Session III: Nutrition and feeds

Advances in yak nutrition research

H. Linghao, L. Shujie and Ch. Shatuo

Qinghai Academy of Animal and Veterinary Sciences, Xining 810003, Qinghai, P.R. China

Summary

The yak nutrition research group at the Qinghai Academy of Animal and Veterinary Sciences has been studying yak digestion metabolism, energy metabolism, nitrogen metabolism and nitrogen supplementary techniques since 1985. The research includes the following aspects: 1) Rumen digestion metabolism, in terms of parameters such as pH, TCA-P, NH₃-N and TVFA, was tested in 1 to 3-year-old yak with permanent rumen fistula and raised under different conditions. The results show that each parameter is affected by the different conditions under which the yak are raised. The higher the nutrition level, the higher the main parameters but the lower the pH value. 2) Fasting energy metabolism was tested in 1 to 3- year old yak with closed circuit respiratory masks and feeding at different altitudes. The results indicate that the fasting heat production (FHP) of the yak was stable at the altitudes of 2300–4300 metres above sea level (masl). The zone of thermo neutrality was 8–15°C, and, at relatively lower altitudes (e.g. 2300 masl), the FHP = 302.13 kJ/kgW^0.75, MEm = 458 kJ/kg W^0.75 and Km = 0.66, which are similar to that in cattle at similar altitudes but different at higher altitudes. 3) Intake, digestion and utilisation of protein in grasses during different phenological periods by growing yak were systematically studied under natural grazing conditions. 4) Protein requirement for maintenance and growth were estimated and the utilisation of non-protein N in yak was investigated. 5) Nutrient fluctuation in grasses on the alpine frigid meadow was systematically surveyed. From the point of views mentioned above, the protein balance between different phenological periods and between animal and pasture are discussed. Concerning protein shortage during the cold season, some nitrogen direct supplement methods are suggested. It was found that to maintain a proper ratio of energy and protein, molasses-urea block could be given to yak for 200 days during the cold season. Using this method, the digestible protein of yak can be increased by 35%, which basically meets the level for maintaining the energy-protein balance and reduces body weight loss remarkably. During the warm season, indirect nitrogen supplementation using nitrogen fertiliser can also have a beneficial effect.

Keywords: Energy, metabolism, nitrogen, rumen, supplementary techniques, yak

Introduction

There are about 13 million yak in China, which account for about 90% of the world's total yak population. Yak, a unique livestock species domesticated on the Qinghai-Tibetan Plateau, is the dominant livestock on the Plateau because of its ability to adapt to anoxic environment and grass shortages at higher altitudes over 2500 masl. Yak provide raw material for local herders' production activities. However, research on yak, especially on nutrition, remains far behind that on other livestock due to natural and social constraints, which have resulted in both blind production strategies and inefficient research programmes, such as that related to yak breeding for example (Cai and Wiener 1995). It is well known that nutrition research is very important to plan feeds and feeding. The yak nutrition research group at the Qinghai Academy of Animal and Veterinary Sciences has been studying systematically digestion metabolism, energy metabolism, nitrogen metabolism and nitrogen supplementary techniques related to yak nutrition since 1985, with significant results.

Rumen digestible metabolism

Rumen digestible metabolism of growing yak under different feeding conditions

Rumen parameters of 1 to 3-year-old castrated and fistulated yak were compared under conditions of barn, grassland and meadow pasture. It was found that these parameters do not vary among age groups, but the total volatile fatty acid (TVFA), trichloro acetic acid-protein (TCA-P), ammonial nitrogen (NH₃-N) and number of ciliate protozoa do vary depending on pastureland conditions, such as barn, grassland and meadow pastureland (Bi et al. 1989; Xie et al. 1989; Liu et al. 1992). The better the nutrition condition, the greater the parameter values but the smaller the pH value (Tables 1 and 2).

Table 1. Nutrition levels under different feeding conditions.

Table 2. Items of rumen fluid under different feeding conditions. 

