characteristic of the selected population.

The use of the laws of inheritance and variability of traits in each population during selection can lead to desirable changes in correlative relationships. As practice shows, lines of hens with a small live weight that require less feed and housing space but lay many large eggs are economically most expedient. Modern selection methods have produced lines of Leghorns possessing such qualities with sufficiently high heritability, repeatability, and correlation of the specified properties.

Heritability, variability, and correlations of quantitative traits are influenced by many factors and are most pronounced under feeding and poultry management conditions that correspond to the genotypically determined characteristics of the species, breed, or line of poultry. With insufficient and inadequate feeding or the violation of optimal management conditions, heritability decreases, primarily for traits associated with the reproductive abilities of poultry, which are of great economic importance, such as:

• egg production;
• hatchability;
• viability, etc.

Heritability coefficients are used to develop methods of selection and mating. Improvement of traits with a high degree of heritability, such as chick weight, egg weight, etc., can be achieved by selection based on the phenotype. Selecting fast-growing and fast-feathering chicks at an early age and raising them under rational conditions with subsequent use for reproduction allows for improving the meat qualities of poultry by the [—11] generation. The development of broad-breasted turkey breeds is largely due to selection based on pectoral muscle development. These examples show the possibility of rapidly increasing the highly heritable meat qualities of poultry during mass selection through selection based on phenotypic traits, mainly the live weight of young birds and conformation.

When developing meat lines and evaluating crosses for poultry breeding, it is necessary to take into account egg production, the yield of hatching eggs per hen, and the viability of the parent flock, alongside high meat qualities of broilers and feed efficiency. However, the insufficient correlation between highly heritable traits associated with meat productivity and lowly heritable egg production and hatchability does not allow breeding work in meat poultry farming to be based solely on mass selection by phenotype.

When breeding chickens for egg production, mass selection by choosing large hatching eggs leads to obtaining layers that produce heavier eggs than the birds of the initial population. But the correlation between heritable traits — the weight of eggs and chickens — leads to an increase in the weight of layers in a flock producing larger eggs, which is not always desirable for specialized egg-producing farms due to less efficient feed conversion by large chickens.

However, the study of such traits as live body weight of the bird and egg weight shows their variability and makes it possible to isolate for reproduction in the population chickens that lay large eggs but do not have significantly increased live body weight, and on this basis create breeding groups of chickens characterized by the following parameters:

One example of the practical application of genetic research results is the use of chickens carrying the dwarfism gene. Since it was established that dwarfism is a recessive trait, dwarf chickens have been crossed with Cornish roosters to produce broilers. According to the All-Russian Research and Technological Institute of Poultry Breeding, at 8 weeks of age they weigh about 1.5 kg with an expenditure of about 2.3 kg of compound feed per 1 kg of gain. White mini chickens obtained using chickens carrying the dwarfism gene weigh about 1.4 kg and lay eggs weighing 56 g or more.

Increasing egg production through mass selection methods is based on the evaluation and selection of chickens by constitution and exterior traits correlated with the intensity of sexual maturity, for example, the size of the comb in young pullets at a certain age, the condition of the pubic bones, molting of laying hens, and some others. An experienced poultry breeder can confidently select for further reproduction, based on these traits, chickens that lay more eggs, and in the next generation stock the flock with more productive birds. With individual recording of egg production, bird selection will be even more accurate.

However, the application of mass selection in this direction has limited significance due to the mismatch in many cases between the phenotype and genotype of the bird. Most often, chickens with an egg production of 280—300 or more eggs yield very few daughters that repeat the high productivity of their mothers; selection of roosters based on the origin from the best chickens, but with unknown heritability of this trait in them, is even less reliable.

Obtaining stable selection results is achieved through the use of selection and mating based on genetically determined and economically significant traits. At the same time, evaluation by phenotype with the mandatory selection of only healthy birds is part of the breeding and genetic work system.

In breeding work, selection and mating of birds based on the pedigrees of the best males and females have become widespread. Soon, however, it turned out that males and females of the same origin do not always pass on high productive qualities to their offspring. Evaluation and selection based on pedigrees have entered the practice of breeding work, but do not have decisive importance.

