7 PERFORMANCE OF HYBRIDS OF YAK WITH BOS TAURUS AND BOS INDICUS CATTLE

Overview

Hybridizing of yak with Bos taurus cattle, and in some countries also with Bos indicus, is done by using the local cattle in natural service or by using semen of "improved" breeds like the Holstein-Friesian (and many other breeds) in artificial insemination (to produce "local" or "improved" hybrids, respectively). The hybrids are less well adapted to the harsh conditions and high altitudes typical for yak and are kept at intermediate elevations. The "improved" hybrids especially need better management and feeding than yak.

The first hybrid (F1) females, especially those with "improved" breeds of sire, reach sexual maturity sooner and are first mated a year earlier than the average yak. F1 females can return to heat several times in the same season, if not pregnant, and will generally calve every year. Calf survival is similar to that in the yak. Consequently, overall reproductive rate of the hybrids, especially the "improved" ones, is higher than in the yak. F1 males are sterile. Although their sexual functions are intact, their semen does not contain sperm. The reasons for this have been under investigation for a long time, and although there are a number of possible causes known, there is, as yet, no resolution. Sperm production does not resume until the third backcross at the earliest (15/16 yak or cattle), and often not until the fourth backcross is reached. In practice, there are few backcrosses after the first, since they have no role in the livestock economy.

F1 hybrids generally grow faster and become larger than the yak - and also larger than some of the local cattle. Backcrosses to the yak, or to breeds of local cattle, are smaller than the F1. "Improved" hybrids are larger than the "local" hybrids.A larger size of hybrid animal also leads to greater meat production, although dressing percentages and other attributes of the carcass are usually fairly similar to those of the yak. The hybrid animals, from crossing yak females with "improved" breeds of cattle, however, are capable of slaughter at much younger ages than the traditional yak steer, or the "local" hybrid steer.

Milk yield is higher in hybrids than in the pure yak and often higher than in the local types of cattle (for example, the small hill cattle of some countries). Milk yield of the hybrids is especially increased if their sires are from "improved" breeds. Backcrosses give less milk than the F1. The fat percentage of the milk of hybrids is usually lower than that of yak.

Strict interpretation of this and similar results in terms of heterosis is, however, difficult, since the different types of hybrid, the pure yak and cattle breeds are not usually kept and managed together under identical conditions. This applies with even greater force when "improved" breeds of cattle are involved. Holstein-Friesian or Simmental cattle, for example, are not kept at high altitudes alongside the yak and certainly not managed in the same way as the yak. Nor are the reciprocal hybrids produced by mating females of the "improved" cattle breeds, say, the Holstein-Friesian, to yak bulls. The presence of both the reciprocal hybrids alongside both the parental types is required for any strict interpretation of the role of heterosis. The circumstantial evidence for heterosis from hybridization of yak and other species of cattle, both local and "improved" breeds, is considerable. The strongest part of that evidence is the poorer performance of the first backcross (from F1 females mated to bulls of yak or of other cattle) relative to the F1. This is as expected if heterosis plays a part in enhancing the productivity of the hybrids.

Introduction

As indicated in Chapter 3, the hybridization of yak with cattle of other species has been practised since earliest times. Originally such hybridization occurred with cattle of the local breeds, generally referred to as "yellow cattle" (Bos taurus) in China and with cattle of both Bos taurus and Bos indicus cattle (zebu) species elsewhere. This practice is still extant. Nevertheless, in recent decades, encouragement has been given by scientists and officials to expand the use of "improved" breeds of cattle for such hybridization. This has been facilitated by the introduction of artificial insemination and the use of frozen semen. As referred to earlier (Chapter 3), the use of A.I. is, inevitably, restricted to more accessible areas. Moreover, the expense of acquiring and maintaining bulls of the "improved" breeds of cattle for semen production in the yak-rearing territories (as well as the poor survival of such bulls) means that, except in a few localities, most hybridization of yak continues to be done with the locally available cattle.