Items	Grassland		Meadow
	Green	Withered	Green	Barn
Dry matter (%)	1.88b	1.70b	1.83b	–
pH	6.79bc	6.87ab	6.93a	6.77c
TVFA (x10–2 mol/L)	4.57a	2.15b	3.90a	2.70b
NH3-N (x10–2 mol/L)	16.48a	3.88c	7.68b	8.26b
TCA-P (g/L)	3.17b	9.62a	2.79b	7.10a
Ciliate number (x108)	4.31a	2.22b	3.25b	6.13a

It is worth mentioning that the pH, TVFA and NH₃-N in yak fluctuated twice during the daytime when yak were fed twice a day, but did not change so much on the lower productive pasture.

Comparison of TVFA under different feeding conditions

The concentrations of acetic acid, propionic acid and butyric acid were tested under green grass, yellow grass, withered grass and barn feeding conditions (Liu et al. 1992; Xie et al. 1992). The results follow the general law that the absolute amount and value of high-efficient acid is higher under better nutrition conditions, which is also true for water buffalo and yellow cattle. However, the results show that the ratio of high-efficient acids in yak is much higher than for other ruminants (Tables 3 and 4).

Table 3. Rumen total volatile fatty acid (TVFA) under different grazing conditions (1 × 10^-2  mol/L). 

Items	Grazing conditions			Barn
	Green grass	Yellow grass	Withered grass	Maintenance	Regular	High level
TVFA	5.4 ± 0.5	1.8 ± 0.3	0.6 ± 0.2	3.2 ± 0.7	3.4 ± 0.3	4.1 ± 0.5
Acetic acid	2.5 ± 0.2	a little	a little	1.6 ± 0.4	1.5 ± 0.1	1.8 ± 0.2
Propionic acid	1.4 ± 0.2	1.2 ± 0.2	0.2	0.8 ± 0.2	1.0 ± 0.1	1.2 ± 0.2
Butyric acid	1.4 ± 0.3	a little	a little	0.8 ± 0.2	0.7 ± 0.1	1.0 ± 0.1
Acetic acid (%)	46.3	–	–	45.8	44	42.8
Propionic acid (%)	26.2	–	–	22.7	29.7	28.4
Butyric acid (%)	25.8	–	–	24.2	20.8	23.3
C2/C3	1.8 ± 0.10.1	–	–	2.0 ± 0.3	1.48	1.5

Table 4. Compositional changes in total volatile fatty acid (TVFA) among ruminants (%).

Animal	High concentrate and low rough feeds
	A	B	C
Cattle	6–75	15–21	5–14
Sheep	65–71	15–24	8–15
Deer	54-–70	15–24	10–17
Buffalo	54.7	27.44	16.7
Yak	43.6–44.3	29–31	20.6–21.3

Rumen fluid volume and speed in growing yak

Taking polyethylene glycol (PEG) as fluid dilution, the volume and moving speed of rumen fluid were observed to be 33.8 L and 3.26 L/h (CV<5%) from 4 (150 kg BW) castrated fistulated yak (Liu et al. 1991). This was equivalent to 67%, which is 48% of that found in yellow cattle.

Protein degradability of feedstuff in yak

Degradability of 11 feedstuffs was determined using a nylon bag method fitted to the rumen fistulae (Xue and Han 1998). The result shows that the protein digestibility of garden pea, highland barely, formaldehyde fish meal, skimmed dry acidification milk, corn, garden pea straw, meat and bone meal are higher in yak rumen, but lower for fish meal, wheat bran, rapeseed meal and formaldehyde rapeseed meal (Table 5).

Table 5. Protein degradability of feedstuff in yak rumen.

Feedstuff	Crude protein	Dry matter
Corn	0.68	0.68
Garden pea straw	0.56	0.30
Rapeseed meal	0.44	0.49
Garden pea	0.85	0.73
Fish meal	0.49	0.50
Skimmed dry acidification milk	0.79	0.74
Meat and bone meal	0.55	0.54
Highland barley	0.83	0.82
Formaldehyde fish meal	0.81	0.59
Formaldehyde rapeseed meal	0.38	0.32
Wheat bran	0.48	0.49

Energy metabolism

Fasting heat production (FHP) of growing yak at different altitudes

Under natural temperature in summer, the FHP of growing yak does not differ (P>0.05) between the altitudes of 2261 masl, 3250 masl and 4272 masl; but does vary (P<0.05) among age groups (Table 6). Table 6 shows that the FHP of yellow cattle increases significantly (P<0.05) with the increase of altitude. This is an important difference between yellow cattle and yak (Hu et al. 1992). The relationship between body weight and FHP is expressed by the equation FHP = 920W^0.52 kJ/day (n = 25, r = 0.8469; P<0.01).

