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Milk production


General considerations

There is, at present, no breed or strain of yak developed especially for milk production. All breeds are kept, to a greater or lesser extent, to produce milk, in addition to their other uses and products. Milk yield is closely related to pasture growth and quality and, in general terms, the amount of milk produced by the yak cow is considered as no more than the amount needed for the normal growth and development of its calf. In this respect, the milk yield of yak is more akin to that of animals in the wild than to the milk yield of dairy cattle. Even though the milk may be taken from the yak cows at the expense of the calf, milk and milk products of yak are important for the herdsmen and their families, in China and in most other yak-keeping countries. In commercial terms, milk is perhaps the most important of the yak products. The F1 hybrids with other cattle produce substantially more milk than the pure yak - the actual amount depending on the cattle breed used to produce the hybrid. The F1 hybrid has, therefore, considerable value to the herdsmen in the right situation.

When considering estimates of the milk production of yak, account has to taken of the milk consumed by the calf - which can only be estimated - and the quantity extracted by the herders. As a rule, yak females are not milked for the first month after calving, though perhaps only the first two weeks in some areas. During that time, the calf takes all the available milk, including the colostrum, on the day or two days after calving. As in other cattle, the quality of the colostrum of yak cows is much better than the milk produced thereafter (Liu Haibo, 1989; Zhang Rongchang, 1989). Table 6.12 shows that the total solid content of the colostrum from yak is about twice that of the later milk, while milk protein content can be three times as high and fat content between two- and three-fold of that in the later milk.

Table 6.12 A comparison of composition between colostrum and normal milk in yak [Source: adapted from Liu Haibo, 1989 and from Zhang Rongchang, 1989]

Species

No.

Milk type

Milk solid (%)

Fat (%)

Protein (%)

Lactose (%)

Ash (%)

Density kg/litre

Yak

17

Colostrum

32.0 - 34.5

13.2 - 15.8

15.1 - 17.1

1.7 - 1.9

0.9 - 1.1

1.03 - 1.07

Yak

33

Normal milk

16.9 - 17.7

5.5 - 7.2

4.9 - 5.3

4.5 - 5.0

0.8 - 0.9

1.03 - 1.04

After the initial period when the calf obtains all the milk provided by the dam, it is estimated that the calf takes about a third of the available milk if the yak cow is milked twice daily and about half the milk with once-a-day milking. Yak females produce about a third more milk, in total, if stimulated by milking twice daily compared with once a day (39 percent more in a study by Cai Li in Sichuan, and 26 percent more in an investigation in Qinghai (Lei Huanzhang et al., 1983)).

There is no generally agreed upon method of assessing yak lactation milk yield. Production over a lactation period of 180 days has been proposed and estimated from the yield on three to five successive days and the use of coefficients based on the month in which the milk yield is measured. The coefficients, in turn, are based on the fact that yield is higher in months of high pasture growth than either at the beginning or end of the grass-growing season. To the estimate of yield derived from hand milking has to be added an estimate of milk consumed by the calf. Though such methods of estimating yield are attractive in principle, they suffer from the further difficulty that the coefficients for different months vary greatly for different locations and dates of calving; hence the absence of general agreement on the use of these methods.

Table 6.13 Milk yield and fat percentage of yak females of different breeds at various locations of China (± SD)

Province

Yak breed or yak location*

Month of measurement

No.

Average Daily yield (kg)

Estimated lactation yield (kg) (in days)

Fat (%)

Source

Gansu

Tianzhu White

5 - 10

223

2.3 ± 0.5

304 (135)

6.8 ± 1.3

Zhang Rongchang, 1989

Gansu

Plateau (in Shandan)*

5 - 11

21

2.6 ± 0.6

464 (180)

5.4 ± 1.5

Zhang Rongchang, 1989

Gansu

Gannan

5 - 10

15

1.8 ± 0.2

315 (177)

--

Research Co-operative Group, 1980-1987

Qinghai

Plateau

6 - 10

181

1.4 ± 0.3

214 (153)

