3 BREEDING, CROSSBREEDING AND HYBRIDIZING OF YAK

Overview

Pure-breeding is the predominant practice with yak. Apart from a scheme involving selection in crossbreds of wild yak with domestic yak in a process of breed development (see Chapter 2), no information has become available on rigorous selection programmes consistently applied for the improvement of the performance of yak in China. However, some selection schemes appear to be under consideration both in China and other countries. The dearth of organized selection schemes is not surprising with an absence of written records of performance and pedigrees and because of the location of yak in harsh environments and remote regions. Herdsmen in some areas, such as those of the Jiulong yak, have a traditional system of selection for replacement bulls. The Jiulong scheme considers the performances of the sires and maternal performance, as well as the physical appearance of the individual. It has to be remembered that the capacity to survive must be one of the chief attributes in the genetic makeup of the yak. This characteristic is likely to be under constant pressure from natural selection.

There is circumstantial evidence that some inbreeding is likely to have occurred with yak as a result of traditional pure-breeding methods and, in some countries, because of insufficient interchange of breeding stock across national boundaries. This can be expected to have harmful effects on the performance of yak.

Crossbreeding among the different types and breeds of yak does not appear to be systematic, but, on theoretical grounds, should be advantageous. Crossing domestic yak with wild yak is receiving widespread attention and favourable results are reported, with indications of heterosis. Crosses of domestic yak with wild yak are also thought to provide a basis for selection in new breed formation (cf. Chapter 2).

Hybridizing of yak with other species of cattle (mainly Bos taurus but also Bos indicus in some countries) is widely practised. Bulls of local breeds of cattle are used for natural service. But for hybridizing of yak with relatively high-yielding "exotic" breeds of cattle, the use of AI with frozen semen is normal, as the bulls of these breeds have not, in the past, survived for long in the mountainous regions. Hybridizing of yak with cattle is advocated in several countries as a means of increasing milk and meat output from the mountainous regions. Only the first generation of hybrids (F1) is favoured, as later generations of backcrosses have poorer performance (and hybrid males are sterile). However, the F1 females can usefully be mated to males specially chosen for "meat" production. There are both economic and biological limits on the extent to which interspecies hybridization can be carried out. The biological limit is set by the low reproductive rate of yak and by survival rates. A large proportion of the female yak population is required simply to replace the pure-bred yak - even if the size of that population were to remain static and not increase, as seems often desired by herders.

Pure-breeding

Ways of improving yak productivity by selection might be of great importance to the people who depend on yak for their livelihood. As discussed earlier, the yak is the dominant domestic animal in the alpine regions and the mountain plateaux of western China and adjacent areas to the south and north - dominant in economic, though not necessarily in numerical, terms. The yak also has great importance in Mongolia and several other countries (see Chapter 11, part 2). It is an integral component of the socio-economic system of people in many remote areas and, often along with sheep and goat, it is the main contributor to the livelihood of the herdsmen and their families. And yet, several factors militate against systematic breeding programmes.

The first of these constraints on improvement by genetic selection is that yak are still widely regarded, especially among Tibetan people, as a symbol of wealth. The more yak a family or a village owns, the richer and stronger it is considered to be. To maintain or increase the number of yak can take precedence over improvements in quality, or even overall productivity. Thus, animals are often kept until they die rather than culled for reasons of poor productivity. This can lead to overstocking of pastures and to a potential reduction in the output from the herd as a whole (see Chapters 12 and 13). "Quality" of the herd can become more of a consideration in situations where "competing" families or villages already own similar numbers of animals. Observation also suggests that smaller herds are sometimes of better quality because more pasture resource is available for a given number of animals and greater individual care is given to the animals by the herders.

A second important reason why genetic selection by herdsmen, or by extension officers acting on their behalf, is impeded is the absence of the necessary performance and parentage records - although herdsmen will often claim to know the parents of yak, especially bulls. It is doubtful if the accuracy of this knowledge is ever tested. In some nucleus breeding herds set up recently on the state farms in Qinghai, Tibet, Gansu and Sichuan, pedigrees but not performance were recorded.

Third, survival of the yak in a harsh, even hostile, environment is of paramount importance, perhaps of higher priority than any other single performance trait (though it is unlikely that this matter has been quantified). In terms of selection for survival under these conditions, natural selection is almost certainly more effective than any current procedure devised by man.

In relation to selection for the main products from the yak - milk, meat and fibre - the only convincing evidence of changes resulting from selection applies to fibre, where selection of a "fibre line" in the Jiulong breed appears to have produced far higher yields than in contemporary animals not selected for this trait (Cai Li et al., 1980). Because fibre traits are quite strongly inherited and much more so (at least in other species) than milk yield or growth traits, selection progress is relatively easier to achieve with fibre production traits.

The milk yield of yak is very low, relative to other cattle, particularly those specializing in milk production. It has been suggested that the amount of milk produced by yak is only the quantity that would normally be needed to rear its calf. Thus, yak calves that receive only some of their dams' milk, because the rest is taken for human consumption, grow significantly less well (see Chapter 6). An incentive to select for a higher yield in yak is most likely to arise only where there is an expanded market for milk destined for sale.

In respect to meat output from yak, three problems arise that may create conflict with opportunities for selection for growth rate or "size" (meat production), even if these traits were somehow measured. One is the fact that a significant proportion of each year's growth of the animals during the warm season is lost over the period of nutrient deficiency in winter and early spring. This makes it difficult to see what an appropriate selection strategy should be. If the strategy were to be the increase in the size of the adult animal, say at the end of a growing season, the selection process would be delayed to late in life and hence would make slow progress. A second constraint is that when milk is taken from yak for human consumption and the calf is left short, the precise effect on each individual calf is difficult to estimate (even though an average effect of rearing practice is known). And in any case, there is variation in the quantity of milk produced by the dams. Selection among calves for growth rate therefore would be less accurate than in a totally uniform rearing system. A third problem is the opportunistic nature of the disposal of surplus stock that frequently occurs. The lack of a regular marketing strategy for well-grown animals, combined with the relatively rudimentary nature of the current marketing system, particularly in the remote areas, works against selection for "meat".