Interpreting the results of hybridization (the question of heterosis from yak-cattle hybrids)

The purpose of any crossbreeding or hybridization is to combine some of the good qualities of the breeds or species being crossed or hybridised (from additive genetic effects). Further, there is a possibility that the performance of the hybrids will exceed the average of the two parental types (from the action of non-additive genetic effects). Any such superiority of the crossbred or hybrid above the parental means is described, in performance terms, as heterosis - or hybrid vigour. In the case of hybrids of yak with Bos taurus or Bos indicus cattle, there is a difficulty in apportioning credit between additive effects and heterosis. This arises firstly because the pure-bred animals of both species are rarely, if ever, kept together under identical conditions of environment and management. Secondly, the reciprocal hybrids between the species (i.e. progeny of yak female with cattle male and of cattle female with yak male) are either not both produced (and in the case of exotic "improved" cattle never produced) or, if produced, then not kept alongside the parental species under the same conditions. Yet, the presence together of all these four types (both parental species and the reciprocal hybrids) is a requirement for any strict assessment of the magnitude of heterosis.

In general, the hybrids occupy a different niche from that of the pure yak in the economy of the pastoral mountain regions, mostly at the lower elevations. Part of the explanation for this may arise from a less well developed physiological adaptation of the hybrids to high altitude, for example in terms of their pulmonary haemo-dynamics (see Chapter 4). As shown in Chapter 3, hybrids are more prevalent in environmentally more favoured areas and at the somewhat lower elevations in the range of yak territory. Local types of cattle, in turn, are most prevalent in still less demanding and agriculturally mixed areas of these regions. As for females of the "improved" breeds of cattle (e.g. Holstein-Friesians), they are not kept in typical yak territory as they do not survive in these harsh conditions, nor are such females mated to yak bulls to provide the reciprocal hybrids. Moreover, as noted in Chapter 5, hybridization is often done early in the breeding season with yak cows at their first oestrus (between June and September), so that, on average, hybrid animals are born earlier in the year than are the pure yak. The purpose of doing this, generally, is to reduce the chances of being left with cows that have not conceived to cattle bulls or to A.I. and still provide an opportunity to expose these cows to yak bulls from October to the end of the breeding season. This procedure increases the overall reproductive efficiency of the herd (Zhou Minhai, 1998). But in terms of comparing the performance of hybrids with that of yak or cattle it introduces a further complication. Date of birth affects the subsequent performance of the animals because they have a better start before the following winter. Such an advantage can be seen over the first two years of life, and possibly longer.

Nevertheless, the comparisons of the performance of hybrids with that of yak and, where possible, cattle are highly suggestive of a part played by heterosis. This arises because the performance is not only better than that of the yak when kept alongside them (although without certainty that they are treated alike or strictly contemporary) but also, in some cases, better than that of the local cattle (again with reservations about the similarity of treatment of each). A few comparisons of the different types (pure yak, pure cattle and both types of hybrid) have been claimed from Mongolia and some early studies in Russia (see Chapter 11, part 2) where a heterosis effect is strongly asserted.

In relation of hybridization with "improved" breeds of cattle, the argument for the occurrence of heterosis is stronger, if rather academic. It arises from the assumption that females of the improved breeds would not survive for long under the conditions tolerated by the yak at high altitudes (any more than did the bulls of these types - see Chapter 3). Further it must be assumed that females of such breeds of cattle (e.g. Holstein-Friesian or Simmental) would not lactate satisfactorily without much additional feeding, even if they were to survive for a time. Therefore, the presumption is that purebred females of the improved breeds would yield effectively nothing if kept like typical yak. On these assumptions, the performance of their hybrids with yak is clearly above the average of the parental types (in fact, providing a good example of what geneticists call "over-dominance"). This result, however, has its roots in genotype-environment interaction - and the argument is based on assumptions and not on direct experiment.

Less controversial evidence for heterosis arises from the poorer performance of backcrosses (B1) relative to the F1. It is again rare to find results, as the theory requires, on adequately large numbers of animals involving both the reciprocal F1 hybrids and both types of backcrosses (derived from both types of F1 mated back to both yak bulls and to bulls of the other cattle species). Nevertheless, the reported results from backcrosses are as would be expected if heterosis is a feature of this interspecies hybridization.

Some results from inter-species hybridization will now be presented. Although they show, without exception, that the hybrids between the species out-perform the yak, any interpretation of the results in terms of heterosis should bear in mind the considerations discussed above. Figures 7.1 - 7.3 illustrate different hybrids.

