This experiment with crossbred beef cattle to evaluate rice straw versus Elephant grass as source of forage and of presenting the diet as a fermented total mixed feed (FMTR) and as plus 3% Tra fish oil was implemented. Twelve crossbred cattle (Brahman x Zebu) of 352 ± 2.96 kg were arranged in a completely randomized design with 4 treatments and 3 replications and a feeding period of 90 days. Tra fish oil and fermented total mixed ration were the main treatments Elephant grass (EG) versus rice straw as a source of forage and balance the concentrate which was fed separately or as a fermented total mixed feed (FMTR) EG: Elephant grass + concentrate; RC: Rice straw + concentrate; FMTR Fermented mixed Elephant grass + concentrate; RCO: Rice straw + concentrate + 3% Tra fish oil The growth rate was improved by 43% and feed conversion by 26% when rice straw placed Elephant grass as the source of fiber in the diet. Fermenting and offering the complete feed as a mixture (FTMR) led to a 10% reduction in feed intake by 10% with a similar order of increase in weight gain and improved feed conversion; however, the dietary modification that had the most impact on growth and feed conversion was the addition of 3% Tra fish oil, which supported 30% better growth rate and 15% better feed conversion than the diet without the Tra fish oil. It is proposed that these major improvements in beef cattle performance were the direct result of the fish oil in reducing rumen methane production as reported in our earlier paper (Thu N V and Dong N T K 2021). Key words: beef production, climate change, fermentation, lipid supplement, ruminants