Nonetheless, in the regions where yak products are in great demand in the marketplace, it seems that herdsmen have acquired both the knowledge and skill to improve production traits - even though it may be done unsystematically and perhaps unconsciously. This is a possible reason why some breeds are held in higher esteem than others. But different breeds are rarely compared with each other in the same place and at the same time. So it is difficult to quantify the extent of any genetic differences in performance of the breeds, as distinct from differences in their looks.

Selection objectives for the chief yak breeds in China

In general, there are no clearly defined breeding objectives and no developed breed structure among herdsmen. Chinese animal scientists, however, decided towards the end of the 1980s to develop breeding objectives for the principal yak breeds. The intention was to provide technical assistance for a more systematic approach to yak breeding and to aim for earlier maturity, to improve the animal's shape for meat production and to develop strains for either milk or meat, or for meat and hair production. The criteria to be adopted therefore stressed body size, growth rate, dressing and meat percentages, milk yield and fat percentage, as well as the yield of hair - both coarse and down, but with an emphasis on the down.

The criteria proposed were approved in Sichuan and Qinghai for the Jiulong, Maiwa, Plateau and Huanhu breeds of yak (Zhong Guanghui et al., 1995; Wen Yongli et al., 1995) and a corresponding scheme was developed in Gansu for the Tianzhu White breed in 1985 (TAHVS and DAS, 1985). Some information and comments about these schemes appear below, but first though, attention is drawn to a selection procedure used by herdsmen. The procedure in the Jiulong breed is regarded as traditional because it occurred before the advent of the recent provincial schemes and also had particular involvement from the late Professor Cai Li and his colleagues (1980; GAAHB and YRO, 1980a, b).

A "traditional" selection procedure used by herdsmen in the Jiulong area of Sichuan

Selection of yak by the herdsmen in the Jiulong area is relatively systematic. Herdsmen pay more attention to choice of yak bulls for breeding than they do to the cows. The guiding principle for the herdsmen is to check the ancestors (the parents) first and the bull second. Selection of replacement males starts in the herd with calves from cows that have good conformation and high milk yield over two parities of calving. The herdsmen require that the sire of the males being chosen as replacement bulls should have copious hair and a large number of progeny. The bulls being selected should have good conformation. In particular, the herdsmen require that the horns of the selected bulls stretch outward from a rough base and that there is a long distance between the horns. The forehead, head, muzzle and mouth have to be broad; the neck thick and the lips thin and long; withers should be high and brisket wide; the back, loin and rump should be wide and flat; the tail hairy; forelimbs straight and hind legs curved; the scrotum should be shrunken. Acceptable coat colours are black or black with some white specks on the forehead and at the extremities of the body (e.g. legs or tail), but not on the body itself.

It is of interest that selection of bulls in the Jiulong area is made in three stages. The first is a pre-selection at the age of one to one and a half years. There is a second selection from among the first group at the age of three years and a final selection at the age of four to five and a half years. (The relative importance given to different traits at each stage is not specified). Bulls that are culled are castrated and used for meat or draught purposes. After initial mating with cows, bulls that are found to have been defeated in the normal competition for mates, which occurs among the bulls, and males found to have physical defects or bad conformation are then also culled. The herdsmen aim to have two or three successors to an excellent, dominant bull that has been working in the cow herd.

In 1979, in accordance with newly instituted breeding plans, nearly 7 000 reproductive bulls and cows (about a third of the total) were evaluated on physical conformation and body weight (GAAHB and YRO, 1980b). As a result, four adult yak bulls were identified that met or approached the predetermined performance levels. However, by the time the bulls were identified they were too old for use. Clearly, this was an uncertain start to selective breeding and was more akin to a process of population screening (a search for exceptional individuals) than a process of continuous genetic selection. This particular scheme could not be continued, but consideration was subsequently given to selection of yak at various locations and in different counties where the Jiulong yak are kept (Cai Li, 1989). A standardized evaluation scheme for the Jiulong yak was drafted and approved to assist individual evaluation and selection (Zhong Guanghui et al., 1995). Finally, a nucleus herd with 412 breeding animals was established in the centre of the Jiulong yak territory and 106 individuals were maintained on a state farm to implement a breed-improvement programme (Lin Xiaowei and Zhong Guanghui, 1998).

The traditional selection methods for Jiulong yak appear to have produced over a period of many decades, perhaps centuries, an improved breed of yak that is highly regarded. Clearly, the criteria applied contain elements that are related to important aspects of production in the yak. However, a cautionary comment should be added, lest it be thought that these methods have to be unreservedly commended because they have tradition and herdsmen's experience on their side. Geneticists would wish to suggest that there is great scope for improving these procedures, even in the absence of sophisticated indices of breeding value and modern computational procedures. To start with, they would ask how closely related the physical appearance of the yak, so much emphasized by the herdsmen, is to actual performance of the herd - in terms of, say, growth, milk yield or reproductive rate. Usually the relationship is not high. A geneticist would also wish to encourage the herdsmen to pay most attention to those characteristics of the yak that provide the greatest economic return irrespective of whether the products from the yak are for home or commercial use. For that reason, it would also be urged that the number of criteria considered for selection be restricted to an essential minimum. Improvement of the important traits is diluted, or even lost, when a lot of attention is paid to less important, even trivial, matters - as may be the case now.