Figure 7.1 "Local" and "improved" F1 hybrids (Both F1 animals have yak dams. That on the left has a yellow cattle sire; the one on the right has a Holstein-Friesian sire. The animals are castrates, saddled as pack animals.)

Figure 7.2 "Improved" hybrid (Pian Niu) being milked

Figure 7.3 "Black" Pian Niu (F1 from yak dam and Holstein sire)

Reproduction and fertility

The female hybrids - F1 and backcross generations - have normal fertility. The males, by contrast, are sterile until there have been several generations of backcrossing to either the yak or to cattle.

Female

Oestrous

Oestrous in the F1 females is seasonal, as in the yak, and is affected by climate and nutrition. Sexual maturity in the F1, however, occurs at least a year earlier than is typical for yak. Thus the F1 females are usually mated at the age of 25 - 28 months (in the third warm season of their life), and they calve for the first time around the age of three years. Breeding a year earlier does occur under more favourable conditions, and has been regarded for many years as a potential advantage enjoyed by such hybrids in Kazakhstan (Denisov, 1938).

Importantly in terms of overall reproductive rate, oestrous can occur several times in a season if the F1 female has not previously been mated or is not pregnant. This differs from the majority of pure yak cows in China - although repeated display of oestrous in yak is more commonly reported from some other countries, e.g. Mongolia (see Chapters 5 and 11). Also, signs of oestrous in the F1 females are more obvious than in the pure yak.

Conception rate

Among 211 F1 females in heat, Cai Li et al. (YRO and XF, 1983) found, by rectal palpation, that 185 had normally developed follicles - a proportion slightly higher than in contemporary yak (see Chapter 5). Nonetheless, in ordinary mass mating with yak bulls, the conception rate (75 percent) of F1 hybrid cows (F1 from yak dams and yellow cattle sires) was somewhat lower than that of yak at the same location (shown in Table 5.5). But in a specific trial conducted by Cai Li et al. (YRO and XF, 1983) on the Xiangdong Yak Farm using yak bulls, 155 F1 females had a conception rate of 70 percent at their first oestrous of the breeding season, but a further 25 percent conceived at a second oestrous. When using frozen semen from Holstein-Friesian bulls by A.I., the overall reproductive rate of the F1 hybrid was found to be substantially better than that of pure yak. Zhang Rongchang (1989) reported a similar observation using Hereford semen (Table 7.1).

Gestation length

Cai Li et al. (in YRO and XF, 1983) recorded gestation length in hybrids of yak with local cattle as being 278 days (SD 9.7) for 110 F1 cows with male calves and 271.3 days (SD 11.1) in 98 such cows with female calves. Denisov (1938) measured an average gestation length of 282 days for F1 females mated back to Schwyz cattle bulls and 265 days for those mated back to yak bulls. Corresponding results from the Datong Yak Farm (Zhang Rongchang, 1989) showed a gestation length of F1 cows backcrossed to Hereford bulls (by A.I.) of 282.7 ± 7.8 days when carrying male calves (n19) and 277.4 ± 9.6 days with female calves (n6).

Table 7.1 Reproductive parameters of F1 and pure yak cows served by frozen semen from Hereford bulls through A.I. on Datong Yak Farm (1975 - 1978) [Source: Zhang Rongchang, 1989]

Reproductive and survival rate

Survival of calves to the age of six months is very similar in F1 hybrids and first backcrosses to that in pure yak. Thus, relative to the yak, the F1 hybrid has a better overall reproductive rate over a lifetime. This arises from a combination of factors: the likelihood that the hybrid will be mated for the first time a year earlier, the greater probability that the F1 hybrid will re-mate in the same season following failure to conceive at an earlier heat period and its greater capacity to calve every year.

The equally good survival of the first backcross (B1) in observations by Cai Li (1989) requires comment since it contrasts with the views expressed by herdsmen who regard this backcross as having poor survival. This viewpoint, however, has to be seen in the context that herdsmen generally wish to keep as much milk as possible from the F1 dams for their own use or for sale. Accordingly, the B1 calves are neglected by their owners and often allowed to die. This neglect is further encouraged by the poor performance of backcrosses relative to the F1 and by the complications inherent in systematic backcrossing programmes, which would be needed to produce successive generations.

