14 YAK NUTRITION - A SCIENTIFIC BASIS by Long Ruijun^[16]

Overview

As discussed in previous chapters, the yak is largely dependent on natural pastures for its survival. Thus, its nutritional state varies seasonally as the supply of supplementary feeds is limited. In most herds, only very weak animals and some pregnant or lactating yak are given access to feeds in addition to grazing. Low calving and growth rates are attributed to the poor nutritional condition of the yak, in the cold season particularly. The traditional way of maintaining the animals is to allow them to put on as much fat as possible during the warm season; fat that is then used over the long cold season as an energy reserve to allow survival beyond the early spring. The tragedy of large numbers of animals dying because of snow disasters is frequent on the Qinghai-Tibetan Plateau. Nowadays, the yak population is increasing rapidly, causing rangeland degradation and, hence, further increasing the gap between feed supply from natural pastures and the animals' feed demand. Thus, the malnutrition that the yak has to suffer is likely to become worse in the foreseeable future. A good understanding of yak nutrition under grazing conditions, which could help to alleviate some of the problems, is still rather inadequate. But this chapter provides some of the evidence that is accumulating and points to several gaps in understanding and the need for validation of some of the preliminary findings.

Feed intake

Generally, yak consume less feed than other cattle, probably because of their smaller rumen capacity. Yak prefer fresh, high-quality forages, and both housing and high temperature can reduce feed intakes. Dry matter intake (DMI, kg per day) of the growing yak under indoor feeding can be estimated as DMI = 0.0165 W + 0.0486 (W is body weight in kilograms), and that of the lactating yak as DMI = 0.008W^0.52 + 1.369Y (W^0.52 is metabolic body weight, Y is milk yield, kg per day).

Ruminal digestion and metabolism

The rumen of the yak is far smaller than that of other cattle. Outflow rate of rumen fluid ranges from 3.1 to 3.5 litre per hour, hence lower than in cattle. The outflow rate of digesta from the yak rumen stays comparatively constant, ranging from 11.5 percent to 14.9 percent per hour. Total volatile fatty acid (VFA) production in the yak rumen increases with the animal's age. The proportions of propionic acid and butyric acid to total VFA in the yak are higher than those in other ruminants.

The concentration of NH₃-N in the yak rumen varies with the diet composition and feeding behaviour. Mature forages can promote lower NH₃-N concentrations in grazing yak than can young forages. Both feed type and feeding behaviour affect degradability of dietary nutrients in the yak rumen.

Energy nutrition

Lactating yak cows have better utilization of dietary energy than dry yak cows when they are given oat hay at the same level under indoor feeding conditions. An increased feeding level leads to the decreased digestibility of dietary energy in dry cows. The thermoneutral zone of the growing yak is estimated as 8° - 14°C. The fasting heat production (FHP) of the growing yak can be estimated as FHP = 916 kJ per kgW^0.52 per day. The metabolizable energy requirement for maintenance (ME_m) in growing yak is around 460 kJ per kg W^0.75 per day. Metabolizable energy requirement in the growing yak can be estimated as: ME (MJ per day)=0.45W^0.75 + (8.73 + 0.091 W) DG (DG is kg per day).

Protein nutrition

There is no difference in the digestibility of dietary nitrogen between lactating and dry cows. A relatively lower excretion of endogenous urinary nitrogen in yak suggests the possibility that the animal has evolved a mechanism to recycle more nitrogen to the rumen than ordinary cattle.

Yak can use non-protein nitrogen as efficiently as other ruminants. The endogenous purine derivative excretion in the yak is only 40 percent of that in cattle but is similar to that in buffaloes. The value of creatinine excretion for the yak when fasting is much lower than for buffaloes and cattle. Rumen degradable crude protein requirement for maintenance (RDCP_m, g per day) in growing yak is around 6.09W^0.52 g per day. The crude protein requirements for daily gain (DG RDCP_g g per day) in growing yak can be estimated as RDCP_g = (1.16/DG + 0.05/W^0.52)^-1. Thus the total crude protein requirement of growing yak could be calculated as RDCP (g per day) = 6.09W^0.52 + (1.16/DG + 0.05/W^0.52)^-1.

Mineral nutrition

Mineral nutrition is poorly documented. But the existing information suggests that mineral deficiencies may occur, varying from one yak-raising area to another. Seasonal deficiency of specific elements could be a common issue throughout the Plateau owing to an uneven seasonal supply of feeds. Mineral and trace element deficiencies can cause some problems to yak, but appropriate supplementation will generally improve the conditions.

