Milk yield and greenhouse gases emission in mountain dairy cattle systems: an ambiguous relationship

The study aimed to analyse the relation between milk production and the emission of greenhouse gases (GHG: methane, CH4, nitrous oxide, N2O, carbon dioxide, CO2) in the north eastern Alps dairy system (Top Value – Interreg ITA-AT). The GHGs emission was computed through a cradle-to-farm gate life cycle assessment. Functional units were 1 kg of fat- and protein-corrected milk (FPCM) and 1 m2 of land. Herd and manure management, on-farm feedstuffs production, purchased feedstuffs (plus land use change) and materials were included into the system boundaries. Data originated from 75 farms (38±25 LU, 20.9±5.4 kg fat protein corrected milk – FPCM/cow/day), associated to 9 cooperative dairies in the eastern Alps. Mean GHG emission was 1.3±0.2 kg CO2-eq/kg FPCM and 0.6±0.2 kg CO2-eq/ m2. Methane was the first contributor (57%; enteric and manure fermentations), followed by CO2 (29%; fossil fuels and land use change) and N2O (14%; fertilisers application). Milk yield was negatively correlated with CH4 (r=-0.88, P<0.001) and N2O (r=-0.34, P<0.01), but not with CO2 emission (r=0.10, P=0.38). Considering instead land surface as functional unit, milk yield was positively correlated with all GHGs emissions (r ranged 0.26-0.46, P<0.05). Therefore, high-productive farms mitigated their GHG emission per unit of FPCM with respect to low-productive farms especially through a reduction in biogenic CH4 emissions, but showed a trade-off in terms of pressure on land surface. In addition, no mitigation was observed for CO2, an emission that adds new net carbon into the atmosphere. As the share of CO2 emission on total GHGs emission was nearly 30%, its mitigation could give a notable contribution towards a net zero-carbon system, starting from off-farm feedstuffs (42% of total CO2 emitted, land use change considered) and on-farm energetic sources (41%). The results of this study highlighted that mitigation strategies aimed to a net zero-carbon balance should consider not only milk yield, but also the origin of the GHG emission, in particular when grassland-based mountain dairy systems are taken into account.