5.6 ± 1.2

Research Cooperative Group, 1980-1987

Qinghai

Huanhu

6 - 10

96

3.0 ± 0.3

487 (153)

6.4 ± 1.4

Zhang Rongchang, 1989

Qinghai

Guoluo*

7 - 11

20

1.0 ± 0.2

162 (153)

6.6 ± 0.7

Li Quan et al., 2000b

Sichuan

Maiwa

4, 7

20

1.8

365 (150)

6.8

Chen Xiafei et al., 1981

Sichuan

Jiulong

7, 8

93

2.8 ± 0.2

414 (150)

5.7 ± 1.0

Cai Li et al., (1980a)

Tibet

"Plateau"*

7 - 9

19

2.7 ± 0.3

280 (105)

--

Zhang Rongchang, 1989

Tibet

Alpine*

8 - 10

41

0.92


6.4

Research Co-operative Group, 1980-1987

Tibet

Jiali **


48

0.8 ± 0.2

148 (180)

6.8 ± 1.3

Ji Qiumei et al., 2000a

Tibet

Pali **


25

1.0 ± 0.2

200 (180)

5.9 ± 0.7

Ji Qiumei et al., 2000a

Tibet

Sibu**


36

0.9 ± 0.1

180 (180)

7.5 ± 1.4

Ji Qiumei et al., 2000a

Yunnan

Zhongdian

5, 7, 11

81

1.1 ± 0.2

132-302 (180-210)

6.2 ± 1.6

Zhang Rongchang, 1989

* Location of yak within the province - for Tibet: "Plateau" here refers to Pali breed, Alpine to Jiali.
**Yields for these breeds are based on varying proportions of yak females in the lactation following calving and a lactation in a second year without calving again ("half lactation"). The "full lactation" yields for these breeds as shown in Table 11.5 (Chapter 11) are Jiali:192; Pali: 215; Sibu: 216.

Factors influencing milk yield

Breeding

Table 6.13 provides the estimated milk yields and fat content in milk of different yak breeds at various locations in China. These yields are the amounts milked by hand with an adjustment for milk taken by the calves. Table 6.13 indicates that, in general, the milk yield of the yak is low, but the fat content is relatively high. The results also suggest that the Huanhu yak in Qinghai had the highest milk yield and yak in Tibet had mostly the lowest.

Table 6.14 Milk production in crossbred* and domesticated yak [Source: adapted from Jialin et al., 1998a]

Month

Crossbred yak (n=20)

Daily milk yield (kg)

Fat content (g/kg)

Monthly milk yield (kg)

Mean

SD

Mean

SD

Mean

SD

May

1.1

0.02

47.5

3.6

32.9

6.0

June

1.7

1.2

53.5

8.7

49.8

6.7

July

2.1

0.17

49.4

4.9

60.4

5.2

August

2.3

0.28

57.7

5.4

68.9

8.5

Significance

Milk yield over 120 d (kg)

Mean = 212.2

SD = 20.2

Mean daily milk yield (kg/d)

Mean = 1.7

SD = 0.16

Mean fat content (g/kg)

Mean = 52.0

SD = 2.9


Crossbred yak (n=10)

Daily milk yield (kg)

Fat content (g/kg)

Monthly milk yield (kg)

Mean

SD

Mean

SD

Mean

SD

May

1.1

0.21

50.9

14.6

33.3

3.5

June

1.1

0.21

50.9

14.6

33.3

3.5

July

1.7

0.10

47.6

8.5

51.2

3.0

August

1.8

0.28

68.8

6.7

54.6

8.5

Significance

Milk yield over 120 d (kg)

Mean = 184.5

SD = 10.5

Mean daily milk yield (kg/d)

Mean = 1.5

SD = 0.10

Mean fat content (g/kg)

Mean = 53.5

SD = 4.1

** Wild yak semen was used to inseminate domestic yak cows to produce the crossbred.

As stressed in respect of other breed comparisons in this book, some caution is needed in interpreting the variation in milk yield among the breeds: The "breed" differences may be associated with different herd sizes, forage availability, daily milking times (once a day or twice a day) and with differences in management among locations. Also, as noted elsewhere in this book, even estimates for the same breed can vary among different studies because different conditions apply to the observations.