Feeding

Forages on natural grassland are in surplus in summer but deficient in winter under the traditional grazing system. The nutritional status of yak can be improved by ensuring adequate protein intake in summer - but yak suffer deficiency of crude-protein and of energy from grass in winter. The use of feed supplements seems vital if the productive and reproductive potentials of grazing yak cows on the Qinghai-Tibetan Plateau are to be developed. Concentrate and urea block supplements are effective in improving the productivity of grazing yak and maintaining the body weight of animals in winter.

Introduction

The yak, like other grazing ruminants, has a highly developed and specialized mode of digestion that has evolved to maximize the utilization of carbohydrates from cellulose (Van Soest, 1987) and thus allow better access to energy in the form of fibrous feeds than that occurring in the non-ruminant herbivores. The yak has adapted, uniquely among cattle, not only to the high cellulose diet of the Qinghai-Tibetan Plateau but also to its extremely harsh climate and, as a result, has developed nutritional and metabolic features that probably differ from those of other cattle species. Yak nutrition is, however, poorly documented compared with some other aspects of yak science, such as biology and ecology characteristics, productive and reproductive performances and aspects of breeding and interspecies hybridization. Knowledge of yak nutrition has been very limited owing to the remoteness and poor infrastructure of yak territories, difficulties of on-farm research and lack of scientific information exchanges.

Until the 1990s, most of the research on digestion and metabolism of protein and energy, as well as supplementation strategies, had been conducted by the Yak Nutrition Research Group of Qinghai Academy of Animal Science and Veterinary Medicine, whose findings were collected in the publication Recent advances in yak Nutrition (Hu, 1997). Other researchers (Gansukh, 1997; Long et al., 1997, 1998, 1999; Dong et al., 1997, 2000a,b; Shi et al., 1997) have made contributions to the better understanding of the feeding and nutrition of the yak. Although some nutrition and feeding habits of yak still remain unclear, compared to those of ordinary cattle, the results have been used to improve yak performance on the farm. It is considered that a satisfactory performance of the indigenous animals could be achieved by effective nutritional intervention in the harsh ecological zones.

The aims of this chapter are: (i) to summarize the current advances in yak nutrition research as contained in various scientific reports and (ii) to suggest future research requirements for establishing better guidelines for yak-feeding systems.

Feed Intake

As already noted in Chapter 13, voluntary intake (VI) of the yak varies with the season and sward heights, from 18 to 25 kg of fresh forage in summer to 6 to 8 kg per day, or even much less, of wilted grass in cold-season grazing conditions. Other factors affect the intake levels, including feed types, feeding conditions, environmental climate, as well as age, size and sex of the animal.

Feed types

Han et al. (1990a) fed castrated yak (two to three years old) in barns seven diets and found that the dry-matter intake (DMI) of roughage decreased with the increasing content of concentrates in the diets (Table 14.1). Dong et al. (2000a) studied the digestion and metabolism of protein and energy in lactating yak given different diets and found that concentrates in the diets decreased the DMI of yak. Preference for fibrous feeds may result in higher intakes of roughages than of concentrates by yak, and a relatively faster passage of high-quality roughage (such as oat hay) leads to reduced mean retention time of digesta (Han, 1996) and thus results in higher intake. In both studies, the authors stated that the intakes of yak were less than those of other cattle, possibly because of the smaller rumen capacity of yak (Liu, 1991).

Feeding conditions

Liu et al. (1997) reported that the DMI of two-year-old yak (as a percentage of body weight) varied from 3.7 percent in the late growing period of forages to 3.4 percent in the mature period under grazing conditions, while that of three-year-olds ranged from 3.7 percent to 3.1 percent. Feed intake per unit of body weight under grazing was greater than in the indoor feeding. Possibly unsuitable housing and restriction to a given diet may be the main factors that reduced the feed intake of the indoor-fed yak.

Climate

Climatic factors, especially temperature, have a profound effect on feed intake and digestibility in the yak. Growing yak increased their intake levels at lower temperatures irrespective of whether they were feeding indoors (Han et al., 1990a) or grazing on natural pasture. The faster rate of passage of feed particles at lower temperatures (Liu et al., 1997) would provide more rumen space to be filled by food.

Greater milk production on cold, cloudy days (cf. Chapter 6) may be partly attributed to the higher forage intakes of lactating yak on such days. The yak can feed normally on grasslands when the temperature is as low as -30° to -40°C, or even lower in a harsh winter. In contrast, the yak moves and grazes less at higher temperatures (cf. Chapter 4), and, consequently, feed intake falls.

