Allocation in field crop and livestock life cycle assessment

Field crops and livestock are significant contributors to a variety of environmental impacts, commonly assessed using life cycle assessment (LCA). In LCA, allocation is used to partition these impacts of production when processes produce multiple co-products, such as different parts of a field crop, or multiple animal products produced from a single livestock species. The choice of allocation method can have a large impact on estimated impact assessment results. However, the International Organization for Standardisation (ISO) standard for LCA is not applied consistently with respect to the choice of allocation methods and the differentiation between co-products, wastes, and recycled products in agri-food supply chains, which undermines their rigor and utility with respect to informing credible, evidence-based decision making. Therefore, the goals of this paper were (1) to assess current food sector allocation methods in published research against the ISO standard, and (2) to provide recommendations for decision-making regarding allocation methods that align with the ISO standard, and that can be consistently applied to model the different co-products and wastes associated with field crop and livestock production systems. Published agricultural LCA studies from the last 5 years were reviewed to determine the allocation methods used. A total of 126 peer-reviewed agri-food LCA studies which reported on allocation methods between co-products were included in the review. Based on these studies, and other relevant journal articles and reports, recommendations were developed for allocation methods that align with the ISO hierarchy. Taken together, physical, natural science-based allocation approaches were the most common in the literature, and a large variety of such methods were applied. Economic allocation was also found to be commonly performed in agricultural LCAs (and equally as common as natural science for crop-specific LCAs), often justified because it represents the motivation behind producing each co-product. However, the ISO standard indicates a clear prioritization of natural science-based approaches. Therefore, we provide recommendations in line with this prioritization – namely, biophysical methods that represent pathways of causality in agri-food systems. The results of the literature review showed many options for natural science-based biophysical allocation for the co-products of crop and livestock production. Preferentially, these are based on internal causality – the real flows of material and energy within a system, particularly metabolic partitioning of energy within an animal, or construction cost partitioning within crops. Otherwise, external causality, such as the nitrogen content of the products, is recommended. In this paper we provide detailed recommendations for a representative sample of multi-functionality scenarios in crop and livestock product systems. These can be used to enable consistent comparisons of the environmental impacts of different food production pathways. Multi-functionality in LCA refers to processes that produce multiple co-products. Allocation between agricultural co-products was often based on natural science approaches. We recommend allocating based on internal causality (e.g., metabolic partitioning) where possible. Economic allocation is not in line with ISO 14044 unless physical or causal relationships cannot be determined. Consistent, biophysical allocation methods can enable rigorous comparisons and decision support.