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Simple Summary: The yak is the primary income source for millions of people in remote areas of western China and other parts of Asia, providing a basic subsistence commodity for herdsmen. Yaks depend on natural grazing, but the grass trace elements may be insufficient for the yaks' requirements. Our prior research demonstrated that the levels and sources of copper, manganese, and iodine could influence in vitro rumen fermentation for yaks. The aim of this study was to optimize yak feeding standards by examining the impact of diets containing different levels of copper, manganese, and iodine on yak growth. Results showed that diets containing 15.00 mg/kg of copper, 50.00 mg/kg of manganese, and 0.50 mg/kg of iodine had the most positive effect on the growth of growing yaks. Our study will facilitate yak diet formulation design and promote yaks' industry development. Copper, manganese, and iodine are part of a yak's required trace elements. However, knowledge about their dietary requirements is scarce. Therefore, an experiment was conducted to evaluate rumen fermentation, blood parameters, and growth performance and screen out the optimum levels of trace elements in yaks' diet. Here, 18 three-year-old castrated yaks were randomly divided into four groups, which fed with diets containing basal (CON: 4.40, 33.82, and 0 mg/kg) and low-level (LL: 10.00, 40.00, and 0.30 mg/kg), middle-level (ML: 15.00, 50.00, and 0.50 mg/kg), and high-level (HL: 20.00, 60.00, and 0.70 mg/kg) copper, manganese, and iodine for 30 days. With the increase in trace elements, yaks' daily weight gain (DWG), rumen pH, ammonia nitrogen, microbial protein (MCP), and volatile fatty acids levels and serum triglycerides and urea nitrogen levels showed firstly increasing and then decreasing trends and reached the highest values in ML, and serum ceruloplasmin and total superoxide dismutase (T-SOD) activities showed continuously increasing trends. Yaks' DWG, rumen MCP, butyrate, and valerate levels and serum triglycerides, urea nitrogen, ceruloplasmin, and T-SOD levels in ML were significantly higher than CON. Therefore, the recommended levels of copper, manganese, and iodine in growing yaks' diet are 15.00, 50.00, and 0.50 mg/kg (ML), respectively. [ABSTRACT FROM AUTHOR]