To test the accuracy of the exponential value (0.52) in the equation above, a paper-sticking method was used to measure the surface area of growing yak (Hu et al. 1989). The result indicates that the surface area is highly correlated with the 0.52th power of body weight.

Table 6. Fasting heat production (FHP) of growing yak and yellow cattle under natural temperatures.

Altitude (masl)	Age (year)	Number of animals		Heat production (kJ/kgW0.75·d)
		Yak	Yellow cattle	Yak	Yellow cattle
2261	1	3	3	351.544 ± 25.744	292.395 ± 7.563
	2	7	4	305.344 ± 20.920	250.516 ± 3.610
	3	6	4	302.185 ± 17.911	219.061 ± 2.313
3250	1	3	4	328.873 ± 6.249	414.430 ± 7.456
	2	4	4	321.363 ± 6.160	53.260 ± 2.457
	3	4	4	327.737 ± 7.745	357.357 ± 3.164
4271	1	3	2	376.252 ± 23.518	516.372
	2	3	2	324.787 ± 46.044	387.925
	3	3	2	281.144 ± 24.410	359.940

Effect of ambient temperature on FHP

Statistical analyses correlating FHP with environmental temperatures indicate that the FHP is slightly correlated with ambient temperatures ranging between 23°C and –30°C, remains similar between 8°C–15°C, and increases when the temperature is higher than 15°C. FHP exhibits a downward trend when temperatures decrease, and increases again sharply when temperatures drop lower than –20°C. Other physiological attributes such as respiratory rate, heart rate and body temperature remain the same between 8°C and 15°C (Table 7), which indicates that 8°C–15°C is the zone of thermo neutrality for yak (Han et al. 1992a).

Table 7. Regression equation between temperature and fasting heat production (FHP).

Temperature range	Y = a + bX	n	r	Significance
–30°C–20°C	FHP = 891–18.4T	37	–0.2917	P<0.05
–20°C–0°C	FHP = 1188 + 15.5T	40	0.4744	P<0.01
0°C–10°C	FHP = 1155 + 13.8T	46	0.2431	P<0.05
8°C–15°C	FHP = 1080 + 0.7T	52	0.0066	P>0.05
15°C–23°C	FHP = 1070 + 10.5T	48	0.2735	P<0.05

Energy conversion efficiency of growing yak

In barn with different feed levels, 30 energy-balance trails were conducted on six yak aged 2 to 3 years to determine FHP (Han et al. 1997). When dietary concentrate increased from 50–90%, general energy increased from 14.393 MJ to 75.092 MJ, energy digestibility from 60–77%, the metabolic rate from 50–70%, and the precipitation rate from 9–25%. On the other hand, faecal energy loss was reduced from 40–23%, urine energy loss was reduced by half, and heat production remained stable around 46% (above the maintenance level) (Table 8).

Table 8. Energy metabolism of yak under different feed levels.

Using a regression analysis of ME/kgW^0.75 and RE/kgW^0.75, MEm = 458 kJ/ (kgW^0.75. day), metabolic energy efficiency for maintenance Km = 0.66 and energy efficiency for fattening K_f = 0.49 have been obtained. The metabolic energy demand for growth has been measured by means of a factorial method ME (MJ/day) = 1.393 W^0.52 + (8.732 + 0.091W) DG (Han et al. 1997). A forty-day trial was conducted, in which 72:28 (concentrate: rough) feed was given to castrated yak with a body weight of 100–200 kg, in order to test the theoretical value.

The metabolic rate (MR) was estimated once again using a method which compared rough type diet feed (concentrate: rough 28:72) and typical diet feed (concentrate: rough 48:52). No significant difference (P>0.05) was found between MR rough = 0.470 and MR typical = 0.478, though the metabolic rate exhibited a downward trend when the concentrate was converted to coarse (Han et al. 1992b). More studies are needed to decide whether the results are characteristic of yak or due to an inaccurate calculation formula.

