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Introduction of laying hen breeding methods

2021-01-04

Laying hens are selected and bred to determine the combining ability of different strains to select the best hybrid combination to produce high-yield commercial hybrid chickens. Conventional breeding methods include inbreeding breeding method and closed group family breeding method.
 
1. Inbreeding
 
Inbreeding can make genes homozygous, fix both beneficial and harmful genes, and expose and remove harmful genes. The result of inbreeding will cause decline, which will reduce the egg production, viability, and fertility of chickens, so most families and strains have to be eliminated in future breeding. Moreover, in order to determine the combining ability through positive and negative hybridization and select the best hybrid combination, a large number of inbred lines must be established, which requires a lot of chicken coops and funds. For example, if there are 10 inbred lines, if two lines are crossed, 90 cross combinations must be tested; if three lines are crossed, 720 cross combinations must be tested; if four lines are crossed, then 90 cross combinations must be tested. 5040 hybrid combinations were determined. With so many combined measurements, it is difficult to complete without chicken coops, manpower and financial resources. Therefore, a good line is bred and sold at a high price.
 
Although inbreeding has declined, and due to low viability, most families are naturally eliminated due to low fertility. The surviving inbred lines, through the cross between inbred lines, produce obvious heterosis, which is the fundamental reason why inbreds are still used by breeders. In order to avoid the adverse consequences of inbreeding and the economic loss caused by inbreeding, breeders have studied other methods of producing heterosis. This is the closed group family selection method with certain genetic homogeneity and characteristics.
 
2. Breeding method of closed group family
 
Closed flock breeding is a kind of breeding of families with 4 to 5 generations or more according to a certain trait without introducing any extra blood after the introduction of the flock, and avoiding inbreeding. Such chicken flocks have a certain genetic homogeneity and are different from other flocks. Because the closed breeding method is relatively safe and will not cause harm to production, it is widely used. Experts believe that if there are 20 breeding families in a closed line, normal breeding can be carried out to avoid a significant increase in the inbreeding coefficient, and certain genetic progress can be made. If the family of locked lines is too small, several generations of selection will inevitably lead to inbreeding.
 
In order to prevent inbreeding during closed breeding of strains, 5-7 line ancestors should be established in each line to form 5-7 small lines. When each generation reproduces, a small line of roosters mate with another small line of hens. In each generation, 20% to 40% of hens and 2% to 3% of roosters are selected for breeding. When choosing a hen, consider both the performance of the family and the performance level; when choosing the rooster, take into consideration the performance of the family and the performance of the siblings. The more families, the greater the intensity of selection and the faster the genetic progress.
 
When foreign breeding companies select and breed matching hybrid chickens, they generally purchase chicken breeds from various countries and participate in random sampling to determine the best hybrid chickens based on the information they have. On this basis, hybridize, use inbreeding or lock-in method to establish a new line with specialty in a certain trait, namely synthetic line. Each company generally has 40 to 60 such lines. Extensive collection of these strains allows the company to quickly respond to market needs and changes. The most common breeding programs are as follows.
 
The selected lines are crossed with each other according to the random method, that is, random mating.
 
Individuals with the best combination of beneficial traits are selected from the hybrid offspring. These individuals become the line ancestors of the new strain, and then mate with different roosters and the same hen. Two-line hybrids must undergo at least two repeated tests, and then select the parents of the three-line and four-line hybrids. The multi-line hybrid chickens are first tested by the company, and then they will participate in the random sampling test at the international testing station. If successful, the commercial hybrid chickens will be sold to users.
 
Breeding companies generally stock good-performing strains. Therefore, it is generally not bred to produce a fully stereotyped hybrid, but only to replace a certain line in the existing matching combination at a certain time. A basic principle is that special male and female parents must be separated, which can be pure or single-cross. In general, the breeding traits of male parent lines are egg production, egg weight, eggshell quality, fertilization rate, combining ability with the female parent, and breeding rate; the breeding traits of female parent lines include sexual maturity, egg production, Peak and intensity of egg production, stability of egg production curve, hatching rate, fertility rate and survival rate.
 
At present, in order to meet the needs of the market, foreign breeding companies have introduced both white shell and brown shell layers. Because the brown-shelled layer and the white-shelled layer are of equal status. When breeding and improving egg hybrid chickens, the positive and negative repeated breeding method is used most. For this, there must be at least two lines, each selected for different traits. In order to maintain the A line, it is necessary to determine the formation of the parent group based on the production performance of the A line × B line hybrid progeny; to maintain the B line, the parent group shall be formed based on the production performance of the B line × A line hybrid progeny. The essence of the positive and negative breeding method is to identify and fix the genes that can produce heterosis among the tested strains. This method is the same as the closed method when breeding pure lines, so it does not cause high relatives. Compared with the inbred line hybridization method, the cost is not high. But the positive and negative breeding method also has certain difficulties. Because the performance measurement results of the offspring of the hybrids can be determined to determine the group reproduction and succession of the pure line, it is necessary to use two-year old chickens. In addition, breeder farms have to test a large number of hybrid offspring, which will reduce the utilization of pure line breeding eggs.
 
In recent years, many people believe that cultivating new hybrid chickens and multiple hybrid methods are the most promising. The first step is to establish a multivariate hybrid population; the second step is to breed the hybrid population from the group for 2 to 3 generations; the third step is to establish several lines to determine the general combining ability and special combining ability to find the best hybrid combination. If successful, even if a new high-yielding hybrid is cultivated, this is the synthetic line breeding method.


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