Table 6.14 shows that a herd of crossbred yak (domestic yak dam crossed with wild yak sire), milked once daily after suckling of the calf, had a higher daily milk yield and total lactation milk yield than ordinary domestic yak, but there was no overall difference in the fat content of the milk.

Effects of age and parity

Lactation milk yield increases with the age of the female up to about 10 or 11 years old and also with the number of calvings (Zhang Rongchang et al., 1983; Xu Guilin et al., 1983) - although these two factors are rather closely associated with each other. Figure 6.3 shows seasonal changes in yield in six different lactations.

Figure 6.3 Milk yield of yak females in five (warm) months of the year in six separate lactations (Number females per lactation: 1:20; 2:20; 3:13; 4:10; 5:10; 6:18)

There appears to be no evidence to show that yak cows have a lactation peak in relation to calving date - as found in dairy cattle; an overriding effect on milk yield in the yak seems to be that of pasture growth, reflected by month, in each of the six lactations as seen in Figure 6.3. Similar effects of parity and the month in which the cows lactate have also been shown for Jiulong yak (at an elevation of 3 200 m) in a substantial study by Wen Yongli et al. (1994) involving between 65 and 128 animals in each parity group.

Other correlations with milk yield

Lang Jie et al. (1999) reported that in breeding groups of Maiwa yak the correlation of milk production of mother and daughter was positive and linear (r=0.374, P<0.05) - of course, this does not provide an estimate of the heritability (the strength of inheritance) of milk production in yak. These authors also reported that the sizes of various linear body dimensions of yak in their first lactation are positively (and significantly) correlated with milk production (r in the range of 0.32 - 0.56). This shows a tendency for the better-grown females to yield more milk. This correlation was much weaker for female yak in later lactations.

Environmental factors affecting milk yield - the lactation curve

Herbage supply. As referred to earlier, one of the important factors influencing milk yield is pasture production - the quantity, growth status and nutritive value of the herbage. These are, in turn, affected by climate and season (cf. Chapter 13). All lactating yak, irrespective of age, parity or breed type and even location, tend to peak in yield in July and August when grass is at its best in terms of quality and quantity (cf. Figure 6.3). These months are known in yak-producing areas of China as the "golden age". Before July, though the grass has started to turn green and to grow, the amount of grass available is not high. After August, as air temperature falls, the nutritive value declines - as the grass produces seeds and then wilts, and the content of crude fibre of the grass is high. Table 6.15 provides some information on the composition of grass (but not on grass quantity) from samples taken on pastures at an elevation of 3 600 m in Hongyuan county of Sichuan province. The pasture samples were representative of the herbage grazed by the yak. (For more information on composition of different pasture species, see Chapter 13.)

Table 6.15 Percentage composition (on a dry-matter basis) of grass on meadow in different months over the warm season [Source: Sichuan Grassland Institute, 1982]

Date

Dry matter

Crude protein

Crude fibre

Ash

N-free extract

Calcium

Phosphorus

9 June

24

18.8

27.9

5.0

42.0

0.50

0.25

30 June

28

11.8

30.0

5.0

48.6

0.43

0.25

30 July

33

12.4

27.6

6.1

50.9

0.42

0.33

30 Aug.

38

10.0

30.8

8.2

48.9

0.61

0.26

29 Sept.

42

8.1

33.3

5.7

50.5

0.62

0.14

31 Oct.

62

4.8

34.8

5.0

53.5

0.53

0.16

Month of milking. In Hongyuan county, from where the data on grass composition in Table 6.15 were derived, the peak of monthly milk yield was in July for yak cows and in June for hybrids (both F1 [Pian Niu] and backcrosses to cattle [one quarter-bred yak]). The monthly yields and the fat percentage of the milk are shown in Table 6.16.

Table 6.16 Monthly milk yield and fat percentage of milk of yak, F1 (Pian Niu*), and one quarter-bred yak** in Hongyuan county of Sichuan



No.

June

July

Aug.

Sept.

Oct.

