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In general, the reproductive rate of the yak is low under the normal grazing and rearing conditions of the principal yak territories. Female calves born early in the year may show oestrous for the first time when 16 - 18 months old. But those born later in the season will not show oestrous until they are more than two years old. Yak are seasonal breeders with mating and conception restricted to the warm part of the year. In a few areas, under favourable conditions some of the yak may be mated for the first time after they have reached the age of two years or, very exceptionally, even a year earlier. Normally and in most places, yak are not mated for the first time until they are three years old, and often not until four years. Thereafter, female yak are most likely to calve once every two years or twice in three years - producing, on average, perhaps four to five calves in a lifetime. Many yak cows will have only one annual oestrous, irrespective of whether they calved in that year or not. The next oestrous will often not occur until the following year. Under better conditions, in some areas and some countries, yak cows do show oestrous up to three or four times in a season, if they are not already pregnant. Statistics for Hongyuan county of Sichuan province, over the period 1976 - 1980, showed that from nearly half a million female yak of breeding age, 43.8 percent produced calves, which also survived, in any one year. Some breeds, such as Jiulong yak, may do better, and so will yak in areas where improved husbandry is practised. Clearly, the seasonal and general environmental conditions affect the reproductive rate markedly. In exceptionally harsh years, mortality rates of cows and calves can be very high.

Behavioural changes in the yak cow resulting from oestrous are not usually as clear as in other domestic cattle. The duration of oestrous is normally less than a day, although some reports give longer averages, and the range for individual animals is much greater. The average length of the oestrous cycle is approximately 20 days. Gestation length on average is around 258 days, shorter than in other cattle. It is longer when male calves are carried by the dam than it is with female calves. Also, the gestation length is longer, by as much as 20 days on average, when the yak female carries an F1 hybrid calf (having been mated to a bull, or inseminated with semen, of another species of cattle). Abortions and other causes of premature termination of pregnancy are between 5 percent and 10 percent when yak are bred pure, but higher in interspecies hybridization when the calves born are much heavier.

Physiological parameters in oestrous and gestation are similar to those in other types of domestic cattle. Hormonal treatment can be used to induce oestrous and can increase reproductive rate, though the evidence on conception rates following such induction is somewhat conflicting. Conception rates are nearly always much better following natural mating than after artificial insemination.

There is sufficient evidence to suggest that the reproductive rate of yak can be increased by a variety of improvements in management and feeding and by techniques to increase oestrous frequency and the conception rate. Successful detection of first oestrous and good timing of mating can be useful aids particularly when mating is artificially controlled or assisted. Whether the economic rewards from such improvements are a sufficient incentive to incur the costs involved is a separate question.

Male yak start to show mounting behaviour around the age of six months, but sperm have not been found in the ejaculates of yak males before the age of two years. Bulls usually start to mate at three or four years old but then have to establish their position in the mating hierarchy of the herd. Bulls reach the peak of their mating ability around the age of six to seven years. Bulls fight with each other for possession of females, and the dominant bulls have the most mates.

When left to their own devices, yak cows will not allow a bull of another cattle species to approach them, and such a bull has no chance of mating when in competition with a yak bull. In the same way, yak bulls prefer females of their own species. Wild yak bulls, however, will readily mate with domestic yak. All matings of yak cows with Bos taurus or Bos indicus bulls have to be assisted by people, either through keeping yak bulls away from the females or, more often, through restraint of the cow followed by use of a bull or artificial insemination.


The survival and spread of the yak species over the centuries testify to an adequate level of reproduction for this purpose - in the face of an adverse environment. In the context of livestock improvement, however, it is important to consider the limits set by reproductive rate. The reproductive rate affects the opportunities for selection of improved pure-bred stock and also the opportunities for hybridization systems where the yak cow is the dam, which normally depend on "surplus" numbers of pure-bred yak available to sustain hybridization. The reproductive rate also affects the economics of yak production. The purpose of this chapter is, therefore, to document the reproductive performance of the yak under various sets of circumstances. Consideration will also be given to components of the reproductive process, in order to indicate which of these is the most limiting, or most amenable to change in the yak.

The information is derived from investigations on experimental stations or in herds to which the investigators have been given access. Because farms or herds involved in investigations are, in that sense, exceptional, it cannot be known with certainty whether the results obtained are fully representative of those applicable to ordinary herds in remote areas. This problem is not unique to yak but needs to be considered in assessing the results. In situations where the taking of observations and records is not usual, the act of doing so might have led to some improvement in performance.

Some results are also provided on anatomical features and physiological parameters of the yak in relation to reproduction. Male reproduction will be considered in a separate section from the reproduction of females.

Reproduction in the female

Female organs

The structure of the reproductive organs of the yak differs in some respects from those of dairy cattle of the Bos taurus type. In order to facilitate and improve techniques for artificial insemination, Cai Li (1980a) dissected the reproductive tracts of 38 female Jiulong yak over the years 1976 - 1980 and found that:

Cui and Yu (1999) published a study detailing a number of the anatomical features of the reproductive tract of female yak at age one month, one year, two years and seven to ten years old (using a total of 45 animals). Included in their study are the size and weight of the ovaries (on average only a little more than 2 g in weight in the oldest group of 14 cows), the numbers of follicles exposed (on average between three and seven on each ovary but for some reason significantly lower in the two-year old females), and various dimensions of the uterus and uterine horns, oviducts and cervix. The cervix length and diameter (width) recorded by these authors was less than noted previously from the results of Cai Li - possibly because it involved a different type of yak (the Gannan yak) from a different location.

When using artificial insemination, the short corpus and the long septum make it reasonably easy to deposit semen in optimal positions, such as the corpus uteri, uterine horn or its tip, especially as the cervix is relatively free within the pelvic cavity and can be readily held.

Nonetheless, deep insemination of female yak, using a recto-vaginal technique, is more difficult in the yak than in ordinary dairy cows. The histological structure of the reproductive organs of the yak is similar to that in dairy cattle (Qiu Zhongquan and Zhu Qimin, 1981).

Oestrous of the female yak


Generally, first oestrous occurs in the second or third warm season (summer and autumn) following birth, at ages between 13 and 30 months. Generally, puberty occurs earlier in yak with larger body size under better nutritional conditions (Zhang Rongchang, 1989). Magash (1991a) made a detailed study of 104 female yak in Mongolia. The distribution of first oestrous in his study is shown in Figure 5.1. As noted already, oestrous occurred only in the warm season.

Figure 5.1 Frequency distribution of first oestrous in 104 female yak in Mongolia, by age [Source: Magash, 1991a]

The results of Magash (1991a) show that little more than 10 percent of the female yak came into oestrous for the first time in the second summer of their life, and that most females did not show oestrous for the first time until their third summer, when they were more than two years old. Magash pointed out that the 12 females that showed first oestrous when they were 16 - 19 months old had all been born in March or April of the previous year - and had managed to make more growth before the onset of winter than those born later in the season. Those not born until May or June had first oestrous delayed for a year, when they were 25 months old, or older. Magash concluded that the onset of first oestrous was determined more by body development at the beginning of the breeding season than by age. Very similar results, based on observations on yak in the (then) Tuva autonomous republic, were reported by Katzina and Maturova (1989). Yu and Li (2001) found, in 60 yak heifers in Gansu, that the sexual maturity in terms of cyclic activity came when the animals were 33 ± 6.7 months old.