Male

The external sex organs of the hybrid male are normal and so is its sex drive. Mounting and serving of females on heat is normal, but there are no sperm in the seminal fluid. The hybrid is, therefore, a "natural" teaser bull.

First and second backcross generations are also sterile. By the third generation of backcrossing (15/16 yak or cattle blood) some spermatocytes are usually present (or even in the second backcross generation, according to a report by Zhang Rongchang et al., 1991) and the occasional such male is found to be fertile. Fertility is not assured until the fourth or fifth generation of backcrossing (for a number of references on this aspect, see Zhong Jingcheng, 1994, 1996). In practice, entire males of the backcross generations are rarely seen because there is no good reason to keep them. Therefore, precise proportions of bulls showing normal spermatogenesis in successive generations of backcrossing are not available.

The precise causes of this sterility have been, and are still, the object of much investigation and speculation. Tan Chunfu et al. (1990), Zhao Shanting et al. (1990) Xu Zukang et al. (1991), Qing Fufang et al. (1990, 1993) and Liu Hui et al. (1994) found that the development of genital glands and organs in F1 males showed large differences from that in yak and cattle and also great variation among individuals.

What is certain is that no spermatogonia are found in the seminiferous tubules of the first hybrids and of the early generations of backcrosses (Zhang Rongchang et al., 1991). Possible causes for this have been considered in the structures of the X- and Y-chromosomes - these structures differ in certain respects in the hybrids from those found both in the pure yak and in pure cattle (Guo Aipu, 1983). In particular, the arm ratios differ, most notably for the Y-chromosome. It has been pointed out in a review of the subject (Li Jiyou et al., 1994) that the chromosomal arm lengths and the relative lengths of the chromosomes also vary among breeds of cattle. This has led the authors to suggest that fertility of the male yak hybrids might be restored by selecting a Bos taurus cattle breed with similar Y-chromosomal arm length as that of the yak. This is clearly open to investigation.

A tentative hypothesis is advanced by Zhong Jingcheng (1994, 1998) suggesting that male sterility may be due to an imbalance at many chromosomal loci, including autosomal loci. Tumennasan et al. (1997) found that reduced numbers of spermatogonia appear to characterise the testicular tubules of the F1 hybrid, and despite the identical cytological appearance of the two parental karyotypes, synaptic anomalies are seen at meiotic prophase in primary spermatocytes. The impression is gained of better meiotic pairing in the backcross B1 and B2 animals than in the F1; therefore the "Haldane Rule" is followed perfectly by the cattle-yak hybrid, namely, that sterility is confined to the male.

It has also been found that the proportions of different types of cells in the anterior pituitary gland differ in the hybrids from those in yak or cattle. A consequence is a reduced production of FSH - an essential hormone for the satisfactory function of the tubular epithelium of the testes. Yet, simple, frequent injection of FSH into the blood stream of four well-fed B2 hybrid calves did not result in the production of sperm - just a high libido in the calves at the age of 18 months (Xu Zukang et al., 1991).

Production characteristics

Body size

Birth weights of hybrids of yak with yellow cattle can be as much as 50 percent heavier than pure yak calves (shown in Table 6.1). Reports from other countries (outside China) also suggest substantial increases in birth weight in hybrids of yak females with males of local breeds - for example an increase of around 15 percent for Buryatia (Katzina et al., 1994) and 30 - 40 percent for Mongolia (Zagdsuren, 1994a). The actual results will depend greatly on the types of yak and the breeds of Bos taurus or Bos indicus used and, of course, on the local husbandry conditions. But the substantial increase in birth weight, generally found to occur, is responsible in large measure for the increase in calving difficulties in yak cows with hybrid calves compared to those with pure yak calves (cf. Chapter 4). These difficulties are further accentuated by the even bigger size of calves from sires of the "improved" breeds of cattle (Table 7.2). Further investigations might show whether sire breeds differ in the incidence of dystokia caused in the cows to which they are mated - as the experience from crossbreeding among cattle breeds elsewhere would suggest.

Within locations (Table 7.2) there is a marked difference between the birth weights of the pure yak calves and the F1 hybrids from "improved" breeds. There is relatively much less variation among the breeds of sire in the average birth weights of their hybrid calves. There appears to be a clear trend of increasing average birth weight of calf with increasing average body weight of the yak dam. However, caution is required in drawing that conclusion because the different body weights of yak dams are confounded with the different locations - although, taken at face value, the trend shows, for yak, a relationship between size of dam and birth weight of calf which would be generally accepted for cattle.

