Conditions and Implementation Methods for Purebred Selection and Breeding of Yellow Cattle
The yellow cattle is an indigenous ordinary cattle breed of China. Its number of raised heads in China ranks first among all large livestock or among cattle. Its raising areas cover almost the whole country. In agricultural areas it is mainly used for draft, in semi-agricultural and semi-pastoral areas it is dual-purpose for draft and milk, and in pastoral areas it is dual-purpose for milk and meat. In terms of body shape and performance, differences arise from different natural environments and raising conditions, and it can be divided into 3 major types: northern yellow cattle, Central Plains yellow cattle and southern yellow cattle. The coat of yellow cattle is mostly yellow, and the breed may have been named for this reason, but there are also red-brown and black, etc. The head is slightly heavy and thick, the horn shape varies, and the base of the horn is round. The constitution is robust, the structure is compact, the muscles are well developed, the limbs are strong, and the hoof quality is firm. There are about 25 breeds of yellow cattle in China, and the top five are: Nanyang cattle, Qinchuan cattle, Luxi cattle, Yanbian cattle and Jinnan cattle, collectively known as China's five major improved yellow cattle breeds.
Purebred selection and breeding is also called selection and breeding within the breed. It refers to the method of continuously improving the quality and production performance of the cattle herd within a cattle breed through selection, matching and rearing. Many yellow cattle breeds abroad and many local yellow cattle breeds in China were developed through this method; existing yellow cattle breeds still need to continue implementing the purebred selection and breeding system to increase the production benefits of yellow cattle. In the yellow cattle industry, it is still necessary to rely on purebred breeding to provide breeding bulls, as well as to provide large numbers of feeder cattle for fattening.
(1) Conditions for purebred selection and breeding. Purebred selection and breeding is generally carried out under the following circumstances.
1. Local meat-type breeds. Such a breed has relatively uniform body conformation and appearance and relatively high production performance, and has stable heritability. In order to further improve its production performance and promote greater uniformity in its body conformation and appearance, the method of purebred selection and breeding is needed to consolidate and enhance certain excellent traits. This method is also called improved breed selection and breeding. Examples include China's excellent local breeds such as Qinchuan cattle, Nanyang cattle, Jinnan cattle, Luxi cattle, Yanbian cattle, Fuzhou cattle, Jiaxian red cattle and Bohai black cattle, etc. Through purebred selection and breeding, some defects in appearance and structure are gradually corrected and their production performance is improved.
2. Excellent yellow cattle breeds. Such breeds have been developed by people into specialized yellow cattle breeds, have very high meat production performance, and have stable heritability. In order to increase population numbers, maintain breed characteristics and continuously improve quality, planned purebred selection and breeding is needed. Examples include foreign yellow cattle breeds such as Charolais cattle, Limousin cattle, Angus cattle, Piedmontese cattle, Simmental cattle, Hereford cattle, Shorthorn cattle and other improved breeds. All need to adopt the method of purebred selection and breeding to maintain pure breeding and expand the population, so as to meet the needs of promotion and crossbreeding.
3. Local meat-type breeds. Although such breeds do not have high economic value and cannot fully meet people's needs, certain traits and characteristics, such as adaptability to the environment, disease resistance, tolerance of coarse feed, and certain or relatively prominent advantages in some aspect of production performance, require that a necessary number be retained within a certain region as a gene pool for crossbreeding. This method is also called conservation selection and breeding.
4. Meat-type hybrids. After such hybrids enter the stage of intercrossing and fixation, purposeful selection and matching are needed to fix their excellent traits, so that the quality of the whole herd is further improved and tends toward uniformity. Although the breeding work at this stage is not selection and breeding within one breed, it is similar to the method of selection and breeding within a breed; this selection and breeding method is also called breeding within the group.
(2) Mating methods for purebred selection and breeding groups. It mainly includes two aspects: consanguineous breeding (inbreeding) and strain breeding.
Consanguineous breeding: Consanguineous breeding refers to mating combinations between bulls and cows that are related by blood. Inbreeding is also called close breeding. The parameters reflecting the degree of close relationship are the inbreeding coefficient and the relationship coefficient. The inbreeding coefficient is a measure indicating the reduction in the proportion of heterozygous genes. Consanguineous breeding can not only purify certain excellent genes, but can also make certain recessive unfavorable genes homozygous and exposed, appearing as undesirable traits and thus producing undesirable individuals, such as smaller body size, weaker constitution, reduced production performance, abnormal development, etc. The methods to prevent inbreeding depression are: first, use close mating only 1 time or 2 times, then use moderate or distant mating to maintain its excellent traits; second, strictly select and prevent mating of bulls and cows that share common defects; third, strictly cull those undesirable individuals with mutant types and recombined homozygous inferior genotypes.
(3) Strain breeding. Strain breeding is an advanced stage of breeding work and is a commonly used breeding method in purebred selection and breeding. Its characteristic is to purposefully cultivate differences in type within the cattle herd, so that the beneficial traits of the herd continue to be maintained and expanded into the offspring. For individuals or groups that perform outstandingly in a certain aspect, using the method of homogeneous matching can continue to maintain the excellent traits of the breed in this aspect. Using this method, several strains are established within one breed, each strain has its own characteristics, and later through combination (crossing) between strains, the entire cattle herd can be improved in many aspects. Therefore, strain breeding can not only achieve the purpose of maintaining and consolidating the excellent characteristics and traits of the breed, but can also combine these excellent characteristics and traits in individuals.