In China, the majority of yak are mated for the first time at the age of three years - in the fourth warm season following birth, but under favourable conditions some yak may be mated a year earlier. Such conditions prevailed among 197 primaparous Jiulong yak cows in Sichuan province studied by Cai Li et al. (GAAHB and YRO, 1980): 32.5 percent of them calved first at three years old, 59.9 percent at four years, 6.1 percent at five years and the remaining 1.5 percent (three yak) at six years old. As shown in the next chapter, the three-year-old Juilong yak had reached about 78 percent their mature body weight (here the weight at six years old) (see Table 6.8).

In this context, Katzina and Maturova (1989) noted that in the Tuva autonomous republic (an area at more northern latitudes, but an elevation of only 1 500 - 2 500 m) female yak reached fertile oestrous at approximately 90 percent of mature body weight compared with 60 percent for Bos taurus cattle in that region. First mating at the age of two years, though it also occurs in China, is more common among yak in some other countries (see Chapter 11, parts 2 and 3).

Breeding season

Yak are seasonal breeders. The onset and end of the period in the year when female yak come into oestrous is affected by climatic factors, grass growth and both latitude and altitude. When temperature and humidity start to rise, the ground begins to thaw and grass starts to grow. The female yak then improve in body condition and gain weight - following their long period of deprivation and weight loss over the winter - and they come into season. On the northwestern grasslands of Sichuan this occurs around June (Hu Angang et al., 1960). At the higher elevation of Nakchu prefecture in Tibet, the breeding season may not start until July. Similar observations are reported from Kyrgyz where it was noted, rather precisely, that the annual onset of the breeding season started on 25 May at an elevation of 1 400 m and became progressively later until at the altitude of 2 700 m oestrous started on or after 22 June (Denisov, 1958) - though it would be surprising if these precise dates applied to every year.

Zhang Rongchang (1989) summarized information from different sources on the effect of elevation on the time of onset of oestrus in yak. For example, at an elevation of 1 400 m yak started to show oestrus around 29 May; at 2 100 - 2 400 m it started 10 - 15 June; at 2 700 m it started 19 - 22 June; and at 3 000 - 3 800 m it started on 25 June. In Nakchu, Tibet, at an elevation around 4 570 m the yak only came to oestrus in early July.

The breeding season reaches its peak in July and August when temperature is at its highest and grass growth at its best. Thereafter, yak oestrous decreases in frequency and stops around November. Two sets of data on the onset of oestrous by month of year are summarized in Table 5.1; one set derived from the Datong Yak Farm in Qinghai province and the other from the Chovosol district in Mongolia.

Table 5.1 Oestrous in female yak according to month of year


Percentage in oestrous from June to November[month]

















Zhang Rongchang, 1979








Magash, 1990

The type of distribution shown in Table 5.1 is fairly typical of the general situation. Accordingly, if mating and conception do not occur, some yak may not return to oestrous in that season. Yak showing only a single oestrous in a season are not uncommon.

The vast majority of all mating takes place naturally between bull and cow at pasture (several bulls competing for the privilege - see section, Puberty and mating). For mating to other species of cattle, the yak cows are normally restrained (as they are also in some areas for mating to yak bulls). For mating to "improved" breeds of cattle (e.g. Holstein or Simmental), artificial insemination is now always used (as the bulls of these breeds do not adapt to the climate and altitude).

Signs of oestrous

Changes in the appearance of the reproductive organs are more obvious than behavioural changes, although these also occur (Luosang Jiangcuo and Chen Yu, 1987; Zhang Zhaowang et al., 1997). The vulva becomes swollen and the vagina reddens. Mucus is discharged from the vulva in a majority of females in oestrous, but a substantial minority show no such discharge. Vagina and cervix dilate, and the female tends to raise her tail and urinate frequently. As in other cattle, female yak in heat search out and ride other females and like to be approached by male yak - but these signs are less pronounced than in Bos taurus cattle. When a specific mating is required by the herder and mass mating is not practised, heat detection usually requires the use of a teaser bull. It is thought that the use of a teaser bull, on female yak in heat, will increase pregnancy rate following either natural mating or artificial insemination. Katzina and Maturova (1989) also make the point that signs of heat in yak are less obvious than in other cattle, and detection by herdsmen is unreliable.

Daily milk yield shows a dip during oestrous although this could not, on its own, be used to accurately indicate the presence of oestrous (Cai Li, 1989b). However, Yu et al. (1993b) have reported that the pre-ovulatory peak of oestradiol 17b and the progesterone profiles in both blood and milk are similar in yak to those observed in dairy cattle. Magash (1991a) has similar findings.

Time of day of oestrous

Most yak start their oestrous in the early morning or in the evening and only rarely at other times of day. Among 633 female yak on the Xiangdong Livestock Farm, Cai Li (1989a) observed that two thirds of the animals started to show heat before 0900 hours when they had started grazing, and most of the remaining third started after 1900 hours when grazing had ended for the day. Similar observations were reported by Lei Huanzhang et al. (1964). Magash (1991a) with records on 73 yak in Mongolia found, by contrast, that only 38 percent came on heat between 0200 and 0800 hours, and 34 percent between 1600 and 2200 hours. This still left a substantial remainder to show oestrous outside those hours - mostly between 2200 and 0200 hours.

Table 5.2, from yet another set of data, shows the distribution of oestrous of yak cows on the Datong Yak Farm. It is difficult not to conclude, from these various studies, that the location of the herd or other environmental factors help to determine the time of day at which oestrus starts.

Table 5.2 Distribution of oestrous according to time of day [Source: Zhang Rongchang, 1989]

Time of day

0600-0900 hrs

1000-1200 hrs

1300-1800 hrs

1900-2200 hrs














Length of oestrous cycle

There is some variation in the length of the oestrous cycle from year to year. For example, the 1 184 observations by Liu Wulin and Liu Shengyu (1982) included in Table 5.3 were collected over a period of five years and showed that the annual mean length of the oestrous cycles varied from 19.2 to 21.6 days.

A feature of all the studies summarized in Table 5.3, except that of Yu et al. (1993a), is the large amount of variation among individuals. The coefficient of variation among these studies ranged from 16 percent to 41 percent. The reason for this is that oestrous in the yak is greatly affected by the environment. When the weather is unfavourable, the onset of oestrous is delayed; while in favourable circumstances, the onset of oestrous in female yak is advanced. The interval between heat periods can vary up to three-fold.

However, in different production systems there are reports of female yak capable of showing oestrous up to three to four times in the same season, as in the report of Katzina and Maturova (1989) for yak in the Tuva region, and Magash (1990) for yak in Mongolia, both referred to earlier.