Table 7.2 Birth weights of calves of yak and different hybrids of yak at various locations (differing in elevation and environment) and body weight of local yak dams (average of male and female calves)

Sources of data: [1] AEA, 1984; [2] YRO and GISP, 1984; [3] Zhao Bingyao et al., 1984; [4] Zhang Jiachuan, 1984; [5] Hu Angang et al., 1960.

Body weights and linear body dimensions of adult females and steers are shown in Table 7.3 for yak and different types of hybrids, derived from observations in small herds in several parts of Sichuan province.

It is apparent from the results in Table 7.3 that the F1 hybrids from Holstein-Friesian bulls (or grade HF bulls) were the heaviest and largest, followed by the hybrids from yak mated to bulls of yellow cattle. The backcrosses were smaller than the F1. The backcrosses to yellow cattle bulls were actually smaller than the local yak, which suggests that the cattle involved in the hybridization may have been very small indeed - but in the absence of this information, this is speculation (but see also Table 7.4). The larger size and weight of steers relative to females corresponds to what was reported in Chapter 6. Similar results are shown in Table 7.4 but with data from all three types of animal, yellow cattle, Maiwa yak and their F1 hybrids (yak female mated to yellow cattle male), cohabiting in another area of Sichuan.

Table 7.3 Body weights and linear body dimensions of adult Maiwa yak F1 hybrids from Maiwa yak females mated to bulls of yellow cattle or Holstein-Friesian (or 75 percent HF + 25 percent yellow cattle) and backcrosses involving these types - in Hongyuan, Ruoergai and Ganzi counties of Sichuan [Pooled data from several farms; Source: Cail Li, 1989]

Note: f = female; s = steer (castrated male); m=male; HF* = Holstein-Friesian or (75 percent HF + 25 percent yellow cattle).

As seen from Table 7.4, the yak in the Ganzi area were somewhat larger and heavier than the local yellow cattle and the F1 hybrids between them were bigger than either of these. This certainly suggests a substantial effect of heterosis, greater among the males than among the females. Unfortunately, the results again preclude a strict estimate of the magnitude of heterosis because of the absence of the reciprocal hybridization (the "counter-hybridization" - female cattle mated to male yak) from these trials, and the lack of certainty that treatment of all the classes was identical. Again, if speculation is in order, one might assume, on the evidence of the adult size of the local yak and the local cattle, that hybrid offspring borne by local cattle females might, if anything, be smaller than the hybrids borne by the yak females. If so, estimates of heterosis (expressed as a percentage deviation from the mid-parent levels) would be lower than the values that might be inferred directly from Table 7.4 (these range for females from 2.3 percent for height at withers to 12.3 percent for body weight and for males from 10.7 percent to 53.3 percent, respectively).

Table 7.4 Body measurements and body weight of contemporary adult animals of Maiwa yak, local yellow cattle and F1 hybrids in Ganzi county of Sichuan [Source: Hu Angang et al., 1960]

Evidence from other studies shows that hybrids vary in size and body weight from district to district, as might be expected from differences between districts in their environment and in the types and breeds of yak and local cattle kept (Qiu Huai, 1957; Zhao Zhengrong, 1957; Zagdsuren, 1994b). (As an example of this, it may have been be noted that the "local" F1 hybrids, as adults, were much heavier in the data used for Table 7.3 than in the data available for Table 7.4)

In the case of hybrids of yak with "improved" cattle, the breed also has an effect - as illustrated in Table 7.5.

The animals included in Table 7.5 are the same as those from the Longri location described in Table 7.2, which gave the birth weights. In relation to these birth weights, the pure yak calves had increased their weights ten-fold by 17 months old, while none of the hybrids did as well as that. However, in absolute terms, the hybrids all gained much more in weight than the yak. The differences among hybrids from different breeds of exotic sire were small by comparison, although such differences can be seen - with the Holstein-Friesian hybrids the heaviest of those tested. Intermediate weights, at three-month intervals, were also available (not shown here). From them it is of interest to note that weight losses over winter were of the same order of magnitude (11 - 12 percent), relative to weight at the beginning of winter, in the hybrids as in the pure yak.

