OVERALL REVIEW OF GENETICAL RESEARCH ON CHINESE PTGS IN FRANCE AND PROSPECTS FOR EXPLOITING CHINESE

CROSSBRED ANIMALS

C. LRGAULT AND J.P.BIDANEL

Institut National de la Recherche Agronomique

Station de Genetique quantitative et appliquee

78352 Jouy en Josas Cedex – FRANCE

SUMMITRY

The present review summarizes the main results obtained in France over 12 years of research on three native breeds of pigs from the People’s Republic of China: Meishan (MS), Jiax-ing (JX) and Jinhua (JH). The first part is devoted to breed evaluation regarding reproduction, growth, feed efficiency, body composition, meat quality, behaviour and resistance lo diseases. On the basis of an estimation of Dickerson’s crossbreeding parameters between Meishan and Large White breeds, the second part of the review presents and discusses several strategies for taking advantage of prolific Chinese breeds as maternal components in crossbreeding schemes under different production systems.

I -INTRODUCTION

A limited number of native breeds of the People's Republic of China exhibits exceptional reproductive ability. They could then be incorporated as a component of maternal lines in European breeding programs in order to improve sow numerical productivity. That is why breeding animals from three native Chinese breeds of pigs: Meishan (MS), Jiaxing (JX) and Jinhua (JH) were introduced into France in 1979. Later, Chinese pigs have also been imported by several other countries: Hungary, Japan, the Netherlands, Great-Britain, the USA,….

Chinese pigs represent a valuable resource for basic research in many fields of biology and pig industry including reproduction, growth, body composition, meat quality, nutrition, resistance to deseases and behaviour. Several research programmes have been set up in France in order to evaluate these three lines and to assess their economic value as a component of the maternal genetic type in various crossbreeding systems. Due to the extremely narrow genetic base of these lines, any extrapolation to the whole breeds is of course not advisable. Nevertheless, these programmes have provided useful results concerning their performance under various production systems and their physiological characteristics.

The first purpose of the present review is to evaluate these three local breeds in comparison with usual European breeds such as Large White (LW) and French Landrace (LF) on the basis of the main results obtained in France over the last 12 years. The second purpose is to define and discuss the different strategies of utilisation of these breeds under various production and crossbreeding systems. Most of these results have already been widely discussed during the First Symposium on Chinese pigs held Toulouse in July 1990.

Source: Proceedings of the International Symposium on Chinese Pig Breeds in Harbin, China. 1992. pp. 10-24.

II. BREED EVALUATION

A first review of experimental results obtained on Chinese Pigs in France has already been published (Bidanel et al., 1990a; 1990b; 199]a). In the present review, the emphasis will be placed un recent development of these investigations.

A-Reproduction

Figure 1. Distribution of age at puberty (days ) in Large While and Meishan gilts (Dcspres et al 1992).

       Compared to LW animals, MS pigs are characterized by an earlier growth pattern of1he reproductive tract in females, accessory glands in males and gonads in both sexes (Prunier et al. 1987). This low age at puberty could result from an earlier start in sexual development due to an increase in hypothalamic and pituitary secretions and also from the lack of a waiting phase in the ovary development generally observed in European gilts in the weeks preceeding puberty (Prunier et al., 1987).

       Fl Chinese x European gills also reach puberty very early, between 85 and 110 days of age under normal temperate climate (Legault and Caritez, 1983) and between 136 and 150 days of age under tropical climate (Canope et al. 1990).

      An experiment was designed to estimate the effects of an early age at first farrowing on body development, longevity and productivity of sows (Gauthier et al. 1992). Sixty-one Fl French Landrace x Meishan crossbred gills (LF ´ MS) were mated either at 5 (MS) or at 7 months of age (M7) and compared over five successive parities to 29 control F1 Finch Landrace x Large White crossbred gilts (LF ´ LW) mated at 7 months of age. The highest longevity as well as the best reproductive performance were observed in LF ´ MS sows mated at 5 months. The delay in body development observed over the first reproductive cycle became non significant in the second and later parities. The numerical productivity estimated over the periapt from 150 days of age to the weaning of the fifth litter was 29.6, 26.7 and 23.4 pigs weaned / sow /year for M5, M7 and control sow', respectively. The excellent longevity of Fl sows as compared to herd-contemporaries or the French national reference is illustrated by figure 2a under. temperate climate (Gauthier et al., 1992) and by figure 2b under tropical climate (Delate et al 1991).