Duration of oestrous

The duration of the oestrous period is not easily determined in the yak because the symptoms of oestrous are not always clear. Estimates from northwestern Sichuan suggest 12 - 16 hours (Cai Li, 1989a, b), while a report from yak in Shandan, Gansu province suggests 1.6 ± 0.8 days (Liu Wulin and Liu Shengyu, 1982). In a very small proportion of yak, oestrous may last four to five days, but one to two days is not unusual. Thus, a study with 41 well-fed and closely monitored female yak showed that 26 of them had an oestrous lasting 24 hours or less and three yak had oestrous up to 72 hours. More than 80 percent of these animals ovulated within 24 hours after the end of oestrous (Yu et al., 1993a).

Table 5.3 Various estimates of the length of the oestrous cycle (days)

Number of observations




1 184



Liu Wulin and Liu Shengyu, 1982




Liu Wulin and Liu Shengyu, 1982




Zhang Rongchang et al., 1979




TLRI, 1978




Yu et al., 1993a



(10 - 28)*

Katzina and Maturova, 1989




Purevzav and Beshlebnov, 1967



(10 - 27)*

Magash, 1991a

* Range

Zhang Zhaowang et al. (1997) reported a duration of oestrus of 12 - 48 hours for Tianzhu White yak. There is a tendency for the proportion of yak with heat periods of one to two days to increase later in the breeding season (August/September) when air temperature begins to decline. Katzina and Maturova (1989) reported an unusually long average duration of 3.7 days (from a range of 1 to 6 days) for yak in the Tuva region. In this context, it is interesting that the majority of their yak conceived in September. By contrast, Purevzav and Beshlebnov (1967) recorded substantially shorter heat periods in Mongolia. Among 54 Mongolian yak, 26 were noted to be in heat between only 0.5 and 6.5 hours, 17 yak in heat between 6.5 and 12.5 hours, 7 between 12.5 and 18.5 hours and only 4 yak with a longer oestrous duration. To account for some of the differences between the different studies, it is difficult not to conclude that the observational criteria of what constitutes the length of oestrous must vary among the studies.

Postpartum anoestrous

The average duration of postpartum anoestrous at the Xiangdong Livestock Farm in Sichuan province was found to be 125 days. That figure, however, was subject to much variation. At this farm, females that had calved did not usually show oestrous again in the year of calving (Cai Li, 1989b). The exceptions were cows, which had calved early in the season - before June - and which had acquired good body condition and good fat deposits over the summer. Postpartum anoestrous periods were found to be much shorter (70.5 [SD 18.5] days) for yak in good body condition than for those in a poorer body condition (122.3 [SD 11.8] days) (Liu Wulin and Liu Shengyu, 1982).

The anoestrous period following calving has been reported as related to the month of calving: 131 days, 124 days, 90 days and 75 days for females calving in March, April, May and June, respectively. (As reported earlier, only a few of the yak calving later in the season return on heat in the same year.)

Magash (1990) provided results on the interval between calving and first postpartum oestrous for female yak in Mongolia. These show clearly a relationship with month of calving - the earlier the calving, the longer the interval. However, in these results there was a considerable amount of variation around the average intervals. The results of Magash are shown in Table 5.4.

Table 5.4 Interval between calving and first postpartum oestrous in yak according to month of calving [Source: Magash, 1990]

Month of calving

Number of animals

Interval (days)







69 - 188





59 - 172





40 - 145





30 - 106





30 - 188


In the same study, Magash (1990) reported that, as might be expected, the service period following calving (on average only eight days longer than the oestrous interval) showed a very similar seasonal pattern to that seen in Table 5.4 for the interval between calving and first postpartum oestrous. Magash (1990) also showed for Mongolian yak that the interval between calving and the first postpartum oestrous was longest in cows that had calved only once (around 120 days) and that it then declined to its lowest interval in females between fifth and seventh calving (around 85 days) and increased again thereafter for older cows. Wang Minqiang et al. (1997) reported a calving interval of 459 days in a study of 439 parities of 161 cows on the Datong Yak Farm. This interval, they observed, was 25 - 48 days shorter among the cows of third to sixth parities than the interval for cows of both the earlier and later parities.

Erdenebaatar et al. (1997) attempted to shorten the interval to postpartum oestrous in the cows that had calved early in the year (from May to June) by various hormone treatments given early in the postpartum period (around 24 - 32 days after calving). But the results were not as expected, possibly due to failure, it was thought, of the activating hypothalamic-pituitary axis to support follicle development.

Proportion of cows in oestrous and calving

The proportion of female yak that come into oestrous in any one season depends on the previous calving history of the females as well as on their individual body condition. Female yak of reproductive age can be divided into three categories: those that have calved in the current year and are lactating and suckling a calf ("full-lactating yak"); those that calved in the previous year, are not pregnant but may or may not be still suckling their last calf (Yama or "half-lactating yak"); and those that previously had a calf but not for at least two years and are not lactating (Ganba).

On the Xiangdong Livestock Farm (from June to mid-September), Yama had the highest proportion of females in oestrous during that period (112/161), Ganba came next (217/408) and "full-lactating cows" had the lowest proportion (90/629) (YRO and XLF, 1983; Cai Li, 1989a). Similar observations were made by Zhang Rongchang (1979) in a study at the Datong Yak Farm in Qinghai (84.3 percent of Yama, and 36.6 percent of cows suckling a calf of the current year). Relative to the full-lactating cows with a calf at foot, the Yama and Ganba classes have had a better opportunity to recover from the drain on their body resources, consequent on calving and lactation, and more than 95 percent of them show oestrous (Ling Chenbang et al., 1982).

In ordinary production herds in the mountainous regions of China, a general average figure is that 50 - 70 percent of yak cows of suitable age show oestrous in any one year and that such female yak are mated and calve twice in three years, or once every second year. In one study, Cai Bolin (1981) found that 28.8 percent of Maiwa yak cows of reproductive age gave birth every year, 51.3 percent every second year and the rest, 19.8 percent, every third year. In another survey, by Lu Caijie (1982) in Tongren county, Qinghai, only 11 percent of yak cows gave birth every year, 75 percent twice in three years and 14 percent in every second year. The majority of yak in Damxung, Tibet calved every second year (Tang Zhenyu et al., 1982).

Use of hormones to induce oestrous

Various studies have shown that oestrous can be induced in yak and that the reproductive rate can be increased by that means. For example, at the Xiangdong Livestock Farm, Cai Li (1980b) gave an intramuscular injection of an analogue of LRH early in August - approximately one month after calving had ended - to induce oestrous in yak cows that had calved and were nursing a calf. Table 5.5 provides a summary of the results from the three-year investigation.