Table 7.5 Body weights and linear body dimensions, at 6 and 17 months of age of yak and hybrids with different "improved" breeds of cattle at Longri, Sichuan (means and [SD]) [Source: Cai Li, 1989]

Like the yak, hybrids of yak with cattle show similar responses to the seasonal cycles of grass growth, with rapid growth during the warm season followed by severe loss over the winter and recovery the following year (Xiao Zhiqing, 1984). Also, the hybrids are subject to the same seasonal effects on birth weight. For example, the birth weights of F1 hybrids from yak mated to Holstein-Friesian on the Xiangdong Livestock Farm in Sichuan increased as the season progressed. Thus, 25 calves (sexes pooled) born in April weighed 21.3 kg [SD 2.6] on average at birth, 28 calves born in May 23.5 kg [SD 4.0] and 31 born in June 24.3 kg [SD 3.6].

Meat production

A number of studies have also examined the meat output, at slaughter, of hybrids of yak derived from mating of yak with both local cattle and the different exotic breeds shown earlier (Tables 7.4 and 7.5). Numbers in the breed groups examined tend to be small and there are no consistent differences between the breeds of sire in dressing percentage or the yield of meat. Nor do these differ from pure yak. There are differences in carcass weight, but only in so far as this is related to slaughter weight.

In spite of the small numbers involved in some of these studies, it seems that, potentially, there is a large difference in growth rate during the first and second summer of life between yak on the one hand and its hybrids from the "improved" breeds of sire on the other (Table 7.5) and that this difference will be reflected at the time of slaughter. Of the animals described in Table 7.5 that were slaughtered at 17 months old, it was found that the hybrids had reached slaughter weights around 50 percent greater than the yak at the same age. The hybrids had already attained an adequate degree of finish at this age relative to the yak - at the end of their second summer. Thus the hybrids provided a proportionately much greater yield of meat from the carcass than the less mature yak at that age. The loin-eye area was also two thirds larger in the hybrids than in the yak. Zhang Rongchang (1989), quoting evidence for Altai in the former Soviet Union, noted that hybrids (Simmental or Shorthorn sires) had reached up to twice the slaughter weight of the yak at 21 months of age (though only three animals per group) and were also much fatter but had very similar dressing percentages (see also Chapter 11, part 2 - CIS countries).

Katzina et al. (1994) also reported slaughter weights of hybrid animals at 18 - 20 months old that were 25 - 30 percent heavier than for yak at that age in the Buryatia region of Russia. They reported further big increases in the growth of the hybrid if slaughter was delayed for another year. The Schwyz breed (cf. Brown Swiss) has been used in this hybridization with the yak in addition to the use of local breeds of cattle - and subsequently also the Jersey and Galloway breeds (but see Chapter 11, part 2 - section on Buryatia for more recent changes in practice).

When slaughter occurred at the age more normal for yak steers, above the age of 8 years - in a study conducted in Sichuan - it appears that there was little or no difference in meat production between yak and F1 hybrids (with local cattle as the sires). Zagdsuren (1994a), however, has shown that F1 steers from yak cows mated to local Mongolian (cattle) bulls slaughtered at six and a half years old had higher carcass weights and dressing percentages than either of the parent breeds. Carcass weights of these F1 steers were 49 kg heavier than those of pure yak and 19 kg heavier than those of pure Mongolian cattle.

Supplementary feeding of corn meal, urea and bone meal during summer, in addition to grazing, and supplementary feeding of concentrates during the winter, have both been shown to give a marked response in weight gain (Langjie Zeren et al., 1987) and in the proportion of meat in the carcass of hybrids of yak with, for example, the Holstein-Friesian (Li Xuewen et al., 1983). Under similar circumstances, pure yak showed very little such response. It is questionable whether such supplementary feeding is economically worthwhile in terms of meat output alone - the answer must depend on market circumstances. It is generally considered that supplementary feeding over winter is not cost-effective (but see the following section for the effect of supplementary feeding on milk yield and see Chapter 14 for a more detailed discussion of supplementary feeding).

