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