Copper (Cu) is essential for the health of livestock, however, the optimal source and level of dietary Cu for yaks are uncertain. To fill this important gap, we designed an in vitro study to examine the effects of three Cu sources, namely Cu methionine (Met-Cu), Cu chloride (CuCl2) and tribasic Cu chloride (TBCC), at five levels, namely 5, 10, 15, 20 and 25 mg/kg DM (includes Cu in substrate), on rumen fermentation in yaks. In vitro dry matter degradability (IVDMD) and amylase activity were greater (p < 0.05) with added Met-Cu than the other two Cu sources, and ammonia nitrogen (NH3-N), microbial protein (MCP) and propionate contents were greater with Met-Cu and CuCl2 than with TBCC. Total gas production and lipase activity were greater with Met-Cu and TBCC than CuCl2 (p < 0.05), which meant that the metabolizable energy yield was greater in the two former Cu sources than the latter, but CH4 production did not differ (p = 0.92) among Cu sources. IVDMD and lipase activity were greatest (p < 0.05) at 15 mg Cu/kg DM in the substrate and MCP, isobutyrate, butyrate and isovalerate contents, and amylase and trypsin activities were greatest or second greatest at 10 and 15 mg Cu/kg DM. It was concluded that Met-Cu was the best source of Cu and 10 to 15 mg Cu/kg DM was the optimal level for yaks, at least under in vitro conditions.