      The high litter size of MS gilts is mainly due to a low embryonic mortality, their ovulation rate being comparable at the same age and inferior at the same oestrus number with those of LW gilts (Bolet et al., 1986; Bazer et al. 1988a, Bidanel et al. 1990c). The physiological mechanisms underlying the high conceptus survival of MS gilts are discussed by Terqui et al. (1992).

      In the French experiment, MS females reached the same high levels of prolificacy as those obtained in China, with a superiority over LW or LF of about three piglets born per litter (Legault and Caritez, 1983; Bidanel et al., 1989). On the other hand, JX and JH French lines do not appear superior to European contemporaries, at least in the first three parities (figure 3). After the third parity, JX saws farrow larger litters than LW or JH, but remain at a lower level than MS sows.

Figure 3. Comparative variation of litter size at birth (total number born) according to parity in Lange White, Jinhua, Jiaxing and Mcishan sows.

      The aim of a recent study (Mandonnet et al., 1992) was to test the hypothesis of a major gene contributing to litter size in pigs, using all available data on pure MS and crossbred MS ´ LW sows. The total number. of piglets born and the number of piglets born alive per litter ware recorded in 1903 and 1660 litters, respectively. Prolificacy was considered as a trait of the dam. Under the hypothesis of a major locus with fixed A and B alleles in MS and LW breeds, we were expecting an AB homogeneous Fl population and backcross and F2 populations in which the 2 alleles were segregating: 1/2 AB and 1/2 AA (or BB) for the backcrosses, 1/4AA, 1/2AB and 1/4BB for the F2. This hypothesis was tested versus a sporadic inheritance by use of a segregation analysis method. Non significant likelihood ratios led us to conclude that prolificacy differences between MS and LW breeds could not be explained by one major gene and that the high prolificacy in MS was probably due to a very long term selection in favour of litter size.

      MS and to a lesser extent JX sows also have excellent maternal abilities, as shown by the large litters they are able to foster up to weaning (Depres et al., 1991). After adjusting for number born alive, MS and JX exhibit an advantage in survival rate over LW of 5 and 3%, respectively (Bidanel et al.,1990a). These good mothering abilities are partly due to behavioural characteristics (Meunier–Salaun et al., 199l), but also to the high number of teals in both breeds: 16.1 in MS, 18.8 in JX (Bidanel and Legault, 1986). Nevertheless, MS have a lower milk production than LW sows (Legault and Caritez, 1983; Bidanel et al., 1989). Weaning–oestrus interval (5.2+ 0.8 vs 7.5+ 0.9 days) as well as weaning fertilization interval(12.4+ 1.8 vs 12.4+ 1.5 days) were reduced in MS as compared to LW saws (Despres et al.,1992).

      B-Growth and feed efficiency

      MS, JX and above all JH are much lighter at birth than LW (between 900g and 1000g for MS, around S50g for JX and 650 g for JH vs. 1100-1300g for LW-Legault et al., 1982; 3idanel et al., 1990d; Le Dividich tt al., 1991). MS piglets have a greater percentage of body dry matter and protein, are more mature and have higher (+75%) plasma concentrations of free fatty acids than LW. This would explain their better resistance to cold (Le Dividich et al., 1991). The size disadvantage of MS, JX and JH with regard to LW keeps on increasing all through growth (figure 4). It reaches 26, 32 and 41 kg respectively at 154 days, 58,100 and 120 kg at first farrowing and 80, 130 and 150 kg at adult weight.

Figure 4. Growth pattcrn of Large White I.W), Mcishan (MS), Jiaxing (JX), and Jinhua (JH) fcmalcs (Bidancl et al., 1990a).

      Feed efficiency of purebred Chinese animals has so far not been measured over the whole post – weaning period. Extrapolations from the performance of crossbred pigs lead to a respective disadvantage of 0.9, 1.4 and 1.6 points of food conversion ratio for MS, JX and JH, respectively, compared to European controls (Legault et at., 1985). However, he real difference is certainly larger, as iced efficiency presents non-negligible heterosis effects. The ability of MS pigs to digest a fibrous diet was studied by Fevrier et al. (1988). It was found that MS digest a fibrous diet more efficiently than LW, but retain less nitrogen.

      Feed intake is similar in LW and MS in young animals (up to 30 kg live weight). After-wards, the increase with weight in feed intake is lower in MS animals (Bonneau et al., 1990). The difference in growth rate and body composition between the two breeds is mainly associated with a much lower daily protein and muscle deposition in MS animals.