More recent provincial schemes: the example of the Tianzhu White

The Provincial Administration of Standardization in Gansu adopted criteria in 1985 to standardize the assessment of grading for the Tianzhu White yak and to evaluate breeding value as an aid to selection (TAHVS and DAS, 1985; Zhang Rongchang, 1989). The aim was to improve the breed for meat and hair.

Scores are allocated for aspects of general conformation, the body, testes for males and udder for females, legs, feet and the coat. Calves and adults are graded to somewhat different criteria. Weight and height classes are designed according to age and sex of animal and assigned to four grade classes. The use of selected breeding bulls is recommended, and newborn animals may be assigned a grade on the basis of the grades of their parents. Breeding bulls, in turn, are classified into four grades on the basis of the grades attained by their offspring. There are eight nucleus herds with a total of about 400 breeding animals maintained in the central area of this breed and 40 multiplier herds with approximately 20 000 individuals in surrounding areas (Zhang Haimin and Liang Yulin, 1998).

On the face of it, this scheme, like the "traditional" Jiulong scheme, pays considerable attention to aspects of the animals' appearance. This may well detract herders from considering more single-mindedly the performance aspects that matter most, namely, in line with the objectives for this breed, meat and hair production and the underlying factors of reproduction and vigour. Also, as has been found elsewhere with breeding schemes, if too many traits are considered there is a likelihood that none are improved (unless combined in highly sophisticated, statistically complex and computerized schemes).

In spite of reservations about the selection schemes, there appears to have been significant progress in the Tianzhu White yak since the 1980s. For example, the body height of adult breeding bulls and cows older than four and a half years increased from 108.1 cm and 104.3 cm (average of 17 males and 88 females) in 1981 to 110.2 cm and 104.7 cm (20 males and 44 females) in 1987 and to 114 cm and 112.9 cm (98 males and 826 females) in 1997. Corresponding body weights changed for males and females from 189.7 kg and 171.4 kg to 199.2 kg and 179.6 kg and to 202.8 kg and 192.7 kg respectively over those same years (Zhang Rongchang, 1989; Wang Yuchang and Wang Yanhong 1994; Zhang Haimin and Liang Yulin, 1998). However, these data were collected in a simple survey on various farms over a period of years. It is not possible, therefore, to distinguish any contributions from genetic improvement from those in management and feeding (or simply from year effects). There is a presumption, though, that management and feeding practices have remained largely unchanged over this period.

Other schemes

Sarbagishev et al. (1989) referred to an organized breeding programme in Kyrgyzstan based on specifications for yak males and females that were concerned primarily with conformation, growth rate and body size. Pedigrees were included and breeding values constructed. The improvement scheme was spread over a number of stock-breeding farms.

But the main scientific effort towards genetic improvement of productivity of yak, in many countries, has been directed at hybridization with Bos taurus and, to a lesser extent, Bos indicus cattle, rather than to selection. Some consideration has also been given to introducing, by crossbreeding, genes from wild yak into the domestic yak population as a means of improving productivity (see the following section). Lei et al. (1994) reported a scheme that uses performance criteria of individual yak and the potential benefits of introducing wild yak.

In the late 1980s, the first Wild Yak Frozen Semen Station was established on the Datong Yak Farm in Qinghai with three wild yak bulls (two captured from the Qilian mountains and one from the Kunlun mountains (Lu Zhonglin and Li Kongliang, 1994; Bo Jialin et al., 1998). Another Yak Frozen Semen Station is now in operation at Damxung in Tibet (Zhang Yun, 1994). These are the only A.I. centres in China specific to yak. By 1995, 8 700 crossbred animals of the wild yak with domestic yak had been produced in Qinghai and Gansu that served as the base herds for further selection and breeding of the new improved yak strain of Datong yak (Bo Jialin et al., 1998). The scheme used in this development of a "new" breed is described in Chapter 2.

Zhang Yun (1994) reported that there were ten yak bulls from the Sibu and Jiali yak breeds in Tibet and 28 semi-wild yak (F1, or backcrosses) at the Damxung station, though this number had been reduced to 17 in use. At the time of Zhang's report, 50 000 doses of semen had been produced and 2 000 yak cows inseminated - as well as a much larger number of yellow cattle to produce hybrids with the yak.

As yet there is no information on progeny records from these A.I. bulls. The full potential of using such information in selection procedures for improved performance of yak has not yet been realized. However, Zhang also suggested that the distribution of yak semen from this station could play a significant role in counteracting adverse effects of inbreeding, which have been thought to occur in yak in some areas. (The need to introduce yak "blood" from outside sources, to counteract inbreeding in the yak population of different areas, is also referred to by Pal in relation to India [see Chapter 11, part 2].)

Group breeding schemes

Because of the potential advantages of group-breeding schemes in promoting genetic improvement, especially when the participating herds are each relatively small, consideration is being given to setting up such schemes for yak. At present, as far as is known, these remain in the planning stages.

In the early 1990s an "open-nucleus" herd was established at Longri farm in Hongyuan county. This set-up included a small trial to check problems in the recording of accurate pedigrees for purposes of estimating genetic parameters (Zhong Guanghui, 1998). The nucleus herd to promote the improvement of Maiwa yak consists of 12 breeding bulls, tested for their performance, and 180 breeding cows (Lin Xiaowei and Zhong Guanghui, 1998). Records of growth, milk and reproduction have been collected continuously.

Consideration of inbreeding in yak

Inbreeding has harmful effects on nearly all aspects of livestock performance. Inbreeding reduces, for example, reproductive capacity, growth rate, adult size, and milk production and increases mortality, especially among the newborn and young. The amount of harm is usually quite closely related to the degree to which inbreeding occurs. It is a matter that should be considered in relation to yak because the traditional pattern of breeding may encourage inbreeding (cf. Chapter 5). In this system, bulls compete for mates and, in due course, these bulls are often replaced in the hierarchy of the herd by their offspring. This makes it inevitable that some inbreeding occurs. Inbreeding can be much reduced if bulls are exchanged across herds and greater distances - even then the problem may not be avoided but only postponed if two villages, for example, were consistently to exchange breeding stock only with each other. Controlled mating, whereby the herdsman decides on the mates for a particular bull, is similar in that it may reduce or postpone inbreeding, but rarely avoids it for long.