Table 5.5 Reproductive rate of yak cows with induced oestrous and normally occurring oestrous (control) [Source: Cai Li, 1980b]




Calved next year (%)

Calved and surviving (%)



























The results from Table 5.5 show that the induction of oestrous by hormonal treatment, followed by mating, was not very successful in terms of calves born in the first year of the trial (1976). But the proportion of cows in which oestrous had been induced that then calved and the number of calves that survived to the end of the year had improved markedly in 1977 and 1978. (Calf survival was not, apparently, reduced by the increase in calving rate). Results of a study by Magash (1991a) with yak in Mongolia agree in showing, albeit on much smaller numbers of animals, that the oestrous rate in female yak can be increased by hormonal induction, compared with untreated controls. But, in his trial, there was, generally, a reduced conception rate following artificial induction of oestrous (similar to the result in 1976 of Table 5.5). Thus, following a single hormonal treatment, the conception rate recorded by Magash was around 56 percent compared to 75 percent in the controls. Following two hormonal treatments, the conception rates in the treated and control groups were 78 percent and 86 percent, respectively. In another report, Magash (1991b) noted also that the success of oestrous induction increased from June to August, approximately doubling over that period (the actual oestrous rates depending on the method of induction). This is analogous to the increase in oestrous rate, which occurs naturally over that period, as was noted earlier (Erdenebaatar et al. 1997).

Other researchers at various locations in China have used different hormonal treatments to induce oestrous in the yak. These trials have shown, for the most part, that the onset and timing of oestrous can be controlled in the yak, as in ordinary cattle species (e.g. Shao Binquan and Zhao Yanben, 1984; Yang Tingyou, 1984; Liu Zhiyao and Shuai Weiwen, 1985; Shao Binquan et al., 1986; Yu and Liu, 1996). The use of triple hormone injections was usually the most effective in these experimental situations. In one set of trials, Chinese traditional medicine ["injecting Herba Epimedii compound"] (Ma Tianfu, 1983) produced an increase in the number of animals on oestrous compared with the control group (44 percent vs. 18 percent).

Zhang Yun (2002) reported that among 80 yak cows synchronized for oestrus by different hormonal treatments, 62 showed oestrus within seven days. Seventy-six of these cows were inseminated by A.I. and 54 of them (71 percent) became pregnant, though only 44 of them calved. From a study in Mongolia, Magash (1997) reported that oestrus synchronization was achieved by using the PGF2-a alone. Davaa et al. (2002) used four different treatments to induce oestrous: PGF2-a, progesterone sponge, PMSG and FSH - but with only small numbers for the last two. Overall, 28 (53 percent) of the 53 cows and 8 (31 percent) of the 26 heifers showed synchronized oestrus. These authors also pointed out that the success of synchronization and of subsequent conception were both markedly affected by the body condition of the animals - the better the condition, the better the success of the treatment. Other factors, such as age of cow, interval after calving and the type of hormone preparation, also influenced oestrus induction. (In this trial, the FSH treatment [four cows and four heifers] appeared to be the least successful of the four treatments.)

A general comment on reproduction in yak

From the evidence just presented, the conclusion might be drawn that the yak is reproductively poor - calving relatively late for the first time and not regularly thereafter. This, however, is entirely a description of the situation prevailing in the high mountainous regions and the high mountain plateau where the vast majority of the world's yak are found. It has to be borne in mind that in these regions, with a short summer growing season followed by a harsh, prolonged winter, there is a severe shortage of feed for the animals over several months, coinciding with the time when cows should be pregnant. The lack of sufficient feed over winter leads to loss of weight and body condition, both often severe, in the female yak. These substantial losses are only made up over the next summer season - provided that the summer is not marred by lack of rain. It is highly likely therefore that the principal reason for variation in the age of the first onset of calving and the subsequent frequency of calving is due to variation in the nutrition of yak cows over the winter period. Evidence from the effects of supplementary feeding trials supports this. Further evidence is deduced from the reported reproductive rate of yak in North America (see Chapter 11, part 3). The yak there, although a relatively small population, are given sufficient hay and other feed throughout the winter to eliminate weight loss. The majority of such female yak will then breed for the first time a year earlier than in traditional yak territories and thereafter calve annually. Under the conditions of the traditional yak regions, such supplementary feed may, of course, be neither available nor cost effective, even if it can be procured (but see also Chapters 8 and 14). The point needs to be made that the relatively poor reproductive record of the yak, compared to, for example, dairy or beef cattle elsewhere, is not a consequence of heredity but of environment. The ability to induce oestrus by hormonal treatment, as shown by the results presented earlier, is clearly of interest. As also shown, however, body condition affects those results. More investigation is certainly needed to discover whether conceptions thus induced, in the absence of adequate winter nutrition, might not further exacerbate the problems of yak reproduction in subsequent years.

Gestation and parturition

Pregnancy rates

Conception following mating at first oestrous of the season is generally high. Among 68 female yak on heat, Cai Li (1989a) found by rectal palpation that 53 of them had well-developed follicles and 15 did not, due to diseased reproductive organs. In a trial (Liu Zhengkui, 1981) with 265 yak that had calved previously, 72.4 percent became pregnant following the first oestrous of the season, another 23.4 percent following the second, and 3.4 percent and 0.8 percent following the third and fourth cycles, respectively. Zhang Zhaowang et al. (1997) observed a 76.5 percent (727/950) conception rate to a single service in Tianzhu White yak. In an investigation with 342 yak in Mongolia, Magash (1990) found that 70.5 percent were pregnant after a first service, 19.3 percent conceived to a second and 4.6 percent to a third service, giving an overall pregnancy rate of 94.4 percent. It appears in that part of Mongolia, at least, that yak which do not become pregnant at a first service are able to return to oestrous up to three times in the same season, as already referred to earlier. Conception to first service improved as the breeding season advanced, and was best in September.

In one particularly well-maintained group of yak on grassland in Gansu province where the yak had also been given some supplementary feed in late winter and early spring, a conception rate of 94.3 percent was achieved (Yu et al., 1993a). A pregnancy rate of 74.9 percent, following insemination with frozen semen at first oestrous of the season, has also been recorded by Cai Li (1989b) in trials with 621 yak.

Many studies, in addition to those quoted, have shown that once mating has occurred, whether naturally or by artificial insemination, pregnancy rates above 70 percent in female yak are not uncommon, provided matings have been to yak bulls (Table 5.6 and also: NIAVS and Datong Yak Farm, 1965; Du Fusheng, 1981, 1987a, b; TLRI, 1978; Luosang Jiangcuo and Chen Yu, 1987). There is, however, a marked difference between pregnancy rates resulting from pure-breeding of yak and those from hybridization. When the yak female is inseminated with semen (or mated by a bull) of other species of cattle, antagonisms appear to arise and the pregnancy rate falls. As apparent from Table 5.6, the proportion of calves born and surviving was more than halved when yak cows were mated to Bos taurus bulls. The situation was even worse, and dramatically so, when A.I. was used with semen from such bulls - only three calves survived from the 217 such cows initially available for service. A combination of circumstances led to this disappointing result, including in this particular situation poor detection or occurrence of oestrous, poor conception rate and high loss of foetuses from the few pregnant cows. It is possible that advances in methods and AI technology (Zhang Rongchang, 1979; Ling Chenbang et al., 1982; Li Shihong, 1985), developed since the time when the trial recorded in Table 5.6 was conducted, might have led to an improvement on that situation.