      C-Body composition and meat quality

      Direct estimations of body composition of MS as compared with European breeds were made on relatively small samples by Poilvet et al. (1990) and by Bidanel et el. (1991b). However, most of the available results on carcass traits come from extrapolations based on performance of Chinese crossbreds. These extrapolations are much less subject to be criticized than those for feed efficiency, as carcass traits have an essentially additive genetic variability. Indirect estimations of killing out percentage give a disadvantage of 2-3 percentage points for MS, no disadvantage for JX and an advantage of 3-4 percentage points for JH over LW (Legault et al., 1985; Bidanel et al., 1992). Direct measurements confirm the low killing out percentage of MS 5rrnales:-3.4 points compared with LW according to Poilvet et al. (1990); -4.5 points compared with Pietrain according to Bidanel et al. (1991b). The three Chinese breeds have a much worse body composition than usual European breeds. The estimated carcass lean content reaches 29-30% for JH and 33-35% for MS and JX, against about 53-54% for LW or LF and 62% in Pi4train (Legault et el., 1985; Bidanel et al., 199lb; 1992). It is associated with a much thicker 4<in and a higher bone weight (Poilvet et el., 1990; Bidanel et al. 199lb). The important bone content of MS is mainly due to their high head weight (+0.72 kg over a European crossbred control, Legault et el., l985; +1.55 kg over Pietrain, Bidanel et al., 199lb). A similar phenomenon exists for JX, but not for JH (Legault et el., l985).

      According to Bonneau et al. (1990), the lower potential for muscle growth of MS pig is already present at birth via a reduced number of muscle fibers. The smeller postnatal increase in arose-sectional area of existing type II fibers in MS pigs further enlarges the difference between the two breeds. The more oxydative metabolism of MS muscle is observed from birth onwards while the lower glycolytic activity is acquired during postnatal growth. Another approach for determining the distribution of fibres consists in measuring the distribution of lactate dehydrogenase (LDH) isozymes because LDH is a major glycolytic enzyme and the proportion of isozyme 5 is high in glycolytic fibres (white fibres) and low in oxydative fibres (red fibres). Such a study conducted by Bousset et al. (1990) over six different muscles from three genotypes (MS, LW and F1) led to the conclusion that significant but not fundamental differences exist between genotypes, in good agreement with the results from Bonneau et at. (1990).

      Several studies have shown that MS pigs have more intra muscular fat than LW (3.27% vs 1.91%; Poilvet et el., 1990) or than Pietrain (3.07% vs 2.28% according to Bidanel et al., 1991b). These observations were confirmed in various crossbred animals including MS or JX (Touraille et al., 1989; Gandemer et al., 1992). The observed divergences in muscle fat content were essentially due to breed variation in neutral lipids content. Differences in polar lipids content and in fatty acid composition were much less marked, even if signifcant deviations in favour of MS sometimes appeared.

     The technological quality of the meat tends to be enhanced with increasing proportions of MS genes in crossbred products. The strongest influence is on meat colour. Chinese crossbreds have a significantly lower reflectance (i. e. a darker coloured meat) that European centrols (Legault et a1., 1985; Bidanel et al., 1992). Higher ultimate pH and water holding capacity of crossbred Chinese were also obtained, but differences wert not significant For all muscles.

      A summary of results obtained in crossbreeding with MS is shown in figure 5 (for average daily gain and feed conversion ratio) and figure 6 (for estimated lean content of the carcass and meat quality index).

Figure 5. Variations of average daily gain and feed conversion ratio according to the percentage of Mcishan gencs in the darm, the sire being Pietrain (Bidancl et al., 1992)

      Several studies clearly established the superiority of crossbred Chinese pigs for organoleptic quality of the meat. Meat from various 1/2 and 1 /4 Chinese crossbred pigs was judged more tender, more juicy and more tasty than meat from purebred European pigs (LW x LF) in several taste panel and consumer tests (Touraille et al., l989; Gandemer et al., 1992 and figure 7). This superiority seems to be highly connected with their higher intramuscular fat content (Gandemer et al., 1992). However, the overall acceptabili1y of the meat from crossbred Chinese pigs was not better than that of European controls, probably due to the excessive amount of visible fat. A superiority of Chinese crossbred over European controls animals was also observed for dry ham sensory properties (Rousset el al., 1990).

Figure 6. Variations of meat quality index and estimated carcass lcan percentage according to the percentage of Mcishan genes la the dam, the sire being Pietrain (Bidanel et al., 1992).