The absence of the pedigrees of animals in yak herds has made it impossible in the past to know the actual extent to which inbreeding has occurred. However, recently, microsatellite markers were used to analyse the genetic structure of different yak breeds/herds in China and other parts of the world and hence to estimate a general inbreeding effect. An assumption is made that the fewer alleles found at any one locus in a breed or herd, the higher will be the degree of inbreeding in that population. These investigations may help clarify the inbreeding issue specific to yak herds or breeds (cf. Chapter 15). However, the actual effects in yak are not known since this requires comparison of the performance of groups differing in their degree of inbreeding. This, in turn, requires performance records linked to pedigrees. For the time being, the probability of harmful consequences of inbreeding in yak is therefore inferred from known, corresponding effects in cattle, sheep and other livestock.

In some countries, such as Bhutan, Nepal and India (see Chapter 11, part 2), concerns about the effects of inbreeding have been expressed by those on the spot. The yak populations in these countries have become relatively closed. This is a consequence of reduced interchange of breeding stock across national boundaries relative to former times that, in turn, increases the likelihood that related animals are mated to each other. The effects of inbreeding must be suspected whenever the general performance of the stock is known, or thought, to have declined relative to an earlier era, and when other systematic changes in husbandry practices, such as overgrazing, for example, cannot account for it. Thus Kozlovskii (1960) stated that yak in the Gorno-Altai region were becoming closely inbred, which, if true, could well account for the earlier view of Denisov (1935) that the yak of that area were inferior, at that time, to those of other regions. Kozlovskii advocated, by way of remedy, the introduction of unrelated yak males and/or of hybridization with other cattle.

Inbreeding occurs whenever animals that are more closely related to each other than "average" are mated to each other. For example, if a son or sons of a popular bull are used in a herd as his replacement, they, in turn, are liable to mate with some of their half-sisters or cousins. Moreover, such bulls are likely to serve other less closely related females, but related through common ancestors more generations back (grandparents or great-grandparents). Mating of full siblings to each other, or parents to their offspring, which is regarded as close inbreeding, can easily occur if steps are not taken to avoid it. Pal et al. (1994), writing in relation to yak in India, stated that farmers may use the same male to serve females of two to three successive generations.

Inbreeding also occurs as a consequence of selection, even though selection is widely and correctly advocated and practised for the genetic improvement of livestock. Selection has the inevitable consequence of bringing about an increase in inbreeding, simply as a consequence of restricting the number of animals that become parents of the next generation. The objective in selection schemes must ensure that the beneficial effects of selecting superior stock outweigh the harmful effects of the consequent inbreeding. This consideration is nowadays a routine part of large-scale and long-term breeding plans, such as cattle improvement programmes involving the widespread use of a few bulls through artificial insemination.

The reason for having dealt with the topic of inbreeding at some length is that experience suggests that the effects of inbreeding are easily ignored because they are not readily recognized in the short term. However, the circumstantial evidence for inbreeding is strong in some yak populations, and the potential for inbreeding should not, therefore, be ignored when yak are allowed to mate.

Crossbreeding within the yak species

No systematic crossbreeding appears to be practised among the different breeds or local populations of yak. This is not surprising considering the relative isolation of different communities and the distances separating them. But it is more surprising that it does not seem, so far, to have played more than a minor role in investigations to find out whether hybrid vigour would result from such crossbreeding. There is a likelihood that hybrid vigour would result, although the magnitude cannot be predicted. The likelihood of heterosis from breed crossing can be argued from the relative isolation, over a long time, of discrete populations of domestic yak in different localities and from the likelihood that breeding practices within herds have led to inbreeding (although, again, some would dispute this). Crossing under these circumstances could have merits. From past experiments in China where Jiulong yak and Tianzhu White yak were introduced to other localities for crossing with the local yak, the crosses were at least heavier and larger than the local yak (Ren Chen Luoerri et al., 1995; Liang Hongyun et al., 1997). However, in the absence of results from the pure-bred animals of the introduced breed in the same locality, it is difficult to know to what extent this improvement represents the effects of heterosis or the consequence of bringing in "superior" genes from the new breed. Table 3.1 gives some of these results for crosses with the Jiulong yak.

Table 3.1 Improvement of the yak in Luhuo county in Sichuan by crossing with the Jiulong yak [Source: Zhong Jincheng, 1996]

Further support for the potential usefulness of crossbreeding comes from the attention paid more recently to crossing of domestic yak with wild yak and the claims of improved performance from such crossbreeding.

In the results presented from such trials, it is also not possible to differentiate clearly between the additive genetic effects (e.g. the fact that wild yak are larger than domestic yak) and the occurrence and magnitude of heterosis as a result of the crossing; but some results from such crosses are shown in Table 3.4.

Size of pure wild yak

Measurements were made in the 1960s on five adult male wild yak by the Agriculture and Animal Husbandry Department of Tibetan government (Study Group [Qiangtang], 1978). These animals had been caught in the Qiangtang area of northern Tibet. Their measurements are shown in Table 3.2.

Table 3.2 Body dimensions and weight of five male wild yak from Tibet

Some wild yak calves caught by staff of the Animal Husbandry Institute of the Yushu Tibetan autonomous prefecture of Qinghai province were compared with domestic yak calves under the same conditions of feeding and management (Xu Guilin, 1985). Table 3.3 shows the weights and weight gains of the two groups. It can be seen from these results that the wild yak calves were 86 percent heavier than the domestic yak calves at three months of age but, relative to their weight, grew more slowly (though not necessarily less in absolute terms) so that by the age of 16 months the wild yak were only 63 percent heavier than the domestic ones.

