Evaluating Carbon-Negative Spirulina Feed Supplementation as a Supply Chain Carbon Removal Strategy for More Sustainable Dairy Systems

Reducing the greenhouse gas (GHG) intensity of dairy production remains challenging because enteric methane and upstream supply chain emissions persist despite existing mitigation strategies. This study evaluated a supply chain decarbonization approach in which Spirulina (Arthospira platensis) produced through a carbon-removal supply chain (GeoSpirulina) was incorporated into dairy rations to enable net supply chain carbon removal within cradle-to-farm-gate boundaries. A carbon-focused life cycle assessment (LCA) was combined with a commercial-scale feeding trial to determine an appropriate inclusion rate and assess production outcomes. The LCA integrated primary activity and production data from the commercial dairy farm with previously published GeoSpirulina production and soil organic carbon sequestration data derived from primary Icelandic production and field studies, supplemented by secondary background datasets and published dairy emission estimates. GeoSpirulina was produced using geothermal energy and coupled with soil organic carbon sequestration associated with the application of residual biomass as a soil biostimulant, resulting in a modeled net-negative production footprint. Under the defined LCA assumptions, supplementation with 2 g cow−1 day−1 of GeoSpirulina reduced the modeled milk GHG intensity from approximately 1.9 to 0.06 kg CO2e kg−1 at the farm gate. Scenario analysis identified active vitamin B12 concentration as a key driver of the modeled carbon balance. Milk yield and group-level feed efficiency were unaffected, whereas butterfat (+4.7%, p = 0.002), fatty acids (+4.4%, p = 0.011), and somatic cell count (−71%, p = 0.007) improved without detectable adverse effects. These findings suggest that carbon-negative feed ingredients may provide a complementary sustainability strategy for dairy production by embedding measurable carbon removal within agricultural supply chains while maintaining production performance. Although this approach counterbalances rather than eliminates biological emissions, it may contribute to broader dairy decarbonization and sustainable food-production strategies alongside direct greenhouse gas mitigation interventions.