Mapping the carbon footprint of milk production from cattle: A systematic review.

Recently, many studies have assessed the carbon footprint of bovine milk production. However, due to the complexity of life cycle assessment, most studies have analyzed research farms or "representative" farms, which do not capture farm variability. Furthermore, the lack of consistency in most studies means that we can seldom compare the footprint between different countries. To address this issue, we performed a systematic review of the literature, removing inconsistencies in life cycle assessment papers, namely the functional unit, allocation to milk, and global warming potential. We analyzed papers that accounted for many farms to address the variability of production systems within the countries. We found 21 papers from 19 countries; footprint recalculations were necessary for 16 papers. New Zealand, Uruguay, the United Kingdom, Australia, and the United States had a footprint <1 kg of carbon dioxide equivalents (COe) per kilogram of fat- and protein-corrected milk, whereas 5 countries had a footprint >2 kg COe·kg fat- and protein-corrected milk. The change in functional unit resulted in a small effect on the final footprint, whereas the global warming potential change was dependent in on the greenhouse gas profile for each country. Countries where milk is produced mainly as a pasture-based system had most of their footprint (>50%) associated with the emission of methane from enteric fermentation, whereas other countries (especially from Europe and North America) had a significant share of emissions from manure management, feed production, and fertilizer use. This different greenhouse gas profile allow decision makers to tailor mitigation options specific for each country. The choice of the allocation method had a strong influence in the final footprint. We suggest that for future studies, authors adhere to the International Dairy Federation guidelines. When this is not possible, we suggest a set of extra information to be reported, allowing recalculations as done in this review.