Data from the Datong Yak Farm in Qinghai province where semen from Hereford bulls was used by A.I. to inseminate 117 yak showed a conception rate of 43.6 percent from 1975 to 1978 (Zhang Rongchang, 1989). Similarly, frozen semen of cattle was used for A.I. with yak in Hongyuan county of Sichuan province and the average conception rate was 44.9 percent (12 526 yak) with some variation over the years and, on average, 25 percent of the cows mated ended up with a surviving calf (Table 5.7).

Table 5.6 Effect on success of mating, conception and calving in yak cows of different types of mating [Source: YRO and XLF, 1983, 1984]

Mating of yak female*

No. females of breeding age available

Mated (%)

Conceived of those mated (%)

Foetuses lost of those pregnant (%)

Calving of those pregnant (%)

Cow with surviving calves of those pregnant*** (%)

Yak male - natural service







Bos Taurus male - natural service







Bos taurus male - A.I.







Yak male - natural service after failure of A.I.**







* Natural mating to yak bulls: July-October; A.I. with Bos taurus semen: July-August; re-mating, to yak bulls, of cows which failed to conceive to A.I.: September-October.

** Note months of re-mating (see above).

*** Calves surviving to six months of age.

Table 5.7 Conception and calving in yak cows serviced by A.I. with frozen cattle semen in Hongyuan county of Sichuan [Source: adapted from Liu Shenqing, 1989]


No. of females mated

Conceived of those mated (%)

Foetuses lost of those pregnant (%)

Calving of those pregnant (%)

Cows with surviving calves of those pregnant (%)














3 774






2 841






3 212






1 626






12 526





Effect of age and physiological state

The three types of female categorized in terms of their current and previous calving history differ in pregnancy rate (see Table 5.8 and also: Ling Chenbang et al., 1982; Cai Li, 1989a). The age of the female also has an effect, with five- to six-year-old females being the best, on average, although not by a large margin. By the age of nine to ten years the conception rate starts to fall away. Some results are shown in Table 5.8.

(An explanation is required in respect of the youngest age group - the yak females aged three to four years old [this includes the summer following their fourth birthday when, strictly, the females may be four and a half years old]. Some of these animals will have calved previously at two or three years of age - that is the relatively small proportion of yak females that had achieved this because of particularly good body condition and other favourable circumstances earlier in their life. If they had not become pregnant again after their first calving, they were also categorized as Yama or Ganba, even though they were only three to four years old.)

Table 5.8 Conception rate of three types of yak female according to age

Type of female

3 - 4 years

5 - 6 years

7 - 8 years

> 9 years









Yak cow with calf*



























* Yak cow nursing calf born in current year.
** Yama = cow calved in previous year - with or without calf.
*** Ganba = cow that has calved, but not for at least two years.

The gravid uterus

Cai Li (1980a) dissected the reproductive organs of 38 female yak on the Xiangdong Livestock Farm (Sichuan), 17 of which had been pregnant between one and four months. Eleven of these 17 had the foetus implanted in the left uterine horn and six in the right. The size of the ovary and the oviduct on the side of the gravid horn was substantially larger. However, the maternal caruncles and cotyledons were about equally developed in both uterine horns. Up to one and a half months after conception, the gravid and empty horns had the same diameter; and up to that time the foetal placenta can invade both horns equally. Thereafter, the septum dividing the horns becomes indistinct and disappears as the foetus grows. The gravid horn sinks below the pelvic brim after two months of pregnancy. In early pregnancy, diagnosis of pregnancy by rectal palpation depended mainly on the size and shape of the two ovaries and on changes in the shape of the septum.

Mohanty et al. (2002) at the National Research Centre on Yak in India examined 16 placentas immediately after their expulsion following the birth of the calf and found that the average placental weight was 1.6 ± 0.5 kg and the total numbers of cotyledons in pregnant and nonpregnant horns were 45.0 ± 7.1 kg and 27.3 ± 4.1 kg, receptively. Total cotyledon area was 1494 ± 327 sq cm and the average calf birth weight was only 12.4 kg, giving a ratio of total cotyledon area to birth weight of calf of 123.0 ± 23 sq cm per kg. The authors suggested that malnutrition in the third trimester of pregnancy seems to be the major cause of low calf birth weight and low placental weight.

As in other cattle species, the progesterone level in milk increases during gestation, but in mid-cycle it falls greatly in barren cows (see Figure 5.2). Progesterone level can therefore be used in pregnancy diagnosis in yak and with a high degree of accuracy between about days 18 and 24 after mating (Xue Liqun, 1983; Magash, 1991b). Yu et al. (1993c) confirmed a rapid rise of progesterone levels both in blood and milk of yak starting around day two of pregnancy.

Figure 5.2 Progesterone levels in milk of 15 pregnant and 8 barren yak females between 12 and 30 days after mating [Source: Xue Liqun, 1983]

Gestation length

The gestation length of female yak is shorter than that of Bos taurus cattle, particularly when a pure yak calf is carried. Female yak bred pure have, on average, a shorter gestation length than when carrying a F1 hybrid calf. For yak with pure-bred calves, Denisov (1938) reported an average gestation length of 258 days; Lei Huanzhang et al. (1964) recorded an average gestation length in 36 yak of 260 days (range 253 - 278 days) for those carrying male calves and 250 (226 - 283) days for those with female calves on the Datong Yak Farm. Dubrovin (1992) reported an average gestation length of 258 days for 800 yak cows in the Caucasus; Katzina and Maturova (1989) noted an average gestation of 259 days (228 - 280 days) for yak in the Tuva region; and Yu et al. (1993) reported an average of 254 days [SD 2.7 days, a range of 248 - 258 days] for yak in Gansu province. Joshi et al. (1994) gave an average of 258 days for the gestation length of yak in Nepal.

The average gestation length of yak cows with F1 calves (cattle bull mated with yak cow) was found by Cai Li (1989a) to be around 270 days (273.2 with a SD of 12.7 days for 371 cows with male F1 calves and 268.6 with a SD of 10.2 days for cows with female F1 calves). In Cai Li's study, the breed of bull used also appeared to affect gestation length. This also was noted by Zagdsuren (1994) for yak in Mongolia. Denisov (1938) recorded an average gestation length of 276.2 days for female yak mated to Schwyz cattle.

On the Datong Yak Farm, the average gestation lengths of yak cows carrying male and female hybrid calves sired by the Hereford were 277 ± 6.2 days and 261 ± 7.6 days, respectively (Zhang Rongchang, 1989) and longer for yak cows with pure yak calves (260 days on the Datong Yak Farm; Lei Huanzhang et al., 1964). The longer gestation lengths for the hybrid calves are associated with substantially higher birth weights relative to pure yak calves (cf. Li Kongliang et al., 1989) and this, in turn is a cause of an increase in dystokia when yak cows carry hybrid calves (see the parturition section that follows).

