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ABSTRACT The objective of this study was to evaluate the effects of mineral and protein-energy (MPES), exogenous fibrolytic enzyme supplements (ES), combination of MPES + ES, and straw without supplement (WS) on digestibility, fermentation kinetic parameters, cumulative gas production, methane, CO2 production, and volatile fatty acid concentration of rice straw of low and high nutritional value, estimated by in vitro techniques. The experimental design was randomized and factorial 2 × 4: two straws (low and high nutritional value) incubated with four supplements (MPES, ES, MPES + ES, and WS) and their interactions. Four experimental periods were used, totaling four replications per treatment over time. Data were analyzed by PROC MIXED of SAS. The in vitro dry matter and organic matter digestibilities of the rice straw with high nutritional value was improved by MPES, while the combination of MPES + ES supplements inhibited the digestibility of this straw. Dietary carbohydrate and nitrogen increased through MPES and MPES + ES supplements resulted in an increase in NH3-N concentration and a decrease in CO2 production due to the microbial mass formation. However, this increase was not enough to improve organic matter degradability parameters, cummulative gas production, gas production kinetics, and acetate:propionate ratio and reduce methane emissions. The straw with high nutritional value showed greater content of nitrogen fraction a, effective degradability, cummulative gas production, and methane and CO2 productions comparing with low-nutritional value straw. The use of MPES and MPES + ES supplements can be used as strategy to mitigate CO2 in ruminant production systems that use rice straw.