This study aimed at assigning gaseous emissions from ruminants to animals or feeds they have consumed. Three adult rumen-cannulated German Holstein steers and 3 forage types were used in a 3x3 Latin square design. Forages were corn silage (CS, 366 g dry matter (DM)/kg, 70.7 g crude protein (CP)/kg DM), alfalfa silage (AS, 411 g DM/kg, 246 g CP/kg DM) and grass hay (GH, 881 g DM/kg, 79 g CP/kg DM). Each period consisted of 10 d where animals received 10 kg DM/d of one of the forages as sole feed and the last 3 d of each period were used for sampling. A defined amount of forage was put in a closed vessel and gas samples were obtained using evacuated headspace vials after 0, 10, 20, 30, and 40 min. Additional samples were taken 3 h after filling following the same procedure. Samples from the gaseous phase of the steers´ rumen were taken 3 h after offering feed in the morning. In 10-min intervals, 4 samples were obtained with a syringe through the closed lid of the rumen-cannula and filled into headspace vials. This was repeated on 3 consecutive d. Samples were analyzed for carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) using gas chromatography. Data were analyzed using SAS 9.4 and a mixed model taking forage type and period as fixed effect as well as their interaction. For the rumen samples, animal was added as random effect. Within each period, day was taken as repeated measurement. There were large differences in the amount of CO₂ and N₂O emitting from the forages. Most N₂O came from AS (9.9 µg/kg fresh matter (FM) · h; P < 0.05) and only small amounts from GH (0.20 µg/kg FM · h) and CS (0.04 µg/kg FM · h). Highest CO₂ concentrations were measured in CS (P < 0.05). After 3 h, emissions from forages were strongly reduced. Methane was not detected in any forage sample. Animals fed CS showed slightly lower (P < 0.05) CH₄ concentrations in the rumen gas sample than when fed AS or GH (16.9% vs. 20.6% and 20.3%). Big differences were found for N₂0 with 0.857 ppm for AS and 0.246 and 0.171 ppm for CS and GH (P < 0.001). Results indicate that fermented forages rich in CP may release climate-relevant N₂O. The N₂O emitting from ruminants seems to originate from the consumed feedstuff and is probably not synthesized in the rumen.