The wide range of gestation lengths quoted by some authors seems surprising, as do some of the differences among studies. The wide ranges, when not otherwise stated, will include the differences in gestation length attributable to sex of calf and any differences that may be associated with age of dam (though none such are reported). It is also possible that there are differences attributable to breed of yak, which would be, in these circumstances, usually confounded with location (although, again, there are no specific reports of breed differences in gestation length for the yak, though such differences are documented for other types of cattle). In order to explain some of the more protracted gestation lengths, one must perhaps wonder whether both females with yak calves and those bearing inter-species hybrid calves are included together in the ranges, although this is not apparent from the reports. These considerations would still leave unexplained the very short gestation lengths reported at the lower end of the range. The problem with the ranges quoted is that they provide no clue to whether the extreme values are isolated cases and might include premature parturition.


Almost all births take place during the day and only very few occur at night when the yak cows are normally at the herders' campsite. When the time for parturition approaches, the female yak looks for a sheltered spot, such as a depression in the ground or a ditch, at a distance from the herd. Typical behaviour of the yak during labour includes lying on her side and standing up again for delivery when a pure yak calf is carried. When a hybrid calf is carried, the female will deliver the calf while lying on her side - presumably on account of the larger size of the calf and the longer time needed for the delivery (Cai Li, 1989b).

Dystokia is a rare occurrence in female yak with pure-bred calves; for example, in Gannan, Gansu, it was only 1.8 percent among 1 929 yak cows (Lu Huaijiang, 1995). Normally, herdsmen offer no assistance. The umbilical chord is broken by the act of mechanical stretching as the cow gets up or the calf falls down after delivery. Inflammation of the chord is rare. Yak cows with hybrid calves, however, require help for delivery and dystokia occurs to some extent. For example, there were 28 cases among 861 such calvings (3.3 percent) over a period of ten years in one study in Sichuan. In another study, Yuan (1991) reported that 20 out of 63 yak cows that were carrying Holstein hybrid calves retained their afterbirth, whereas normally such retention is less than 5 percent in yak.

Zhang Rongchang (1989) suggested that to reduce the incidence of dystokia it might be helpful, when producing hybrids, to use the yak cows of larger body size, with well-developed rump, which had already calved in previous years. As found in studies with cattle elsewhere, it appears that the relationship of calf size at birth to maternal size may be a critical factor in dystokia.

Twins are rare in yak; in general, they account for about 0.5 percent of all births, though higher rates have been recorded occasionally.


The dam generally licks the newborn calf for about ten minutes, after which the calf attempts to stand up and suck. Again, differences in behaviour have been observed between dams delivering pure-bred yak and F1 calves. Some results are presented in Table 5.9. They show that time intervals are markedly longer when a F1 calf is involved than when pure yak calves are born.

Table 5.9 Intervals between successive events at parturition according to type of calf [Source: Qi Guangyong, 1984]

Nature of events

Interval (range in ' minutes and '' seconds) between events

Pure yak calf

F1 hybrid calf

Appearance of calf to end of parturition

3' - 6'

45' - 107' (with help)

Calf out to calf being licked

0'2'' - 0'5''

0'3'' - 0'7''

Calf out to calf starting to stand up

14'2'' - 21'30''

60'0'' - 99'14''

Calf out to first sucking

15' - 22'

74' - 103'

Duration of first sucking

3'0'' - 5'30''

5'32'' - 11'21''

In a more recent study, Zhang Zhaowang et al. (1997) found in Tianzhu White cows that parturition was completed in 3 - 40 minutes and that the newborn calves stood up for suckling after about 20 - 30 minutes - both periods somewhat longer than in the study shown in Table 5.9. The placenta is ejected usually between half an hour and six hours after parturition

In the period shortly after birth, the dam is intensely protective of her calf and will attack any person coming close. She may not, however, do so directly. For example, the dam may retreat, as though afraid of the person, and then attack from the side or from behind. This is a time for people to take special care! (Cai Li, 1989b)

Bonding of dam and calf depends mainly on smelling and licking. Longer times of parturition and dystokia militate against such bonding and thus place F1 calves at a disadvantage to pure yak calves - at least on average. Yak cows can distinguish their calves by smell from among quite large groups of calves.

Calving season

The calving season is obviously connected with the time in the previous year when oestrous and mating occurred and is therefore prone to the same environmental and physiological constraints. Table 5.10 shows the distribution of calving from March to August at various locations.

On the basis of the distribution of the month in which yak cows in Mongolia were mated, as Magash reported by (1990), it can be estimated that a small proportion of the yak cows calved in March, probably around 25 percent in April, many more than that in May, the peak month, and a declining number in June and July.

Table 5.10 Percentage distribution of calving in different months of the year at different locations

Location [source]








Qinghai - Datong[1]







Qinghai - Datong[2]








Gansu - Zhangxian[3]







Sichuan - Ganzi[4]






Yunnan - Zhongdian[5]







* Including August.
Sources: [1] Lei Huanzhang et al., 1964; [2] Liu Zhengkui, 1981; [3] Xiong Zaiyue, 1982; [4] Hu Angang et al., 1960; [5] Jiang Ruisheng and Bai Yinhua, 1985.

Denisov (1958) reports from (what was then known as) "Kirgizia" that the calving season for a herd of 597 yak extended from February to December but with only five calves born in February, rising to 116 in March, 253 (42.4 percent) in April and 113 in May, with numbers tailing off rapidly thereafter.

When hybridizing yak to produce F1, it is common practice to attempt the hybridization - irrespective of whether by natural mating or A.I. - in the first half of the breeding season and to follow this by the use of yak bulls, to catch cows that have not conceived and have returned on heat. Thus, it is not unusual for the hybrid calves to be born earlier in the season than the pure-bred ones.

Calf survival - prenatal and postnatal

Abortions and other causes of premature termination of pregnancies account for perhaps 5-10 percent of all pregnancies, as was shown in Table 5.6 with observations from Sichuan. A similar incidence was reported from observations on 971 female yak in Nagqu, Tibet with an abortion rate of 5.4 percent, and a rate of 5.2 percent among 1 929 yak cows in Luqu, Gansu (Lu Huaijiang, 1995). On the Datong Yak Farm, Qinghai, 85.9 percent of 2357 pregnant yak cows calved normally (NIAVS and Datong Yak Farm, 1965).

As already indicated, the normal calving rate is lower when interspecies hybridization is carried out. Among 1 348 such yak cows (carrying hybrid calves) in northwestern Sichuan, 20 percent lost their calves during pregnancy and, in another study with 158 young pregnant yak females, 14 lost their hybrid calves before normal parturition (Ling Chenbang et al., 1982). Calf survival is generally high when the calves are allowed to suckle and the dams are not milked, but survival can fall greatly when the cows are also milked, as shown in Table 5.11.

Table 5.11 Survival of calves according to rearing method [Source: Wu Derun and Ma Juru, 1985]

Rearing Method

No. of cows

Calves born (%)

Calves surviving of those born*** (%)

Reproduction rate*** (%)

Dam milked*

1 366




Dam not milked**

2 542




Year of records:

*1975 - 1976

**1977 - 1979

***of cows mated with calf surviving to six months old

Since the two rearing methods were conducted in different years (Table 5.11), it is possible that a year effect might have affected the results. The apparent difference between the groups in the proportion of calves born is difficult to explain unless it is a "random" year effect, or unless it is a carry-over effect from the previous year. Such an effect could have arisen if the herder's decision to milk or not to milk is repeated across years so that cows milked in the current year were also those milked in the preceding year, and those not milked were also not milked in the year before (information on this point is not given). It would be surprising if a year effect negated the large rearing effect on calf survival after birth, as similar results to those shown in Table 5.11 have also been obtained in other sets of observations (NIAVS and Datong Yak Farm, 1965; GAAHB and YRO, 1980).

