Methods for Breeding Introduced Pig Breeds and Key Points for Applying New Breeding Technologies

The continuous development and progress of science and technology have made it possible for people to explore natural laws and reveal biodiversity. The greatest advancement in pig breeding in the 20th century was breaking through 800 g/day in daily weight gain, achieving a lean meat rate of over 60%, a feed-to-meat ratio of 2.8:1, and slaughter pigs reaching over 90 kg body weight at 156 days, producing truly lean pigs. In recent years, with the continuous development of breeding theory and practice, pig breeding in countries with developed @#@227@#@ industries has gradually transitioned from breed selection to specialized composite line breeding. Targeting the different uses of each line in the crossbreeding system, genetic improvement focuses on key traits, followed by inter-line composite crossbreeding; combining the advantages of each line improves the production performance and economic benefits of commercial pigs. To develop composite lines, a complete crossbreeding and propagation system must first be established. On this basis, parental lines are developed through selection of core group parents, then crossbreeding combinations are determined through combining ability tests, and parent stock or breeding herds are produced to supply commercial piglets. According to analysis, establishing this complete system in pig production, although requiring certain investment, is the most economically advantageous (Chen Runsheng et al., 1997). Furthermore, without a complete crossbreeding and propagation system, breeding work will have no foundation, and the production performance of introduced breeds will be difficult to maintain and improve. This is the purpose of establishing a new crossbreeding and propagation system.

1 Pig Breeding Objectives

1.1 The factors determining pig breeding objectives are mainly market and consumer demand, while animal breeding objectives refer to breeders selecting superior breeding individuals to ensure that the production population achieves maximum economic benefits under expected production and market conditions (Meuwissen, 1998). At the same time, breeding objectives are influenced by factors such as region, culture, customs, and market demand. Breeding objectives differ across different goals, different regions, and different periods. The development of breeding objectives has gone through a long process from focusing on biological characteristics of livestock and poultry to pursuing maximum economic benefits. Since the 20th century, the main objective traits for pigs have been growth rate, feed efficiency, and carcass lean meat rate. Although the above objectives have been met or exceeded, they have also brought some problems. First, intramuscular fat content (IFM) has declined; for example, from 1978 to 1992 in Denmark, the IFM of Landrace, Yorkshire, and Duroc pigs decreased by 50%, with Landrace and Yorkshire at only about 1%, and Duroc also declining from 4.15% to 2.05%. Second, the proportion of porcine stress syndrome has increased, pork quality has declined, and the incidence of PSE meat and DFD meat has increased.

1.2 In the 21st century, pig breeding objectives must be continuously adjusted according to market and consumer demand. From the producer's perspective, high-quality lean pork needs to be produced at the lowest cost and in a manner acceptable to the public. Breeding objectives have three different directions: first, improving the potential of already high production performance levels (including lean meat rate, lean tissue growth rate, feed efficiency, carcass quality and conformation, annual piglets per sow, and uniformity); second, maximizing the full expression of these potentials in actual production (including disease resistance, adaptability, and stress resistance of the pig herd). There is no signal that selection limits have been approached, so there is reason to speculate that in the next 10 years of BLUP selection, the possible combined effect of the Meishan pig breed and the estrogen receptor gene could produce genetic improvement of 4 more live piglets per litter, with average daily gain during the 30-100 kg growth stage reaching as high as 1 kg. The third breeding direction is quality breeding. Because in the international market with oversupply, picky buyers, and fierce competition, continuous innovation in high-quality pigs, high-quality meat, and specialty meat is the most promising; for example, Taiwan, China has begun establishing a production system for excellent black pigs in Taiwan, China.

1.3 For Chinese people who regard pork as an important food, there will inevitably be increasingly higher demands for pork quality. Therefore, pigs bred in the 21st century should have the following characteristics:

1.3.1 High reproductive and nursing capacity of Chinese pigs;

1.3.2 High production performance and feed conversion capacity;

1.3.3 Commercial pigs with high lean meat rate, good meat quality, abundant intramuscular fat, thin and uniform backfat, and little abdominal fat;

1.3.4 Good constitution, strong disease resistance and stress resistance, sound immune function, and good vitality.

2 Application of New Breeding Technologies

Since the 1990s, with the rapid development of biotechnology and information technology, animal breeding internationally has gradually entered the molecular level. The use of joint breeding has enabled the expansion of superior individual traits, greatly accelerating breeding progress and substantially improving pig production performance. According to predictions by developed countries such as the United States and the United Kingdom and by FAO, in the 21st century, globally commercialized livestock and poultry breeds will be developed through molecular breeding technology, and the contribution rate of breeds to animals will exceed 50%.

2.1 Molecular Biotechnology

The main applications of molecular biotechnology in pig breeding are as follows:

(1) Pig gene mapping technology (Genemepping), whose main goal is to find DNA markers linked to loci of important economic traits (such as lean meat rate, milk production, disease resistance, etc.) and use them for marker-assisted selection to improve livestock and poultry breeds, enhancing selection effectiveness and genetic improvement. Currently, on the 19 pairs of pig chromosomes, there are nearly 3,000 markers.

(2) Detection and utilization of quantitative traits and major genes in pigs, mapping of quantitative trait loci (QTL), etc. Commonly used methods are segregation analysis, candidate gene method, and genome scanning. Currently, the porcine stress syndrome gene, candidate genes for litter size, etc. have been applied in breeding.

(3) Marker-assisted selection for quantitative traits. In pig breeding, for traits with low heritability (such as reproductive traits), expensive measurement costs (such as disease resistance), phenotypic values difficult to measure early before development (such as lean meat rate), or sex-limited expression (such as milk production), using marker-assisted selection (MAS) can improve selection accuracy and genetic progress, and enhance breeding efficiency.

2.2 Joint Breeding

The purpose of joint breeding is to solve the problems faced by small population selection. Modern science and technology such as computers, networks, molecular biotechnology, and the latest methods of genetic breeding theory are applied to pig breeding to increase the number of core populations, improve selection intensity, slow inbreeding depression, accelerate genetic progress, and improve the genetic level of pigs. The main traits measured are: age at 100 kg body weight, litter size, backfat thickness, loin eye area, and meat quality traits. The breeding structure takes a pyramid form, consisting of the core group, breeding group, and commercial group. In the core group, purebred performance selection and improvement are mainly carried out; the main purpose of the breeding group is to expand the number of superior breeding stock and produce crossbred parent stock; the commercial group mainly carries out commercial pig production of crossbred or purebred pigs (Shephe 1997). The organizational structure consists of an internal decision-making level, a middle management level, and a bottom management level. In summary, joint breeding can improve genetic progress and the reliability of breeding value estimation, and reduce the rate of inbreeding increment.

2.3 Computer Information Technology

First is the establishment of genetic evaluation systems. Selection is a key link in breeding work, and correct selection must be based on accurate assessment of livestock and poultry genetic quality. In the early 1950s, American scholar Charles R. Herelerson proposed the best linear unbiased prediction (BLUP) method, which can greatly accelerate genetic improvement progress. For example, since Canada began applying the animal model BLUP method in 1985, the improvement rate of backfat thickness has increased by 50%, and the improvement rate of age at 100 kg body weight has increased by 100%-200%.

Second is the application of computer image analysis systems. In pig breeding practice, images can be used to analyze backfat thickness and loin eye area measured in vivo by B-ultrasound, without waiting for measurement after slaughter. This not only reduces measurement costs but also increases selection intensity and improves selection accuracy.

Third, through the application of information network technology, information sharing has been achieved, benefiting all breeders or companies.