Assessing the carbon footprint of converting confinement dairy production to seasonal grazing in a dairy herd: a scenario study

• Herd simulation improves GHG modelling accuracy during system conversion • Grazing is not a GHG mitigation option and does not raise overall climate impact • Grazing reduces enteric CH 4 emissions and lowers emissions from replacement heifers • High pasture yield and low N fertilizer use on pasture can further reduce emissions Understanding how conversion from confinement to grazing systems affects dairy production and greenhouse gas (GHG) emissions is essential for informed decision-making for emission mitigation in dairy production. Here, we combined a dairy herd simulation model with a farm GHG tool to assess the impact of converting a conventional confinement dairy farm to seasonal grazing on the carbon footprint (CF) of milk production. We hypothesized that the CF would decrease after conversion due to reduced enteric methane (CH 4 ) emissions and improved animal health on pasture. We modelled a 6-month grazing season with cows on pasture 6 hours daily, where grazed grass made up 20 % of daily dry matter intake. Herd simulations were performed to assess the effect of grazing on animal health, and the resulting effects on feed intake, milk yield, and herd replacement. These outputs informed a farm GHG tool, which calculated CF from key emission sources including enteric fermentation, feed production, energy use, and manure management. Results for CF indicated that the benefits from grazing, including reduced enteric CH₄ emissions and improved animal health, were slightly outweighed by increased emissions from nitrogen fertilization. Consequently, the CF increased slightly, from 1.115 to 1.119 kg CO₂-eq per kg fat- and protein-corrected milk, marking a 0.3% increase. Sensitivity analysis showed that longer walking distance, no health effect, purchased concentrated feed, and decreased pasture nitrogen fertilization changed CF by -1.1 to +1.4 % after conversion. Our findings suggest that a well-managed seasonal grazing system does not substantially increase GHG emissions as compared to the same farm under confinement production. This indicates that grazing is neither a significant GHG mitigation strategy nor a major contributor to climate impact.