Type and number of environmental impact categories used in livestock life cycle assessment: A systematic review

Life cycle assessment (LCA) is an important tool to evaluate environmental 'hot spots' in livestock systems and recommend production improvements. However, it is common for livestock LCA to investigate only a narrow subset of environmental impacts to simplify results for decision-makers, which makes it difficult to fully understand the tradeoffs among environmental impacts and identify the most relevant mitigation options. We completed a systematic review of the livestock LCA literature to better understand the impact categories included and the progress made towards more comprehensive LCA. Our search of publications between 2000 and 2016 identified 173 relevant peer-reviewed papers. Nearly all the publications (98%) included climate change as an impact category and almost one-third of the publications (28%) focused solely on that one category. Biodiversity, ionizing radiation, and particulate matter were the least common categories addressed. Cattle LCA, including dairy or beef, were the livestock species most frequently evaluated. Our analysis shows that while the number of multi-category livestock LCA (LCA with 4 or more impact categories) increased over time, LCA including 1-3 impact categories (which we define as “simplified LCA”) increased at a higher rate than multi-category LCA. Simplified LCA therefore remain the most prevalent in the literature. Publications that included multiple impact categories were better able to identify environmental impact tradeoffs among livestock production systems and management scenarios. To compare results across livestock LCA studies, it is necessary to increase the standardization of system boundaries, functional units, impact frameworks and mandatory inputs. The optional steps of normalization and weighting in the life cycle impact assessment can also help decision-makers prioritize which environmental impacts to address. More work that includes a greater number of impact categories in livestock LCA is sorely needed to more fully understand and to harmonize the communication of the environmental performance of livestock production systems.