Effect of choice of reference flow and energy correction formulas on results in life cycle assessment in dairy production

Life cycle assessment (LCA) is increasingly important for the environmental assessment of dairy systems. While efforts to standardize procedures are being made, many studies define the functional unit and reference flow in a different way even though they all refer to energy corrected milk (ECM). The reference flow should be the amount of ECM at the farm gate to account for losses and milk fed to calves. The calculation of raw milk to ECM consists of the calculation of energy of raw milk and the scaling to the energy content of ECM. While the different formulas to calculate the energy content of raw milk differ only slightly, no consensus exists on the energy content of ECM, as it has been an arbitrary choice in all instances. Calculating the feed demand based on milk yield is also sensitive to the ECM calculation. Different energy demands for the same amount of ECM can lead to different calculate feed intakes, and consequently different resource efficiencies and environmental impacts. Consequently, when no information on the definition and calculation procedure of ECM is given, LCA results may face a severe uncertainty. We evaluated the effects of different settings on carbon footprint of milk in a calculation example and found an uncertainty of 33 % to either side of the results. In order to provide valid LCA results, the definition and calculation procedure of the functional unit and reference flow must be transparently disclosed.