Systems of Breeding and Mating in Poultry

Systems of Breeding and Mating in Poultry: Inbreeding, Outbreeding & Crossbreeding

Breeding and mating systems in poultry are important tools in genetic improvement programs for manipulating homozygosity, heterozygosity, and the expression of desirable traits. Depending on the breeding objective, different systems such as random mating, inbreeding, assortative mating, outbreeding, and crossbreeding may be used. Understanding these systems is essential for developing improved poultry lines, exploiting hybrid vigor, maintaining genetic diversity, and producing birds with desirable production and reproductive characteristics.

Breeding systems can be classified according to whether they aim to increase homozygosity or heterozygosity. They include panmixia (random mating), breeding for increased homozygosity, and breeding for increased heterozygosity.

Uncontrolled Breeding/Random Mating

Random mating means that any individual of one sex has an equal chance of mating with any other individual of the opposite sex in the population. It is the most commonly used method in selection experiments because it keeps inbreeding to a minimum. Random mating is used to develop a control population, which is required to compare and measure the effects of other breeding systems.

Controlled Breeding Methods

These can be of two types: breeding for increased homozygosity and breeding for increased heterozygosity.

Breeding for Increased Homozygosity

Homozygosity in a population can be increased by either practicing intense inbreeding or assortative mating.

Inbreeding (Disassortative Type)

Inbreeding is defined as mating between individuals that are more closely related to each other than the average relationship between all individuals in a population; i.e., inbreeding means the mating of individuals that are related to each other by ancestry.

Objectives of Inbreeding

• To develop closely inbred lines for further crossbreeding.
• To develop more homogeneous lines (for nutritional studies such as bioassays). The individuals receive common genes from each of the parents because of common ancestry; i.e., homozygosity increases progressively as a result of inbreeding.

Advantages

• Inbreeding eliminates undesirable recessive genes from the breeding stock.
• Since desirable genes are often dominant, good-quality animals/poultry are often prepotent, i.e., they have the ability, whether male or female, to perpetuate a given set of characteristics in their offspring. Since a single sire is used to breed a number of dams, the term is usually applied to sires.
• Inbreeding is the only way to increase prepotency.
• Through inbreeding, distinct lines or families can be developed from the foundation stock.
• Inbreeding combined with selection over periods of time has resulted in many valuable breeds of domestic animals.
• Inbreeding combined with selection is practiced to develop elite inbred lines of poultry for producing hybrids.

Disadvantages of Inbreeding

• Undesirable traits appear with increasing frequency as the intensity of inbreeding increases.
• Growth rates are reduced by inbreeding.
• Reproductive efficiency is reduced.
• Inbred animals/poultry have lower vitality.

There are three distinct inbreeding methods that can be carried out consistently for several generations. They are:

• Close Inbreeding: Mating between siblings or parents and progeny (incest) to form inbred lines. Full-sib mating or backcrossing of the progeny to the younger of their parents is often practiced. Heterozygosity is reduced to 12% or less in 10 generations.
• Strain Formation: Developing a small group of birds within a breed and variety with a specific characteristic in view. This is a mild form of inbreeding, e.g., the Babcock strain of SCWL (Single Comb White Leghorn), developed to lay heavier eggs.
• Line Breeding: This is inbreeding within an ancestral line and is the most intensive form of backcrossing. Line breeding is backcrossing to the same parent for several generations in succession.

Assortative Mating/Mating Based on Phenotypic Resemblance

Assortative mating refers to mating likes with likes. When practiced among selected individuals, the response is usually greater than with random mating. However, this method of mating leads to inbreeding in the long run. In this case, not identical genes, but genes with similar effects are brought together. This can be practiced when the heritability (h²) of the trait(s) in question is high.

Breeding for Increased Heterozygosity

Outbreeding is practiced to increase heterozygosity in the population.

Outbreeding

Outbreeding is defined as the mating of individuals that are less closely related than the average relationship within the population. Crosses among breeds, varieties, and strains/lines are different types of outbreeding. Its main purpose is to overcome the deleterious effects of inbreeding that arise due to small flock size and, at times, to introduce new desirable characteristics. In outbreeding, however, care should be taken not to introduce germplasm from an inferior flock.

Terms Used in Outbreeding:

• Crossbreeding: Crossing among breeds is usually called crossbreeding.
• Strain Crossing: Crossing between strains is called strain crossing.
• Incrossing: A cross between two inbred lines belonging to the same breed is called an incross; if they belong to different breeds, the progeny is considered crossbred.

Topcrossing is a method of outbreeding in which inbred males of one line are crossed with females of an outbred population. Commercial crosses used for egg and meat production may result from crossing two or more lines, strains, or breeds. When a cross involves two lines, it is a single cross; a three-line cross is a 3-way cross; a four-line cross is a 4-way cross; and so on. All crosses except single crosses are known as multiple crosses. Most commercial layers and broilers developed in this country are single crosses. However, commercial chicks sold in the market from parent and grandparent hatcheries working in collaboration with foreign countries are more often the products of multiple crosses.

The methods of outbreeding are:

• Single Cross/Two-way Cross: Two different populations (inbred lines, strains, or breeds) are crossed to produce a first filial (F₁) generation, which is used purely for commercial purposes and not for breeding. F₁ progeny usually exhibits hybrid vigor, especially when inbred lines are involved. When two inbred lines of the same breed are crossed, the progeny is said to be an incrossbred.
• Multiple Crosses:
  • Three-way Crosses: In this F₁ crossbred females are mated to males of a third line, strain, or breed to utilize the hybrid vigor of the dams.
  • Four-way or Double Crosses: Two different single crosses, AB and CD, are crossed to obtain the final progeny. This is usually practiced in poultry breeding for crosses between inbred lines of low viability, since only a relatively small number of birds from lines A, B, C, and D need to be maintained.
  • Other Multiple Crosses: Depending on the number of pure lines entering into the crossing, multiple crosses may be 8-way, 16-way, 32-way, or other higher-order crosses.
• Backcross: Crossing F₁ females with males of one parental population. This is especially useful when F₁ females are better mothers than females of the parental population (maternal influence). This is commonly practiced in pig breeding.
• Rotational Crossbreeding: Crossbred females are mated with males from either of the populations; the males are alternated in each generation. This is also usually practiced in pig breeding.
• Topcrossing: Mating of inbred males with females of non-inbred populations.
• Crossing for Reciprocal Recurrent Selection
• Grading Up: Grading up involves backcrossing to the same superior breed for several generations so that a population of mixed breed can be changed to a “pure breed.”
• Crossing for Production of a New Breed: Similar to grading up, this aims at increasing homozygosity. Different breed types have been crossed to produce modern-day breeds of poultry and livestock so as to combine desirable traits from multiple sources. These foundation crosses have to be subjected to inbreeding combined with selection to consolidate them into true-breeding populations (breeds). E.g., Cornish was developed from Aseel, Malay, and English Game breeds. The Campbell breed of duck was developed by crossing Aylesbury and Indian Runner.
• Diallel Mating: This is a mating system in which crosses are made in all possible combinations. Suppose there are 4 strains/families; 4 × 4 crosses will be made to identify the best combining ability, especially in terms of Specific Combining Ability (SCA) and General Combining Ability (GCA). SCA is the ability of a strain to “nick” or combine well with another particular strain in the crossing program. GCA is the ability of a strain to combine well with all the other strains.