Table 3.3 A comparison of the body weights and weight gains at various ages of five wild yak and 19 domestic yak kept under the same conditions of feeding and management [Source: Xu Guilin, 1985]

Crossbreeding of wild yak with domestic yak

Some results from the crossing of wild yak with domestic yak are available. Provided the progeny from such crosses of domestic with wild yak have not been given preferential treatment over the domestic yak alongside them (and that may be a matter in question), the results suggest that the crosses have an advantage. Lu Hongji et al. (1987), for example, showed that the birth weight of crosses between domestic and wild yak were more than 30 percent heavier at birth than domestic yak calves. By age six months, the advantage in favour of the cross had increased to more than 50 percent. Calves with only one quarter wild-yak blood were 16 percent and 35 percent heavier at birth and six months of age, respectively.

Staff at the Lanzhou Institute of Animal Husbandry and Veterinary Science of the Chinese Academy of Sciences used some frozen wild yak semen to inseminate female domestic yak on the Datong Yak Farm of Qinghai province (Lu Hongji et al., 1987). They also produced some backcrosses of the F1 to local domestic yak (to produce 0.25 percent wild yak) and mated some local domestic yak to males of the Jiulong (domestic) breed of yak (cf. Chapter 2, Datong breed). The results are shown in Table 3.4 and suggest that crossing to the wild yak increased body weights and weight gains over the first six months of life. These weight gains were greater, relative to the birth weights, in the crosses with wild yak than in crosses with the Jiulong. The local domestic yak showed the lowest relative weight gains to six months old. There were no measurements beyond that age. Some of the wild-domestic crossbred yak at the Datong farm are illustrated in Figure 3.1.

Table 3.4 Body weights (kg ± SD) of local domestic yak and crosses with Jiulong yak and wild yak [Source: Lu Hongji et al., 1987]

In the 1990s, there was intensive use in Qinghai of the wild yak semen by A.I., or the use of semi-wild yak bulls with natural mating, to try to improve the domestic yak productivity and "rejuvenate" the yak population. Some comparable data from observations of the F1 (half wild yak blood), B1 (one quarter wild yak blood) and local yak under the same feeding and management system in southern Qinghai are shown in Table 3.5 (Yan Shoudong, 1998). It was found that the body measurements and weights of the semi- (F1) and quarter-wild yak (B1) were higher than those of domestic yak within the same age groups. As seen from Table 3.5, birth weight, height, length and heart girth of the F1 were greater than of the domestic yak calves and particularly at 18 months old, the measurements of both the F1 and the B1 were greater than of domestic yak.

Interest in the use of the wild yak to improve production of domestic yak was exemplified by the presentation of a number of papers on this topic at the first, second and third international congresses on yak, held in China in 1994, 1997 and 2000 (Zhang Rongchang et al., 1994; Yang Rongzhen et al., 1997a; Han Jianlin et al., 2002; Zhao Bingyao and Zhang Jianwen, 1994). It was noted in those papers that, historically, herdsmen in the Gannan area of Gansu drove their domestic yak females into regions where wild yak lived, in order to allow natural mating with wild yak bulls. The crossbred progeny would later be selected to improve the domestic yak population. Based on this popular experience, more systematic studies using frozen semen from wild yak bulls are in progress. Substantial numbers of first-cross and backcross (25 percent wild yak) offspring have been born and are reported to grow significantly larger than the local domestic yak. The benefits of wild yak blood, as noted in these studies, have also carried over into crossing with the local yellow cattle. When yak bulls that had 50 percent wild yak blood were mated to yellow cattle, the resulting F1 hybrids were of the order of 20 percent larger at six months old than comparable F1 hybrids of yellow cattle with domestic yak. Yang et al. (1997b), Lu and Zhao (1997), Yan Shoudong (2002) and Amarsanaa et al. (2002) presented similar findings using wild yak to increase the growth and the related meat production of the domestic yak.

It is not known, from any of the studies previously referred to, what is the relative importance of the role of heterosis and of the additive genetic contribution from the wild yak to its cross with the domestic yak, as discussed earlier in relation to crosses among domestic breeds of yak.

Results of studies in another area of Gansu (Lu Zhonglin and Li Kongliang, 1994) suggested that substantial increases in body size, hair production and meat output were achieved in first crosses of wild with domestic yak, relative to the latter. Milk yield was found to have increased by more than 10 percent. Yan Ping et al. (1994) reported, more specifically, that the fleece weight of adult females was 1.76 kg, 1.65 kg and 1.47 kg for half-wild, quarter-wild and domestic yak, respectively. These authors also found that, importantly, the proportion of the undercoat was increased substantially with the introduction of wild yak blood - but the strength of the fibres was not affected. The use of wild yak to improve domestic yak performance through a process of crossing and selection was also reported to be under investigation in Qinghai (Lei Huanzhang et al., 1994).

But clearly, only the additive genetic contribution from the wild yak genes will be useful in the actual process of subsequent selection (though the cross will retain some of the advantages from the initial heterosis). It is the perceived advantages of the introduction of wild yak blood into domestic yak populations that led to a project to develop a new breed from such a crossbred foundation (see Chapter 2).

Breed conservation

Taking into account the size of the present domestic yak population as a whole, it would be difficult to argue that conservation measures are a matter of urgency at this time. This might change if social or economic pressures were to reduce the extent of yak keeping - as is already evident in some areas, such as Nepal - or if predicted changes in global climate (over decades and centuries) have the effect of restricting the future distribution and size of the yak population.

