Mid term effects of grazing intensity on yield formation and forage quality in grassland ecosystems in Inner Mongolia with special reference to water supply
Zusammenfassung der Projektergebnisse
Summary of results (1st and 2nd project phase; 3-6 pages) 1st project phase (2004-2007) The Preliminary Experiment An area of 0.7 ha was chosen and a block design with 4 replicates was established. Three treatments were compared: there was one hay cut in treatment I (TI) in the end of the P3 - Mid term effects of grazing intensity on yield formation and forage quality in grassland ecosystems in Inner Mongolia with special reference to water supply. 96 season, TII was cut every 6 weeks and TIII every 3 weeks. For the cut frequency experiment, the increase in herbage productivity and crude protein content in response to frequent defoliation in 2004 and 2005 shows that the steppe grassland is resistant to defoliation stress in the short term, even under drought conditions. Climatic fluctuations, especially in precipitation rate, had a fundamental influence on the grassland sward. The presented results underline the importance to refer to degradation dynamics on different time scales, that is to distinguish between short-, mid-, and long-term effects of grazing stress. Herbage productivity was substantially reduced and short-term effects of defoliation stress, whether due to cutting or grazing, was likely to cover effects on herbage quality. These results are in line with results obtained in other semi-arid grasslands. In conclusion, short-term responses of the investigated semi-arid grassland sward to defoliation stress were positive with regard to increased aboveground biomass productivity and N uptake. Significantly negative shortterm responses were not observed for any of the chosen parameters. This implies that the evaluated grassland ecosystem is resistant to defoliation or grazing stress respectively, in the short-term and that the process of grassland degradation therefore results from midand/or long-term grazing stress. Furthermore it became apparent that the mean annual precipitation and its intra-annual distribution have a substantial impact on this grassland ecosystem. The presented results underline the importance to refer to degradation dynamics on different time scales, that is to distinguish between short-, mid-, and long-term effects of grazing stress. The CP-content is a reliable indicator for short-term effects of defoliation stress. For other herbage quality parameters the strong influence of climatic fluctuations and seasonal dynamics was demonstrated, which interfere to a great extent with short-term effects of defoliation. However, in the long-term herbage quality parameters might be useful indicators for changes caused by defoliation stress, as it is likely that herbage quality is influenced by substantial changes in the plant community, e.g. a shift from grass to shrub vegetation. From the animal nutrition point of view the quality of the investigated herbage is low and only suffices minimal nutritional requirements. Due to high N-yield variation in the cutting frequency experiment, we added in 2005 an additional N-fertilization treatment in order to analyze the hypothesis of a cutting frequency x N availability interaction. So, the plots existent for the cutting frequency experiment were subdivided and one half was fertilized with 80 kg N/ha as ammonium nitrate, simulating the nitrogen status of urine spots of sheep in the grazing situation. The differences due to fertilization were significant for all treatments, and T III showed the highest DM yield. However, N-fertilization had a marginal effect on the increase in DM yield. Since 2005 was a dry year, weather effects may have confounded the effects of N-fertilization. P3 - Mid term effects of grazing intensity on yield formation and forage quality in grassland ecosystems in Inner Mongolia with special reference to water supply. 97 Fertilization nearly had no significant effects on proportions of Leymus chinensis and Stipa grandis in the community (except for Leymus in treatment II), but the percentages were significantly different between 2005 and 2006. From the results based on plant functional groups (PFGs), fertilization also had no remarkable influence on either PFGs, but the PFGs were notably different between years. In treatment II&III herbs were notably decreased in 2006, and the xerophytes were markedly increased in all treatments at 2006. The grazing experiment The grazing experiment was established in 2005 with 2 systems (traditional and mixed) and seven grazing intensities (0, 1.5, 3.0, 4.5, 6.0, 7.5, and 9.0 sheep/ha are GI0, GI1, GI2, GI3, GI4, GI5, and GI6, respectively). Each system included grazed areas and areas for hay making. The grazing intensity (GI) affected the aboveground biomass production significantly in both years negatively (P<0.001). Neither in 2005 nor in 2006 was the annual DMY affected by management system (S) or by interaction S*I. The interaction was significant in the hay making area, which was attributed to the heterogeneity of the experimental area, since no different management was applied in this first year. Forage quality parameters were strongly influenced by stocking rate. In 2005 and 2006, organic matter (OM) contents decreased (P<0.05) and crude protein (CP) contents increased (P<0.05) continuously with intensified grazing. In vitro digestibility of OM (CDOM) was also affected by stocking rate (P<0.05), but no clear response to stocking rate was detectable in the first year. In 2006, CDOM of forages tended to increase with stocking rate. The response of neutral detergent fibre (NDF) contents to intensified grazing differed between the years. Whereas in 2005 NDF content of forages increased with stocking rates (P<0.05), in 2006 no significant effect of stocking rate could be detected. Generally, small system related effects on forage quality were detectable. After two experimental years only CP content and CDOM of herbage mass tended