Grass-based veal production offers a sustainable and welfare-oriented use for male dairy calves. Multispecies pastures are often linked to lower methane emissions, yet this relationship has not been investigated in veal calves. This study evaluated the in vitro gas production of rumen fluid from six-month-old male calves reared for three months under four grass-based systems: indoor hay feeding, and grazing on lowland, mid-elevation, or alpine pastures. Three genotypes were compared – Brown Swiss (dairy), Limousin × Brown Swiss (crossbred), and Swiss Fleckvieh (dual-purpose) – with four calves of each genotype per system (n = 48). All calves received hay ad libitum and standardised feed supplements. Access to pasture totalled 611, 769, and 771 h for the lowland, mid-elevation, and alpine pasture systems, respectively. Rumen fluid was collected by intubation (day 175 ± 9) and incubated in vitro to assess total gas, methane (CH₄), and ammonia (NH₃) production from a standard feed, and in vitro organic matter digestibility (IVOMD) was calculated. Absolute CH₄ production in samples from extensive systems (mid-elevation and alpine pastures) was up to 49 % higher than in intensive systems (indoor and lowland pastures). IVOMD was greatest in indoor samples, while NH₃ formation significantly differed between systems. Rumen fluid from Swiss Fleckvieh calves yielded less CH₄ than that of crossbred calves, with no genotype × system interactions. Overall, rumen fluid from more intensive, less botanically diverse systems produced lower in vitro gas emissions. The methodological influence of time between sampling and incubation was quantified and discussed.