Effects of Different Grazing Intensities on Vegetation Characteristics of Cultivated Pastures in Southern China
WAN Li-qiang, LI Xiang-lin*, HE Feng,CHEN Wei-wei, WAN Jiang-chun, ZHAO Yun, WU Wei-da
Institute of Animal Sciences, Chinese Academy of Agricultural Sciences, Beijing 100193, China
Abstract [Objective] The paper was to study the effect of different grazing intensities on vegetation characteristics of artificial pasture by plot rotation grazing experiment. [Method] Grazing intensity was divided into 2 treatments of heavy grazing and light grazing, and each treatment contained 6 plots, with randomized complete block design. A blank control was set near test plot. Ninety disease-free and healthy 2-year-old Yunling Black Goats with consistent body condition were selected and divided into 2 groups: heavy grazing (55) and light grazing (35). The goats were conducted rotation grazing in 6 plots, and the average stocking rates were 12.6 goat /hm2 and 8 goat /hm2. The plant height, coverage, underground biomass and pasture plant nutrition of 2 treatments were measured, respectively. [Result] The cocksfoot height had significant difference with control under grazing conditions (P<0.05), while the heights of ryegrass and white clover in control plot had no significant difference with that in grazing plot (P>0.05). Grazing intensity had different effects on coverage of different plants; grazing utilization reduced the coverage of cocksfoot and ryegrass, which had little impact on coverage of white clover; viewed from the whole plant communities, the coverage in control plot was greater than that in grazing plot. The underground biomass in 0-20 cm soil depth in grazing plot accounted for about 90%; with the increase of grazing intensity, underground biomass was decreased. The crude protein (CP) and crude fat (EE) content of forage decreased, while acid detergent fiber (ADF) and neutral detergent fiber (NDF) increased. [Conclusion] The research provided scientific basis for determination of suitable grazing system and sustainable utilization of pasture resources.
Key words Grazing intensity; Artificial pasture; Vegetation characteristics
The correlation between pasture grazing intensity and vegetation characteristics is an active plot of grazing ecology [1], and the researches are mainly concentrated on the effect of livestock grazing intensity on the aspects of pasture productivity, plant community diversity, grass height and vegetation succession [2]. The main parameters of pasture vegetation structure reflect the changes of "quality" and "quantity" of pasture and grass group [3]. In order to ensure the sustainable utilization of pasture, as well as integrated and efficient development of animal husbandry, the extensive pasture animal husbandry development thought of traditional one-sided pursuit of livestock number and light grass heavy livestock should be abandoned, and the grazing capacity and stocking rate should be determined by scientific methods [4]. Therefore, the determination of optimal grazing intensity not only depends on livestock grazing behavior, but also depends on the structural characteristics of pasture vegetation [5]. However, there are few reports about the effect of goat grazing on artificial pasture in southern China. Through plot rotation grazing experiment, the impact of different grazing intensities of goat on vegetation characteristics of artificial pasture was studied in the test, so as to provide scientific basis for determination of suitable grazing system and sustainable utilization of pasture resources.
1. Materials and Methods
1.1. Natural situation of survey plot The survey plot is located in Yunnan breeding sheep field 7 km away from the county, which is in the west of Northeast Xundian County in Kunming City, Yunnan Province, E103°11′, N25°40′, with altitude of 2100 m. The annual average temperature is 13.4 ℃, and the annual rainfall is 965mm, with rain and heat over the same period. Rainfall is concentrated during June and October, accounting for 82% of annual rainfall. The soil is brick red soil, with pH of 5.0. The organic matter content in 0-10cm soil layer is 4.61%. The total nitrogen content is 0.74%, and the available phosphorus content and available potassium content are 24.81 and 90.77mg/g, respectively. The plot has low temperature and less rainfall in winter, which is warm and dry in spring.
1.2. Experimental design The survey plot was artificial cultivation pasture with mixed sowing of perennial ryegrass, cocksfoot and white clover, and the weed mainly was artemisia. Grazing intensity was divided into 2 treatments of heavy grazing and light grazing, and each treatment contained 6 plots (each plot is about 0.73 hectares), with a total of 12 rotational grazing plots, randomized complete block design. In addition, the control plot with area of about 1 hectare was set near survey plot, and the forage inside the plot was under natural growth state. The plot was completely enclosed without any grazing. Ninety disease-free and healthy 2-year-old Yunling Black Goats with consistent body condition (average body weight is 46.5 kg) were selected and divided into 2 groups: heavy grazing (55) and light grazing (35). The goats were conducted rotation grazing in 6 plots, and the average stocking rates were 12.6 goat /hm2 and 8 goat /hm2.
