G. Yanjun, L. Ruijun Zh. Degang and Ch. Jiangang
Department of Grassland, Gansu Agricultural University, Lanzhou 730070, Gansu, P.R. China
The seasonal dynamics of concentrations of total extractable condensed tannins (CT), crude protein (CP), acid detergent fibre (ADF) and crude ash (CA) of five shrubs growing in the Qilian regions of Gansu Province, China, were determined from June to September 1999. The dry matter (DM) disappearance of these shrubs in yak rumen was also estimated by in sacco method. The results show that different feeding shrubs have different concentrations of CT, which declined markedly with the degree of maturity. The concentrations of CT vary between 58.1–102.6 g/kg DM in June. In August, almost all decline to below 50 g/kg DM. The CP contents, ranging from 13.9–23.3% in August, also decline with the degree of maturity. The changes in the ADF and CA contents are inconsistent among the shrubs, except for the gradual increase of ADF in Caragana jubata and gradual decrease in Spiraea alpina throughout the season. DM digestibility of Salix mystillacea, Spiraea alpina and Dasiphora fruticosa are 69.9%, 63.0% and 65.7%, respectively, in August, which is higher than that in June and July. There are negative relationships between 48 h DM degradability and the CT content (r = –0.24, P>0.05) and ADF content (r = –0.065, P>0.05), respectively. Based on this study, it is suggested that shrubs are good feed sources in the alpine region, primarily because of their high protein content, although the CT content has negative effect on the DM digestibility. It is suggested that the best utilisation season of shrubs should be after August.
Keywords: Alpine region, condensed tannins, shrubs, utilisation, yak
Alpine shrub and grass tussocks are one of the main grazing pastures of yak in China. They cover about 116,400 km2 and are mainly distributed in the area above the forest line at an altitude of 3000–4000 metres above sea level (masl). There are lots of edible shrubs for yak and Tibetan sheep in these areas. These include Dasiphora fruticosa, D. glabra, Spiraea alpina, Caragana jubata, C. brevifolia, Salix alpina, S. bolia, S. cupularis, S. mystillacea, Lonicera syringantha and Hippophae tibetica. They play an important role in reducing the grazing pressure on pastures and are an important source of protein for yak (Pu 1999). The calorific value of shrubs in alpine grassland has been reported to be much higher than that of herbs (Long et al. 1993). However, most shrubs contain anti-nutritional components such as tannins (Jackson et al. 1996), which influence the palatability of shrubs. Consumption of a large quantity of feeds rich in condensed tannins (CT) has been shown to reduce feed intake (Barry 1989). But low concentrations of CT will increase the quantities of essential amino acids (EAA) absorbed from the small intestine of ruminants without affecting volatile fatty acid (VFA) (Waghorn et al. 1987). This is due to CT-protein bonds being stable in rumen (pH 5.5–7.0), but unstable in abomasums (pH 2.5–3.5) and in small intestine (pH 8.0–8.5) for releasing the protein (Johns and Mangan 1977).
Many researches have been done in tropical and sub-tropical regions in estimating the feeding value of shrubs rich in CT. However, little is known about the CT and the seasonal dynamics of nutrients of alpine shrubs. The objectives of this study were to measure the seasonal dynamics of total extractable CT, CP and ADF in the leaves of alpine feeding shrubs, and to determine the DM disappearance rate in sacco of these shrubs in yak rumen.
Leaves were sampled from five native feeding shrubs on 20 June, 20 July, 20 August and 20 September 1999 from alpine shrubs and grass tussocks in the Tianzhu Tibetan Autonomous County of Gansu Province, China. The shrubs were Dasiphora fruticosa, Spiraea alpina, Caragana jubata, Salix mystillacea and Hippophae tibetica. Plant samples were oven-dried at 65°C for 48 h and milled through a 1.0 mm screen for chemical analysis and through a 2.5 mm screen for in sacco degradability study.
Three young female yak fitted with rumen fistula were used in the study. The animals were fed twice a day on a diet consisting of 100% oats hay (CP content 6.3% and ADF 36.4%).
Plant tissue (0.2–1 g fresh weight) was macerated in a glass homogenizer with 70% v/v acetone: water (tissue weight: volume, 1:3), containing 0.1% w/v ascorbic acid. The concentration of CT was measured following Vanillin-HCl method described by Broadhust and Jones (1978). Catechin was used as the standard.
DM and CA contents were determined as described by Association of Official Agricultural Chemists (AOAC 1984). ADF was determined according to the method described by van Soest et al. (1991). CP was determined following Kjeldal method.
Analysis was carried out according to the procedure described by Mehrez et al. (1977). Samples weighing 2.5 g were transferred into a nylon bag and incubated in three rumen fistulated yak for 48 h. On removal, nylon bag was thoroughly washed and dried at 65°C for 48 h for further analysis.