to be affected by S (P<0.1). CP concentrations in the MS (13.6%) were higher compared to TS (12.6%) and, CDOM of herbage mass in the MS (64.5%) were above TS (63.4%). The measurements of LAI in the grazing experiment showed the expected results as influenced by the grazing intensities. With increasing grazing intensities, more defoliation occurred and consequently less foliage intercepted the light, decreasing the LAI consistently. The difference between systems was not significant, as well as the interaction between systems and grazing intensity. The low values of LAI for the swards are a result of the low plant density and low leaf appearance rates at the end of season. In conclusion, the grazing experiment after one year of establishment demonstrated the importance to have assembled several levels of grazing intensities, varying from the very P3 - Mid term effects of grazing intensity on yield formation and forage quality in grassland ecosystems in Inner Mongolia with special reference to water supply. 98 light to the very heavy intensity. The heterogeneity in DM yields of the experimental area influenced the distribution of herbage on offer between grazing intensities. However, the swards were already stressed according to the different grazing intensities. The differences imposed by the levels of grazing intensity overcome the effects of heterogeneity of the experimental area, as well as in 2006 grazed areas could recover in the mixed system, where one hay cut was taken in August 2005 and changes in botanical composition are expected. Results from sample collection in 2006 confirm these preliminary field observations. In this experiment it was hypothesised that when herbage on offer is abundant, i.e. in the low grazing intensities, the in vitro estimated organic matter digestibility (OMD) will underestimate the in vivo OMD, because herbage ingested by grazing sheep will differ from herbage on offer due to selective grazing. But when herbage on offer becomes limited, i.e. in the high grazing intensities, then herbage ingested and herbage on offer will not differ very much from each other anymore, because the grazing sheep are not able to select, but forced to ingest all herbage offered. Therefore the in vivo OMD and in vitro OMD will converge in the high grazing intensities. The differences between in vivo and in vitro OMD might be used as an index for the selectivity of the grazing sheep in the grazing experiment. Herbage on offer significantly decreased with increasing grazing intensity, as it was expected. But the OMD was not significantly influenced by grazing intensity, neither the in vivo OMD, nor the in vitro OMD. Therefore the hypothesised convergence of in vivo and in vitro OMD in high grazing intensities was not observed. This result suggested that herbage ingested and herbage on offer were similar and that selectivity was not very pronounced in 2005. This argument is supported by the results that firstly, the quality structure of herbage on offer was rather homogenous, especially the vertical quality structure, and that secondly, the green herbage proportion of herbage on offer remained constant across the grazing intensities. A difference in selectivity between grazing intensities was therefore not observed in 2005. 2nd project phase (2007-2010) Analysis of short-term effects of season, grazing management, and grazing intensity on herbage mass, herbage accumulation, relative growth rate, and herbage quality (results from 2005-2006 only of grazing plots): Sampling time was the main factor affecting yield and quality. Stocking rate also showed considerable effects on yield. Herbage mass decreased linearly from ungrazed (UG) to very heavy grazing (GI6) by 85% and 82% in 2005 and 2006, respectively. Herbage accumulation was also affected by grazing intensity, and was highest at GI1 and clearly reduced at GI6. P3 - Mid term effects of grazing intensity on yield formation and forage quality in grassland ecosystems in Inner Mongolia with special reference to water supply. 99 Grazing effects on relative growth rate indicated grazing tolerance of plants in the short-term, since up to high grazing intensity, relative growth rates remained stable. Precipitation also determined plant responses to increasing levels of grazing. The year of higher rainfall generated higher grazing tolerance of plants and higher herbage growth than the drought year. Despite considerable reduction of herbage mass, consistent short-term responses of herbage quality to grazing in 2005 and 2006 were reflected only in terms of crude protein and acid detergent lignin. Herbage crude protein content was highest at GI5 and GI6, while lignin was lowest at GI5 and GI6. Neither productivity nor herbage quality was affected by the management system, suggesting that both systems may be applicable on typical steppe in the short-term. Mid-term effects of management system and grazing intensity on end-of-season standing biomass (ESSB), aboveground net primary production (ANPP), relative difference in ANPP between 2005 and 2008 (ANPPDiff), relative growth rate (RGR), and sward characteristics (litter accumulation, soil coverage) (results from 2005-2008 considering production units, i.e. hay-making and grazing plot): Each grazing intensity treatment was considered a production unit comprising two adjacent plots, one for hay-making (single-cut system) and one for grazing. Hay-making and grazing alternated annually in the MIXED SYSTEM (MS), while in the TRADITIONAL SYSTEM (TS) the same plots were used either for hay-making or for grazing. Effects of management system, grazing intensity, and year on end-of-season standing biomass (ESSB), aboveground net primary production (ANPP), relative difference in ANPP between 2005 and 2008 (ANPPDiff), relative growth rate (RGR), and sward characteristics (litter accumulation, soil coverage) were analyzed. Litter accumulation of production units was affected by grazing intensity (P<0.001) and decreased from GI0 to GI6 by 83%. Correspondingly, soil