A bird usually has many daughters and sons, so its evaluation by the quality of offspring is significantly more reliable than by the productivity of a few ancestors. In addition, the bird is evaluated by the quality of the family — brothers and sisters, half-brothers and half-sisters obtained from the sire or dam being tested. This significantly increases the number of animals involved in breeding evaluation and increases the reliability of its results.

Studying the interaction between natural selection, which maintains population stability, and artificial selection, which develops it in a desired direction, opens up possibilities for developing coordinated plans for breeding work to increase the productivity and viability of the bird. The effectiveness of using population genetics methods is especially great in poultry farming due to the creation of large breeding farms that employ highly qualified geneticists, use modern scientific achievements, and conduct population analysis using statistical methods with the application of computer technology.

Since individuals within a population have their own individual characteristics, a close connection between these methods of evaluating selection results and evaluation and selection based on constitutional robustness is necessary. Population genetics and the application of modern selection methods allow for breeding birds in closed flocks. For this, the best specialized lines are used without importing birds from other farms. In this way, even without the use of close inbreeding, high genetic uniformity and differentiation of bird groups with good productivity can be achieved.

To obtain highly productive hybrids and crosses, lines possessing desirable qualities—high vitality, productivity, and efficiency of feed utilization in specific environmental conditions—are necessary. However, only by crossing certain such lines does the effect of heterosis arise and the most valuable hybrids and crosses are created. This is due to the compatibility of certain lines, genetically determined by combining ability, which results in the manifestation of the heterosis effect.

There are two types of combining ability:

• General combining ability, expressed as the heterosis effect when crossing a compatible line with various others.
• Specific combining ability, the realization of which makes it possible to obtain the heterosis effect when crossing a line only with a specific other one that is compatible with it.

In poultry farming, specific combining ability is of the greatest importance, as it is used to develop meat and egg lines of paternal and maternal forms. In practice, evaluation for general combining ability is often carried out based on the quality of progeny obtained from crossing the line under test with several others or with poultry from the selected population. Specific combining ability in this case is evaluated by traits expressed in the progeny from the crossing of individual lines under test.

In some cases, specific combining ability is evaluated based on the progeny of individual pairs, and in this respect, it has individual significance for the male and female. The development, mass reproduction of compatible lines, and their crossing are essentially the ultimate goal of the complex work involved in creating hybrid poultry, the use of which has enormous economic significance in industrial poultry farming for increasing egg and meat production.

Currently, a great deal of data has been accumulated on the high productivity of hybrid poultry. The egg production of hybrid hens reaches 240–280 eggs, and for the best layers, over 300 eggs per year. The weight of hybrid broilers is more than 1.6–1.7 kg; the survival rate of the stock is 97% and higher, and the feed expenditure per 1 kg of eggs or meat is approximately 2–2.5 kg. Hybrid poultry differs from the purebred stock from which it originates by a number of anatomical and physiological features. Experiments have established a genetically determined superiority of heterotic poultry in terms of the level of metabolic and secretory processes, activity of endocrine glands, content of nucleic acids, and certain hematological parameters. Studies have shown that in domestic hybrid hens obtained from interbreed and interline crossing, the egg formation period, determined by the time between consecutively laid eggs, is shortened, and the length of cycles, egg production, and the vitality of layers are increased.

X-ray studies have confirmed that the egg formation process in hybrid hens is accelerated compared to purebred ones. Some hybrid hens have laid two eggs on certain days. Heterosis can be expressed in one or, in some cases—which is more desirable—in several economically useful traits.

Table 15. Productivity of hybrid hens (according to data from the Poultry Department of TSHA)

Moscow hens, line G Leghorn line Hybrid hens Indicators Heterosis effect Hatchability, % 90.5 91.3 93.5 Survival of young poultry, % 93 93 95 Egg production for 500 days of life, 214 216 230 pcs. Without heterosis effect Average egg weight at 12 months 58 59 58 of age, g Live weight of hens, kg 2.3 1.8 1.9

In meat poultry farming, the most important indicators of the heterosis effect are growth rate, live weight, meat quality, vitality of the poultry, egg hatchability, and feed efficiency. In foreign poultry farming, the beginning of work on selection for heterosis is linked to attempts to apply the techniques used for creating hybrid corn to the hybridization of poultry.