Nov.

Total

Yak

Milk yield (kg)

13

40.4

40.6

37.3

27.0

20.0

11.0

176.5

Fat percent

6


5.6

6.6

6.5

7.6

7.8

6.8

(F1)

Milk yield (kg)

12

64.7

61.3

43.1

29.5

17.9

11.0

227.5

Fat percent

5


5.1

5.4

5.8

6.8

6.6

5.9

¼-bred yak

Milk yield (kg)

10

48.5

40.9

40.9

29.5

19.4

6.7

185.9

Fat percent

7


6.0

6.0

6.0

7.0

7.6

6.6

* The one half-bred yak from local Bos taurus (yellow cattle) bulls used for crossing with yak.
**The one quarter-bred yak are backcross offspring of the F1 (Pian Niu) dams mated to bulls of the yellow cattle.

The milk yields of the cows represented in Table 6.16 are lower than those represented in Figure 6.3, but both show the seasonal decline in milk production. It is also seen that, as found in dairy cattle, that the percentage of fat in the milk increases as the season advances and the quantity of milk declines.

Similar results to those presented on the period of peak lactation have been obtained from various locations, all showing that the supply of grass is the major factor influencing yield and that parity of cow has a much smaller effect. Another aspect of this, from a more northerly region (the Vostochnyi Sayan area of the Russia Federation), derives from a study of Katzina (1993). She reports that yak which had calved early in the year, in April, attained their maximum udder volume in the third month of the lactation, while those that did not calve until June attained their maximum udder volume of yak in the first month of lactation. Thus, both groups would have their largest udder volume around the same month of the year.

Month of calving. The study by Katzina (1993) also showed that yak that had calved in April had a maximum udder volume, which was far greater (two-and-a-half-fold) than that of yak that calved in June. The April-calving yak, in that part of the Russian Federation, lactated for six to seven months to produce around 360 kg milk while those that did not calve until June lactated for only four to five months to produce 150 kg of milk on average.

An interrelationship between the month of calving and the month of milking is shown particularly well in Table 6.17, with data from Mongolia. This re-affirms the month of milking as the most important variable for milk production - reflecting the importance of growth and quality of the available herbage.

Table 6.17 Monthly milk yield of adult cows (based on quantity milked plus amount of milk taken by calf) according to month of calving and month of milking [Source: State and Cooperative Farms: Bat-Erdene, 1993]

Month of milking*

Month of calving

March

April

May

June

1

65.2 (41 - 91)

70.1 (49 - 94)

70.1 (58 - 100)

112.6 (82 -128)

2

63.6 (47 - 85)

63.3 (48 - 81)

101.5 (67 - 154)

118.3 (79 - 132)

3

64.3 (41 - 90)

105.5 (71-130)

108.5 (78 - 146)

102.0 (71 - 111)

4

107.9 (76 -158)

107.3 (93 - 132)

109.3 (80 - 153)

87.2 (62 - 97)

5

115.8 (87 - 174)

99.8 (71 - 126)

82.0 (64 - 109)

65.6 (37 - 94)

6

104.7 (73 -151)

83.5 (68 - 107)

61.9 (44 - 90)

48.1 (27 - 84)

7

84.5 (59 - 119)

58.8 (40 - 84)

40.5 (25 - 90)

29.7 (16 - 31)

8

60.4 (42 - 90)

52.9 (25 - 73)

33.5 (16 - 48)


9

44.7 (25 - 84)

31.3 (16 - 43)



10

26.1 (16 - 45)




* Month of milking counted from month of calving (e.g. calving in May, first month of milking is May, second month of milking is June etc.) July, as the month of milking, is shown in bold

Weather. Xu Guilin et al. (1983) recorded that the weather conditions on the day of milking affected yield on that day. Thus, in comparison with milk produced on a clear day, milk production on a cloudy day was 1.9 percent higher and on a rainy day 7.7 percent greater. Zhang Rongchang (personal communication, 2000) also found that variation of milk yield due to the weather condition can be up to around 8 percent. According to the Qinghai Provincial Meteorological Bureau, the optimum temperature for yak is around 5° - 13°C (Zeng Wenqun and Chen Yishi, 1980). Yak appear to produce less milk when the ambient temperature is too high especially when this is associated with strong solar radiation and a lack of wind on a clear day. Conversely, more milk is produced during cloudy or rainy conditions. It is not immediately obvious whether the small increases in milk production associated with the weather result from an increased intake of herbage or of water, or some other factors related to the environment - or from associated variation in the metabolic rates of the yak (the latter possibility being favoured by Zhang Rongchang, personal communication, 2002).