Milk production

As noted in Chapter 3, one of the main purposes of hybridizing the yak with cattle of other species is to provide the herdsmen with additional milk for their own use or for sale. The larger quantity of milk made available is a consequence of the higher yield from such hybrids relative to the yak but also because all or most of the milk from the F1 is taken by the herdsmen. (As noted earlier, the progeny from the F1 females (the backcrosses) are reared for meat production and not for further breeding; in some cases, these calves are (or used to be) taken from their mothers at or soon after birth.)

Table 7.6 shows the daily milk yield and its fat percentage for different types of F1 hybrids (yak females with local cattle and yak with Holstein-Friesian or part Holstein-Friesian). (The source of the information is the same as for Table 7.3.).

Table 7.6 Daily milk yield and fat percentage of yak cows and F1 and backcross cows involving the yak and its hybrids with bulls of local cattle and with Holstein-Friesian (or 75 percent HF and 25 percent local cattle) bulls - in Hongyuan and Ruoergai counties of Sichuan province ten females per group [Pooled data from several farms; Source: Cai Li, 1989]

B = backcross, f = female, m = male

The F1 hybrids from the predominantly Holstein-Friesian bulls are seen (Table 7.6) to have yielded a much higher daily amount of milk than the yak. But the backcrosses in this case have regressed in yield, as would be expected if the initial F1 advantage is partly attributable to hybrid vigour. In the case of the ordinary F1 hybrid (F1 involving local cattle), the yield is higher than that of the pure yak, but the backcrosses, perhaps surprisingly, gave less milk per day than the yak cows. However, no milk yields are reported for the local cattle, which, as in comparable results of Joshi (1982) from Nepal, may have had lower milk yields than the yak.

Zhou Minhai (1998) reported that F1 cows with Holstein-Friesian sires gave 714 kg milk in a first lactation and those with sires that were themselves crossbred (Holstein-Friesian crossed with local cattle) gave 659 kg milk. These yields were significantly higher (P<0.01) than the reported first lactation yield of 388 kg for F1 cows from yak mated to "local" cattle sires and 244 kg for pure Maiwa yak cows (the number of cows in these studies involved is not known).

Other results from Hongyuan county in Sichuan also show that F1 hybrids from yak dams mated to Holstein-Friesian bulls (by A.I.) can produce milk in significant quantities - and much more than contemporary yak. Moreover, they start to do so a year earlier than is typical for yak - at the age of three instead of four years. If calving of the F1 females is delayed by a year to the age of four years, the first lactation yield is higher. Results for Holstein-Friesian and a mixture of other hybrids are shown in Table 7.7.

Table 7.7 Milk yield in 149 days of different types of F1 hybrid by age at calving (and parity) [Sources: YRO and GISP, 1984; Xu Guiling et al., 1983]

HF = Holstein-Friesian or crosses of HF with local cattle breeds
* half-lactation" - a second lactation in the year following first calving without a further pregnancy
** various breeds of exotic cattle sire used

It is also of interest from the results of Table 7.7 that the hybrids, like the yak, have the capacity to give milk in a second year without another intervening pregnancy - although, again as in the yak - the yield in the second year is reduced relative to that in the first.

How much heterosis is shown in respect of milk yield by hybrids of yak with Bos taurus or Bos indicus cattle cannot be clearly deduced from the literature - quite apart from the over-riding problems referred to at the start of this chapter. While the reduced yield of backcrosses relative to the F1, as in the results shown earlier, is consistent with heterosis in the F1, Zagdsuren (1994a), for example, presented results from Mongolia of F1 cows (yak mated to Mongolian cattle) that gave substantially more milk than those of either of their parent species (heterosis indeed, if the different groups were treated alike) - though with a fat percentage almost average between the two (see also Chapter 11, part 2). In contrast, Jain and Yadava (1985) present results from F1 hybrids of yak with hill zebu bulls in Himachal Pradesh which suggest that the daily milk yield of the hybrids was only half-way between that of the parent species (no heterosis) - but again no information is given on whether the three types of animals (ten animals in each group) were treated alike. It is quite possible that they were not, as all the animals were not from the same place. Surprisingly, the milk from this F1 hybrid is reported as having had a significantly higher fat content (7.32 percent) than that of the yak (6.45 percent) (in spite of a higher daily milk yield in the hybrid) and much higher fat percentage than that of the hill zebu cows (4.17 percent). This result is difficult to explain and at variance with other investigations.