To obtain the lines, close inbreeding mating of the brother-sister type was used for 4–5 generations. In this process, the vitality and productivity of the poultry experience significant depression. However, experiments and practice have shown that close inbreeding over several generations and the selection for further reproduction of only a small number of combinations that can withstand its "pressure" make it possible to maintain the level of productivity characteristic of the poultry of the original lines. Testing inbred lines for compatibility and crossing the best of them made it possible to obtain the heterosis effect for one or more economically important traits. Already in the first years of hybridization, a number of compatible inbred lines were developed in the USA, and the realization of the resulting hybrids was measured in hundreds of millions of chicks.

But as data accumulated, it became clear that the development of inbred lines in poultry farming is significantly more complex than in plant production,

S.I. Smetnev 97 and is possible only on the basis of large farms, with the expenditure of significant funds and time. Organizational and economic difficulties arise in connection with the need for a significant number of matings to evaluate the compatibility of lines when producing three- and four-line hybrids. Summarizing the data, it can be considered that with inbreeding, in 37.5% of cases, hatchability decreases by 14–17%, the vitality of chicks—by 11–12%, and of hens—by 7–8%, and egg production—by 8–17%. It must be noted that all these traits are weakly inherited. Indicators of live weight and egg weight—traits with a high heritability coefficient—decrease to a lesser extent. A significant increase in inbreeding leads to inbreeding depression with a weakening of the constitution and a decrease in the vitality and productive qualities of the poultry.

New opportunities for obtaining hybrid poultry without the need for expensive and time-consuming work on developing inbred lines have been opened up by the development of reciprocal recurrent selection and its widespread use in practice. Reciprocal recurrent selection is based on the evaluation of the combining ability of lines and parental pairs to obtain offspring with a high heterosis effect. The lines, families, and individuals selected for breeding are studied for compatibility in a system of reciprocal matings—males and females of one line with males and females of another—and evaluated based on the quality of the hybrid offspring. Poultry with the best combinations is propagated for the purpose of obtaining sire and dam lines and for use in mass crossings for egg or meat production (p. 127).

To increase the efficiency of reciprocal recurrent selection, it is necessary to solve theoretical and practical tasks related to this method of breeding work. For example, when evaluating the compatibility of poultry based on the quality of hybrid offspring, time intervals arise for further selection and mating in the system of reciprocal matings, which slows down the achievement of breeding results. To accelerate this, research is being conducted on the use of evaluating lines and pairs based on the offspring's egg production over 270 days of life, which correlates with annual productivity. As a result, it becomes possible to carry out two successive cycles of reciprocal matings within 1—1 1/2 years with an evaluation of the breeders based on the quality of the hybrid offspring. During the same period, expanded reproduction of the poultry with the best combinations and their transfer to production are carried out.

A known difficulty is the need for a significant number of crosses to test lines in four-way crosses; for example, from 10 lines, one can obtain dozens of simple and several thousand combinations of a four-way cross. In this regard, tasks arise for the development of methods for predicting the productivity of crosses to ensure the most well-founded selection of lines and pairs when crossing. Scientists suggest the potential for using immunogenetic methods for this purpose, as well as the need for other research in this direction. Searches for new methods of selection for heterosis are underway. For example, at the Department of Poultry Science of the TSHA, a method was developed for obtaining hybrid poultry using artificial insemination and comparative evaluation of purebred and hybrid offspring from the same hens and roosters.

Fundamentals of poultry breeding in relation to the interaction between genotype and environment. The genotype, by determining the norm of reaction of the organism and the direction of trait development, finds expression in the phenotype, which arises under the specific influence of feeding and keeping conditions in the process of development, selection, and mating of poultry possessing predetermined desirable qualities. In the production of eggs and meat, poultry adapted to specific conditions is of the greatest economic importance; therefore, the task of selection includes the creation of breeds, lines, and the production of hybrids for specific conditions of their use. At the same time, it is necessary to ensure appropriate environmental conditions for the created genotypes. In breeding work, one must keep in mind the relatively small influence of external factors on highly heritable qualities, but a much more significant influence on such essential traits as egg production, egg fertility, poultry viability, etc. Therefore, it is advisable to conduct selection, especially for these indicators, under conditions corresponding to its use in industrial poultry farming.

