The utilization efficiency of the feed depends on the proportion of energy and protein it contains. In order to determine, in vitro, the effect of protein level (CP) and energy (ME) of the feed on the extent of digestibility (IVDMD), the volume (mV), rate (R) and Lag phase (L) of gas production; the fast (FFF), medium (MFF) and slow (LFF) fermentation fractions; methane production, global warming potential indicator (GWPI) and environmental impact index (EII), five diets were formulated with different ratios of metabolizable energy and crude protein (ME: CP) 2.8:16 (MM), 3:17.2 (AA), 2.8:14.5 (AB), 2.6:17.2 (BA) and 2.6:13.8 (BB). The R was higher (p<0.05) in the diets with the highest energy content (AA, AB), and the mV was lower (p<0.05) for the diet low in ME and high in CP (BA). The AA diet had the highest (p<0.05) values of FFF and MFF, while the highest proportion of LFF (p<0.05) and lowest IVDMD (p<0.05) was observed for the BA and BB diets. In these same diets, the highest proportion (p<0.05) of CH₄ was found and the BB diet caused the highest (p<0.05) GWPI and EII. It is concluded that diets with a higher content of non-structural carbohydrates have better fermentative kinetics and digestibility, and generate lower proportions of CH₄. The GWPI may be a good estimator of the environmental impact since it considers the amounts of CO₂ and CH₄ produced in the rumen.