Feeding legume and cereal straw-based rations has no adverse effect on heifer performance, while reducing methane emissions by 5.60% and methane-related energy loss by 5.97%. Supplementation of 3% SSF in TMR enhances growth rate by 13.45% and significantly improves body weight gain, nutrient digestibility, and rumen fermentation. Additionally, SSF biomass in TMR reduces methane emissions and energy losses by 13.14%, redirecting saved energy towards heifer weight gain. Despite varied feeding regimes, ruminal microbial diversity remains stable, with Methanobrevibacter and Methanobacterium populations lower in SSF-supplemented groups (T2 and T3). Feed costs are comparable across groups, but cost per kg of weight gain is lower in the SSF group.