Animal responses to herbage allowance: Forage intake and body weight gain of sheep grazing the Inner Mongolian steppe - Results of a six-year study.

An increasing human population and the growing demand for food of animal origin are leading to an intensification of sheep production and widespread overgrazing of the grassland steppe in Inner Mongolia. The aim of this study was to analyze the effect of herbage allowance (HA) on OM intake (OMI) and BW gain (BWG) of grazing sheep. In July to September 2005 to 2010, a grazing experiment was conducted in the Xilin River Basin using 15-mo-old female Mongolian fat-tailed sheep (31.5 kg BW [SE 0.2]). Six HA classes were tested on 4 experimental plots per HA class that were alternately used for grazing and haymaking each year (i.e., = 2 grazed plots per HA class and year). Mean HA ranged from 15.4 (SD 4.0) to 1.5 kg (SD 0.8) herbage DM/kg BW in HA class 1 to 6, respectively. In 6 sheep per plot (4 sheep in 2009 and 2010), OMI and BWG were determined. Titanium dioxide was used to determine fecal excretion, and digestibility of ingested OM was estimated from CP concentration in feces. Fecal grab samples were collected during 5 d each in July, August, and September. The animals were weighed monthly. Daily OMI of sheep ranged between 68 and 89 g/kg BW and was not affected by HA class ( = 0.373), so that total OMI per hectare was exponentially decreased with increasing HA (root mean square error [RMSE] ≤ 0.31 g/d; ≤ 0.003 for the slope estimates). The BWG of individual sheep increased with increasing HA in 2 of the 6 yr (RMSE 18.4 g/d; ≤ 0.175 for the positive slope estimates). Nevertheless, BWG per hectare strongly decreased with increasing HA (RMSE 0.25 g/d; ≤ 0.006 for the slope estimates). These data support the common practice of farmers to manage the grassland at low HA to allow for greater animal performance per unit of land area.