Construction cost of plant compounds provides a physical relationship for co-product allocation in life cycle assessment

PURPOSE: Although, according to the ISO 14044 standard, economic allocation should be the solution of last resort, it is frequently used in LCA studies of agri-food systems because of the inadequacy of solutions preferred by ISO. We propose a new way of allocating impacts to plant co-products according to the underlying physical relationships between the co-products. We compare this method to two current allocation methods. METHODS: (i) We surveyed 24 LCA studies from peer-reviewed scientific journals and scientific reports for the 2000–2013 period that attributed impacts to plant co-products used as animal feed to establish the prevalence of allocation methods in recent LCA studies. (ii) We calculated allocation factors for 21 plant co-products used as ingredients for animal feeds according to three methods: energy allocation based on lower heating values (En), economic allocation based on average prices for a 5-year period (Ec), and allocation based on plant physiological construction cost of plant compounds (Cc). RESULTS AND DISCUSSION: (i) In the studies surveyed, Ec was used in 19 out of 24 cases. En was used in three studies, system expansion and mass allocation were used in two studies, while nitrogen content was used in one study. (ii) Compared to En and Ec, Cc yielded higher allocation factors for protein-rich co-products and lower factors for lipids. Whereas the difference between En and Cc was modest (up to 5 percentage points), the difference between Ec and Cc was more variable and sometimes large (up to 18 percentage points). CONCLUSIONS: Although, according to the ISO 14044 standard, Ec should be the solution of last resort, it is frequently used in LCA studies of agri-food systems because of the inadequacy of solutions preferred by ISO. For plant co-products, Cc is an attractive option, as it is based on the physiological mechanisms involved in plant growth rather than on a common property of the co-products.