Age, size and sex of yak

As shown in Table 14.2, the DMI varies with the age and size (body weight) of the yak. There is a good linear correlation between the DMI of yak and their body weight (W) or metabolic body weight (W^0.75 or W^0.52). Han et al. (1990a) found it was much better to use body weight than metabolic body weight to estimate the DMI for growing yak. On this basis, Liu et al. (1997) deduced the equation: DMI (kg per day) = 0.0165 W + 0.0486 (r = 0.959) for growing yak.

Table 14.1 Dry-matter intakes of growing yak from various diets under indoor feeding (± SD) [Source: Han et al., 1990a]

For lactating yak, extra feed is needed to meet requirements for milk yield. Therefore, milk production must be taken into account when calculating DMI. The following equation (Dong et al., 2000a) can better describe the relationship: DMI=0.008W^0.52+1.369Y (where Y is kg per day of standard milk of 4 percent fat content [r=0.992]).

Table 14.2 Feed intake of grazing yak on natural grassland at various growth stages of forages (±SD) [Source: Liu et al., 1997 ]

Note: Means with different superscripts are significantly different (P<0.01).

Feed digestion and metabolism in the rumen

For ruminants, a number of factors, including gastrointestinal size and capacity, rumen fill, rumination, digestive capacity, absorptive gut surface and quality of available forage affect feeding strategy (Van Soest, 1987). Yak, standing apart from other ruminants, have their own rumen characteristics and digestive capacity under different feeding strategies.

Rumen volume

Liu et al. (1991) determined the volume of the yak rumen contents (l) by using polyethylene glycol (PEG) as a marker and found that for a yak of 150 kg body weight, the rumen content varied from 32.3 to 35.8 l (Table 14.3). When the rumens of these animals were filled with water after slaughter the mean-water content was 34.8 l (Liu et al., 1991). The maximum size of the yak rumen, reported by Liu Haibo (1989), was approximately 66.8 l. Compared with cattle, yak had a much smaller rumen volume (Liu et al., 1991; Han, 1990a).

Table 14.3 Rumen fluid content of yak and outflow rate of rumen fluid (±SD) [Source: Liu et al., 1991]

Outflow rate of rumen fluid and digesta

Outflow rate of rumen contents is positively correlated with the protein degradability but negatively correlated with the synthesis of microbial protein in the rumen. Many factors, such as feed availability, air temperature, composition of diet, state of feed (solid or liquid) and size of feed particles, can affect the outflow rate (Han et al., 1996).

Table 14.4 shows that dietary intake is positively related to the outflow rate of rumen fluid. A lower outflow rate of rumen fluid with a higher proportion of roughage in the diets indicated that lower quality feeds required more time to be fermented and degraded in the rumen (Han, 1996). The relatively higher intakes under grazing conditions may lead to faster outflow of rumen fluid than under indoor-feeding conditions (Han, 1996). No effects of the level and source of dietary nitrogen on the outflow rate of rumen fluid were observed in yak, although a quicker outflow of rumen fluid with increasing levels of nitrogen in the diet occurred in buffalo (Han, 1990a).

Table 14.4 Outflow rate of rumen fluid and digesta in yak fed with various diets under indoor feeding (±SD) [Source: Han et al., 1996]

Note: Within columns, the means with different superscript capital letters are significantly different at P<0.01; the means with different lower case superscripts are different at P>0.05.

* CP = crude protein

Outflow rate of rumen fluid in the study of Liu et al. (1991) ranged from 3.1 to 3.5 litres per hour, somewhat lower than in cattle.

Irrespective of the composition of the diet, nitrogen level and source, and feed intake, the outflow rate of digesta from the yak rumen remains comparatively constant, ranging from 11.5 to 14.9 percent per hour (Table 14.4).

