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Endocrine changes and their relationships with bodyweight of growing yak

T. Yongqiang,1 Zh. Xingxu,1 W. Minqang,2 L. Zhonglin2 and Zh. Rongchang2

1. Department of Veterinary Medicine, Gansu Agricultural University, Lanzhou 730070, Gansu, P.R. China
2. Lanzhou Institute of Animal Science and Pharmaceutics, Chinese Academy of Agricultural Sciences, Lanzhou 730050, Gansu, P.R. China


Concentrations of growth hormone (GH), insulin, thyroxin (T4) and 3,3',5'-tri-iodothyronine (T3) in blood samples of growing yak during different seasons were determined by radio-immunoassay. Changes in bodyweight of the growing yak and compositions of grass were also measured. The seasonal changes in hormones were significant (at least P<0.05). In the same season, variation in hormonal concentration was affected by the growth stage of the animal. The bodyweight gains varied in different seasons, with significant increase from May to September and decrease from January to May. Correlation analysis indicated that T4 had a significant positive correlation with bodyweight of the growing yak (r = 0.323, P<0.05), but other hormones did not have significant correlations with bodyweight. The results show that annual cycle of weight loss and gain is attributable to the seasonal changes in feed supply and that the seasonal changes in the concentrations of the assayed hormones were indirectly dependent on grass growth.

Keywords: Body weight, correlation, growth, hormone, yak


Yak (Bos grunniens) is one of the unique domestic animals living on the Qinghai-Tibetan Plateau. Its dominance in this area is attributed to its great adaptation to high altitude, anoxia condition and feed shortage in cold seasons. It provides important products for the herders who live in the area. Due to cyclic nutrient deficiency or unbalanced nutrient supply within short growing seasons for herbage growth, annual cycle of weight loss in cold season and weight gain or compensatory growth in warm season in yak is a common phenomenon.

We knew that endocrine activity regulates animal nutritional balance and growth. Many reports have demonstrated that nutritional status profoundly affects circulating concentration of growth hormone (Blum et al. 1985; Breier et al. 1986; Ellenberger et al. 1989; Breier 1991). Insulin is known to be responsible for catabolic processes such as fat mobilisation and reduced protein accretion (Gregory et al. 1982; Blum et al. 1985). Thyroxin and tri-iodothyronine, hormones of the thyroid gland, are major regulators of metabolic rate, growth and development of animals (Kahl and Bitman 1983). However, information on hormonal profiles in relation to yak' growth is very limited in the literature. This study attempted to identify seasonal changes in plasma concentrations of GH, insulin, T3 and T4 in growing yak at different ages under natural grazing conditions.

Material and methods

Ecological condition

Experimental yak were located at the Datong Yak Breeding Farm of Qinghai Province at an altitude of 3500–4800 metres above sea level (masl), with average annual precipitation of 463.2 to 636.1 mm, annual mean ambient temperature of between 2 and 4°C and no absolutely frost-free period throughout the year. The growing period of natural herbage, which germinates in late May and starts to wither in September, is only about 130 days. Herbage consists of alpine and mountainous meadows and is divided into cold (November to May) and warm (June to October) season pastures.


A total of 60 healthy yak were allotted to three groups as follows:

Group A (GA) 30 yak, 0.5–1.5 years old

Group B (GB) 20 yak, 1.5–2.5 years old and

Group C (GC) 10 yak, 2.5–3.5 years old.

Each group had equal numbers of males and females. The animals were herded all year round and no pen or supplementary feeds were available in winter. Blood samples, grass samples and bodyweight measurements were performed at two-month intervals from September 1997 to July 1998.


Blood samples were obtained from each yak before grazing in the morning. The samples were placed in tubes at room temperature for 3–4 hours until clotting and then centrifuged at 2000×g for 20 minutes to separate serum. The serum was stored at –20°C until assay. Yak bodyweight was determined with platform scale. On the grassland, five plots of 1 m2 grassland were randomly selected and harvested. Grass samples were dried naturally at room temperature and placed in plastic bags until assay.