Preservation of some of the remarkable traits of the yak in terms of its adaptation to a harsh environment and to long periods of severe deprivation should, nonetheless, be of interest to animal breeders worldwide. There are parts of the world where these characteristics could assist in establishing animal production and other parts where such resilience, on the part of the animal, could lead to better utilization of natural resources. Currently, however, the gene pool of the domestic yak as a whole is not endangered.

A different situation seems to exist for some of the more localized, and to an extent differentiated, populations or breeds of yak. The total numbers in some of these breeds is not large and hybridization with Bos taurus and Bos indicus cattle further reduces the proportion of the yak population available for its replacement.

For example, the Jiulong yak, possibly the best producer among the yak breeds, numbers 50 000 animals (Zhong Jincheng, 1996; Lin Xiaowei and Zhong Guanghui, 1998). The total numbers, however, tell only a small part of the story. Starting from a small herd, the Jiulong breed of yak has been a closed population for hundreds of years. Throughout the breed's history, herdsmen are said to have avoided introducing outside blood. Moreover, the system of selection practised by the herdsmen (previously described), and the natural competition among bulls for dominance makes it virtually certain that the effective size of the population is small and that inbreeding occurs (though the extent of this is a matter for debate). Thus, if the particular properties of the Jiulong are worth preserving and are not to be lost through genetic drift, special measures may be required. This was recognized by Chinese experts some years ago and led to the setting up of a random-breeding herd of 100 yak females and 20 males maintained per generation (Zhong Jincheng, 1996).

Table 3.5 Body weight and measurements of F1 and B1 of wild yak crossed with domestic yak in southern Qinghai [Source: Yan Shoudong, 1998]

This was managed in the Hongba area of Jiulong county and was the responsibility of the Animal Husbandry Station there. Income from the sale of milk and culled animals met some of the costs. There was also a subsidy from local government to assist the project. This type of approach is clearly commendable as one way forward in terms of breed conservation. A random-breeding herd has, however, a further potential advantage in that it can also serve as a yardstick against which to measure progress from any genetic selection in other parts of the breed population.

Other yak breeds may be in a similar situation to the Jiulong, with total numbers not large and the size of the "effective" breeding population possibly quite small. The Tianzhu White breed, in an area of Gansu province, could be one and its conservation is being considered (Wang Yuchang and Wang Yanhong, 1994). Zhang Haimin and Liang Yulin (1998) indicated that the number and proportion of the pure white yak in Tianzhu have increased as a result of the protection programs; for example, in 1952, the proportion of pure white animals was 20.3 percent and in 1981 it was 31.5 percent. It increased to 44 percent in 1998. Interestingly, the price of a white tail was double that of a black one in 1998 (120 yuan per kg compared to 60 yuan per kg).

Local breeds may have special merits or special characteristics that could be lost in the absence of positive action to maintain such breeds. Investigation of the need for conservation in the yak should therefore receive some attention even if local rather than general action may be called for. A useful start might be an up-to-date census of the yak population, its various types and breeds and current breeding practices. In combination, such information would help to indicate the (genetically) effective size of the different breeding populations, both in China and elsewhere. A census of numbers alone, as regularly practised in some countries such as Mongolia, though helpful, is not enough for this particular purpose.

Too often in matters of conservation, action has been delayed until damage to the breed, or even extinction of the species, has become imminent. This must not be allowed to happen with the yak.

The genetic approaches using chromosomal and protein polymorphisms, mitochondrial DNA RFLP and sequencing, and microsatellite genotyping (referred to in Chapter 2) to estimate genetic distances among breeds should go some way towards determining priorities for breed conservation (Han Jianlin, 1996, 2000). (The technology is discussed in more detail in Chapter 15).

For the wild yak, it is widely accepted that conservation is a matter of importance and urgency. Accounts, from as recently as the nineteenth century, testified to vast herds of wild yak in the Kunlun mountains of Tibet and Qinghai. These are no longer seen. Miller et al. (1994) estimated that wild yak of all ages and both sexes may still have numbered around 15 000 in the early 1990s, and this is also the number quoted more recently by Schaller (1998). Miller and Schaller (1996) claimed an estimated 7 000 - 7 500 wild yak remained in the Chang Tang Wildlife Reserve in Tibet at the time of their survey. But this number does not necessarily give an accurate picture of the threat confronting this wild species. Wild yak in China are included in the country's wildlife-protection legislation, but, according to Miller et al. (1994), the Departments concerned have inadequate resources for enforcement. The factors that have led to a dramatic decline in wild yak numbers over the past century still operate, even if to a lesser extent. These factors include excessive hunting, partly for food, the encroachment of the infrastructure of modern society, such as roads, and the increasing competition for grazing land from domestic livestock (Miller et al., 1994).

Hybridization of yak with cattle of other species

Ancient documents show that yak have been hybridized with ordinary cattle (Bos taurus) for at least 3 000 years. Documents from the eleventh century China, in the Zhou dynasty, suggest that hybridization of yak with cattle by the Qiang people gave benefits that nowadays would be called heterosis (or hybrid vigour). The name Pian Niu and variants of it have been used for these hybrids from earliest times (Ceng Wenqiong and Chen Yishi, 1980; Xie Chenxia, 1985; Cai Li, 1989). However, many other names exist (see section on local names). In some areas, such as northern India, Nepal and Bhutan, hybridizing with Bos indicus cattle also occurs.

Systematic hybridization of yak with other cattle has been recommended and practised for many years - and certainly as long as hybridization by plant breeders has been in fashion. The hybrids find a special niche with herdsmen in providing extra milk and as draught animals, usually at somewhat lower altitudes than the typical yak country. Hybridization is carried out primarily with yak females mated to bulls of local cattle.

This is regarded as the normal hybridization and, in China the F1 is called "true Pian Niu" (or simply Pian Niu). The reciprocal hybridization of female cattle to yak bulls is also practised and regarded as "counter-hybridization" with the progeny called "false Pian Niu" (see Figure 3.2) and many other local names.

