Simulation of enteric methane (CH₄) emissions and nitrogen (N) excretion from cows fed grass silage-based diet indicated that there may be a trade-off between reduced N excretions and increased CH₄ emissions when certain dietary manipulations were considered. The study aimed to examine effects of changing dietary forage and CP contents on CH₄ emissions and N excretions from lactating dairy cows. Twelve post-peak lactating Holstein cows were randomly assigned to a 2 × 2 factorial arrangement of 2 forage (alfalfa hay) levels [37 (LF) vs. 53% (HF)] and 2 CP levels [15.2 (LP) vs. 18.5% (HP)] in a 4 × 4 Latin square design with 4 periods. Cows were fed and milked twice daily. During the first 14 d, cows were housed in a free-stall barn, where enteric CH₄ emissions were measured using the GreenFeed system from d 8 to 14 in each period. Cows were then moved to metabolic cages, where total feces and urine output (kg/cow/d) were measured using total collection approach for 3 d. Treatment effects were analyzed by a mixed model with the random effect of cow. Dietary forage content was negatively associated with DMI (P = 0.09) and milk yield (MY, P = 0.02) and positively related (P < 0.05) to CH₄ emission per unit of DMI (HF = 20.4 (0.83) vs. LF = 17.8 (0.84) g/kg) and MY (HF = 13.7 (0.80) vs. LF = 11.6 (0.80) g/kg). Dietary CP content did not affect CH₄ emissions but was positively related to milk urea nitrogen content, urine, urine N (UN), and total manure N outputs (MN) per kg of DMI or MY (P < 0.01). Dietary forage content was positively associated with only UN (HF = 170 (8.5) vs. LF = 152 (8.5) g/cow/d, P < 0.01). Fecal N output was not affected by dietary CP levels. Dietary forage content was positively associated with only UN (HF = 170 (8.5) vs. LF = 152 (8.5) g/cow/d, P < 0.01). The enteric CH₄ emissions and MN per unit of milk yield were positively correlated (r = 0.67, P < 0.01). There were no interactions between dietary CP and forage contents on CH₄ emissions or N excretion. Overall, increasing dietary forage and CP contents independently increased CH₄ emissions (g/kg of MY) and total manure N excretions (g/kg of MY), respectively.