DESTiny SHOWS THAT DAIRY PASTURE FARMS CAN BE GHG NEUTRAL

Dairy production systems worldwide contribute significantly to anthropogenic greenhouse gas (GHG) emissions, accounting for about 4% of global emissions which presumably poses both a climate change and sustainability risk. The emissions originate mainly from enteric methane (CH₄) fermentation, nitrous oxide (N₂O) from manure and soil management, and carbon dioxide (CO₂) from energy and feed production. Standard carbon accounting methods for dairy systems report emission intensities ranging from 1.02 to 1.40 kg CO₂e per kg fat-and-protein-corrected milk (FPCM) In pasture-based systems however, some or all of these emissions can be offset by net carbon accumulation in soils and biomass, as well as the animal itself. The offset success depends on management practices, soil type, climate conditions, animal productivity, and baseline carbon levels.

The Dairy Environment Sustainability Tool (DESTiny) is a system dynamics model designed to integrate biogenic fluxes and emissions. The model in calculation includes on-farm cycling processes (photosynthesis, feed inputs, respiration) and direct emissions (enteric, manure, soil, fuel/electricity). By combining emissions and cycling processes, the model enables the quantification of net annual GHG balances. In this study, empirical data from 12 South African pasture-based dairy farms were used to: (1) quantify net carbon balances across management levels; (2) identify practices that boost carbon accumulation; and (3) evaluate whether lower-input systems can maintain productivity while sequestering carbon. The hypothesis was that farms that optimize feed efficiency, milk quality, conservation tillage, and forage self-sufficiency will achieve better net carbon balances than others.

Twelve pasture-based dairy farms on the Garden Route were selected. Farms were grouped as low-, moderate-, or high-input based on fertiliser use, purchased feed, stocking rate, conservation tillage, and forage self-sufficiency. Eleven of the 12 farms exhibited negative net GHG balances, i.e. sequestered more carbon than emitted. Farm balances ranged from –15.2 to +6.76ton CO₂e per year, and carbon intensity ranged from –2.21 to +0.53kg CO₂e per kg FPCM (median –0.83kg CO₂e per kg FPCM). Low-input farms showed the most negative intensities (median –1.09kg CO₂e per kg FPCM), followed by moderate-input farms (–0.94kg CO₂e per kg FPCM). High-input farms varied widely and included the only net source. External inputs (mostly purchased feed) and enteric methane each contributed about 40% of gross emissions. Farms achieving the greatest carbon accumulation potential typically combine high feed efficiency, strong milk solids, legume-rich pastures, conservation tillage, and near-complete reliance on home-grown forage. The results also showed that management decisions matter far more than input intensity, and that well-managed pasture-based dairies in this region generally maintain a negative net carbon flux.

In conclusion, the results showed that regenerative pathways focussing on feed efficiency, nitrogen cycling, and pasture productivity can be achieved across various production systems. While long-term validation is needed to confirm the permanence of soil carbon, the size of the identified sink underscores the importance of including biogenic carbon accounting in sustainability assessments. By supplementing existing IPCC and LCA inventories, this approach offers a more comprehensive view of the livestock-climate relationship, thereby providing a solid basis for evidence-based policies and sustainable dairy transitions.