Short-term effects of defoliation on herbage productivity and herbage quality in a semi-arid grassland ecosystem of Inner Mongolia, P.R. China

The vast grassland steppes of Inner Mongolia have been heavily degraded due to overgrazing in the last decades, so that the consequences of grassland degradation are today the main ecological and socio-economical problems in the region. The major objective of the presented work was to describe short-term effects of defoliation using (i) plant related parameters and (ii) herbage quality parameters. A field experiment was carried out, simulating different levels of grazing stress by cutting. Additionally, a grazing experiment was established and the digestibility of herbage ingested and herbage on offer was compared between different grazing intensities to test for differences in selective feed intake. The field experiment was carried out in 2004 and 2005. A natural grassland sward was exposed to three different cutting frequencies. The biomass yield, as well as nitrogen uptake, and with it the crude protein content, were significantly increased due to higher cutting frequency. Short-term differences in root mass and root carbohydrate reserves, as well as differences in species composition and tiller density were not significant. The herbage quality parameters cell wall content and -composition, the organic matter digestibility (OMD) and the non-structural carbohydrate content showed significant changes, but these were not consistent, neither between treatments nor between years. The herbage quality parameters were more influenced by climatic and seasonal fluctuations, than by the tested cutting frequencies. The increase in biomass yield and crude protein content in frequently cut treatments demonstrated, that the grassland steppe was resistant in the short-term to high defoliation intensity even in extremely dry years like 2005. The grazing experiment started in 2005 and the data obtained in this year were analysed to answer the question, if OMD of ingested herbage was underestimated by in vitro determined OMD of herbage on offer, what can be expected due to selective feed intake of grazing animals. When herbage on offer decreases, one would expect that OMD of herbage offered and OMD of herbage ingested converge, as grazing animals have less opportunity to select. As expected, herbage on offer significantly decreased with increasing grazing intensity. However, OMD was not significantly different between grazing intensities, the expected effects of selective grazing were not assessed. The observation that the animals did not graze selectively, or were not able to, was supported by two other observations: firstly, the vertical quality structure of herbage on offer was very homogenous in 2005 and secondly, the necrotic material proportion of herbage offered was relatively constant across all grazing intensities. High defoliation intensities have positive effects on productivity and to some extent on herbage quality of the grassland vegetation in the short-term. Nevertheless it became clear, that climatic fluctuations, especially the amount of precipitation, have a fundamental influence on the grassland ecosystem.