Laboratory analyses

GH, T4, T3 and insulin were measured by radio-immunoassay. Dry matter (DM), crude Protein (CP), minerals (M), crude lipid (CL), crude fibre (CF) and total energy (TE) in the grass samples were measured by conventional analyses.

Statistical analysis

Statistical analysis was conducted by SAS software and all the results were expressed in mean ± SE. The data were analysed by PROC ANOVA using multiple comparisons for seasonal hormone changes in growing yak at different ages. The correlation analysis was done with a view to estimating correlations between blood hormones and bodyweight in growing yak.


Seasonal hormone changes

The seasonal changes in hormones are presented in Table 1. GH had three peaks occurring in January, May and September, with a maximum value being in January (3.58 mg/L) and minimum value in March (1.52 mg/L). Only the differences among seasons in GA were significant (P<0.05). No differences existed among groups in the same season (P>0.05). The results show that GH concentration in yak is hardly affected by age, but is mainly affected by nutrition.

Average insulin concentration peaked in September (8.63 IU/mL) and was lowest in March (5.37 IU/mL). It varied significantly (P<0.01) in GA and GB among seasons, being higher in July and September in both GA and GB than in other months, but lower in March in GA and in November in GB than in other months (P<0.05). Within the same seasons there were significant (P<0.01) differences among the three groups, being higher in GA than in GB and GC in both September and November.

The changes in T3 and T4 concentrations were basically consistent. T3 concentration had its highest value in July (2.68 mg/L) and lowest value in May (0.60 mg/L). Within the same season, T3 concentration in GA was significantly higher than those in the other two groups in March and November (P<0.05). T4 concentration had the highest value in September (62.56 mg/L) and the lowest value in November (28.32 mg/L). There were significant differences among seasons in the three groups (P<0.05). T4 concentration was higher in July in GA (P<0.05), in July and September in GB, but was lower in March and November than in other months (P<0.05). In GC, T4 concentration was higher in September but lower in March and November than in other months (P<0.05). In addition, there were significant differences between the three groups in September; T4 concentration in GA being lower than those in GB and GC (P<0.05).

Seasonal bodyweight changes

Yak bodyweight showed a clear seasonal pattern (Table 2). From September to November, the daily bodyweight gains of three groups exceeded 300 g, and from January to May there was a gradual bodyweight loss. From March to May, the daily bodyweight loss was around 220 g, but from May to July, there was a rapid bodyweight gain at 500 g per day (Table 3). In GA there was an unexpected daily bodyweight gain of 140 g from November to January at a time when yak expected to go through a period of weight loss. This may have resulted from the fact that the animals were young (pre-weaning) and at the stage of high intensity of growth. Variance analysis shows that the changes in bodyweight of the three groups were significant (P<0.01).

Table 1. Hormonal change of yak in different seasons.



Change in groups






Growth hormone (mg/L)


3.82 ± 0.31

3.43 ± 0.13

3.84 ± 0.12



1.07 ± 0.15

1.79 ± 0.32

1.71 ± 0.46



2.27 ± 0.29

3.54 ± 0.24

3.17 ± 0.21



2.04 ± 0.54

1.57 ± 0.22

2.39 ± 0.13



2.58 ± 0.65

2.36 ± 0.35

3.41 ± 0.22



2.38 ± 0.33

1.51 ± 0.44

1.84 ± 0.23

Insulin (IU/mL)


6.0 ± 2.1

6.1 ± 3.6

6.7 ± 3.4



4.6 ± 1.8

5.0 ± 4.8

6.5 ± 5.2



6.4 ± 2.8

6.1 ± 2.9

6.0 ± 2.2



7.8 ± 3.4

7.7 ± 3.7

7.8 ± 3.0



11.1 ± 2.0

8.3 ± 4.2

6.5 ± 2.8



6.8 ± 0.8

4.2 ± 4.2

4.4 ± 4.8

T3 (mg/L)


