Hybrids produced by crossbreeding of different populations of pigs are often superior to pure breeding populations in terms of viability, growth potential, and production performance. This is the hybrid advantage of pigs. The utilization of heterosis has become increasingly important for the development of modern hog production. In China, the use of hybrid superiority is being achieved through the use of "dual localization of sows, improved reproduction of boars, and hybridization of boars" to "sow hybrids". The development of ternary crossbreeding of boars with high product lineage and ternary hybridization of commercial pigs. This is a correct guideline for the production characteristics of pigs, which makes extensive use of heterosis, and gives full play to the potential for yield increase. The advantage of hybridization depends mainly on the parents used in hybridization and their interactions. If the parental population lacks good genes, or the parental purity is poor, or the two parental groups have little difference in the frequency of the major economic traits, or in the major traits, the dominant and epistatic effects of the genes in the two parental groups are small. Or the hybrids lack the breeding and management conditions to fully exert their heterosis, they can not show the ideal heterosis. It can be seen that there are a series of necessary measures for the utilization of heterosis in pigs, including the following six key technologies. 1 Selection and purification of hybrid parental populations This is one of the most fundamental aspects of hybrid superiority utilization. Hybrids must obtain good, high-yielding, dominant and epistatic effects from their parents in order to produce significant heterosis. “Preferred selection†means that the frequency of good, high-yielding genes originally made in the parent population is increased as much as possible. "Purification" means that by selecting and inbred, the genotype frequencies of the homozygote of the parental population in the major traits are increased as much as possible, and the inter-individual variability is reduced as much as possible. The importance of purification is no less than selection, because the purer the parental population is. The difference in gene frequency between the two hybrids can be greater, and the practice of not using simple hybridization based on pure breeding is wrong. Pure breeding and hybridization are the two processes that promote each other, complement each other, and are based on each other in the process of advantage utilization of hybrids. The preferred method of purification is strain breeding. Its advantage is that the strain is smaller than the variety, easy to select and purify, which is beneficial to shorten the breeding time, and is conducive to improving the consistency of the parent group. Can better meet the requirements of modern pig production. For example, China's Xinhuai pigs, Guanzhong black pigs, and Xiaomeishan pigs are all available good pigs. 2 Selection of Cross Parents Cross parents should be selected according to the parent and the female parent. The selection criteria are different and the requirements are different. 2.1 The choice of female parent: (1) The species or strain with a large quantity and adaptability in the region should be selected as the female parent, because the quantity of female parent needs is large, and the breeding source problem is very important; it is easy to adapt in the region. Grassroots promotion. (2) Selection of breeds or strains with high fecundity, good maternality, and high milking power should be used as the female parent, which is related to the survival and development of hybrid offspring during embryonic and lactation periods, thus affecting the performance of heterosis, and at the same time with the production of hybrids. The cost reduction also has a direct relationship. (3) The size of the female parent should not be too large, and the size of the female parent should be too large to waste food and increase the cost of feeding. The above items should be flexibly applied according to the actual situation in the area. 2.2 Selection of male parent: breeds or strains with fast growth rate, high feed utilization rate, good carcass quality, and the same type of hybridization requirement as 2.3 should be selected as male parent. Those with these characteristics are generally highly cultivated varieties, 2.4 such as Landrace, Great White, and Duroc. The traits of these traits are highly heritable, and the excellent characteristics of 2.5 sires are likely to be inherited to hybrid offspring. As for the problem of adaptability and breeding stocks, 2.6 can be considered as a secondary consideration. 2.7 Because the number of male parents is small, 2.8 appropriate care costs are not 2.9. 3 Prediction of cross-breeding effects The cross-breeding effects of different populations vary greatly. In the end, it must be determined through the determination of combining ability. However, combinatorial power measurement is costly and time-consuming. There are many varieties of pig breeds. It is not possible to conduct a hybridization test between the two. Therefore, before the determination of combining ability, there should be a rough estimate. Only those hybrid combinations that are expected to be larger than others will be formally determined by others. This may save a lot of manpower and material resources, help the development of heterosis utilization, and estimate the effectiveness of hybridization. The following points. 3.1 Where there are large differences between populations, heterosis is often greater. In general, hybrids that are distant from each other in the distribution area, have large differences in sources, and have different types and special characteristics can obtain greater heterosis because the major traits of such populations often have large differences in gene frequencies, and thus the hybrids The advantage is also greater. 3.2 Long-term isolation of interspecific populations generally results in greater heterosis. There are two main types of isolation: one is geographical isolation, such as Lantang pig in Zijin County, Guangdong; the other is the isolation of breeding methods. Some are consciously closed group breeding, some are unconscious habits. 