A 90 percent survival rate is typical for pure-bred calves, e.g. 1 328 out of 1 470 calves surviving among Jiulong yak in Sichuan (Cai Li, 1989b), and 1 818 out of 2 025 pure-bred calves on the Datong Yak Farm, Qinghai (NIAVS and Datong Yak Farm, 1965). In contrast to the greater problems before and during parturition experienced with F1 calves, the hybrid calves, once delivered, have a slightly better survival rate after birth than pure yak calves. Results at the same locations showed survival for F1 calves to be about 2 percent better than for the pure-bred calves.

Detailed observations (Ouyang Xi et al., 1984) on 20 yak calves on a farm at an elevation of 3 500 - 4 100 m showed that neonatal survival was also related to the maintenance of body temperature in the calf. The fall in temperature in the first hour after birth (average fall 0.38oC) was significantly correlated (r 0.69) with birth weight (the greater the weight, the less the temperature loss) but much less strongly correlated with ambient temperature. Thus, the body condition of the dam during pregnancy affected calf survival through its effect on birth weight. The body temperature of the calves returned to normal, after three hours on average.

To maintain the temperature of cows and newborn, a simple greenhouse made from clear plastic sheeting has been introduced to yak-raising areas in Gansu, Qinghai and Tibet to keep the animals inside during the colder times of the day.

Other factors influencing reproductive efficiency and calf survival

Inbreeding effects

Chen Youkang et al. (1994a) studied the effect of inbreeding on reproductive efficiency in 70 yak cows. They found that in this experimental herd, the inbreeding coefficient went up from the base generation (taken as zero inbreeding) to 0.11 in generation 1 and 0.19 in generation two. The interval between generations zero and one was 1.82 years and that between generations one and two was 2.61 years. At the same time, calf survival went down from 94 percent to 81 percent respectively for the two generations. The decline in conception rate and increase in calf mortality, which together were responsible for a marked reduction in reproductive efficiency, was attributed to the increasing inbreeding. Inevitably, different generations are born in different years. Unless special measures are taken to create an overlap of generations within year, a year effect could be confounded with the inbreeding effect. The inbreeding effect, showing a decline in reproductive efficiency is, however, in the direction expected (see Chapter 3).

Supplementary feeding

Yak cows given supplementary feed during pregnancy showed a small increase in the number of calves born, probably through reduced embryonic loss, and a somewhat greater increase in calf survival. This result is derived from an experiment conducted at Longri Farm in Aba county of Sichuan (part of a project undertaken by the Southwest Nationalities College [now University] with support from United Nations Development Programme and FAO). Results from this trial will also be referred to in Chapter 6, in relation to growth and milk yield; the design is described here only in outline.

Three groups of females, equivalent to each other in all respects and which had previously had normal opportunities to mate, were allocated to three feeding regimes. One group was given hay from mid-December to the end of April. The amount eaten by the cows in mid-winter varied around 4 kg per animal per day, but the intake fell to less than 1 kg, on average, in April. This treatment was repeated for two years. A second form of supplementation was practised in the first of the two years by allowing a group of cows access to grass paddocks that had been closed off in the autumn. The yak in this group were allowed into these paddocks with standing wilted grass for 45 days from the beginning of April - the latter part of pregnancy for most of the animals. The third group of yak cows received no supplementary feeding and were subjected to the management normal for the area. This group acted as the control. The results, in respect to the number of calves born and calf survival, are shown in Table 5.12.

Table 5.12 Percentage of Maiwa yak cows calving and calf survival from three groups of cows [Source: Wen Yongli et al., 1993]


Treatment group*

No. cows

Calving (%)

Calf survival (%)

Calves surviving Per 100 cows mated







Paddock grass





















* For details, see text.
a) fed hay from mid-December to end of April;
b) allowed access to conserved grass paddocks from 1 April for 45 days; and
3) unsupplemented, control group

In relation to the results in Table 5.12, it is tempting to suggest that the provision of hay may have stopped before the final stage of pregnancy, when foetal growth is at its most rapid. However, as indicated earlier, the cows began to eat less in the latter part of the feeding period as the first green shoots of grass started to appear on the pastures. Of course, the overall effect of feed supplementation cannot be judged by only the small increase, about 8 percent, in the proportion of surviving calves per 100 cows mated. Other effects, shown in Chapter 6, need to be considered. Unfortunately there is no information available on possible carry-over effects on the subsequent conception rate of the supplemented and control groups of cows. Results presented in Chapter 14 (Table 14.17) provide further evidence on the positive effects of supplementary feeding on reproductive rate and other aspects of performance.

Length of reproductive life

Exceptional female yak may live to an age of about 24 years, but 15 - 16 years is the normal upper limit for reproductive activity. The peak reproductive ability is considered to be between the fifth and the ninth year of life. Maiwa yak females reproduce normally for about ten years with a total of five to seven calvings (Cai Bolin, 1981) and Zhongdian yak cows showed their best reproductive performance between the ages of five and 11 years (Duan Zhongxuan and Huang Fenying, 1982). In one study by Ding and Chen (1994) 82 percent of females of reproductive age were ten years old or younger. Chen Youkang et al. (1994b) found five cows older than 20 years among 437 yak cows giving birth on the Xiaman Farm in Sichuan, but the survival of their calves was poor. Zhang Zhaowang et al. (1997) noted that the normal practice among herders of Tianzhu White yak is to use them for breeding up to the age of 15 years (see also Chapter 11, part 1).

Reproduction in the male

Male organs

The anatomical and histological structures of the genitalia of the yak bull are virtually the same as those of other bovine species, apart from the small, hairy scrotum - an adaptation to the cold environment (Qing Fufang et al., 1990, 1993; Xu Kangzhu et al., 1991). Pan Heping and Yan Ping (1997), Yan Ping et al. (1997) and Doyoddorji and Batbayar (1997) showed variously that the sizes and weights of testes and epididymis, as indicators of spermatogenesis, increased greatly from a young age right up to five years old. Doyoddorji and Batbayar (1997), for example, recorded that the weights of the right and left testes of 14 two-year-old Mongolian yak were 44.9 ± 3.6 g and 48.9 ± 2.9 g, respectively (similar to the weights of testes of 18-month-old yak in Pan Heping and Yan Ping's survey.). By the age of five years, the testis weights (from three bulls) were almost four times greater, and the epididymis weights had doubled. The Pan Heping and Yan Ping studies also included crosses between domestic and wild yak, but in terms of the sizes of testes and epididymis, or the increases in size, there was relatively little difference between the crossbred and the pure-bred males.