0.56 ± 0.47

0.74 ± 0.31

0.94 ± 0.19



1.56 ± 0.21

1.09 ± 0.18

1.17 ± 0.38



0.7 ± 0.23

0.56 ± 0.37

0.54 ± 0.32



1.67 ± 0.42

2.47 ± 0.29

2.90 ± 0.22



2.58 ± 0.28

2.36 ± 0.48

2.21 ± 0.41



1.77 ± 0.51

1.29 ± 0.18

1.14 ± 0.49

T4 (mg/L)


40.38 ± 18

47.38 ± 14

45.25 ± 18



36.24 ± 15

32.00 ± 15

28.10 ± 16



47.29 ± 19

53.37 ± 14

51.85 ± 18



63.84 ± 14

61.64 ± 12

57.93 ± 10



43.85 ± 19

71.68 ± 13

72.15 ± 13



32.79 ± 9

24.47 ± 8

27.69 ± 10

Table 2. Bodyweight change of yak (kg).


Bodyweight of three groups






59.39 ± 16.78

126.52 ± 10.33

151.37 ± 11.05


89.94 ± 12.28

149.75 ± 14.36

171.17 ± 10.11


98.21 ± 15.32

146.59 ± 12.75

168.33 ± 18.61


95.78 ± 11.88

140.10 ± 9.54

157.72 ± 10.93


84.72 ± 14.29

129.11 ± 9.86

143.01 ± 12.97


115.81 ± 14.60

159.14 ± 10.54

179.32 ± 15.48

Table 3. Daily bodyweight change of yak (g).


Daily bodyweight gain




25 September–25 November




26 November–24 January




25 January–26 March




27 March–23 May




24 May–26 July




Grass yield and nutritional composition

Grass yields increased gradually from May to July and stabilised from July to September, then began to decline, dropping to the lowest level in May of the following year (Table 4). Crude protein (CP) content fluctuated between seasons, with a maximum value of 12.92% in July and a minimum value of 5.47% in March. CL declined from 3.56% in September to 1.28% the following May. In contrast, CF had the lowest value of 16.5% in July and the highest value of 27.52% in November. TE, CL and CP were consistent and showed a single-peak with the highest values from July to September and the lowest values the following May. The period of the highest yield and quality for the grass was from July to September in the Datong Yak Breeding Farm, the yield and quality of grass declining gradually from November to May.

Table 4. Yield and nutrients of herbage.








Yield (kg/m2)







CL (%)







CF (%)







CP (%)







DM (%)







M (%)







TE (MJ/g)







CL= crude lipid; CF= crude fibre; CP= crude protein; DM= dry matter; M= minerals; TE= total energy.

Correlation of bodyweight and hormones

Table 5 showed the correlations between bodyweight and hormones. Among the hormones studied, T4 concentration had a significantly positive correlation with bodyweight (r = 0.2509, P<0.01), but the correlation of T3 and GH concentrations were not significant (P>0.05). Insulin concentration had very marked positive correlations with T3 and T4 concentrations (P<0.01). T3 and T4 concentrations were also significant (P<0.01).

Table 5. Correlation matrix of hormones and bodyweights.

Body weight




Growth hormone (GH)






















Growth hormone (GH)



** = Values show ray significant correlations


The bodyweight of yak increased mainly from May to November and the absolute bodyweight gain was highest from May to September when the daily bodyweight gain of two or three year-old yak was 500 to 600 g (Table 3). Daily bodyweight gain was 300 to 400 g from September to November, although grass on pasture gradually became withered. The bodyweight of one-year-old yak continued to increase from November to January, mainly because these animals were not entirely weaned. The bodyweight declined from November to next May and the bodyweight loss was most severe with daily weight loss of 180 to 250 g from November to the following May, the bodyweight loss gradually increasing because grass reserve reduced gradually.