Volatile fatty acid production

Production of volatile fatty acid (VFA), an original energy substrate, can reflect the fermentative capacity of the rumen. This fermentative capacity is determined to a considerable degree by dietary composition and, therefore, is greatly influenced by feeding behaviour (Van Soest, 1987). The effect of dietary composition and feeding level on fermentation in the yak rumen under indoor-feeding conditions was reported by Xie et al., (1992). According to this report, the production of propionic acid and ratio of acetic acid to propionic acid for a maintenance diet (72 percent of concentrate and 28 percent of roughage, and the intake just meeting maintenance requirements) were significantly lower than those of a roughage diet eaten to appetite (22 percent of concentrate and 78 percent of roughage). The production of total VFA and acetic acid from a concentrate feed eaten to appetite (44 percent of concentrate and 56 percent of roughage) were greater than those from the roughage feed, but the ratios of acetic acid to propionic acid of these two feeds were similar (Table 14.5). A difference in fermentation was observed between indoor feeding and grazing conditions by Liu et al. (1992). Yak grazing fresh forages in the early growing period of grassland produced more total VFA and propionic acid than yak fed indoors with a mixture of concentrate and wheat straw and yak grazing withered and wilted forages. And total VFA production in the rumen was higher for indoor-fed yak than for yak grazing on withered and wilted forages. It was found in both reports that the production of the more efficient energy sources, propionic and butyric acids, were higher in the yak than in other ruminants, such as cattle, deer, goats and buffalo, regardless of whether the animals were fed indoors or grazing. Gansukh (1997) working with yak calves between the ages of 30 and 120 days found that the average amount of total VFA frequently increased with age.

NH₃-N concentration

NH₃-N (ammonia nitrogen), one of the fermentative products of feed in the rumen, is the source of microbial protein. As with VFA production, the concentration of NH₃-N in the yak rumen varied with diet composition and feeding behaviour. Bi et al. (1989) found that more NH₃-N can be provided for grazing yak by alpine steppe (dominated by grass) than by alpine meadow (dominated by sedges). According to Yan (2000), mature forages can provide less NH₃-N to the grazing yak than the young forages. Xie et al. (1989) reported no significant difference in rumen NH₃-N concentration between two- and three-year-old yak under indoor-feeding conditions, although the values were lower than those from yak grazing on the alpine steppe and higher than those from yak grazing on the alpine meadow as already described.

Dry matter and protein degradability

Nutrient degradability in the yak rumen varies with the type of feed and feeding behaviour. Xie et al. (1990) determined in sacco degradability of several protein feeds in the yak rumen under indoor feeding and grazing conditions (Table 14.6). The crude protein degradability of plant materials was significantly higher than that of animal materials, and fishmeal and bone meal had higher crude protein degradability than blood meal among animal materials.

Both dry-matter and crude-protein degradability of bone meal and sesame cake were higher under grazing than under indoor feeding, although no significant differences between feeding situations were observed among other feeds.

Table 14.5 Volatile fatty acid production in the yak rumen at various indoor feeding levels (1×10^-2 mol/l) (± SD) [Source: Xie et al. 1992]

Note: Within rows, the means with different superscripts are significantly different (P<0.01); means with the same superscripts are not different (P>0.05).

Energy nutrition

Dietary energy digestion and metabolism

Differences in the efficiency of the utilization of dietary energy are due to the gastrointestinal capacity for fermentation and the proportion of the diet that is catabolized in that fermentation (Van Soest, 1987). The former depends on animal characteristics that include sex, age and physiological state (growth, lactation, etc.), and the latter is highly related to diet composition and feeding level.

Long et al. (1998) observed that lactating yak had better utilization of dietary energy than dry yak cows when they were fed at the same level with oat hay under indoor-feeding conditions. A further study with lactating yak (170 - 200 kg) conducted by the same authors indicated, however, that yak cows had a lower efficiency of metabolizable-energy utilization for milk production (averaging 0.46) than dairy cattle and this in turn suggests that it may be one of the features of energy metabolism developed by the yak under long-term natural selection to concentrate the available energy to withstand the harsh environment and thus ensure survival.

Table 14.6 Dry matter (DM) and crude protein (CP) degradability of various feeds in the yak rumen under indoor feeding and grazing conditions [Source: Xie et al 1990 ]

¹ CV = coefficient of variation

Table 14.7 Features of the energy metabolism of yak fed on various diets [adapted from Han et al. 1990]

BW, Bodyweight; Conc, Concentrate; Straw, Wheat straw; GE, Gross energy content of diet; FE, Faecal energy; UE, Urine energy; CH₄E, Methane energy; HP, Heat production; DE, Digestible energy; RE, Retained energy; DG/d, daily weight gain.

Table 14.7 demonstrates that shifting the dietary proportions of concentrate and wheat straw has a profound effect on efficiency of feed energy utilization in growing yak. The ratios of retained energy in body tissues to gross energy content of diet range from minus 31 percent to plus 26 percent when changing animal diet from I (lowest amount of concentrates) to V (highest amount of concentrates). The lower metabolic hormone levels of growing yak reflected a slower growth rate related to an adverse eco-environment (Han, 1994). Another experiment (Dong et al., 2000b) indicated that digestibility of dietary energy decreased by 2.9 - 6.8 percent with increasing forage feeding levels in dry cows.

