Effects of dietary fat supplementation on enteric methane mitigation and farm profitability: A case study of an organic and a conventional dairy herd

• Enteric methane mitigation depended on the basal diet and supplemental fat level; • Methane intensity declined due to lower feed intake, higher fat intake, and more milk; • Gross margin declined in both herds, mainly due to increased feed cost; • Financial incentives are needed to encourage farmers to adopt fat supplementation. Dietary fat supplementation is widely recognized as a mitigation strategy for enteric methane (CH 4 ) emissions in dairy production, yet limited information exists on its impact at the herd level and its implications for farm profitability. Here, we used a herd simulation model to assess the impact of fat supplementation on herd-level CH 4 intensity (g CH 4 /kg ECM) and gross margin. The simulation was conducted in digital twins of two real dairy herds - one organic and one conventional. For each herd, we simulated four scenarios combining two levels of supplemental fatty acids (FA), namely 10 (FA10) and 15 (FA15) g/kg DM, with two application periods: the full lactation period and from week 5 of lactation. The results showed that supplemented FA reduced herd-level enteric CH 4 intensity by 3.8 to 7.2 %. This reduction was influenced by both the basal diet's FA content and the amount supplemented. The gross margin decreased across all scenarios in both herds, with a greater reduction observed in the conventional herd. Supplementing FA from week 5 of lactation had a minor effect on both herd CH 4 intensity and gross margin compared to full lactation application. Sensitivity analysis indicated that the change in ECM as a result of fat supplementation influenced both herd CH 4 intensity and gross margin, whereas the variation in milk price impacted gross margin, highlighting the need for cautious practice. Our findings emphasised the importance of analysing the cost-effectiveness of emission mitigation strategies to support informed farm decision-making.