Relative emissions intensity of dairy production systems: employing different functional units in life-cycle assessment

Abstract

This study aimed to assess the merit and suitability of individual functional units (FU) in expressing greenhouse gas emissions intensity in different dairy production systems. An FU provides a clearly defined and measurable reference to which input and output data are normalised. This enables the results from life-cycle assessment (LCA) of different systems to be treated as functionally equivalent. Although the methodological framework of LCA has been standardised, selection of an appropriate FU remains ultimately at the discretion of the individual study. The aim of the present analysis was to examine the effect of different FU on the emissions intensities of different dairy production systems. Analysis was based on 7 years of data (2004 to 2010) from four Holstein-Friesian dairy systems at Scotland’s Rural College’s long-term genetic and management systems project, the Langhill herd. Implementation of LCA accounted for the environmental impacts of the whole-farm systems and their production of milk from ‘cradle to farm gate’. Emissions intensity was determined as kilograms of carbon dioxide equivalents referenced to six FU: UK livestock units, energy-corrected milk yield, total combined milk solids yield, on-farm land used for production, total combined on- and off-farm land used for production, and the proposed new FU–energy-corrected milk yield per hectare of total land used. Energy-corrected milk was the FU most effective for reflecting differences between the systems. Functional unit that incorporated a land-related aspect did not find difference between systems which were managed under the same forage regime, despite their comprising different genetic lines. Employing on-farm land as the FU favoured grazing systems. The proposed dual FU combining both productivity and land use did not differentiate between emissions intensity of systems as effectively as the productivity-based units. However, this dual unit displayed potential to quantify in a simple way the positive or negative outcome of trade-offs between land and production efficiencies, in which improvement in emissions intensity using one FU may be accompanied by deterioration using another FU. The perceived environmental efficiencies of different dairy production systems in terms of their emissions intensities were susceptible to change based upon the FU employed, and hence the FU used in any study needs to be taken into account in the interpretation of results.

Effect of Nutritional Variation and LCA Methodology on the Carbon Footprint of Milk Production From Holstein Friesian Dairy Cows

“…However, differences in LCA methodology, allocation methods (for milk and meat) or functional unit hinder comparability (Baldini et al, 2017) and a meta-analysis of 30 published LCA's with 87 carbon footprints from pasture, mixed, and confinement dairy systems found no average footprint differences per kg of FPCM (Lorenz et al, 2019). Choice of functional unit is also important when expressing results from LCA studies assessing environmental impacts arising from differing dairy systems and when considering effects of intensification (Ross et al, 2017, Salou et al, 2017. As far as the authors are aware, allocation methods assessing feed components and their effect on dairy system carbon footprints are not available in literature.…”

A comparison of milk yields and methane production from three contrasting high‐yielding dairy cattle feeding regimes: Cut‐and‐carry, partial grazing and total mixed ration

“…Recent increases in TMR‐fed cattle have contributed to the global rise in milk and meat production, but may also be linked to the rise in atmospheric methane emissions (Thornton, Jones, Ericksen, & Challinor, ; Wollenberg et al., ). Life cycle assessments are used to assess the emissions’ intensities of different livestock management systems; however, these assessments require many field measurements to parameterize the models (Ross, Topp, Ennos, & Chagunda, ).…”

Exploring trade-offs among indicators of performance and environmental impact in livestock areas

“…Although gains in livestock N use efficiency and productivity normally contributed to the reduction of GHG emissions on a product basis, they were translated into higher potential environmental impacts from a land basis perspective. Changing outcomes with different functional units was also found by Ross et al (2017) who reported contrasting results when comparing the impacts of milk production on the basis of fat-and-protein-corrected milk yield and another on the basis of land unit. Our results are supported by literature on the need to account for impacts in a multi-metric approach because of mixed effects.…”

“…Although this statement is common sense in a great part of the literature, the question of efficiency is actually multifaceted. Ross et al (2017) reported that the environmental efficiency of dairy systems depends largely on the functional unit to which impacts are referenced: The same system had lower and higher emissions with a change from productto land-based functional unit, respectively. This highlights the need to account for impacts attributed to both the unit of produce (e.g.…”