Effects of winter and cessation of milking. As air temperature starts to fall after October and winter approaches rapidly, hand milking of the yak females is normally stopped. As indicated earlier, a yak does not dry off in consequence of this and continues to secrete a little milk for its calf. This happens irrespective of whether a female yak is pregnant or not. Only a cow that has lost her calf during the warm season will dry off when hand milking ceases. Similarly, a female that is pregnant but has been isolated from her previous calf will dry off when hand milking stops. Other pregnant females with a calf still at foot will not go dry until their one-year-old calf is removed prior to the next calving. Finally, a cow, even though not pregnant at the onset of winter but still suckling a calf, will continue to lactate through the following warm season and, normally, will be hand milked again. The "half-lactating" female yak (as it is called) will stop being milked at the end of her second warm season and will then go dry, irrespective of whether she is pregnant or not.

Supplementary feeding. Some trials have been conducted with supplementary feeding of conserved grass - silage, hay or straw - during winter months to study the effect on the subsequent lactation. In this context, Long et al. (1999a) reported a trial with 33 Tianzhu White yak cows at grazing that were given supplementary feeds of 1.0 - 1.5 kg highland barley straw or oat hay per animal per day, from the beginning of December to the end of April. This supplementation resulted in an increased lactation length (P<0.01) over the whole milking period, but there was no difference attributable to the type of straw used. Milk yield was also increased, but not significantly, perhaps because the supplementary feeding was stopped before calving began (see also Chapter 13 and Table 13.17).

Zhou Shourong (1984) conducted a trial with nine yak cows in Sichuan in the winter months. He found that supplements of 4 kg of silage per head per day led to a rise of milk output from 150 to 350 g per day, after only seven days of supplementary feeding and to 500 g of milk after 15 days of feeding.

However, the difficulties and costs involved in conserving grass as hay (other material is usually not available) may make supplementary feeding uneconomic over the winter and spring as a means of promoting milk production at that time of year. Silage, as used in the trial referred to above, is not a frequent option as it freezes solid in the cold winter conditions of the region if not very heavily and expensively protected. It is relevant to note in the context of supplementary feeding, that the yak cows are not in good condition during the winter and the additional strain of milk production may make matters worse for them. Also, herdsmen are reluctant to hand milk during the bitter cold of winter.

Nonetheless, there is at least one theoretical reason why supplementary feeding over winter is worth further consideration even if milk is not taken from the cows at that time. Interest centres on the possibility that such feeding may improve the condition of the cows and lead to improved calf production, reproductive rate and milk yield in the following warm season. The supplementary feeding carried out experimentally at the Longri Breeding Farm in Sichuan should be seen in that context (Table 6.18).

Table 6.18. Milk yield over a period of 184 days, fat percentage in milk, and weight loss over winter of Maiwa yak cows of three groups: a) fed hay from mid-December to end of April, b) allowed access to conserved grass paddocks from 1 April for 45 days, and c) unsupplemented, control. Least squares means and standard deviations) (Source: Wen Yongli et al. 1993)

Year

Treatment group

No. cows

Milk yield (kg)

Fat (%)

No. cows

Weight loss (kg)

Mean

SD

Mean

SD

Mean

SD

1989/90

Hay

54

229.0

53.3

6.0

1.3

58

33.5

14.8

Paddock grass

55

220.6

54.7

5.8

1.6

58

34.5

15.5

Control

113

218.3

49.9

5.4

1.6

110

35.5

14.5

1990/91

Hay

50

235.4

53.6

5.9

1.3

59

39.9

18.0

Control

137

224.1

47.6

5.4

1.2

150

42.7

17.4

As seen from Table 6.18, the supplementary feeding appears to have had small but positive effects on milk yield and, unexpectedly (because of the increase in milk yield), on the fat percentage of the milk which is reported to have increased by approximately 0.5 percent (from less than 5.5 percent to nearly 6.0 percent). The estimate of milk yield is derived from measured amounts milked three times per month and does not include, as far as is known, milk sucked by the calves.