3.3 The heredity is low, and the traits with more severe recession during inbreeding have greater heterosis. Because the genes that control this type of traits have greater non-additive effects, the population average will increase with the increase of heterozygote frequency after hybridization. 3.4 Populations with small coefficient of variation of major economic traits generally have better hybridization effects. Because of the uniformity of the population, it can reflect the homozygosity of the genotypes of its members to some extent. 4 Combining ability to determine the combining ability is the degree of heterosis that the population can obtain by crossing, that is, the quality and effectiveness of the hybridization. Since the combining ability among different groups is very different, as long as people have not yet found a short cut that can accurately predict heterosis, the determination of combining ability through hybridization tests is still a necessary method for selecting ideal hybrid combinations. 4.1 Determination of combining ability: The formula for determining combining ability is: heterosis rate = [(hybrid generation average - parent average)/parent average] l100%. 4.2 The following points should be noted when determining the combining ability: (1) Pigs often do the determination of the combining ability of fattening performance. In the hybridization test, the selection of the experimental pigs, the start and end of the test, the arrangement of the prediction period, and the feeding level Feeding methods, as well as weighing and recording, should be carried out in accordance with the provisions of the fattening test. (2) Each test must have the pure parental group of all parents involved in the cross as a control. (3) Pay attention to the representation of the respective groups in the test group and the control group and minimize the sampling error. (4) The determination of combining ability should be conducted under the conditions of feeding and management similar to the promotion area. 5 The purpose of hybrid crossbreeding is to match the genes of each parent together to form new and more favorable genotypes. There are many ways of crossbreeding pigs. Here are four commonly used hybridization methods in China. 5.1 Binary crossbreeding: Also known as simple crossbreeding, the male and female pigs of two breeds or strains are used for crossbreeding, and all hybrid offsprings are used as commercial finishing pigs. Advantages: Simple and easy to use, when screening hybrid combinations, only one test of combining ability is needed; all offspring heterosis can be obtained and the offspring adaptability is strong. Therefore, this is a hybridization method widely used in China. The disadvantage is that no maternal and paternal hybrids can be used. Because both parents are purebred, the hybrids are all used for fattening. 5.2 Three-way crossbreeding: From the hybrids obtained from the binary crossbreeding, good individuals are selected as female parents and then hybridized with boars of another breed. The boar cultivar used for the first hybridization is called the first male parent, and the boar used for the second hybridization is called the second male parent. Advantages: All offspring heterosis and maternal heterosis can be obtained, which can not only give full play to the advantages of hybrid sows in reproductive performance, but also make full use of the advantages of first and second parents in finishing performance and carcass quality. . Its effect is generally better than that of binary hybridization and is becoming the development direction of China's pig production. Disadvantages: The three-way crossbreeding system is more complex, not only to maintain the purity of three parental species, but also to retain a large number of hybrid sows, requiring two times the determination of combining ability. 5.3 Bisexual crossbreeding: In the hybrid generation, excellent female parents are selected, and the female breeds and the pure breed boar of another breed are alternately bred by turns. Advantages: You can always keep 1 maternal parent and 2 purebred male matings. Since both sows and commercial pigs are hybrids, they should show heterosis. In addition, the method is relatively simple, as long as two boars of two breeds are kept, and the production group of hybrid gilts can be replenished in time, and it is not necessary to carry out pure breeding of the parent species, as long as the source problems of the boars of two breeds can be solved. Disadvantages: After one generation of hybrids, the heterosis rate of their offspring was slightly reduced. 5.4 Four-way crossbreeding: Also known as two-hybrid, based on two binary crossbreedings, one of the binary crossbred males is used as a male parent, and the other is a female parent of a secondary crossbred female parent. After a simple cross-breeding, the resulting four-way hybrid pigs were all used for commercial fattening. Advantages: The offspring can integrate the heterosis of the mother line and the paternal line, with the highest heterosis rate. Disadvantages: The breeding system is complex, not only to maintain the purity of the four parent species, but also to keep a large number of binary hybrid sows and boars. At present, some foreign large-scale pig farms use this hybrid method to raise finishing pigs. 6 Feeding management This is an important part of the use of hybrid superiority. Because of the heterogeneity and size of the heterosis, which is closely related to the living conditions of the hybrid pigs, crossbreeding pigs should be given appropriate breeding and management conditions to ensure the full performance of the heterosis. Although the hybrid pigs have improved feed utilization capacity, they can perform better than purebreds under the same conditions, but high production performance requires a certain material basis. Under conditions where basic conditions cannot be met, heterosis cannot be expressed. And sometimes it is not as good as low-grade purebred. To sum up, as long as we master the above key technologies, we can make full use of the heterosis and greatly increase the economic efficiency of the pig industry.
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