1.3. Determination items and methods Plant height: the natural height of 9-15 plants in internal and external plot (0.5m × 0.5m) was measured and recorded according to different species.
Coverage: the plant coverage was measured by quadrat grid method (needle point method) according to different species.
The above indexes were measured for one time before and after each rotation grazing, respectively.
Underground biomass: the soil drilling with diameter of 6 cm was used to take samples. Using diagonal method, 9 drills of samples were taken in pasture before the grazing test; another 9 drills of samples were also taken in each grazing plot and control plot after grazing. The sampling depth was 0-30 cm, and every 10 cm was adopted as a layer and placed into bags. The samples were air-dried and sieved, and dried to constant weight at 70 ℃.
Pasture plant nutrients testing items include crude protein (CP), crude fat (EE), neutral detergent fiber (NDF) and acid detergent fiber (ADF). The dried pasture plants collected were detected in analysis center of Beijing Academy of Agriculture and Forestry.
1.4. Data processing Excel 2003 and SAS 9 statistical software were used for data processing and statistical analysis.
2. Results and Analysis
2.1. Height of pasture plants under different grazing intensities The height changes of pasture plants under different grazing intensities were shown in Table 1. As shown in the table, the height of cocksfoot had significant difference in control, heavy grazing and light grazing treatments (P < 0.05), while the difference between heavy grazing and light grazing treatments was not significant (p>0.05). The height of white clover had no significant difference among various treatments (p>0.05). The height of ryegrass in heavy grazing group showed significant difference with that in control group in October, and it also had significant difference among light grazing groups (P < 0.05). In addition to the cases, the height of ryegrass had no significant difference in September and November (p>0.05). The height of weeds in control was significantly different from that in grazing treatment in September (P < 0.05), and it had no significant difference among various treatments in October and November (p>0.05).
2.2. Coverage of pasture plants under different grazing intensities The impact of different grazing intensities on coverage of pasture plants were shown in Fig. 1. As shown in the fig, the coverage of cocksfoot in control plot showed decreasing trend with the prolongation of time, and the change trend in heavy grazing and light grazing treatments was consistent, which decreased first and slightly increased after October (Fig. 1-a); the coverage of white clover in control plot and grazing plot showed decreasing trend with the prolongation of time, but the change curve was slightly different, and the coverage of white clover in heavy grazing treatment was obviously smaller than that in control and light grazing treatments (Fig. 1-b); the coverage of ryegrass in control plot was always greater than that in grazing plot, and the ryegrass coverage in control and grazing plot generally showed a downward trend, in which the decrease in control group was more obvious; the coverage decreased from 44% to 26%, and the coverage in grazing plot had small changes (Fig. 1-c). For total coverage of pasture plant community, the total coverage after grazing utilization was smaller than control plot; with the prolongation of time, the total coverage of plant community in control plot showed decreasing trend, the total coverage of plant community in grazing plot first decreased and slightly increased, maintaining at a relatively stable level (Fig. 1-d).
2.3. Impact of grazing on underground biomass of pasture plants The underground biomass of pasture plant is mainly distributed in 0-10 cm soil layer (Table 2). The underground biomass within 0-10cm soil layer in control, light grazing (8 goat /hm2) and heavy grazing plots (12.6 goat /hm2) after grazing accounted for 70.67%, 64.08% and 73.94% of total underground biomass of pasture plant in 0-30cm soil layer. The underground biomass of plant in 10-20 cm soil layer accounted for about 20%, and underground biomass of pasture planted in 0-20 cm soil layer in control and grazing plot all reached about 90%. After feeding by different grazing intensities, the underground biomass of plant community in grazing plot decreased, which was 291.29 g/m2 in heavy grazing plot, lower than control plot (545.74 g/m2) and light grazing plot (524.08 g/m2).
2.4. Changes of forage nutrition before and after grazing The crude protein and crude fat content of cocksfoot and white clover were decreased after grazing utilization, while the neutral detergent fiber and acid detergent fiber were increased; the crude protein, crude fat and acid detergent fiber of ryegrass was decreased after grazing, while the neutral detergent fiber was increased after grazing utilization (Fig. 2).