Fasting heat production

Basal metabolism is generally defined as the heat production of a completely quiescent animal in a post-absorptive state, within a thermoneutral environment. Although this state can be achieved with human beings, it is extremely difficult to achieve with animals like the yak. Consequently, the term "fasting metabolism" has been adopted for them. Table 14.8 shows a profile of fasting heat production (FHP) (kJ per kg W^0.75 per day) in growing yak that remained fairly constant compared to that in the Qinghai yellow cattle. The comparatively stable FHP in yak may be related to the ability of yak to take in more oxygen, particularly at the higher altitudes (see Chapter 4). At the elevation of 2 261 m, the absolute FHP for the growing yak was higher than that of the growing Qinghai yellow cattle, but the reverse was the case at higher altitudes. Clearly, basal metabolism of animals living at higher elevations, like the yak, is lower than that of animals living at lower elevations. In Table 14.8, there is a significant difference between age groups in both yak and Qinghai yellow cattle (i.e. the FHP value for a one-year-old calf is higher than that for a three-year-old heifer). This difference is also found in other species of animals.

Hu (1994) suggested that the respiratory quotient (RQ) of the growing yak determined by metabolism significantly declined with increasing altitude, from 0.744 (2 261 m) to 0.696 (3 250 m) and 0.545 (4 272 m). But with no significant difference among age groups, the values less than 0.7 indicate disordered metabolism (perhaps ketosis). Lower atmospheric pressure and oxygen contents in the air at higher altitudes may be the main factors that lead to reductions in the respiratory quotient of growing yak. Corresponding information from other species would be interesting but appears not to be available. Hu (1994) also indicated that ambient temperature has a great effect on FHP and other physiological indices in the yak (see Tables 14.9 and 14.10). But the FHP remained fairly constant and, correspondingly, the body temperature, heart rate and respiratory rate of yak were stable in the environmental temperature range of 8° - 14°C. So the thermoneutral zone of the yak was estimated as 8° - 14°C (see Chapter 4).

Further work by Hu (1994 and 1997) on the measurement of the relationship of bodyweight (W) and surface area of the growing yak, by using the method of plaster (pasting paper) on the animal's body, showed that the highest correlation existed between surface area and W^0.52. An equation of FHP = 916 kJ per kg W^0.52 per day (n=25 r=0.8469, P<0.01) was obtained.

In the light of the equation, Hu concluded that yak calf's FHP value is clearly lower than that of the dairy cattle calf and Holstein heifer and that the heat lost per 1 kg W^0.52 from yak is lower than for other cattle species. The lower fasting heat production of the yak and its stability at different altitudes are probably an adaptive response to life in an alpine-cold and oxygen-depleted environment and to the nutritional deprivation that yak experience in winter and spring. These features could be the result of long-term natural selection.

Table 14.8 The fasting heat production (FHP, KJ per kg W^0.75 per day) of growing yak and cattle at different altitudes in summer [Source: adapted from Hu, 1994]

* Bodyweight.

Metabolizable energy for maintenance

The energy requirement for maintenance is a useful term for expressing the level of the exogenous nutrient supply. It is defined as the metabolizable energy (ME) input per day at which the animals are in energy balance. Han et al. (1990b; 1991) used a respiration mask method to estimate the metabolizable energy requirement for maintenance (ME_m) in growing yak and found its value to be 460.2 kJ per kg W^0.75 per day. In the same experiment, the authors also estimated the efficiency of utilization of metabolizable energy for maintenance (k_m) in growing yak to be around 0.66.

Table 14.9 Regression equations between fasting heat production (FHP) and ambient temperature (T °C) [Source: Hu, 1994]

Table 14.10 Critical upper-limit temperature (°C) leading to a rise in physiological indices [Source: Hu, 1994]

Energy requirements for standing and walking

As the yak is a grazing animal, two of its most common activities are standing up and moving on. Han et al. (1989) compared the energy requirements for standing and walking in yak with those in Qinghai yellow cattle at an elevation of 3 000 m. Table 14.11 shows that the yak (115.3 kg) generates much more heat (J per kg W^0.75 m) in the course of walking than do the larger Qinghai yellow cattle (170.5 kg) and other cattle or dairy cows. The energy expenditure of the yak is a little higher in the course of standing (V₀) than that of Qinghai yellow cattle. The author attributes the difference in heat production to the difference in body size and breed, as smaller animals are expected to generate more heat during walking (Blaxter, 1962). Table 14.11 also indicates that the higher the speed of moving the more heat is generated both in yak and Qinghai yellow cattle. Correspondingly, the respiratory quotients of the animals in the course of standing and walking with speeds of 1 metre per second or 1.5 m per second are, respectively, 0.68, 0.65 and 0.59 J per kg W^0.75 per minute for yak, and 0.79, 0.73 and 0.77 J per kg W^0.75 per minute, respectively, for cattle.

