An in vitro gas production and degradability study has been conducted to investigate the effects of four treatments including: no supplement (control), 0.065% HMBi/ DM diet (HMBi-1), 0.13% HMBi/ DM diet (HMBi-2) and 0.088% dl-Met/ DM diet (dl-Met) on typical dairy cow diets. Two diets with 17.7% (HCP, high crude protein) and 15.7% (LCP, low crude protein) CP have been formulated by different ingredients for early lactating Holstein dairy cows (DIM 55±7, BW 650 and Milk yield 55±6.4). Results revealed that digestibility of DM were significantly increased by HMBi and dl-Met addition while digestibilities of ADF, NDF and HEMI was decreased for the dl-Met treated diets compared with the diet containing the equivalent amount of Met supplied as HMBi-2. Asymptote gas production (A) has been affected by HBMi and dl-met supplementation (P<0.001) but fractional gas production rate (c) parameter of gas production has not affected by treatments. Regarding fermentation parameters, pH has not been affected by supplements or CP levels. Also, there are linear and quadratic effects of HMBi incremental levels on ammonia-N whereas its concentration decreased with addition of HMBi. It is concluded that supplementation dairy cows’ diet with Met sources can alter rumen fermentation and its activity, which is approved by more degradability of DM in both HCP and LCP diets and enhanced cumulative gas production. Low crude protein diets can reduce excess ammonia-N load in rumen and increase N utilization in dairy industry which will modify animals as well as environment friendly farming.