Effect of the amount of motion on energy metabolism

Two-year-old yak (BW about 120 kg) were compared with yellow cattle when they walked on pasture at an altitude of 3000 masl and at the speed of 1 m/second (s) and 1.5 m/s. It was found that heat production in motion is significantly more (P<0.01) than that when standing (Han et al. 1989) and heat production in motion at 1.5 m/s is significantly more (P<0.01) than that at 1 m/s. Furthermore, yak produce significantly more heat in motion (P<0.01) than do yellow cattle (Table 9).

Table 9. Comparison of heat production in motion at different speeds between yak and yellow cattle (kJ/kgW^0.75minute).

Species	Number	Standing	Motion at V1	Motion at V2
Yak	4	0.345 ± 0.003	1.924 ± 0.047	2.347 ± 0.107
Yellow cattle	4	0.314 ± 0.031	1.479 ± 0.125	1.748 ± 0.106

These observations fit with the law that small animals produce more heat. They also imply that, in spite of its adaptation to local conditions, yak consume a large amount of energy, and thus a scientific grazing pattern appears to be very necessary to reduce the heat loss.

Nitrogen metabolism

The protein requirement of growing yak

Twenty-seven yak, 1–1.5 years old, were fed different rations in two trials to determine N metabolism (Xue et al. 1994). In trial 1, nine yak were given a low N ration with 0.985% to study the degradation of endogenous N and the lowest maintenance N. In trial 2, 18 yak were divided into three groups and then fed rations with 6.7, 10.1 and 13.4% CP, respectively. The purpose was to study protein requirements for maintenance and growth, and to evaluate the effect of dietary protein levels on N metabolism. The results are shown in Table 10.

Based on trials 1 and 2, the following four formulae were developed:

1. Digestible CP requirement for minimum maintenance = 2.012W^0.52 (g/day).

2. Digestible CP requirement for maintenance (DCPRm):DCPRm = 6.61W^0.52 (g/day), obtained with low N ration, or DCPRm = 6.09W^0.52 (g/day), obtained with N balance trial.

3. Digestible CP requirement for growth (DCPRg) = (0.0011548/∆W + W^0.52)^-1 (g/day).

4. Total digestible CP requirement = 6.09W^0.52 + (0.0011548/∆W + 0.0509/W^0.52)^-1 (g/day)

Table 10. Effect of dietary protein levels in feed rations on body weight gain, carcass weight and N digestibility.

Apparent digestibility of grass N

Digestibility of grass under different phenological conditions is shown in Table 11 (Liu et al.1996). It is easy to see that as grass turns yellow and dry, the CP digestibility goes down.

Table 11. Digestibility of grass crude protein (CP) during different phenological periods (g/head per day).

Item	Sprout May–June	Green July–September	Yellow October–November	Dry December–April
Animal number	6	7	6	6
Intake	436.48	318.70	283.65	161.32
Excretion	147.61	128.19	120.08	110.28
Digested	288.87	190.51	163.57	51.04
Apparent digestibility (%)	66.18	59.78	57.67	31.64
DM digestibility (%)	74.34	66.58	62.10	38.45

Biomass and nutrient content in alpine frigid meadow pasture

Alpine frigid meadow is the main pasture category in the Qinghai highland and accounts for 49% of all edible pastures there. To study the nutrient dynamics of the pastures, an investigation in different seasons was conducted, the results of which are presented in Table 12 (Xie et al. 1996a).

The biomass of pasture is highest in August and then goes down month by month, until the lowest value is obtained in May. CP values in June are the highest, and then drop down to 8.4% in October. The content of coarse fibres is just the opposite. There is a strong negative correlation (r = –0.895) between the contents of CP and coarse fibre. Ca and P contents are the same as CP. The gross energy of grass generally does not change so much between seasons (Xie et al. 1996a).

Table 12. Above ground biomass and nutrient contents in grass in different months.

A heavy loss of biomass and grass nutrients was observed during winter and spring. The portion of biomass lost was 77 and 65% for CP. The loss rate for P, 30%, is the lowest among the nutrients. The grass conservation rate during the cold season was 48.3%, which is only 60% of the theoretical rate (80%). Thus, during the cold season, only with enough nutrient supplements could the normal growth rate of yak and other animals be maintained.

