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Body weight growth model of Datong yak in Qinghai

W. Minqiang,1 Zh. Huiling,2 L. Pingli,1 T. Yongqiang,3 L. Jiye4 and L. Zonglin1

1. Lanzhou Institute of Animal Science and Pharmaceutics, Chinese Academy of Agricultural Sciences, Lanzhou 730050, Gansu, P.R. China
2. College of Animal Science and Technology, Northwest Agriculture and Forester University, Yanglin 712100, Shannxi, P.R. China
3. College of Animal Science and Veterinary, Gansu Agricultural University Lanzhou 730070, Gansu, P.R. China
4. Datong Yak Breeding Farm, Datong 810102, Qinghai, P.R. China


This study was conducted to develop a suitable model for describing the growth pattern of the yak. The data used consisted of body weight records of 76 growing yak aged between 5 to 37 months. Three mathematical models were applied to describe the growth curves during this development period:

1. Y1 = 20.105 + 11.250x 0.526x2

used for describing the growth curve of yak aged 5 to 13 months;

2. Y2 = -359.687 + 49.977x 1.249x2

used for animals aged 13 to 25 months; and

3. Y3 = -833.339 + 63.772x 1.019x2

used for animals aged 25 to 37 months.

Keywords: Growth, mathematical model, yak


Yak (Bos grunniens) has a growth pattern which is significantly different from that of cattle (Bos taurus). Suitable growth models for cattle breeds have been developed. In China, some studies of growth models have been done for the Chinese yellow cattle breeds such as Anxi cattle, Yianbian cattle and Fujian cattle (Qiu and Ju 1989; Wang 1990; Liang et al. 1995). Although preliminary analyses have been reported in yak growth behaviour (Lu 1981; Ma and Du 1982; Zhang 1989; Cai 1992; Lu et al. 1994), no model is available to describe its growth pattern. The main reason for this has been the general lack of systematic body weight records. This study was conducted to develop suitable models to describe the growth model of Datong yak in Qinghai and to establish the relationship between age (X) and body weight (Y).

Materials and methods

Environment and animal management of the farm

The grazing consisted of mountain meadow located at an altitude of 2900 to 4800 metres above sea level (masl). Annual average temperature at the study location is 24°C, ranging from 28°C in January to 24°C in August. The farm has an area of 330 thousand hectare, of which about 200 thousand hectare is grazing land. The biomass output changes seasonally, the highest output being in August and the lowest in April. The average biomass output was 1500 kg/ha (Xie et al. 1997). There were 17 thousand heads of yak raised on the farm. The reproductive yak were organised into different herds with about 150 individuals per group. The calving season spans from March to June but is mostly concentrated in May. Calves are weaned the following March at about 10 months of age and moved into heifer herds. The yak generally are sexually mature at about 3 years of age when they are gradually introduced into the breeding herds. Nursing cows are milked once a day.


Experimental animals consisted of growing yak aged 5 and 37 months. Animals were weighed bimonthly throughout the year. Experiment animals were the offspring of nucleus breeding herd organised by the Lanzhou Institute of Animal Science of Chinese Academy of Agricultural Sciences since 1990. Most cows had 3 deliveries at the time of the study. A total of 76 yak with relatively complete records and born in 1997 (G1), 1996 (G2) and 1995 (G3) were involved in the study. G1 had 31 animals (15 males and 16 females), G2 had 30 animals (12 males and 18 females), and G3 had 15 animals (8 males and 7 females). Animals were weighed bimonthly from 26 September 1997 to 26 May 1998, a total of 5 weighing. G1 was involved in recording after weaned at 5 months old.

Age assignment

All animals in the study had similar genetic background. As most calves were born in May, the animals in G1 were around 5 months old at the first weighing (26 September 1997), and 15 months at the last weighing (26 July 1998). The age range of the animals in G2 and G3 were around 17 to 27 months, and 29 to 37 months, respectively.

Formulating the growth curve

A preliminary growth curve was derived based on a total of 17 mean weight records of yak aged from 5 to 37 months. From this, three equations of the general form Y = a + bx + cx2 were used to describe the growth pattern of animals in the three growth periods described above.