In large specialized farms, thanks to the regulation of the microclimate, conditions are created that contribute to obtaining high poultry productivity in the Far North and the south of our country, in varying climates. However, there are zonal features that must be taken into account when developing technology for feeding and keeping poultry in a given area, which will allow for an increase in the efficiency of poultry product production. In this regard, it is very important to use the principles of genetics regarding the interaction between genotype and environment when developing new breeds, lines, and obtaining hybrid poultry that is most profitable in intensive poultry farming and in different zones of the country.

In industrial poultry farming, great importance is attached to developing lines and obtaining hybrid poultry with high viability and productivity under cage housing conditions. Experiments have shown the possibility and practical application of hen reproduction in cages using artificial insemination or mating with cage-housed roosters. The next task is the development of selection methods using modification variability, which arises under the influence of the interaction between genotype and environment, for developing lines and obtaining hybrids adapted to the conditions of intensive poultry farming. In this direction, scientific institutions have for a number of years been conducting work on the reproduction of various hen lines and interline crossing in cages, in comparison with traditional methods of breeding by lines and hybridization when keeping hens on the floor in poultry houses. When reproducing hens of egg lines in cages, it is necessary to take into account specific features

4+ 99 features of individual lines and to use those most adapted to intensive keeping conditions in practice.

The influence of environmental conditions on the genotype is diverse. Among various factors, air temperature is of no small importance. The populations of livestock animals (poultry) subjected to research are characterized by heterogeneity in thermoregulatory ability.

In the foothills of Crimea, with high air temperatures in the summer months, a decrease in egg production is observed in breeding poultry, which also affects annual egg-laying. In studies conducted at the selection and genetic station of the Ukrainian Research Institute of Poultry Breeding by Doctor of Agricultural Sciences V. A. Sergeyev, a positive correlation was established between egg production during the hot summer months and the annual total, and selection for the adaptation of hens to local climatic conditions was initiated.

Over four years of selection and pairing of poultry with high productivity in the summer months, the egg production of Russian White hens changed as follows:

Crossbreeding the created lines made it possible to obtain hybrid poultry with an average monthly egg production of 22 eggs per layer during the hot season.

When raising broilers under high-temperature conditions, it is possible to identify birds that significantly outpace the rest of the stock in growth rates. In experiments at temperatures in poultry houses above 25—30 °C, about 10—15% of individuals were identified that exceeded the average parameters of the batch in live weight. Such birds develop faster in hot conditions. It is advisable to use them for the selection of meat lines intended for regions with long hot summers.

Productivity of the herd is also actively influenced by light and diet. In experiments on feeding poultry with feed containing reduced levels of protein, lysine, and other nutrients over several generations, it was found that individual groups successfully adapt to the deficiency. They retain the basic productive qualities of the original forms. This proves that the poultry genotype determines its adaptability to changing environmental conditions.

• Broiler selection temperature — above 25—30 °C
• Share of fast-growing poultry in heat — 10—15%
• Total population of high-quality poultry — more than 65 million heads

Domestic Gene Pool and Global Productivity Limits

For the successful development of new lines and hybrids, the poultry industry requires a constant expansion of the gene pool. Work is being carried out with the entire diversity of breeds, crosses, and breeding herds that are promising for propagation. Today, dozens of specialized lines of egg-laying and meat-type hens, as well as turkeys, ducks, and geese, are used in farms.

At the same time, in large specialized farms in a number of foreign countries, the growth of egg production has practically stopped, reaching a ceiling of 250—260 eggs. Differences in economic indicators between commercial crosses at competitions are gradually disappearing. A similar trend toward slowing growth rates of productivity is also observed in meat poultry farming, especially in the production of broiler chickens.

The decline in productivity growth rates is caused by the following factors:

• breeding a limited number of breeds and lines supplied by large commercial companies;
• reduction of genetic variability of poultry;
• decrease in the heterosis effect when crossbreeding genetically similar lines.