The hybrids are always mated back to either yak or cattle males. There is no alternative to this as the F1 males are sterile. The herdsmen use, for the most part, the cattle available to them in their area; in China, for example, they are the local, so-called "yellow cattle". The hybrid progeny of the F1 generation are then called "local Pian Niu". However, much investigation has gone into the use of "improved" breeds of cattle of dairy, beef and dual-purpose types. Results of hybridizing with both local and "improved" cattle breeds are given in Chapter 7. The name that is given to the first hybridizing of yak with "improved" cattle breeds is "improved Pian Niu" - in order to distinguish it from the "local Pian Niu".

Information on the production of hybrids between yak and cattle will also be found in Chapter 11 in relation to individual countries.

In the course of experiments in the 1920s and 1930s at Buffalo Park, Wainwright, Canada, aimed at developing a meat animal for the cold northern regions, including Alaska, a small number of hybrids were also successfully produced between yak (male) and female American bison and half-bison (bison crossed with a cattle cross) (Deakin et al., 1935).

Figure 3.1 Pian Niu female (F1 from yak dam and local, yellow cattle sire)

Figure 3.2 "False" Pian Niu female (F1 from local, yellow cattle dam and yak sire)

Local names for hybrids

Names for the first generation hybrids of yak and cattle include the name dzo in Tibetan areas, variants of which extend into Mongolia and other countries, and chauri, the name used in Nepal. The various types of backcross hybrid, both to cattle and to yak, have an especially rich variety of names that differ in different parts of China and elsewhere. Descriptions of these names have been given by, among others, Zhao Zhengrong (1957), Hu Angang et al. (1960), Cai Li (1980), Joshi (1982), Zhang Rongchang (1989) and Pal (1993). The uninitiated traveller may find himself confused by the fact that the local people in China are said to call the hybrids of yak with cattle "improved cattle" - this usage is avoided here.

Distribution of hybrids

In the areas of the Henduan Alpine type of pasture, hybridizing of yak females with cattle males is not widely practised, nor is interspecies hybridization common in the pastoral regions at high elevation to which cattle cannot adapt. Such hybridization is, however, widespread in areas of mixed pastoral and agricultural production at lower altitudes. Table 3.6 shows, by way of example, the relative proportions of pure yak to hybrids and yellow cattle in two such areas in Sichuan. In the main yak-producing areas, hybridization with cattle is normally restricted to only a small proportion of the yak herd (see section, Limits to hybridization).

Table 3.6 Proportions of yak, Bos taurus cattle and hybrids in Ganzi county of Sichuan

The hybrid females are an important source of milk and milk products, for home consumption or for sale, and the males, since they cannot be used for breeding, are used for draught purposes, or are slaughtered for meat (see Chapter 7).

In China, the reciprocal hybridization procedure between yak bulls and yellow cattle females is carried out mainly in the cattle-producing areas of the cold Minshan mountains, especially in the Min county of Gansu province and Pingwu county in Sichuan province. These hybrids do not give much milk and are used mainly for draught purposes.

Hybridization policy

The first generation of hybrids of yak and "ordinary" cattle adapt well to the conditions in which they are used. They have some of the good characteristics of both parental types: resistance to a harsh environment from the yak and extra productivity, milk in particular (but with a lower fat percentage), from the cattle. Backcross hybrids to cattle, however, are less well adapted to the environment, and their productivity is often little better than that of yak - most probably through loss of heterosis (although there is no strict quantification of this). Backcross hybrids to cattle are not therefore favoured - one practice being to dispose of these hybrid calves immediately after birth, in order to have all the milk from the dam available for use or sale by the herders.

The alternative of backcrossing to the yak does, however, provide a particularly good source of animals for meat production. This system is encouraged and practised in China and elsewhere.

Cai Li et al. (YRO and XLF, 1983; YRO and GISP, 1984) showed, from a comparison of two neighbouring and otherwise similar grassland farms in Sichuan, that the output per head of animal, per unit of land and per unit of labour can be seriously reduced if the proportion of B1 hybrids is allowed to become too high. On the Xiangdong Livestock Farm, the proportion of B1 hybrids was not allowed to exceed 5 percent of the total herd and some selection was practised of those retained. On the other farm, Axi Livestock farm, the backcross progeny of the F1 hybrid generation were retained in full. The results of the comparison are shown in Table 3.7.

The use of "improved" breeds.

In China, starting at Datong in Qinghai province around 1939 and in the area now known as the Ganzi Tibetan autonomous prefecture in Sichuan from 1941, some yak were crossed with Dutch Holstein-Friesian bulls. Such hybridization did not become systematic until the mid-1950s when 200 bulls of various breeds were introduced to the yak-producing areas of China (Zhang Rongchang, 1989). The breeds included the Holstein-Friesian, Shorthorn, Simmental; Latvia, Ala-Tau, Kostrome cattle, the Mongolian, Binzhou, Sanhe, Qinchuan, Yinging and others. More recently, Charolais, Hereford, Limousine and others have been added to those available for hybridizing with yak. Mating was tried initially by natural mating, but artificial insemination was also used and continues as the predominant practice (Cai Li, 1989; Zhang Rongchang, 1989). From 1979 to 1985, a yak research team coordinated the hybridizing with such exotic breeds in the five principal provinces with yak in China, and some 32 000 hybrids were produced. As so often happens with fieldwork, relatively little of this work has provided comparative performance results - those available are quoted in Chapter 7.

Table 3.7 Comparison of output of animal products from two neighbouring and similar farms in pastoral areas of Sichuan province (1977 - 1981)

* B1 hybrids here are mostly backcrosses to cattle bulls - very few to yak.
** Output value (yuan; US$1=1.7 yuan) as the mean of 1979 - 1981 calculated according to fixed prices in 1980 as follows: milk 0.33 yuan/kg; cheese 0.56 yuan/kg; hide 8.9 yuan each; hair and down 1.74 yuan/kg; market cattle 85 yuan/head.