Results and discussion

Growth curve and mathematical models

The growth curve of the Datong yak is demonstrated in Figure 1. Three mathematical models were formulated to describe the growth curves:

Y1 = 20.105 + 11.250x 0.526x2

used for describing the growth curve of yak aged from 5 to 13 months,

Y2 = 359.687 + 49.977x 1.249x2

used for yak from 13 to 25 months, and

Y3 = 833.339 + 63.772x 1.019x2

used for yak from 25 to 37 months. The respective coefficients of determination were: R12 = 0.863; R22 = 0.728; and R32 = 0.840. All the coefficients were significant, indicating that the models fit well.

Figure 1: The growth pattern of Quinghai Datong yak.

Characteristics of yak body weight growth model

The yak is a pure grazer and its growth largely depends on the biomass and grass nutrients of the pasture, changing among seasons. Yak experience weight gain during the warm season from June to October and weight loss during the cold season from November to following May. The ratio of body weight gain to loss decreases with increasing age until body maturity (at about 7 years of age for males and 5 years for females) and the ratio tend to be about 1. Hence, the growth pattern of the yak consists of peaks (the heaviest weights) and valleys (the lightest weights) occurring every year. In contrast, cattle under barn feeding or semi-barn feeding usually have a relatively stable growth curve (experiencing less environmental effects) and one peak value of growth during its lifetime. For this reason, one model suffices to describe the growth curve in cattle and the R2 value is usually higher (>0.90) than that of yak.

The relationship of weight gain and age

From Figure 1, it can be seen that G1 calves were continuously gaining weight from 5 to 11 months (the end of September to the end of March), and had only a slight 0.5 kg weight loss at 13 months at the end of May. For the older animals (G2 and G3), the weight loss started from the end of September until the end of May. One reason for this is that the G1 calves were kept with cows and were able to get some milk and this made up for the lack of feed supplement during the cold season. Another reason for the difference in the weight change between G1 and the other two groups is that the younger animals (G1) possess greater potential for growth than that the older counterparts (G2 and G3) as is the case in all species. The growth potential differences were investigated in this study by comparing the peak values of weight among ages. The mean birth weight of calves was 12.5 kg (n = 60, Wang Minqiang 19931994, unpublished data of the Datong Farm). By the age of 11 months, the calves had gained 66.4 kg. From 11 to 17 months, the heifers (G2) gained 68.4 kg. But from 17 to 29 months, the G3 group gained only 33.8 kg. The results showed that the weight gain of the growing yak in the warm seasons before the age of 1.5 years are 2 times that of yak aged between 1.5 to 2.5 years. Compensatory growth may play an important role in the body weight gain of the younger animals.

The relationship between weight gain and seasonal changes

From the end of May to the same time the following year, the G1 animals had grown up to G2 age group and had, therefore, experienced the 2nd growth period. The heaviest weight (147.3 kg) occurred at the end of September and the animals had gained 68.9 kg from 13 to 17 months of age in the warm season, an average daily gain of 0.561 kg. After this period, the animals gradually lost weight as the grass withered until they were 25 months old (from the end of September to the end of next May), losing a total of 33.0 kg during the 8 months. The animals of G3 gained 56.9 kg in the warm season with an average daily gain of 0.463 kg, but they lost an average of 33.5 kg in the cold season. Generally, the yak gain weight only in the very limited 4 months of warm season, but lose weight during the long 8 months of the cold season. Except for the younger animals in G1, the heaviest weight was recorded at the end of September and the lightest weight in the month of May every year.

Optimum yak slaughter age and supplement season

An impressive ratio of weight gain and loss was attained from the study. From birth to 37 months of age (137.7 kg) the animals achieved a net weight gain of 125.2 kg. However, the loss in 3 cold seasons summed up to 67.0 kg, which is more than half of the gain. Two larger weight loss periods occurred in September to November and March to May for G2 and G3 animals, respectively. The weight loss during March to May represents the most significant check on the growth of all animals, but is especially debilitating to the younger animals, which are usually in poor body condition at this time. These results suggest that this is the period during which supplementary feeding could be most effective. The optimum slaughter time should be around October. From the point of economic efficiency, animals should be slaughtered around 2.5 years of age.


The authors are most grateful to all the herdsmen who assisted in weighing the animals. Special thanks are due to Hue Shouhai, Zhao Longquan and Han Kai, the managers of the farm, for managing the experimental animals and ensuring that weighing was done on schedule, even in the coldest seasons.


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