Digestible protein intake of yak during different seasons

From the feed intake (Table 13), nutrient contents in grass (Table 12), and grass protein digestibility in different phenological seasons (Table 11), the digestible protein intake of yak during different periods was calculated (Table 14). This result is important for this study as the following result is mainly formatted according to these data.

Table 13. Feed intake of yak during different phenological periods.

Season	Age (year)	BW (kg)	Intake (kg/head per day)
			By adjust	By 4N-AIA1	Average
Green	2	115.34 ± 2.68	3.98 ± 0.32a	3.86 ± 0.59a	3.92Ba
	3	154.42 ± 1.37	6.00 ± 0.82b	5.50 ± 0.49b	5.75
Yellow	2	125.96 ± 2.28	3.79 ± 0.53a	3.65 ± 0.46a	3.72Ba
	3	168.09 ± 1.10	5.80 ± 0.45b	5.49 ± 0.81b	5.65
Dry	2	120.11 ± 2.40	5.30 ±0.90a	5.59 ± 0.95a	5.45Bb
Sprout	2	122.60 ± 1.67	6.82 ± 0.53	–	6.82A
1. 4N-AIA: 4N (acid equivalent)–acid insoluble ash.

Table 14. Digestible protein intake of yak during different phenological periods.

Item	Green	Yellow	Dry	Sprout
Feed intake (kg/head per day)	3.92	3.72	5.45	6.82
CP% in grass	10.94	7.63	2.96	15.38
Digestibility (%)	60.25	56.36	31.60	69.86
Protein intake (g/head per day)	258.38	159.97	50.98	723.77

Real protein intake and potential requirement of growing yak

From formulae 2 and 4, the digestible protein requirement for maintenance and growth (100 g and 500 g BW gain/day) was calculated (Table 15). During the green season, the protein in grass is sufficient to meet the maintenance requirement of yak, and for them to gain 300–400 g BW daily. During the budding season, the protein in grass is enough to allow a 500 g BW gain/day. During the dry season, however, digestible protein in grass is not enough to maintain body weight (only 55% of the requirement is met), so that yak BW gain is negative during this period, a result that sounds reasonable. It is clear that N supplement is needed to support yak growth and/or maintain body weight during the cold season.

Rumen peptide absorption of growing yak

The result showed that dietary protein is mainly absorbed in the form of peptide in growing yak and the non-mesenteric system is the main site of peptide absorption (Han et al. 2000). Peptide and amino acid absorption levels are related to the intake of digestible crude protein and have no relation to total crude protein. The higher the intake of digestible crude protein, the higher the absorption of peptide but the lower the absorption of amino acids.

Table 15. Protein intake and requirement of growing yak during different phenological seasons.

Item	Green July–September	Yellow October–November	Dry  December–April	Budding  May–June
BW (kg)	115.34	125.96	120.11	122.60
Biomass (g/m2)	80.69	57.92	43.55	49.18
CP intake (g/head per day)	258.38	159.97	50.98	732.77
Requirement for maintenance (g/head per day)	89.94	94.16	91.86	92.85
100 g BW gain	78.82	79.79	79.27	79.50
200 g BW gain	123.96	126.38	125.08	125.64
300 g BW gain	153.20	156.91	154.91	155.78
400 g BW gain	173.68	178.47	175.88	176.99
500 g BW gain	188.83	194.50	191.43	192.75

Nitrogen supplementary techniques

Supplementary molasses—urea block for grazing yak during cold seasons

The blocks contained 40% molasses, 10% urea, 13.5% rape seed cake, 10% wheat bran, 13.5% grass mill, 2% salt, 1% trace elements and 10% binder elements. Samples were left at certain pasture sites for 199 days, and on average 110 g were consumed by each yak per day (Table 16). The blocks were proved very efficient for yak between 2 and 3 years old but not for yak 4 years of age (Xie et al. 1995; Chai et al. 1996).

Table 16. Bodyweight (BW) gain in molasses—urea licks (199 day trial) BW (kg/head).