The cows given supplements lost slightly less weight over winter than the unsupplemented controls (P<0.05). However, the weighing at the end of the winter period was delayed until the end of May, by which time the cows may have started to recover some of their condition, especially because, in 1990, pasture growth at this farm had started exceptionally early. Weight loss alone also does not fully reflect possible changes in body composition, fatness in particular.

Feeding of additive. An imbalance or deficiency of minerals in forages at pasture may limit milk yield of yak. Additives may therefore improve lactating performance of grazing yak. Table 6.19 shows the effect on milk production of a mineral mixture (CuSO4×5H2O, ZnSO4×7H2O, CoCl2×6H2O, NaCl, KI, NaSeO3, CaHPO4×2H2O) (Yuan Youqing, 1994) given as a salt-lick block. The results of such addition during lactation indicate that milk yield of full-lactating yak cows (milking in the year of calving) was increased by 15.5 percent and that of half-lactating cows (those milking into a second season without calving again) by 14.8 percent.

Long Ruijun (1995) has suggested that inorganic phosphate (P) is adequate for grazing yak when a supplement of 0.5 - 1.0 kg of oat hay or highland barley straw was given per day during wintertime.

Data on the effects of individual minerals, including trace elements, on yak productivity are, at present, unavailable. With further investigation, specific trace elements or minerals (or combinations of them) may well be found limiting for yak production as has been found for animal production in other parts of the world.

Table 6.19 Effect of a mixture of mineral additives (provided as a salt block) on monthly milk yield (kg ± SD) of full-lactating and half-lactating cows in different months of the warm season [Source: Yuan Youqing et al., 1994]

Lactating type

Treatment

No. cows

May

June

July

Aug.

Sept.

Oct.

Total

Full-lactating

Additive

10

35.0 ±4.2

45.9 ±4.0

60.4 ±4.2

48.9 ±4.3

44.3 ±4.0

36.4 ±4.1

272.1 ±20.2

Control

10

31.6 ±3.9

41.0 ±4.5

53.7 ±4.0

41.1 ±4.1

36.8 ±4.2

30.5 ±4.0

235.7 ±17.2

Significance


NS

*

**

**

**

**

**

Half-lactating

Additive

7

29.1 ±3.9

33.3 ±4.0

45.6 ±3.8

40.9 ±3.8

28.5 ±3.5

-

183.5 ±14.1

Control

7

27.0 ±3.8

29.0 ±3.2

40.5 ±3.2

35.0 ±3.4

23.8 ±2.8

-

159.8 ±13.3

Significance


NS

*

*

**

**


**

Protein deficiency in forage is potentially one of the most important limiting factors on yak production (especially milk production), and nitrogen supplementation can positively influence the milking performance of cows during lactation. An experiment conducted by Zhang Degang (1998) showed that urea molasses multinutrients block (UMMB) contained 10 percent urea, 0.1 percent mixed minerals and trace elements (Fe, Zn, Mg, Cu, etc.). Given as a supplementary feed (500 g per animal per day) during summertime, it significantly promoted milk yield of Tianzhu White yak and Gannan Black yak in Gansu (P<0.01), relative to control groups grazing only.

The yak with UMMB 500 g per day produced an extra 100 g and 160 g milk per day for Tianzhu White yak and Gannan Black yak, respectively. As an alternative protein source, many species of shrubs' leaf are ideal protein supplements for grazing yak due to their high contents in protein (25 - 35 percent) (Long et al., 1999b; Dong Shikui et al., 2002).