Supplementary feeding of hybrids of yak and the Holstein-Friesian with urea, minerals and some ground corn during the warm grazing season led to an increase in milk yield of 10 percent in full-lactating animals (those that had calved that year) and 20 percent in "half-lactating" cows (those that had last calved in the previous year). This was considered to provide an economic return on the feed (Wen Yongli et al., 1987). Experiments with winter feeding, with silage, hay, roots, concentrates and minerals also raised the milk yield of hybrids of Holstein-Friesian with yak, but the costs of this winter feeding were too high in relation to the extra milk obtained (GISP and WLF, 1985). Moreover, the availability of supplementary feed in winter would be severely restricted in practice to localities where pastures have been improved and where agricultural products are also available.

Some exceptional F1 cows (from Holstein-Friesian sires) when housed and well managed on Institutional farms have been recorded as giving up to 1 800 kg milk (5.7 percent fat), which suggests that a potential for higher yield from yak hybrids exists, given the right circumstances.

Milk let-down

This is more easily and quickly achieved in the hybrids than in the pure yak. About 70 percent of hybrids, and especially those with sires of the "improved" breeds like the Holstein-Friesian, do not need the presence of the calf for milk let-down - the remainder do. Milk from the hybrid females reported on by Zhang Rongchang et al. (1983) came out of the udder about 50 percent more quickly than from the yak cows and the pressure required to extract the milk was lower. As in the yak (see Chapter 6) and in most cattle, the rear quarters of the udder of the hybrid develop better than the forequarters and provide, according to one study (Han Zhengkang and Lu Tianshui, 1980a, b), about 54 percent of the total milk.

Concluding comment on milk yield

Milk yield of hybrids of yak with local Bos taurus or Bos indicus cattle and also those with "improved" Bos taurus breeds is very dependent on the breeds used to create the specific hybrid and on the location and conditions of production. It is not known, for example, if the hybrids, and the "improved" hybrids in particular (with Holstein or other exotic sires) are given preferential treatment relative to yak cows. The results presented here are intended as examples only. The common factor in nearly all the results is that the "local" F1 hybrid (using the local cattle breed for hybridizing with the yak) yield more milk than the pure yak under what are presumed to be similar conditions and that the "improved" F1 hybrid (when, for example, the Holstein- Friesian is used as the hybridizing sire) gives more milk than the local F1 hybrid.

The magnitude of the differences in yield between the groups indicates why herders like "improved" hybrid cows and why such hybrids are "officially" encouraged under appropriate conditions. However, a true evaluation of their worth must also consider the costs and not only the returns (see also Chapter 3 for discussion of limits on the potential for hybridization).

Draught

F1 hybrids of yak females mated to bulls of local cattle, and the reciprocal hybrids, are widely used for draught - both for ploughing and as pack animals. The hybrids are easily tamed and appear to have better heat tolerance than pure yak, which make them very suitable for work. The hybrids produced from mating yak bulls to female (local) cattle ("false" F1 hybrid) are used mainly for ploughing.

Liu Qigui (1981) showed that with a traditional-style plough, two ordinary F1 hybrid steers making a furrow 15 cm wide can till around 0.13 ha per day. With a more modern plough (producing a 24.8 cm-wide furrow), two mature hybrid steers in medium fat condition ploughed 605 sq m in one hour on moderately moist, flat ground. Per unit body weight, the maximum draught power of mature hybrid steers in reasonably good condition has been measured as exceeding that of the pure yak. After ploughing was finished, respiration and pulse rates of the animals returned to normal in less than half an hour.

The F1 hybrid has an excellent memory, which makes it good as a pack animal and for riding. It can find its own way home to the campsite, like an old horse, without anyone in attendance. Herdsmen will ride a F1 hybrid without a saddle, for example when herding sheep, but if the animal is needed for riding over long distances, a saddle is used.

In tests of the F1 hybrid as a pack animal on grassland, the F1 hybrid has been found to carry 75 kg, equal to 16.3 percent of its body weight, and to cover 30 km in six hours - not unlike the performance of the pure yak (see Chapter 6). The respiration and pulse rates had returned to normal in 53 minutes after reaching its destination.

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