Energy requirements for growth

Han et al. (1990b) estimated metabolizable energy requirements for growth (ME_g) in growing yak (n=7) through an energy balance trial on six two- to three-year-old animals by rationing their intakes of concentrate to a series of levels. The daily metabolizable energy requirement of growing yak was estimated as: ME (MJ/d) = 0.45W^0.75 + (8.73 + 0.091 W) DG, where W is the body weight and DG is daily gain (kg), and the efficiency of utilization of metabolizable energy for growth (k_g) in yak is 0.49.

Such a high value might be considered to apply only to diets rich in concentrates. However, similar results were obtained from other trials when animals were given coarse fodders (Han et al., 1992 and Dong et al., 2000a).

Table 14.11 Energy expenditure (EE, J per kg W^0.75 m) of yak and other cattle species moving at various speeds at an elevation of 3000m [Source: adapted from Han et al., 1989]

¹ Qy cattle = Qinghai yellow cattle. *J per kg W^0.75 per minute).
* Quoted by Han et al., 1989

Energy requirements for lactation

Data on energy requirements for lactation are still scarce, as only preliminary studies on dietary energy digestion and metabolism in lactating yak have been conducted by a few researchers (Long et al., 1998; Dong et al., 2000a).

Protein nutrition

The animal needs protein to support its functions, including tissue maintenance, growth of lean tissue, wool and the products of conception and for milk-protein synthesis.

Dietary protein digestion and metabolism

As with dietary energy, utilization efficiency of dietary protein in the yak differs with diet composition and feeding level and with age, sex, body condition and the productive stage of the animal (growth, lactation etc.). Long et al. (1998) reported that when yak cows were fed with oat hay ad libitum (CP: 8.5 percent), there was no difference between lactating and dry cows in crude protein digestibility, although lactating yak tended to consume more feed than dry yak. However, lactating yak with a milk yield of 1.2 - 1.8 kg per day showed a negative balance of nitrogen, while dry yak were in positive balance.

With regard to excretion of endogenous urinary nitrogen, Long et al. (1999a) found that the daily fasting nitrogen excretion of 316 mg per kg W^0.75 per day for yak cows was similar to that found for buffalo (275 mg per kg W^0.75) (Chen et al., 1996). Possession of such a low value suggests that yak could have evolved a mechanism to recycle more nitrogen to the rumen than do cattle.

Non-protein nitrogen metabolism

Non-protein nitrogen (NPN) has been used as an extra source of nitrogen for microbial protein synthesis in dairy and beef production but not so in yak. Xie et al. (1989) found a higher concentration of NH₃-N in the rumen of growing yak fed with ammonia-treated straw than in animals fed with untreated straw (Figure 14.1). Also pH, VFA production and the density of ciliated protozoa in the yak rumen increased correspondingly with NPN ingestion. Clearly, as a ruminant, the yak can use NPN as efficiently as other ruminants. Chai et al. (1996) indicated that when a basal diet (containing 4.2 percent of CP) was supplemented with urea (to give a diet containing 7.3 percent of CP), there was a great increase in the concentration of NH₃-N in the yak rumen that, in turn, improved the microbial protein production (Table 14.12). This shows that the utilization of dietary nitrogen can be improved by adding a source of NPN (such as urea).

Purine and creatinine metabolism

The excretion of total purine derivatives in urine is used to estimate the supply of microbial protein to the host animal in some ruminants (Chen et al., 1990). Long et al. (1999a) investigated the profiles of urinary excretion of purine derivatives and creatinine in the yak and found them to be similar to those of cattle and buffalo, irrespective of feed levels or fasting.

Figure 14.1 Effect of diets on NH₃-N concentration in growing yak rumen [Source: Xie Aoyun, et al 1989]

Table 14.12 Effect of urea additive on degradability of nitrogen and efficiency of microbial protein synthesis in the rumen of the growing yak (n=3, ±SD) [Source: Chai et al. 1996]

* Basal diet, 70 percent pea straw + 28 percent maize + 1 percent fish meal + 0.5 percent salt + 0.5 percent additive; # Urea, 1.09 percent

The endogenous purine derivative excretion (0.22 mmol per kg W^0.75 per day) in the yak is only 40 percent of that noted in cattle but is similar to that found in buffaloes (Chen et al., 1990, 1996). The relationships between total purine derivative excretion in yak urine (mmol per kg W^0.75) and digestible dry matter intake and digestible organic matter intake were significant (P<0.001), but this was not so for creatinine excretion (Table 14.13). The value of creatinine excretion (0.25 mmol/kg W^0.75 per day) for fasting yak was much lower than the findings reported for buffalo and cattle (Chen et al., 1992, 1996).

