The aim was to evaluate the effects of supplementation with nitrogen and nitrogen plus starch in tropical grass hay (TGH) on neutral detergent fiber (NDF) and dry matter (DM) in vitro digestibility, pH, ammoniacal nitrogen (NH3-N), volatile fatty acids (VFA), methane (CH4), and CO2 emissions. The study was divided into two sets, first, evaluating the level of nitrogen to be supplemented: control (CON -TGH, 68 g CP/kg DM), low nitrogen (LN- TGH +90 g CP/kg DM), and high nitrogen (HN- TGH + 130 g CP/kg DM). The second set aimed to evaluate the level of nitrogen and starch to be supplemented: low nitrogen and low starch (LNLS - TGH + LN + 10% forage weight in starch supplementation-SS), high nitrogen and low starch (HNLS- TGH + HN + 10% SS), low nitrogen and high starch (LNHS - TGH + LN + 20% SS) and high nitrogen and high starch (HNHS - TGH + HN + 20% SS). Samples were incubated in serum bottles (50 mL) and each run was replicated 4 times. The measurements of NDF and DM digestibility, NH3-N concentration, and pH were performed at 3, 6, 9, 12, 24, 36, 48, 72, and 96 h, and CH4, CO2, and VFA were evaluated after 24 h. All data were analyzed using a completely randomized design in factorial scheme. Differences across treatments were investigated using five contrasts. In the first set, two contrasts were used to evaluate the nitrogen effect (linear and quadratic). In the second set, three contrasts were used to evaluate the effects of nitrogen, starch, and interaction between them. In the first set, there was linear nitrogen effect on NDF digestibility (p = 0.037), pH (p = 0.001), NH3-N (mg/dL; p = 0.001) and CO2 (g/g digestible DM; p = 0.042). Quadratic nitrogen effect was observed on CO2 production (mL, g/g DM and g/digestible DM; p < 0.050). In the second set, nitrogen supplementation improved undegradable NDF fraction, pH and NH3-H (mg/dL; p < 0.050) levels. The starch supplementation increased propionate, butyrate (mmol/d; p < 0.050), and CO2 production (mL and g/g DM; p = 0.004). There was no nitrogen and nitrogen plus starch effect on CH4 production. In conclusion, nitrogen supplementation increased NDF digestibility, NH3-N concentration, and pH. Moreover, the supplementation with nitrogen (130 g/kg DM) plus starch (20% of TGH) provided greater VFA production. However, despite the benefits of nitrogen or nitrogen plus starch supplementation on nutritional parameters, they were not effective to reduce CH4 and CO2 production.