Carbon footprint of milk from Holstein and Jersey cows fed low or high forage diet with alfalfa silage or corn silage as the main forage source

Abstract

Our objective was to determine the cradle-to-farmgate carbon footprint of fat-and-protein corrected milk (FPCM) for four diets fed to two breeds using measured enteric methane and greenhouse gas emissions during manure storage and after field application. The diets were formulated as 2 × 2 factorial with forage neutral detergent fiber at two levels (19 and 24% of diet dry matter referred to as low forage and high forage diets, respectively) from two sources (70:30 or 30:70 ratio of alfalfa silage and corn silage). Measured emissions were incorporated in a modeled Wisconsin dairy farm with a herd consisting of 118 lactating cows (primiparous), 22 dry cows, and 119 heifers. Emissions were allocated between milk and meat using biophysical allocation, and a sensitivity analysis was performed to determine the impact of alternative allocation methods. Overall, carbon footprint was 1.43 kg CO2-e/kg FPCM for biophysical allocation, and 1.50 and 1.60 kg CO2-e/kg FPCM for economic and no allocation (100% allocated to milk), respectively. Forage source did not influence results. However, low forage-fed cows had 10% greater carbon footprint than high forage-fed cows (1.49 vs. 1.35 kg CO2-e/kg FPCM). Assuming similar herd structure, milk carbon footprint for Holsteins was 4.4% greater than for Jerseys (1.47 vs. 1.41 kg CO2-e/kg FPCM). Accounting for differences in fertility and replacement rate increased the difference in milk carbon footprint between breeds to 10%. Under our study conditions, differences in milk carbon footprint due to enteric fermentation were minimal but the differences became substantial when combining the effects of cow (enteric CH4) and manure (CH4 and N2O) emissions. These differences were exacerbated even further when accounted for the emissions associated with on-farm feed production and purchased feed (CO2 and N2O). This study highlights the need for an integrated approach to assess the effects of dietary manipulations on milk carbon footprint.