Protein requirements for maintenance

Protein requirements for tissue maintenance, growth, lactation and wool and hair growth have been well discussed for many ruminants other than the yak. There is as yet little information to define the protein requirement for maintenance in yak. Xue et al. (1994) used the comparative-slaughter method to measure the rumen degradable crude protein requirement for maintenance (RDCP_m) (n = 9, age: one and a half years) and obtained a value of 6.09W^0.52 g per day for growing yak.

Protein requirements for growth

For growing yak, the protein requirements include that for maintenance (RDCP_m) and for body weight gain (RDCP_g). According to Xue et al. (1994), the latter can be calculated as follows, RDCP_g = (1.1548/DG + 0.509/W^0.52)^-1. Combining the value of 6.09W^0.52 (for RDCP_m), the protein requirement of growing yak can be calculated as RDCP (g per day) = 6.09W^0.52 +(1.16/DG+0.05/W^0.52)^-1, where DG is daily gain (g per day). However, these calculations need more experiments to validate them.

Table 14.13 Feed intake, dry-matter digestibility (DMD), daily intake of digestible dry matter (DDMI) and daily urinary excretion of purine derivatives (PD), creatinine and nitrogen in three yak [Source: Long et al., 1999a]

* Voluntary intake.

Protein requirements for lactation

To date, little information on protein requirements for lactation is available. Only preliminary studies on protein digestion and metabolism in lactating yak have been conducted by some researchers (Long et al., 1998; Dong et al., 2000a).

Mineral nutrition

Compared with energy and protein nutrition, mineral nutrition in the yak is poorly documented. Long et al. (1999c) indicated that inorganic phosphate (P) is sufficient for various categories of yak (calf, heifer, dry cow, lactating cow) (n = 30) during the warm season when grazing was on alpine meadows, with good quality pasture as the sole food. But in the spring and early summer, dietary phosphate failed to meet yak requirements. Dong et al. (2000a) analysed the calcium (Ca) and phosphorus (P) balance of three lactating yak (170 - 200 kg) fed indoors on various diets (Table 14.14) or on an oat and hay diet at different levels (Table 14.15). Either or both Ca and P showed negative balance. Yan Ping et al. (2002) suggested that yak living in the Qinghai Lake area were suffering sodium (Na) and copper (Cu) deficiency but not of other minerals. It seems that there may be a shortage of molybdenum on Tianzhu alpine rangelands (Zhou Zhiyu, personal communication, 2002).

Mineral and trace element deficiency

Effects of mineral or trace element deficiency or imbalance on yak health conditions are not adequately documented throughout the Qinghai-Tibetan Plateau (but see reference to possible Cu and Mb deficiency earlier). However, the symptoms of copper deficiency in yak have been recorded at Whipsnade Wild Animal Park in England, where they noted a difference in the uptake of copper between yak and other Bovidae and local domestic cattle (see chapter 9 on disease). Zhang (1998) demonstrated that milk production of Tianzhu White yak was lifted significantly when animals were supplemented with urea molasses multi-nutrient blocks (UMMB) containing 10 percent urea, 0.1 percent mixed minerals and trace elements (Fe, Zn, Mg, Cu, etc.) during lactation (see Chapter 6).

Though yak living in different regions may face different kinds of mineral problems, deficiencies and imbalances could well exist on the Qinghai-Tibetan Plateau as in other parts of the world. However, up to the present, identification of such has not been made and appropriate supplementation has not been much studied in the various yak-raising areas because, understandably, most attention has been given to the animal requirements for the macronutrients.

Feeding

There is as yet insufficient data to describe the nutritional status of the yak accurately under the normal, year-round grazing conditions. However, an approach for developing appropriate feeding strategies for yak on native grasslands can be made with other existing information. Such information relates to forage availability, nutritive values of native forages, known gaps between the supply of feed and the requirements of the animals and the effects of the limited number of supplementary feeding strategies so far investigated (see Chapter 13).