A general comment on the use of feed supplements for yak (already noted in respect of minerals and trace elements) may not be out of place. Many of the yak-rearing areas are remote and this not only increases the costs of supplying supplements (if not locally available) but also there may not be a ready market for extra produce. The main criteria for needing supplements will be unusually poor performance or ill health of the animals, and, when possible, a demonstration of a positive effect from using supplements in that particular situation. The further point about the need for cost-effectiveness has been made before. There is, however, also the other potential use of supplements (for example feed, blocks) as an insurance against the periodic snow disasters during which many yak can die.

Milk production in a second year without calving again

Many yak do not conceive again in the year of calving. It is thus common in yak for lactation to continue into a second year. The amount of milk given over the months from December to May is very low, declining to perhaps as little as 2-4 litres in the whole of April (in one set of observations). Yield then rises with the onset of grass growth and follows a monthly pattern similar to that already seen (cf. Figure 6.3 and Table 6.16), but the amount produced in the second year is approximately half that given in the year of calving (sometimes up to two thirds and sometimes less); hence the name "half-lactating" commonly given to females milking into a second season without calving again - in contrast to the "full-lactating yak" in the year of calving. Parity differences are maintained in the second season at the lower level of yield. The fat percentage in the milk during the second "half-lactating" season is correspondingly higher than in the first "full-lactating" season, in line with the negative correlation between milk yield (quantity) and the fat percentage in the milk.

Factors influencing milk composition

Yak milk is dense and sweetish and greatly liked by the local people. Although its composition varies slightly among different yak breeds at different locations, generally total milk solid (dry matter) content ranges around 15 - 18 percent, fat percentage around 5.5 - 7.0 percent (see also previous Tables), protein and lactose content each around 4 - 5.5 percent, and ash around 0.7 - 0.9 percent during the main lactating period (Table 6.20). The milk solid, fat and protein contents in milk of the yak and its hybrids are far higher than in other cattle, especially dairy breeds, and close to the levels in buffalo (Zhang Rongchang, 1989).

Typically, the proportion of essential to nonessential amino acids is 0.8:1 as observed in a full analysis of amino acids in the milk of the Tianzhu White yak (Zhang Rongchang et al., 1986) and 0.79:1, 0.67:1 and 0.72:1 in the milk of the Jiali, Pali and Sibu yak (Ji Qiumei, 2000a).

Milk composition varies with seasonal grass growth and climate change as does milk yield. Milk solids, lactose, protein and amino acids in yak milk are at their highest in mid-lactation and fat percentage increases continuously into late lactation (corresponding also to the changes with season, as shown in Table 6.16). Some relevant results on the Tianzhu White yak (Zhang Rongchang et al., 1986) are given in Table 6.21. During the cold season, when milk yield of yak females may be only of the order of 100 g per day, the fat percentage of that milk can be as high as 14 percent. With the growth of forage, protein content in the swards declines from 115 g per kg DM (young grass) to 33g per kg DM (mature grass), and crude fibre in the swards increases correspondingly. The increased crude fibre can offer more acetic acid and butyric acid (the resources of fatty acid) for the mammary gland to synthesize more fat.

Table 6.20 Milk composition and milk yield of different breeds (or yak of countries) at different locations

Breed or yak of country

No.

Milk solid (%)

Fat (%)

Protein (%)

Lactose (%)

Ash (%)

Milk yield** (kg ± SD)

Tianzhu White

17

16.9

5.5

5.2

5.4

0.77

141.7 ± 43.9

Jiulong

13

17.5

6.9

4.9

4.7

0.82

349.9 ± 131.9

Maiwa

24

17.5

6.3

4.9

5.4

0.82

176.5 ± 64.0

Jiali*

21

16.3

6.8

5.0

3.6

0.95

104.5 ± 31.7

Pali*

22

-

5.9

5.7

3.8

-

236.1 ± 74.9

Sibu*

19

-

7.5

5.3

3.5

-

---

Kyrgyzstan (country)

13

17.4

6.5

5.3

4.6

0.87

630

Nepal (country)

12

17.4

6.5

5.4

4.6

0.90

---

India (country)

14

17.9

6.5

5.9

4.7

0.87

238.4 ± 94.4

* Source from Ji Qiumei et al., 2000a and others from Zhong Guanghaui, 1996b.
** Source from Zhang Rongchang, 1989. (The number of animals shown for milk composition does not apply to milk yield.)

