2.2 Experimental design
Twelve adjacent plots (each ca. 4.40 hm2) were established at a grazing experiment site in 2004. These plots were arranged in a randomized complete block design, which included four stocking rate treatments with three replicates of each treatment (Figure 1 ). The stocking rates applied in each treatment were 0, 0.91, 1.82, 2.71 sheep•hm-2•half yr-1, representing no grazing (CK), light grazing (LG), moderate grazing (MG), and heavy grazing (HG), respectively, with 4 grazing sheep in the LG treatment, 8 sheep in MG and 12 sheep in HG. Seasonal grazing started on 1 June and ended on 31 November in each year since 2004. The initial sheep from the same cohort were 2-year old Mongolian wethers and individuals were replaced after 3 years. The daily grazing schedule was from 6:00 am to 6:00 pm. Water and salt were provided freely (Wang et al., 2011).