Table 14.14 Balance of calcium (Ca) and phosphorus (P) in lactating yak (n=3) fed with different diets [Source: Dong et al., 2000a]

Table 14.15 Balance of calcium (Ca) and phosphorus (P) in dry yak (n=3) at different intake levels [Source: Dong et al., 2000b]

Forage availability to grazing yak

Several researchers have reported the feed availability on the Qinghai-Tibetan Plateau (Xie et al., 1996a; Long et al., 1999b). The uneven feed supply throughout a year suggests that grazing yak require feed supplements in the harsh winter on the Plateau. Harvesting oats and some productive perennials or imported crop residues (agricultural by-products) from nearby farming areas could be alternative feed sources to partly alleviate feed deficiency of yak in winter, provided this can be seen as cost-effective.

Seasonality of forage nutrients

Nutritive value and nutrient production of native forages vary seasonally on the Plateau, together with the variation of forage yield. Xie et al. (1996a) found that nitrogen (N) in the dry matter of native forages on the Qinghai-Tibetan Plateau declined from 2.5 percent in June to 1.0 percent in November (Figure 14.2a), while gross energy (GE) dropped from 17.0 MJ per sq m in August to 10 MJ per sq m in November (Figure 14.2b), resulting in N and GE deficiency in the diet of grazing yak. Consequently yak lost much body weight during late winter and spring (cf. Chapter 6) if without appropriate supplementation.

Relation between the nutrient supply of forages and physiological states of grazing animals

Long et al. (1999c) estimated the nutrition situation of yak (n=43) based on the variation of beta-hydroxybutyrate (BHB) values in the serum of yak cows as the grazing season progressed. These results, together with other data shown in Table 14.16, imply generally poor nutrition rather than a specific nutrient deficiency and reflect the quantity of grazing available as winter progressed. This provides some evidence to explain why yak show large weight loss and a loss of condition over winter and early spring (cf. Chapter 6).

Figure 14.2a. Seasonal variation of nitrogen (N) and crude-fibre (CF) contents in native forages [Source: Xie Aoyun et al., 1996a]

Supplementation feeding

Detailed investigations have shown that supplementation can improve the productivity of grazing yak (Long, 1994; Long et al., 1999c; Zhang, 1998; Xie et al., 1996b, 1997; Wang et al., 1997a & b). Most of these studies were concentrated on energy and protein supplementation.

Figure 14.2b. Seasonal yield of crude protein (CP) and gross energy (GE) in the alpine meadow [Source: Xie Aoyun et al., 1996a]

Xiao et al. (1996b) and Wang et al. (1997a) demonstrated that calving rate and calf growth rate can be greatly improved when grazing animals are supplemented with concentrates at 0.14 - 0.25 kg per day (containing 50 percent maize, 30 percent rape cake, 18 percent wheat bran, 1 percent salt and 1 percent minerals) from July to November.

Table 14.16 The nutrient metabolites of cows before and after calving (n=34) (mean±SD) [Source: Long et al., 1999c]

^a b-hydroxybutyrate ^b Inorganic phosphorus

On the other hand, Long et al. (1999c) reported that supplementation with fibrous feeds (oat hay and highland barley straw) had a significant effect on the productivity of yak cows under normal grazing conditions in Tianzhu county, western Gansu province (Table 14.17). Therefore, the practice of supplementation seems important for attaining the productive and reproductive potentials of grazing yak cows and improving their output on the Qinghai-Tibetan Plateau.

Urea blocks can be used as supplementary feed to provide grazing yak with non-protein nitrogen and other necessary nutrients. Xie et al. (1997) reported that supplements of urea blocks in winter (from November to the following May) can reduce dramatically the weight loss of yak calves (Table 14.18) and increase their survival rate (Wang et al., 1997b).

Table 14.17 Effect of supplementation with hay or straw on yak productivity (mean ±SD) [Source: Long et al., 1999c]

^a Grazing nil supplement, ^b Grazing + oat hay, ^c Grazing + highland barley straw; ^d,e,f,g,h values with the same superscript letter (within rows) do not differ significantly. Those with a different superscript letter differ significantly (P<0.05).

Use of the blocks also had a great effect on reproductive performance of adult cows when provided during the summer (June to August) (Zhang, 1998). Urea blocks are an alternative option for maintaining the body weight and productivity of grazing yak in winter when they are facing protein deficiency on the Qinghai-Tibetan Plateau (see Chapter 6 for supplementary feeding in relation to milk production).

Table 14.18 Effect of urea blocks as a supplement on yak body weight during winter (mean ±SD) [Source: Xie et al., 1997]

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