The CP contents of all tested samples are very high, ranging from 8.6–17.2% in August but decline with the change of seasons. From June to September, the CP content of Dasiphora fruticosa declined by 69.9%, Caragana jubata by 49.4%, and Hippophae tibetica by 40.3%. There are no significant differences in CP contents among shrub species (P>0.05). The seasonal changes in the ADF contents are not consistent and remain between 20 and 30%, except for a gradual increase of ADF content in Caragana jubata and a gradual decrease in Spiraea alpina across the seasons. The concentrations of CA range from 3.1–5.9% and show no significant variations among seasons for all shrub species (Table 1).
Table 1. Crude protein (CP) and acid detergent fibre (ADF) contents of five alpine shrubs, % dry matter (DM).
Samples |
CP | ADF | ||||||
June |
July |
Aug. |
Sept. |
June |
July |
Aug. |
Sept. | |
Dasiphora fruticosa |
19.7 |
9.0 |
8.6 |
4.4 |
25.0 |
27.9 |
24.5 |
27.5 |
Hippophae tibetica |
24.4 |
14.3 |
11.6 |
10.7 |
22.2 |
30.0 |
26.3 |
25.4 |
Spiraea alpina |
29.3 |
22.7 |
17.2 |
15.9 |
21.5 |
18.2 |
19.4 |
21.2 |
Salix mystillacea |
22.0 |
19.3 |
10.2 |
10.0 |
29.9 |
26.9 |
23.2 |
22.7 |
Caragana jubata |
24.0 |
18.9 |
14.5 |
11.9 |
26.8 |
25.9 |
31.9 |
44.5 |
Tannins are plant polyphones combining with proteins and other polymers such as cellulose and hemi-cellulose to form stable complexes. They are present in many shrubs and trees in tropical and sub-tropical areas. The data from present study demonstrate that alpine shrubs also contained CT (Table 2).
Table 2. Concentrations of condensed tannins (CT) of five feeding shrubs, g/kg dry matter (DM).
Samples |
June |
July |
August |
September |
Average |
Dasiphora fruticosa |
74.8 |
71.9 |
40.5 |
30.0 |
54.3 |
Hippophae tibetica |
102.6 |
80.8 |
57.6 |
27.3 |
67.1 |
Spiraea alpina |
58.1 |
23.2 |
21.1 |
14.3 |
29.2 |
Salix mystillacea |
91.2 |
72.3 |
47.7 |
22.9 |
58.5 |
Caragana jubata |
69.5 |
47.2 |
38.6 |
13.3 |
42.2 |
The concentrations of CT in these shrubs vary among species. Hippophae tibetica has the highest content of CT in June (102.6 g/kg DM), while the rest of the species have less than 100 g/kg DM in June or 60 g/kg DM in August. The CT contents decline significantly with the maturity of shrubs.
Yak are the main consumers of alpine shrubs in the region of the study. In this study, the DM digestibility of these feeds range from 41.4–75.0% (Figure 1). The average DM digestibility of these shrubs across all the seasons is 67.4% for Salix mystillacea, 62.1% for Spiraea alpina, 45.5% for Hippophae tibetica, 55.1% for Dasiphora fruticosa and 50.1% for Caragana jubata.
Figure 1. Dry matter (DM) digestibility of five alpine shrubs.
Edible plants play an important role in providing fodder for ruminants in most parts of the world. However, many species have evolved with chemicals (secondary compounds) to protect themselves from bacteria, insects, fungi and grazing animals. Condenced Tannins is one group of these chemicals. It may depress intake or utilisation of feed components (high tannin content), or may enhance feed nutritive value (low tannin content) (Barry et al. 1986).
Condensed Tannins content determines whether a shrub can be used as an animal feed. Jackson et al. (1996) suggested that when CT is lower than 50 g/kg DM, the shrub could be edible or fed in medium quantities of the diet in supplementary feeding. In our study, the Condensed Tannins contents of the five main feeding shrubs are below 50 g/kg DM (except Hippophae tibetica) after August. These results indicate that yak could probably consume these feeds safely after August. During June and July, most of these shrubs have high CT contents. It is suggested that farmers should limit the grazing time of yak and Tibetan sheep in alpine shrub lands during these seasons.
There is high variability in the nutrient contents of shrubs attributed to within- or between species differences due to factors such as plant age and plant part, harvesting regimen, season and location. In the present study, the CP content decreased as the plant matured. This result is similar to those reported for Greek browses (Khazaal et al. 1993), while the CP content (85.5 to 172.4 g/kg DM) in August is higher than that from Greek browses (70.5 to 132.9 g/kg DM). Chemicals like fibre and CT are known to influence DM degradability (Nordkvist and Aman 1986; Makkar et al. 1991). However, no significant relationship is found between the DM digestibility and ADF (r = –0.065, P>0.05) and CT (r = –0.24, P>0.05) in the present study.
Alpine shrubs are good supplementary feeds for yak, especially in the alpine regions where protein feed resources are very limited. However, in the present study, the amount of nutrients absorbed by yak was not determined. Thus, potential effect on productivity was not estimated. Research is needed to quantify the utilisation of alpine shrubs by yak, especially in the following aspects: a) voluntary consumption potential; b) potential digestibility; and c) ability to provide by-pass nutrients for absorption in the small intestine.
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