Age	Group	Number	BW at the beginning	BW by the end	BW loss	Significance
2	Control	7	88.37 ± 12.53	65.56 ± 6.06	22.81	P<0.05
	Test	7	85.18 ± 8.79	78.23 ± 11.20	6.89	–
3	Control	7	133.16 ± 8.71	107.85±11.78	25.31	P<0.01
	Test	7	136.56 ±14.69	133.55 ±10.53	3.01	–
4	Control	7	166.35 ± 10.45	145.32 ± 16.13	21.08	P>0.05
	Test	7	159.74 ± 17.66	138.84 ± 20.13	20.90	–

Trial using complex urea licks during cold seasons

These blocks contained 30% urea, 28% salt, 30% (NH₄)₂HPO₃, 7% (NH₄)₂SO₄, 3% molasses, 2% trace elements and a little of binder. More than 80 yak were fed for 167 days with an average daily amount of 34 g for each yak. Only in the 3-year-old group was there an obvious effect (P<0.05). It seems that molasses was necessary for the N supplement (Wang et al. 1997).

Effect of N fertiliser on grass yield and nutrients on alpine frigid meadow

Grass yield and nutrients on alpine frigid meadow increased by 110.2% and 172.27%, respectively, and CP content increased by 126.39% and 196.10%, when N fertiliser was used on alpine frigid meadow at the rate of 50 kg and 80 kg/hectare. From 1 kg of N, 11.8 kg of grass, or 1.35 kg of N in grass, was harvested (in August) (Xie et al. 1996b). However, no influence was found in the other nutritional components, like the total N and N in soil (Table 17).

Table 17. Effect of N fertiliser on the grass yield.

Pasture type	Group	Area (km2)	Yield (g/m2)		Increase (%)	Grass (kg)/N(kg)
			Before	After
Alpine frigid meadow	Control	24.46	30.43	229.60	100	14.68
	Test	14.73	34.94	347.00	151.13
Mountainous dry steppe	Control	30.00	29.91	98.64	100	9.00
	Test	20.00	28.99	170.62	172.97
Average	Control	54.46	30.17	164.12	100	11.84
	Test	34.73	31.97	258.81	157.70

Comprehensive measure to increase N intake under grazing conditions

From the results in Table 15, a model is presented in Figure 1. During the 170 days of the cold season, protein intake was lower than the maintenance requirement, so that 115 g BW was lost every day, or 19.6 kg for a whole year (average value for yak 2–4 years old). In the warm season, the daily BW gain was 200–500 g, totalling 64.5 kg/year. Thus, the net average BW gain/year is about 45 kg, which is reasonable in practice. From this point of view, the N direct supplement measure should be mainly considered during the cold season.

Figure 1. The balance between digestible protein requirement and supplement of grazing yak.

Drawing on the results of the trial about real protein intake, potential requirement and methods for N supplement, a model for the N supplement is presented in Figure 2. Different methods for the N supplement can keep the N intake level at or close to the maintenance requirement during the cold seasons. Supplement methods in the cold seasons can increase the intake level by 35% for yak to meet maintenance requirements. In fact, yak in the test group only lost 15–20 g BW per day during the cold season, and 16 kg in the whole period.

Figure 2. Effect of N supplement on body weight gain.

In addition, an indirect N supplement during the warm season, in the form of N fertiliser, can increase grass N yield by about 9%, and grass yield by 57% (Hu 1997). This is equivalent to 923 g digestible protein per Mu (1 hectare = 15 Mu) of pasture (after taking into account the grass and protein lost during winter and the low digestibility influence during the cold season), and 35 g of digestible protein per day for each yak. Thus, the maintenance requirement can be met.

In summary, the N supplement should be given to animals during the cold season; if possible, during the green season, N fertiliser should be used for winter-spring pasture (50–80 kg of N fertiliser/ha) to increase grass in the same year and next. According to the grass yield and stock rate, animals should be given supplements of molasses—urea blocks and complex urea blocks, and depending on the herd structure, the ratio of the two kinds of blocks should be 1:1. In the critical period or for fattening, some concentrate pellets should also be given.

Acknowledgments

We would like to extend our highest appreciation to Mrs Ma Rui and Feng Yuzhe, and Mrs Zhao Yueping and Zhang Xiaowei of the Qinghai Academy of Animal and Veterinary Sciences, who helped us a great deal on this paper.

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