To better exploit the advantages of hybridization while avoiding the reproductive problems caused by using large "exotic" bulls, an alternative has been devised in parts of China whereby crossbred cattle, instead of large exotic breeds, are hybridized with the yak. For example, the Holstein Friesian or Simmental breed was used to produce F1 breeding bulls by crossing them first, by A.I., with the local cattle. The F1 crossbred bulls, with their relatively smaller body size, were then used on the yak to produce a hybrid F1 through natural mating.

Although the growth and performance of the hybrids for both milk and meat production was highly regarded (see Chapter 7, for performance results) the bulls of these various "improved" breeds (and 75 percent grade bulls of these breeds with yellow cattle) did not adapt to the local conditions and high altitudes in China. Most of the bulls died of mountain sickness or for other reasons within two years of introduction, and many died within the first few months. The bulls introduced in the mid- and late 1950s left fewer than 1 000 F1 and B1 hybrid progeny over a more than 20-year period.

Hybridization of yak with these "improved" breeds of cattle is now carried out by A.I. with frozen semen. This procedure inevitably restricts the utilization of these breeds to the more accessible and well-organized yak herds. In practice in many areas, therefore, the Bos taurus (and Bos indicus) cattle used for hybridizing with the yak will continue to be the local types of cattle.

Hybridization of yak with "exotic" breeds of cattle has also been practised in other countries for a long time (see Chapter 11, part 2), such as in as, some countries of the former USSR. Thus, Denisov (1938) reported on hybrids of yak and Schwyz (Brown Swiss) cattle, and more recently Katzina et al. (1994) added the Jersey and the Galloway and a continuing use of the Schwyz (now of American origin, hence probably the American Brown Swiss) to the list of exotic breeds referred to previously. Several of the breeds referred to are also used in Mongolia (Zagdsuren, 1994).

Limits to hybridization

The relatively low reproductive rate of the yak sets severe limits on the proportion of the female yak population that can be used for hybridizing with cattle if the numbers of the pure yak population are to be maintained, or possibly increased. In practice, it has been found best to restrict production of hybrids to the F1 generation only (whose offspring, in turn, are then slaughtered for meat). The male sterility of the hybrids prevents inter-se crossing systems and allows only the mating of the F1 hybrid back to yak or cattle bulls. Reduced productivity, relative to the F1, makes the B1 and later backcross generations unattractive commercially.

The actual proportion of the female yak population that can be hybridized with cattle depends on the reproductive rate, the replacement rate for cows (depending on the rate of death and disposal of the cows) and the loss of female calves before they reach reproductive age. These factors will vary from region to region and from year to year.

If it were assumed that:

• the yak population remained static in numbers,

• an average reproductive rate for the yak cow is around of 0.5 (equivalent to a live calf every second year),

• 10 percent of cows are eliminated annually and

• 10 percent of calves are lost before breeding age,

then 50 percent of the yak cow population could be available for hybridization. (These assumptions are equivalent, on average, to a yak cow producing, in her lifetime, two female progeny that survive to breeding age.) Any intention to expand the yak population would reduce the proportion that could be hybridized. If an increase of 10 percent in population numbers were required (and, as indicated earlier, herdsmen like to increase the number of animals they own), then only 10 percent of the yak cows could be hybridized with cattle - when the other assumptions remain the same.

Other assumptions would be entirely reasonable. Thus, higher replacement rates for cows and poorer survival of calves would reduce the proportion of yak females available for hybridization. For example, if replacement rates for cows and mortality among calves were both as high as 20 percent, as happens in some situations and some years, no yak cows would be available for hybridization if the reproductive rate of the yak did not exceed 50 percent - even with a static yak population. Matters would be even worse if snow disasters strike in particular years and localities and the rebuilding of the pure yak population becomes the top priority. On the other hand, in some regions and countries, where reproductive rate over a lifetime of the yak may be higher than in the examples given, the proportion of the yak female population available for hybridization can be increased.

The precise proportions of the yak population available for hybridization thus depend on the circumstances in any particular herd or group of herds. The point has been made often (see also Chapter 7) that the production of yak-cattle hybrids can play a useful role in improving the economics of animal production in the mountainous regions and particularly at the lower elevations of the yak territory and in the proximity to markets where the extra produce can be sold. But it also needs to be said that such hybridization is not a panacea. The pure yak must, perforce, remain the major proportion of the total bovine population in the mountainous regions. The attractions of hybridizing yak with cattle should not be allowed to detract from the need to consider genetic and husbandry improvements for the yak itself. In fact, improvements in the productivity and reproductive rate of the yak would also in turn increase the opportunities for hybridizing of yak with cattle, as already apparent in some areas.

There is clearly an opportunity to produce additional hybrids from the reverse process, that of mating cows of other local, domestic cattle species to yak bulls or using the semen of yak for insemination - although it appears that this hybrid (the "false Pian Niu") is traditionally used mainly for ploughing (see Chapter 7). This process also depends on having available a reproductive surplus in the cattle population.

Recently, Professor Jack Rutledge (personal communication, 2002) made a technology-based proposal for trials to produce hybrids from "improved" (e.g. Holstein) cattle and yak by in vitro production of embryos - using oocytes from slaughter cattle and yak semen. The resulting embryos then need recipient dams for their further development to birth. This proposal was conceived in the context of a situation (in a part of the Andes) where such hybrids might become a suitable dairy animal in the absence of either the yak or of a sufficiently productive cattle population or alternative milk producer. Although these procedures may have little immediate relevance to traditional yak-rearing areas, the idea is intriguing (see also Chapter 16)

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