Circling GH was elevated in January and May when both grass production and nutritive value was the lowest, and then decreased in July when grass growth was at high peak. There are many reports of an elevation in the concentration of GH in cattle during restricted feeding (Blum et al. 1985; Breier et al. 1986; Breier et al 1988; Ellenberger et al. 1989). An increase in the half-life of GH in the circulation of cattle influenced the increase in GH during restricted feeding (Trenkle 1976). Furthermore, enhanced levels of GH seem to result from an increase in amplitude vs. frequency of the pulse of GH (Breier et al. 1986; Ellenberger et al. 1989). In the current study, the mean concentration of GH declined when grass was sufficient, which is in agreement with other results (Blum et al. 1985; Hayden et al. 1993). Although circulating levels of GH decreases as an animal matures (Trenkle 1977), a concomitant increase in the quantity of GH binding sites in peripheral tissues has been suggested to maintain GH action (Breier et al. 1991). In addition, during re-feeding, enhanced growth rates and transient increases of nitrogen balance were not associated with a corresponding rise in GH levels. Thus, under certain conditions, circulating levels of GH do not correlate with growth (Joakimsen and Blom 1976; Trenkle 1977; Blum et al. 1985). This is also in agreement with the result of the present study. But in the current study, the results in March and September were not in agreement with previous reports, mainly due to the difference between yak and cattle. As yak inhabits the remote mountains, a major proportion of the variation observed in the present study may have been due to other factors, such as, ambient temperature, seasonal photoperiodism, sampling variances and genetics. As many factors are involved in the regulation of GH secretion, it is necessary to conduct more systematic studies to better understand the GH patterns in the yak.

Insulin levels showed clear seasonal patterns with grass production and composition. In July and September when grass was sufficient insulin concentrations of growing yak reached maximum. In contrast, in March and May when grass was inadequate insulin concentration declined to a minimum. According to Blum et al. (1985), in steers, the levels of circulating insulin declines during periods of food restriction. It probably facilitates catabolic processes such as fat mobilisation and is presumably responsible for reduced protein accretion. The increase of circulating insulin during re-feeding was possibly related to enhanced fat deposition. This also is in agreement with recent reports in cattle (Hayden et al. 1993).

T4 concentration decreased during nutritional deficiency and increased during nutritional sufficiency in growing yak of different ages. A similar reduction in plasma T4 concentration was reported in other studies involving feed-restricted cattle (Blum et al. 1985; Ellenberger et al. 1989; Hayden et al. 1993) and sheep (Blum et al. 1980). T3 concentration showed basically identical pattern, but from the whole change, the level of T4 varied more than did that of T3. However, T3 level has been reported to be more closely associated with shifts in energy status in cattle (Blum et al. 1985). Kahl (1978) reported that T3 increased faster than T4 both in male and female Holstein cattle. The present result agrees with the observations suggesting that T3 should be considered the main biologically active thyroid hormone and that T4 must be converted to T3 before its activity is exerted (Kahl et al. 1983). In addition, the positive correlation between T3 and T4 in our study indicated that the increase of T4 level in May was a transient rise during the process of the conversion of T4 to T3.

Kahl (1978) reported that the positive correlation of thyroid hormone with bodyweight in the Holstein cattle was in accordance with relationships described during normal growth of cattle (Blum et al. 1980; Kahl et al. 1983). Our result indicated that T4 concentration in growing yak was significantly positively correlated with body weight, and that T3 concentration was not correlated with bodyweight. This result is also consistent with results from other studies (Rumsey 1981; Verde and Trenkle 1982) that only the T4 concentration had marked positive correlation with bodyweight gain.

In conclusion, the bodyweight of growing yak increased in warm season and decreased in cold season. This was attributed to seasonal changes in available grass. The seasonal changes in hormones coincided basically with that of the grass grazed, and the T4 concentration was positively correlated with bodyweight in growing yak. Therefore, T4 level could be used to define and evaluate nutritional condition of yak that are more than two years old to determine optimum time for supplemental feeding to prevent loss of bodyweight in the cold season.


The Chinese Agricultural Ministry supported this work. The authors wish to acknowledge the Datong Yak Breeding Farm in Qinghai Province for the logistical support, the Beifang Biotechnique Institute, and the Air-Force Hospital in Lanzhou for technical assistance.


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