3. Discussion
Grazing activity is one of the ways for pasture utilization[6], and the impact of different grazing intensities on pasture grazing system first performs in quantitative characteristics of plant community. After different grazing intensity treatments for certain time, the vegetation coverage, density, height, frequency and aboveground biomass of pasture had certain changes, which all had different degrees of reduction with the increase of grazing pressure. Generally, the changes of community species composition are related to plant palatability and competitiveness [6]. Due to the selective feeding of animals, the excellent forage is fed, and its normal growth is hindered, resulting in changes of vegetation composition and structure [7]. In the ecosystem of pasture, the contradiction between animal and plant is the major contradiction [8]. The feeding and trampling of livestock destruct the pasture environment, which make the dominant position of community species changed. The community diversity is also changed, high degree of grazing will reduce the proportion of excellent forage, and let many annual herbs have their survival space [9]. In optimal grazing intensity, goat feeding promotes the plant growth and optimizes community structure. The suitable feeding of goats can remove aging tissue and enhance photosynthesis, and the excretory behavior of goats accelerates the nutrient cycling of pasture ecosystem. In addition, animals can also spread seeds, and inhibit the competition of adjacent individuals [10]. For any piece of grass, to keep its original production capacity and make the pasture ecosystem be stable and developed, we must set the appropriate grazing intensity of pasture [11]. Obviously, the grazing intensity of pasture has relationship with plant growth and development, improvement and deterioration of pasture environment, and ultimately affects the production capacity of pasture and livestock [12]. The study showed that under grazing stress, various forages in the community had direct losses, and the changes of soil environment caused by grazing was also not conducive to the growth of plant individuals. In the mixed pasture, cocksfoot is the forage with relatively high palatability, so the goats first choose cocksfoot to eat, which makes the average height of cocksfoot significantly decreased. Although white clover is palatable as leguminous forage, due to its low life characteristics, it has better growth after reduction of dominant species competition by grazing, so the average height of white clover in grazing plot had no significant difference with that in control plot. Goat grazing has great impact on the coverage of gramineous forage, this may be because that the gramineous forage has fast growth in growing season, adding with its upright growth characteristics, so it is easy to be fed by goats. With the processing of seasons, the light and temperature condition were dropped, the growth recovery of gramineous forage after feeding was inhibited, so the coverage of gramineous forage after grazing also reduced. In mixing sowing pasture of cocksfoot – ryegrass - white clover, the proportion of gramineous forage was far greater than that of leguminous white clover, and this also made the total coverage of plant community slightly decreased after grazing utilization.
Plant root is the base for plant growth[13], the growth condition of aboveground part has close relationship with underground parts [14]. Some researches believe that the impact of grazing on aboveground biomass of community is immediate, while its effect on underground biomass has certain lag effect [15]. The feeding and trampling of grazing livestock reduce soil porosity, ventilation and permeability, which increase soil bulk density, and the growth resistance of underground part in pasture plant increases [16], so the roots are not easy to grow to deep soil layer, and this correspondingly reduces the underground biomass in deep soil. DONG Quan-ming et al. have found that under different stocking rates of yak, the underground biomass in 0-10cm soil layer accounts for 87%-89.4% of total biomass in 0-30 cm soil layer[17]. The data is 64%-74% in the test, the reason may be related to livestock grazing species, pasture types, forage growth status and grazing intensity differences. However, the test and previous studies all reflect the similar change trends of underground biomass. The feeding of grazing livestock reduces aboveground biomass, and the aboveground is the critical organ for plant to produce nutrients required for its growth [18]. The reduction of aboveground part reduces the photosynthetic efficiency of plant [19], and the growth of aboveground part consumes a large amount of nutrition, which reduces the distribution of nutrients to the underground part [20]. Meanwhile, the changes of livestock on soil physical and chemical properties also affect the growth of underground part [21], causing the decrease of underground biomass.
Forage quality affected by grazing is mainly due to the corresponding changes of plant species and proportions [22], the forage quality under different grazing pressure will have very big difference in theory. However, because the test time was shorter, the livestock did not sufficiently feed. In addition, the nutrition characteristics of forage also have great relationship with seasonal changes. The impact of different grazing pressure on forage quality needs to be further studied.
4. Conclusions
4.1. Grazing utilization has significant impact on cocksfoot height, which has no significant effect on the heights of ryegrass, white clover and weeds.
4.2. Grazing utilization reduces the coverage of gramineous forage, which has little impact on coverage of leguminous forage.
4.3. Underground biomass is mainly distributed in the 0-10cm soil layer, and the underground biomass of this soil layer in control, light grazing and heavy grazing plots account for 70.67%, 64.08% and 73.94% of total biomass in 0-30 cm soil layer.
4.4. The crude protein and crude fat content of forage are decreased after grazing utilization, while acid detergent fiber and neutral detergent fiber are increased.
Acknowledgment
This work was funded by Scientific Research Project of Public Service Industry "Bearing Capacity and Livestock Configuration of Pasture in Different Regions (200903060).
About the Author
WAN Li-qiang (1973- ), male, associate professor, engaged in research on pasture agricultural ecology.
Received: April 11, 2012 Accepted: May 18, 2012
*Corresponding author. Tel: 13264058879; E-mail: lixl@iascaas.net.cn