Net greenhouse gas balances of South African pasture-based dairy farms: results from the DESTiny biogenic carbon model

Abstract

Dairy production is a major source of greenhouse gas (GHG) emissions, yet the full biogenic carbon balance of pasture-based dairy systems remains poorly quantified. The DESTiny framework, a biogenic carbon system dynamics model, was used to calculate net GHG balances for 12 pasture-based dairy farms in South Africa's Garden Route. Farms were grouped as low-, moderate-, or high-input based on fertilizer use, purchased feed, stocking rate, conservation tillage, and forage self-sufficiency. Eleven of the 12 farms exhibited negative net GHG balances. Farm balances ranged from −15,211 to +6,764 t CO 2 e year −1 , and carbon intensity per kg of fat-and-protein-corrected milk (FPCM) ranged from −2.21 to +0.53 kg CO 2 e kg −1 FPCM (median −0.83 kg CO 2 e kg −1 FPCM). Low-input farms showed the most negative intensities (median −1.09 kg CO 2 e kg −1 FPCM), followed by moderate-input farms (−0.94 kg CO 2 e kg −1 FPCM), while high-input farms varied widely and included the only net source. External inputs (mostly purchased feed) and enteric methane each contributed approximately 40% of gross emissions. Farms achieving the greatest carbon accumulation potential typically combined high feed efficiency, strong milk solids, legume-rich pastures, conservation tillage, and near-complete reliance on home-grown forage. These results indicate that management decisions matter more than input intensity and that well-managed pasture-based dairies in this region can maintain a negative net carbon flux, transforming them from traditional emitters into verifiable climate assets.