Table 6.21 Composition of milk of ten Tianzhu White yak in Gansu province in three periods of lactation [Source: adapted from Zhang Rongchang et al., 1986]


Early lactation (12 June) (%)

Mid lactation (25 July) (%)

Late lactation (20 Sept.) (%)

Dry matter

16.1

17.8

16.3

Solids - not-fat

10.9

12.3

10.5

Fat

5.1

5.4

5.8

Protein

5.2

5.7

4.7

Lactose

4.9

5.9

5.0

Ash

0.79

0.75

0.77

Essential amino acid

10.7

11.6

8.1

Nonessential acid

13.7

14.8

9.8

Energy (MJ/kg)

3.5

3.6

3.8

The correlation between the total 184-day milk yield and the average fat percentage was shown to be very small (-0.04) and nonsignificant in a study on 184 Maiwa yak by Wen Yongli and Chen Zhihua (1994) (at the Longri Breeding Farm, referred to a number of times here). This is not a genetic correlation, but suggests nonetheless that if this lack of relationship is confirmed, that milk yield and fat percentage might be more readily improved simultaneously in the yak than is generally the case in dairy cattle.

The yak udder

Yak udders are small and, by the standards of dairy cattle, not well developed. Measurements in Sichuan showed an udder circumference in pluriparous females of 55 cm and an udder depth of between 2 and 3 cm. Sixty females measured in Qinghai (Zang Yinsong, 1985) had an udder circumference of 51.6 cm (SD 12.5), the mammary vein had a diameter of 0.94 cm (SD 0.38), and the teats were 2.2 - 2.3 cm in length and 1.1 - 1.2 cm in diameter. Usually (though not in all studies on the yak) the two rear quarters hold more milk than the two forequarters. The forequarters are generally reckoned to hold about 45 percent of the milk, but other ratios have been reported (e.g. 47.6 percent [Zhang Rongchang et al., 1983] and around 40 percent [Han Zhenkang and Lu Tianshui, 1980]).

The sphincter muscles of the yak's teats are strong and hard squeezing is needed to extract milk. The teats are normally squeezed between the fingers. Especially among Alpine-type yak, some one third of the females are found to have particularly "tense" teats.

In Jiulong county of Sichuan an average yield of 1.4 kg needed almost five minutes of milking time to extract the milk at 80 squeezes per minute - a rate of 0.28 kg per minute (Zhang Rongchang et al. [1983] reported a rate of 0.42 kg per min). When a trial with machine milking was conducted, a negative pressure of 400 mm Hg required to extract the yak milk.

Milk let-down

Normally, milk let-down does not immediately follow on squeezing the teats between the fingers, but requires the presence of the calf to jolt the udder and to suckle briefly. If the calf has died, herdsmen place the skin of the dead calf in front of the dam and allow her to smell and lick it, before milk let-down is achieved. Experimentally, the use of a hot towel to massage the udder has been found to stimulate let-down and even to increase yield (Hu Angang et al., 1960, unpublished).

The time required to stimulate the udder into let-down was found by Han Zhenkang and Lu Tianshui (1980) to be initially around one minute (SD 15 seconds), but a second, somewhat shorter, period of such stimulation was found usually to be needed in the course of the milking process before all the milk can be extracted. These authors also indicated that the amount of milk in the udder cistern of the yak accounted for only about 6.5 percent of the milk produced at a milking and that the milk produced as a reflex to milking accounted for around 80 percent - of which four fifths was let down as a result of the first stimulation of the udder and the remaining fifth after the second stimulation. Residual milk in the udder (that traditionally obtained by "stripping" in the hand-milking of dairy cattle) accounted for about 13 percent, but this is normally left by the herdsmen for the calf. These authors also found that proportion of milk stored in the udder cistern increased with the parity of the cow to around 10 percent by the fourth parity.


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