2.1 | Study area and animal management
The study was conducted over an 83-day period, including a 10-day adaptation period and a 73-day treatment period, from August 1st to October 22nd, 2019. The experiment took place in natural desert steppe located behind the Tan Sheep Breeding Farm (106°58′E, 37°26′N; alt. 1400 m) in the eastern region of Ning Xia of northwestern China. The mean annual temperature of the area is 8.3°C, and average annual precipitation is 282.3 mm, occurring mostly between June and September. The predominant native plant species were Lespedeza sp. followed by Caranana sp. during the experimental period. Other forage is also present in the meadow, including Salsola sp. andArtemisia sp. , but with irregular distributions. A total of 40 hectares of pasture were fenced off into eight equal paddocks used as rotational grazing plots.
All animal experiments received approval from the China Agricultural University Laboratory Animal Care Advisory committee. All experimental procedures and animal care protocols were accomplished in accordance with the guidelines provided by the Institutional Animal Care Advisory Committee for China Agricultural University. Thirty-nine healthy male lambs of Tan sheep with an average initial body weight of 25.10 ± 1.88 kg were randomly divided into three treatment groups (n = 13): a stall-feeding group (control group), a time-limited grazing group (LG), and a grazing group (G). The lambs began to access pasture at 7:00 and were removed at 11:00 and 19:00 for 4h (LG) and 12h (G) treatments, and then were separately housed in individual pens at the end of grazing. All of the lambs from the stall-feeding group were individually penned in steel cages. The lambs were fed their respective diets twice per day at 8:00 and 17:00 h, and the lambs for the LG treatment only received their supplementary feed at 17:00 h. The basal diet was formulated with concentrates and hay (10% alfalfa hay, 15% corn stover, 75% TMR) to meet or exceed the nutrient requirements from the National Research Council (NRC 2007); the ingredients are listed in Table 1. During the 10-day adaptation period, feed was adjusted daily based on the previous day’s intake, allowing refusals of 15%. All sheep were provided with adequate water and salt blocks throughout the experimental period.