Breeding of introduced Pig breeds and Application of New breeding techniques

The continuous development and progress of science and technology make it possible for people to explore the laws of nature and reveal biodiversity. The greatest progress in pig breeding in the 20th century was to make the daily weight of pigs exceed 800g/day, lean meat percentage reach more than 60%, feed meat ratio reach 2.8∶1, and the weight of pigs in 156 days reached more than 90kg, thus breeding real lean pigs. In recent years, with the continuous development of breeding theory and practice, pig breeding in developed countries has gradually transitioned from breed selection to specialized matching selection. According to the different uses of each strain in the hybrid system, genetic improvement of key traits is highlighted, and then inter-line matching hybridization is carried out. The production performance and economic benefit of commercial pigs will be improved by integrating the advantages of each strain. To cultivate mating lines, a complete cross breeding system should be established first, and then parent lines should be bred by breeding parents of core group, and cross combinations should be determined by cross combining ability test, and parents or breeding groups should be produced to provide commercial piglets. According to the analysis, it is economically most advantageous to establish such a complete system in pig production, although it requires certain investment (Chen Runsheng et al., 1997). In addition, if there is no complete crossbreeding system, breeding work will not be supported, and the production performance of introduced varieties will be difficult to maintain and improve, which is the purpose of establishing a new crossbreeding system.

1 Breeding objectives for pigs

1.1 Market and consumer demand are the main factors determining pig breeding goals, while animal breeding goals refer to breeders selecting excellent breeding individuals to ensure that the production population obtains maximum economic benefits under expected production and market conditions (Meuwissen,1998). The breeding target is influenced by geographical area, culture, custom and market demand. The breeding objectives are different for different objectives, different regions and different periods. The development of breeding objectives has experienced a long process from paying attention to biological characteristics of livestock to pursuing maximum economic benefits. Since the 20th century, the main target traits for pigs have been growth rate, feed efficiency and carcass lean percentage. Although the above goals have been met or exceeded, they have also posed a number of problems. One is the decrease of muscle fat content (IFM). For example, the IFM of Landrace, Yorkshire and Duroc pigs in Denmark decreased by 50% from 1978 to 1992, while that of Landrace and Yorkshire pigs was only about 1%, and that of Duroc decreased from 4.15% to 2.05%. Second, the proportion of stress syndrome in pigs increased, the quality of pork decreased, and the incidence of PSE meat and DFD meat increased.

1.2 Pig breeding goals in the 21st century must be adjusted to market and consumer needs. From the producer's perspective, there is a need to produce lean pork of good quality at the lowest cost and in a manner acceptable to the public. Breeding objectives have three different orientations: One is to improve the already high level of production performance (including lean meat percentage, lean tissue growth rate, feed efficiency, carcass quality and body shape, annual litter size and uniformity per sow), and to the maximum extent possible to fully demonstrate these potentials in actual production There was no signal of approaching the selection limit (including disease resistance, adaptability and stress resistance of the pig herd), so it is reasonable to speculate that the genetic improvement produced by the possible combination of BLUP selection, Meishan pig breed and estrogen receptor gene in the next 10 years will increase litter size by 4 live piglets per litter and average daily weight gain of 30~100 kg growth stage will be as high as 1 kg. The third breeding direction is quality breeding. Because in the international market supply exceeds demand, buyers are picky, and competition is fierce, continuous innovation of high-quality pigs, high-quality meat, and specialty meat is the most promising. For example, Taiwan has begun to establish a production system for Taiwan's excellent black pigs.

1.3 For Chinese people who take pork as an important food, it is inevitable that higher and higher requirements will be put forward for pork quality. Therefore, pigs cultivated in the 21st century should have the following characteristics:

1.3.1 To have high reproductive and nursing ability of Chinese pigs;

1.3.2 High production performance and feed conversion capacity;

1.3.3 Commercial pigs have high lean meat percentage, good meat quality, more fat between muscles, thin and uniform back fat, and less abdominal fat;

1.3.4 Have good constitution, strong disease resistance and anti-stress ability, sound immune function and good vitality.

2 Application of new breeding techniques

With the rapid development of biotechnology and information technology since 1990s, animal breeding has gradually entered the molecular level in the world. The use of joint breeding has expanded the excellent individual traits, greatly accelerated the progress of breeding, and greatly improved the production performance of pigs. According to the prediction of developed countries such as the United States and Britain and FAO, the global commercial production of livestock and poultry breeds in the 21st century will be cultivated by molecular breeding technology. The contribution of species to animals will exceed 50%.

2.1 Molecular Biotechnology

The application of molecular biotechnology in pig breeding mainly includes the following aspects:

(1) Genemepping technology for pigs, the main goal of which is to find DNA markers linked to important economic traits (such as lean meat percentage, milk yield, disease resistance, etc.) and use them for molecular marker-assisted selection to improve livestock and poultry breeds. Improve the effectiveness of selection and the amount of genetic improvement. At present, there are nearly 3000 markers on 19 pairs of pig chromosomes.

(2) Detection and utilization of quantitative traits and major genes, and mapping of quantitative trait loci (QTL). The commonly used methods are isolation analysis, candidate gene method and genome scanning method. At present, porcine stress syndrome gene and litter size candidate gene have been applied in breeding.

(3) Marker assisted selection for quantitative traits. Marker-assisted selection (MAS) can improve the accuracy and genetic progress of selection and breeding efficiency for traits with low heritability (such as reproductive traits), expensive measurement cost (such as disease resistance), phenotypic values difficult to determine at early stage before development (such as lean meat percentage) or sex-limited performance (such as milk yield) in pig breeding.

2.2 joint breeding

The aim of joint breeding is to solve the problems faced by small population breeding. Computer, network, molecular biotechnology and the latest methods of genetic breeding theory were applied to pig breeding to increase the number of core population, improve selection intensity, slow down inbreeding decline, speed up genetic progress and improve genetic level of pigs. The main measured traits were age at 100 kg, litter size, backfat thickness, eye muscle area and meat quality traits. The breeding structure is pyramidal, which consists of core group, breeding group and commodity group. In the core herd, selection for purebred performance is mainly carried out; in the breeding herd, the main purpose is to expand the number of improved pigs and produce hybrid parents; in the commercial herd, commercial pig production is mainly carried out for hybrid or purebred pigs (Shephe1997). The organizational structure consists of internal decision-making layer, middle management layer and bottom management layer. In conclusion, joint breeding can improve the reliability of genetic progress and breeding value estimation, and reduce inbreeding increment.

2.3 Computer Information Technology

One is the establishment of genetic evaluation system. Seed selection is the key link of breeding work, and correct seed selection should be based on accurate evaluation of livestock genetics. In the early 1950s, American scholar Charles R. Herelson proposed the Best Linear Unbiased Prediction (BLUP) method, which can greatly improve the progress of genetic improvement. For example, since Canada began to apply the animal model BLUP method in 1985, the improvement speed of back fat thickness has increased by 50%, and the improvement speed of 100 kg body weight day age has increased by 100%~200%.

The other is the application of computer image analysis system. In pig breeding practice, back fat thickness and eye muscle area measured by B ultrasound can be analyzed by image, without waiting for slaughter, which not only reduces the cost of measurement, but also increases the selection intensity and improves the accuracy of seed selection.

Third, through the application of information network technology, information sharing has been realized, so that all breeders or companies can benefit.