Abstract: [Objective] To assess the contents of trace elements in an ecosystem of soil-forage-grazing sheep in the desert steppe of Siziwang Banner, Inner Mongolia, to analyze the surplus or deficiency status of trace elements in sheep among different seasons, and to clarify the nutritional limiting factors of trace elements. [Method]Six healthy young Mongolian ewes weighing （39.65±2.25） kg were chosen as experimental animals from the grazing sheep herd. The weight, feed intake as well as fecal and urinary excretion of the experimental ewes in fresh leaves period （June）, growing period （September） and withering period （November） of forage in this region were determined. The contents of trace elements （iron, copper, manganese, zinc, selenium, molybdenum and cobalt） in soil, forage and ewe serum in different growing periods of forage were evaluated. Feed intake model was used to predict the trace elements intake of ewes in different growing periods of forage; the contents of trace elements in feces and urine were determined by a metabolic trial, and the deposits of trace elements in ewes were calculated. [Result]Extremely significant （P<0.01） differences in ewe weights and feed intakes were observed among different growing periods of forage, and the fecal excretion in withering period was extremely significantly （P<0.01） higher than that in the remaining growing periods. There were no significant （P>0.05） differences in iron contents of grassland soil, forage and ewe serum among different growing periods of forage, and no iron deficiency was observed. The copper content in forage in withering period was extremely significantly （P<0.01） lower than that in the remaining growing periods, and a minus deposit of copper in ewe was found in withering period, which was extremely significantly （P<0.01） lower compared with growing period. While there were extremely significant （P<0.01） differences in contents of manganese and molybdenum in grassland soil and forage as well as the absorbed and deposited in ewe among different growing periods of forage, no deficiency of these two trace elements was observed. Significantly （P<0.05） higher zinc content in ewe serum was observed in fresh leaves period compared with the remaining growing periods of forage; while there were no significant （P>0.05） differences in zinc contents in grassland soil, forage and deposited in ewe among different growing periods of forage, zinc deficiency was observed. The highest selenium content in grassland soil was observed in growing period with extremely significant （P<0.01) differences compared with the remaining growing periods of forage; the selenium contents in forage in fresh leaves period and growing period were below the lower limit of the normal level, and selenium in forage in withering period was absent; while there were no significant （P>0.05） differences in selenium contents in ewe serum among different growing periods of forage, they were all at the lower limit of the normal level. No significant （P>0.05） differences in cobalt contents in grassland soil among different growing periods of forage were found, but they were all lower than the normal level; the cobalt content deposited in ewe in withering period and fresh leaves period was extremely significantly（P<0.01） lower than that in growing period of forage. [Conclusion]Supplementations of zinc and selenium are recommended in appropriate forms for grazing sheep throughout the year in this area, and appropriate supplementation of copper- and cobalt-containing trace element preparations are necessary in winter or spring.

Key words: ecosystem of soil-forage-grazing sheep in grassland, trace elements, seasonal changes, surplus or deficiency analysis