1. Methods and steps for establishing a strain
(1) Creating and selecting the strain ancestor. The primary issue in establishing a strain is cultivating the strain ancestor; only with a strain ancestor can the strain be established. The strain ancestor must be an outstanding excellent breeding bull; it must not only perform well itself, but also be able to pass on its own excellent characteristics and traits to its offspring. If the characteristics and traits of the strain ancestor are not prominent, especially if heritability is unstable, then when matched with homogeneous cows, the offspring produced will not necessarily all have the characteristics and traits of the strain. Therefore, when an ideal strain ancestor has not yet been found in the cattle herd, one should not rush to establish the strain, but should begin by actively creating and cultivating a strain ancestor. When cultivating a strain ancestor, one may select several cows meeting the strain requirements from the breeding cow herd or nucleus herd, match them with a relatively ideal breeding bull, and through rearing and progeny testing of the resulting male calves, select five and keep one, to establish the strain. In order to avoid possible unstable heritability in the offspring of the strain ancestor, in the process of creating and cultivating the strain ancestor, inbreeding matching may be appropriately adopted to consolidate heritability. However, in order to prevent reduced viability of offspring due to inbreeding and genetic defects due to the combination of certain unfavorable genes, father-daughter or mother-son mating should be avoided, and the inbreeding coefficient should generally not exceed 12.5%.
(2) Carefully selecting the foundation cows of the strain. Once there is an excellent strain ancestor bull, individual matching can be carried out with homogeneous cows. The homogeneous cows to be matched must be carefully selected; only cows meeting the strain requirements can be mated with the strain ancestor bull. The foundation cows of this strain must also be of a considerable number. Generally, when establishing each strain, there must be at least 100—150 adult cows available for establishing the strain, because the more foundation cows available for establishing the strain, the more the role of the breeding bull can be brought into play, especially in the case of breeding with frozen semen, when the number of foundation cows of a strain should be greatly increased.
(3) Selecting and breeding the successor of the strain ancestor. In order to maintain an established strain, the successor of the strain ancestor must be cultivated. Generally speaking, the son of the strain ancestor bull can serve as the successor of the strain. Cultivating the successor of the strain ancestor must also follow the requirements for cultivating the strain ancestor bull, selecting an outstanding breeding bull through progeny testing. Because the generation interval of cattle is relatively long, after establishing the strain LK, attention must be paid early to cultivating and reserving the successor of the strain bull. Generally, a strain gradually disappears after continuing for three generations. If there is no need for this strain to exist, there is no need to consider the issue of strain continuation. If a particularly valuable strain needs to be retained, the following methods should be adopted: 1. continue the established strain by selecting and reserving offspring bulls with more kinship to the original strain ancestor as successors of the strain. 2. re-establish a corresponding new strain, that is, re-cultivate a strain ancestor.
2. Combination of strains. The establishment of strains is to increase the internal differences within the strain and maintain the rich heredity of the breed. The combination of strains (that is, crossing between strains) is to increase the uniformity of the breed. Through the combination of strains, the excellent characteristics and traits among strains are mutually supplemented, so as to improve the quality of the cattle herd.
3. Using topcross to prevent inbreeding degeneration. Strain breeding is an important means of selection work. However, inbreeding often produces offspring decline. In order to solve this problem, the methods that can be adopted are: first, control the degree of inbreeding propagation, and the inbreeding coefficient should not be too high; in addition, when inbreeding degeneration occurs, allow inbred bulls to mate with unrelated cows, and obtain heterosis effects within the same breed, so as to achieve the purpose of strengthening the physique of the cattle herd and improving production performance. This is called “topcross”.
4. Several common strain categories. A strain developed from a breeding bull with outstanding performance is also called a single line. In addition, because the methods and purposes of strain establishment have different emphases, there are also the following strain categories.
(1) Inbred line. An inbred line is a group of cattle developed through continuous mating between siblings; its inbreeding coefficient often exceeds the average inbreeding coefficient of the large herd by dozens of times, such as reaching as high as 20% or more.
(2) Group strain. Cattle with similar excellent traits are first formed into a foundation group, without considering whether they are siblings or closely related individuals. Then closed breeding within the group is carried out to consolidate and expand the group with these excellent characteristics and traits. This method of strain establishment is also called the population successive generation method, and the developed strain cattle group is called a group strain.
(3) Specialized strain. A strain that has special performance in a certain aspect and is used specifically for crossing with another certain strain is called a specialized strain.
(4) Local strain. In breeds with a relatively wide distribution and a relatively large number of breeds, different local groups within the breed are often formed due to differences in natural geographical conditions, feed types and management methods in various places, as well as differences in regional selection standards; these are called local strains. For example, in China's yellow cattle breed Bashan cattle, there are local strains such as Xuanhan cattle, Xizhen cattle, Pingli cattle and Miaoya cattle.