Puberty and mating


Yak males start to show mounting behaviour around the age of six months, towards the end of the first warm season in the year of their birth. In the following year, this behaviour continues and intensifies to include searching for female yak and mounting them. No sperm were found in the epididymal fluid of yak bulls before the age of two years in a study by Wang Xiaoxin (personal communication, based on a research report of a former Northwest Animal Husbandry Institute, 1964). Thus, puberty normally occurs in the third warm season following birth, when the male is more than two years old. In practice, bulls start to mate from the age of three years or, more usually, four years onward, reaching their peak ability at around six to seven years old - after establishing their position in the mating hierarchy after four years in the same herd. After the age of eight years, yak bulls start to lose to younger bulls in the competition for females (see section, Breeding season). Cai Bolin (1981) noted that Maiwa yak bulls were used for mating from three years old onward and that their peak ability came around four to nine years old. Zhongdian yak bulls started to be used for mating when four years old and showed peak capacity between the ages of 5 and 11 years (Duan Zhonxuan and Huang Fenying, 1982).

A study of 38 yak bulls at an A.I. stud in Tibet (elevation 4 300 m) supported the belief that the sexually productive life expectancy for a yak bull does not exceed ten years. The Tibet study showed that the ejaculate volume and the concentration and motility of sperm in the semen rose steadily from the age of three to nine years and then declined (Zhang Yun, 1994).

A study by Magash (1990) provided interesting additional evidence from Mongolia on the changes in the mating activity of yak bulls in relation to their age. This is summarized in Table 5.13.

Table 5.13 Mating activity and success of yak bull in Mongolia according to age of bull [Source: Magash 1990]

Age of bulls (years)

Cows mated

No. of mounts per cow

Percentage of cows pregnant




























The results from the Mongolian yak study (Magash, 1990) presented in Table 5.13 indicate that the older the bull (within the age range shown), the more females the yak bull was able to serve - consistent with the courtship behaviour and dominance hierarchy of bulls. Interestingly, the results also show that the younger bulls, with fewer females at their disposal, mount their mates more often. Thus, the three-year-old bulls mounted their mates half as often again as the seven-year-old bulls. It is also interesting that fertilization seemed to be more dependent on the number of services than on the age of the bull, in line with the pregnancy results of the same Magash study quoted earlier, which showed that the overall pregnancy rate of females increased with the number of services.


Bulls stay with the herd only during the breeding season. They spend winter and spring alone. Bulls can pick up the scent of females on heat at a distance of several kilometres - even 10 km have been reported.

Courtship behaviour

When with a herd of females, bulls will fight each other to obtain possession. Only the strongest attain the dominant position in the herd and such bulls have the most mates (as the oldest but virile group did, shown in Table 5.13). However, other strong bulls will also get an opportunity to mate cows, though in smaller number. Old, feeble bulls retain no mating position in the hierarchy of the herd. They then no longer try to mate and leave the herd. Young bulls do not usually win a place in the competition for mates until they are four years old and then only after some experience of fighting in the previous year. (Yak bulls in Mongolia, as judged from the results in Table 5.13, are apparently more precocious and reach that stage a year sooner). Such competition among bulls, to the extent that it introduces an additional element of natural selection, must provide the yak with some advantages in surviving in an unfriendly environment. By also ensuring that old bulls are replaced, generally before their daughters in the herd have reached breeding age, this competition for dominance may also have a role in reducing the chance for inbreeding to occur.

The extent to which inbreeding is avoided must be uncertain, as there is nothing other than human intervention to prevent bulls from mating their siblings or from bulls being succeeded in the herd by their sons.

Mating behaviour

Yak bulls get very excited sexually. Those that have won a mating position in the herd will mate several times a day. The bulls are so intent on mating that when in the process of doing so they will not attack other bulls, unless strongly provoked. It would be interesting to have information on the extent to which dominant bulls, which mate frequently, lose body condition as a result of their sexual activity over the mating season. This is known to occur in some wild animal populations (e.g. deer) and when it happens, it gives less dominant males the chance to mate.

Artificial insemination


Yak bulls can readily be trained to provide semen for artificial insemination, and, once trained, they will retain this capacity into the next breeding season. The bulls can be taught, in as little as seven days, to mount dummy cows and supply semen into an artificial vagina (Du Fusheng, 1987a, b; Li Kongliang et al., 1986; Zhang Yun, 1997). More detail is given in Chapter 8.

Semen quantity and quality

Volume and quality are generally considered good. In one trial with more than 14 collections (Du Fusheng, 1987), the average volume was 2.4 ± 0.9 ml with average sperm density of 2 680 ± 590 million per ml, motility of 0.82 ± 0.05 and a malformation rate of 8.3 percent (6.3 - 10.4 percent). The yak semen retains good fertility after diluting three-fold for fresh use and after pelleting and frozen storage.

Liu Hui et al. (1993) reported that the daily sperm yield in yak bulls is about 4.66 ± 0.47 x 109. This is lower than reported for Bos taurus cattle but higher than for zebu cattle (Bos indicus).

The densities referred to above are more than twice those reported by Zhang Yun (1994) from a station in Tibet (at an elevation of 4 300 m) producing frozen semen from yak bulls (the density values shown are also higher than those usually quoted for bulls of the "improved" cattle breeds [300 - 2 000 million per ml according to a review by Setchell, 1993] although the volume of ejaculate in such bulls is usually greater than in the yak). The sperm concentrations (per ml) reported by Zhang Yun (1994) were in the range 740 - 1 210 million per ml depending on the month of year and the age of bull. Ejaculate volume in these bulls varied with month and age, from 1.2 ml (in March) to almost 3 ml (in August) and from 2 ml in three-year-old bulls to 3.3 ml in nine-year-old bulls. Semen quality was also at its best from the nine-year-old bulls, and in the month of August in that part of Tibet.

Relations with other cattle species

When left to their own devices, yak cows do not allow bulls of other cattle species to approach them, and yak bulls show no inclination to mate cows of other cattle species. The reluctance to mate across the species has been reported from a number of different regions, as, for example, by Bonnemaire and Teissier (1976) in their studies from Nepal.

When forced to graze in herds of mixed species, the antipathy of the yak to members of the opposite sex from the other species of cattle declines, though it may not disappear altogether. When bulls of both species are present at the same time in a herd, a yak cow on heat will only allow a yak bull to mate with her. Bulls of the other species do not win in any competition with yak bulls for yak females.

In situations where a yak cow has been mated by a bull of another species and is then served again by a yak bull in the same oestrous period, the calf is almost always pure yak and not a hybrid - suggesting a preferential fertility for the yak sperm. Thus, to obtain hybrid calves, the yak cows must be kept with the bulls of the other species, and access to yak bulls has to be prevented. One way of doing this, as referred to earlier, is to restrain the yak cow in a mating crate and then allow her to be served by the bull of choice or by artificial insemination.

As was noted in Chapter 3, wild yak bulls readily mate with domestic yak females and produce wild-with-domestic crosses. In this case, there is no preferential fertilization from domestic yak bulls when they are present in the same herd alongside the wild yak bulls.


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