D-Other aspects

       A certain type of neonatal colibaci11osis is due to the adhesion of Escherichia coli K88 on receptor sites of the intestinal epithelium. The lack of receptor, which confers resistance to K88 adhesion is due to a single recessive gee. A limited trial involving MS, JX and LW breeds tend- ed to indicate that, contrary to LW, Chinese piglets lack this receptor, thus suggesting the existence of breed variation in resistance to this type of colibacillosis (Chappuis et al., 1984). The use of genetic resistance to K88 colibacillosis in pig breeding schemes was discussed by Ollivier and Renjifo (1991). In particular, the potential interest of the linkage between K88 and transferrin loci (Bonneau et el., 1990) was highlighted. These loci have recently been located on chromosome number 13 (Chowdhary et al., 1991).

        The general behaviour of 8-week-old piglets from the three Chinese lines was investigated through standard tests (Mormede et al., 1984). When exposed to a new environment, MS piglets had a lower locomotor activity and vocalized less than JX, JH and control piglets. They also had a lower food intake latency abler 24 hours of food deprivation (neophobia test). This behavioural reactivity had complex relationships with the corticotropin, corticoid and epinephrine levels of the animals. The superiority of MS sows regarding maternal behaviour as compared to LW sows would be due to a better adaptative ability of both sows and piglets.

        A comparative study of chromosomes of Chinese (MS and JH ) and European breeds (LW and LF ) showed that the nucleolar organising regions (NOR) were localised on both autosomal pairs n^o8 and 10 in Chinese pigs while the NOR were only expressed on one chromosome of the 8th pair in most European animals (M uramatsu et al., 1990).

Figure 7. Effect of genetic type on eating quality of meat (Gandemer et al., l990).

         finally, let us mention the existence olla European cooperative project on pig gene mapping (Pig Map). The purpose of this project which involves 16 different laboratories is to find genetic markers which art evenly spaced and cover the whole of the pig’s genome. Reference populations for this project are based on crosses between MS and LW breeds in three different countries (Ollivier et a1.,1992).

Ⅲ. PROSPECTS AND DIFFICULTIES FOR EXPLOITING CHINESE BREEDS

         The use of specialized sire and dam lines has become a common practice in modern pig industry. Excellent reproductive performance being higly appreciated in dam lines, prolific Chinese breeds seem to be particularly recommended as maternal or grand- maternal genotypes in breeding schemes. Unfortunately, the utilisation of this extreme genotypes is highly compromised by their poor growth and carcass performance. A precise knowledge of breed differences and crossbreeding parameters allows prospective studies on the different ways of utilizing Chinese breeds in various production systems.

         In France, the performance of MS and JX was extensively studied in both pure and crossbreeding. Dickerson's genetic parameters (Dickerson, 1969) of the cross between MS and LW breeds were estimated for different traits of economic interest. The main results of this evaluation work are summarized in Table l.

Table 1 Breed differences between Mcishan (MS), Jiaxing (JX) and Large White
(LW) breeds and beterosis effects of MS LW and JX LW crosscs (synthcsis of
French results; Bidanel, 1990).

^a extrapolated from the performance of crossbred pigs; ^b approximate value; ^c not estimated.

        The main conclusions of this evaluation work are as follows:

       ．breed differences are, at least in MS, mainly of maternal origin for prolificacy and of direct origin for growth traits. Grand-maternal effects are of little importance.

       ．both MS and JX lines exhibit extremely large heterosis effects for reproductive and growth trails in crosses with European breeds. As a consequence, growth rate of F1MS ´LW females is similar to that of pure LW, their prolificacy being equivalent to that of pure MS.

        ．direct and maternal epistatic recombination losses seem to be negligible. Another central step labor an accurate comparison of various strategies involving Chinese breeds depends on a correct prediction of their relative economic merit. Economic models must hike into account the particularities of Chinese breeds, such as low age at puberty and longevity, as well as their production and maintenance costs, i. e. feed consumption, culling price and costs of selection and multiplication herds (number of animals required, costs of nonbreeding pigs). Another important feature concerns the genetic consequences of the high prolificacy and the early puberty of Chinese breeds. An important increase (up to 50%-3idanel, 1988) in the rate of response to selection can be expected by shortening the generation interval and / or increasing selection intensities. On the other hand, the high sexual activity of Chinese breeds strongly disturbs their growth and might lower the efficiency of standard performance tests of young boars.

Due to the large heterosis effects on prolificacy, the use of Chinese breeds as grand-matermal breeds (i. e. as one of the components of the matermal genotype) is always superior to its use as a single maternal genotype. The short-term interest of Chinese breeds greatly depends on the relative economic impacts of production and reproduction trails, and particularly on the part played by the pork payment system in the global profit. For instance, the EEC market, which puts a strong emphasis on lean weight, is much less favourable to Chinese breeds than the U. S. market, which essentially prices pork. according to live weight. It has been clearly demonstrated that, in France and probably in many EEC countries, there is no conclusive short-term strategy for using MS or JX breeds, al least under the predominant intensive production system (Legault et al., 1985; Gueblez et a1.,1987; Bidanel, 1989).