coverage decreased (P<0.001) from GI0 to GI6 by 43%, indicating an increased vulnerability to soil erosion. We found varying compensatory growth responses to grazing intensity among years, probably because of temporal variability in precipitation. The ability of plants to partially compensate for grazing damage was enhanced in years of greater seasonal precipitation. The ANPP of production units was negatively affected by grazing intensity and decreased from GI0 to GI6 by 37, 30, and 55% in 2006 (P<0.01), 2007 (P<0.05), and 2008 (P<0.001), respectively. The effect of management system × grazing intensity interaction on ANPP (P<0.05) and ANPPDiff (P<0.05) suggested greater grazing resilience of the MS compared to the TS at GI3 to GI6. P3 - Mid term effects of grazing intensity on yield formation and forage quality in grassland ecosystems in Inner Mongolia with special reference to water supply. 100 Effect of management system on species community structure and functioning (results from 2005-2009) Results from the species response to two management systems on a five-year grazing trail in Inner Mongolia steppe, showed: a) the mixed management system has the potential to mitigate species shift in compare with traditional management system, in which significantly negative effects of stocking rate on Leymus chinensis and Cleistogenes squarrosa were detected; b) Compared to traditional system, the mixed system also showed a tendency of higher resilience to precipitation, which is partly attributed to the positive response of Leymus chinensis and Agropyron cristatum. Under the considerations that the seeds play an important role for species persistence and the clear observed phenomena of higher reproductive tiller number in haymaking plot in comparing with grazed plot, the ratio reproductive tiller of dominant species was investigated once in 2007 for both management systems. Results showed: a) grazing reduced the ratio and density of reproductive tiller markedly compared to control and haymaking plots in traditional system; b) there was no carry-over effects of grazing on ratio and density of reproductive tiller in the mixed system. Therefore, in the perspective seed production (as indicated by flower ratio) and long term soil seed bank balance, the mixed system also showed advantages in comparison with the traditional system, in which the grazed and haymaking plot are isolated. Effects of water and N fertilizer supplementation on the short-term recovery of grassland from grazing (2007-2008) A 2-year water and N fertilizer addition experiment was conducted at experimental sites with well defined grazing intensities to investigate the effects of water and N availability on shortterm recovery of vegetation from sheep grazing. In this study, water and nitrogen addition significantly increased grassland aboveground net primary production but had no effect on belowground net primary production (BNPP). Water and N addition also increased the aboveground biomass of dominant species. In parallel with the response of primary production, plant tiller and leaf traits were affected: plant tiller weight and leaf area index was declining but leaf weight ratio was increasing along grazing intensities, where as water and N addition increased tiller weight and leaf area index. Accordingly, during the process of recovery, plant aboveground biomass correlates positively to tiller weight, leaf area index, but correlates negatively to leaf weight ratio. Our study indicates that water and N addition can be useful management tools for accelerating grassland recovery in terms of enhancing primary production and leaf area index. Thus more attention on the water and N P3 - Mid term effects of grazing intensity on yield formation and forage quality in grassland ecosystems in Inner Mongolia with special reference to water supply. 101 management for degraded rangeland should be drawn, such as a small amount of N fertilizer addition in wet years can effectively promote rangeland recovery from grazing.
Projektbezogene Publikationen (Auswahl)
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(2009): Dietary selection of sheep grazing the semi-arid grasslands of Inner Mongolia, China at different grazing intensities. Journal of Animal Physiology and Animal Nutrition, 94 (4), 446-454
Schiborra, A., Gierus, M., Wan, H.W., Glindemann, T., Wang, C.J., Susenbeth, A., Taube, F.
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(2009): Short term management and stocking rate effects of sheep grazing on herbage quality and productivity of Inner Mongolia steppe. Crop and Pasture Science, 60, 963-974
Schönbach, P., Wan, H.W., Schiborra, A., Gierus, M., Bai, Y.F., Müller, K., Glindemann, T., Wang, C., Rave, G., Susenbeth, A., Taube, F.
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(2009): Short-term responses of a Stipa grandis/Leymus chinensis community to frequent defoliation in the semi-arid grasslands of Inner Mongolia, China. Agriculture, Ecosystems & Environment, 132, 82-90
Schiborra, A., Gierus, M., Wan, H., Bai, Y., Taube, F.
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(2011): Differential responses of plant functional trait to grazing between two contrasting dominant C3 and C4 species in a typical steppe of Inner Mongolia, China. Plant and Soil, 340 (1-2), 141-155
Zheng, S.X., Lan, Z.C., Li, W.H., Shao, R.X., Shan, Y.M., Wan, H.W., Taube, F., Bai, Y.F.
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(2011): Effects of grazing management system on plant community structure and functioning in a semiarid steppe: scaling from species to community. Plant and Soil, 340 (1-2), 215-226
Wan, H.P., Bai, Y.F., Schonbach, P., Gierus, M., Taube, F.
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(2011): Grassland responses to grazing: Effects of grazing intensity and management system in an Inner Mongolia steppe ecosystem. Plant and Soil, 340, (1-2) 103-115
Schönbach, P., Wan, H.W., Gierus, M., Bai, Y.F., Müller, K., Lin, L., Susenbeth, A., Taube, F.