In 1he furture, prospects for using Chinese breeds will greatly depend on the possibilities of improving their growth and carcass performance without impairing their reproductive merit. Several different strategies can be proposed to reach this goal. Selecting pure Chinese breeds for production traits could be a first solution. A 50% higher genetic progress could be expected in pure Chinese breeds compared to conventional breeds provided that the genetic parameters are similar and that the sexual activity does not cause any loss in the efficiency of performance test (Bidanel, 1989). However, the coat of this selection would be very high and probably beyond the financial possibilities of most breeding organisations.

Creating a composite line and selecting it for production traits appears to be a more promising solution. In fact, the creation of a 50% Chinese ´ 50% European composite line reduces by half the initial gap for production trails. Further reductions can be expected through selection, which could also be more efficient than in e European-type line because of an earlier puberty, a higher prolificacy and e possible increase in the additive genetic variance (Bidanel, 1988; 19S9). On the other hand, 50% of the favourable heterosis effects on reproduction and growth are lost (in the absence at' recombination loss). From simulation studies, the time necessary to overtake conventional grand-maternal lines should not exceed 3 or 4 years of intense selection for production traits. One should add, however, the time necessary to break down linkage disequilibria involving production and reproduction genes, so that 6 or 7 years are necessary to reach the economic equilibrium. These theoretical predictions seem to be confirmed by results obtained in two French companies which have undertaken the constitution of such lines (Naveau, 1990 Looft et al. 1990). A very high improvement of production traits was achieved without any perceptible reduction of litter size.

Discontinuous crossbreeding schemes, which allow an optimal use of maternal heterosis in Fl sows, may also remain an excellent solution in some situations where the efficiency of lean tissue growth is not a priority such as the production of high eating quality meat (Gandemer et al., 1992)or extensive systems under tropical environment (Delate et al., 1991). Discontinuous crossbreeding schemes might also be interesting under intensive management systems it the use of growth promotors such as porcine somatotropin (pST) to growing pigs were allowed. It has been shown that the effects of pST are much larger in fat genotypes such as MS, thus reducing their handicap for production traits. In an experiment involving lean (Pietrain, PI), fat (Meishan, MS), or intermediate genotypes (Pl ´ 3/4 Large White ´ 1/4 MS), referred to as crossbred (CR), females were injected between 60 and 100 kg live weight with 6 mg/d of pST and compared to control females receiving the vehicle only (Bidanel et al., 1991b). Average daily gain increased similarly in the three genotypes (125 g/d). Feed conversion ratio tended to decrease to a higher extent in MS(-2.0 kg of feed/kg of gain) than in the other two genotypes (-1.1 and - 0.9 kg of feed/ kg of gain for CR and PI, respectively). A significant genotype x treatment interaction was also observed for backfat thickness (BF) and fat, muscle, and bone development. Effects of pST in PI, CR, and MS pigs were, respectively, W.2, -9.6 and -16.1 mm for BF and 3.0, 6.8 and 11.8% for carcass lean con lent. This result could enhance the use of Chinese genes through discontinuous crossbreeding schemes (using highly prolific Fl sows to produce 1/4 Chinese slaughter pigs). However, much uncertainly still remains about the authorization of using pST, particularly within the EEC market.

IV-CON'CLUSION

In spite of an extremely narrow genetic basis, which precludes any extrapolation to the breeds as t whole, French Meishan and to a lesser extent Jiaxing have confirmed under intensive husbandry conditions their exceptional reproductive ability and their very poor lean growth potential. Significant results were abtained over the last 12 years in France, especially in the domain of reproduction. However, much work remains to be done before we are fully able to understand the particularlies of these breeds. From this point of' view, the recent development of research programmes on Chinese pigs in several countries and international cooperation on the subject should lead to much more important progress over the next decade.

Chinese breeds could play an active part in the improvement of the efficiency of the pig industry as suggested by recent development of research and breeding projects in an increasing number of countries. Among different possible strategies for taking advantage of prolific Chinese breeds in various market conditions, the creation of composite lines selected in favour of growth and body composition seems to be by far the most promising. However, the improvement of pure Chinese breeds or the use of discontinuous crossbreeding themes completed by pST administration should also be taken into consideration.

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