A new method of biophysical allocation in LCA of livestock co-products: modeling metabolic energy requirements of body-tissue growth

In agricultural life cycle assessment (LCA), the allocation method chosen to divide impacts among co-products is an important issue, since it may change conclusions about a product’s impacts. We developed a biophysical allocation method to assign upstream environmental burdens and the use of raw materials at farm gate to the livestock co-products at the slaughterhouse based on their metabolic energy requirements.

Biophysical allocation is designed to build a relationship between co-products of a meat-production system and their associated net metabolic energy requirements. A metabolic growth model (Gompertz function) was combined with an energy calculation model to estimate metabolic energy requirements for the growth of an animal from birth to slaughter age. Allocation factors were calculated based on the energy required to maintain and produce body tissues (excluding waste), as a function of their chemical (protein and lipid) and physiological properties. This method was applied for an average beef cow and then compared to other allocation methods (e.g., mass, dry matter, protein, and economic).

At slaughter age, carcass tissues required the most energy (44 %) due to their high quantity of protein; the gastrointestinal tract and liver required about 28 and 5 %, respectively, of total metabolic energy requirements due to their roles in body metabolism. Biophysical allocation considers the energy cost of building and maintaining the tissues, regardless of their final uses. It reflects physical relationships among co-products as well as other allocation methods do. It also reveals the cause-effect relationship between tissues according to the energy required to maintain physiological functions. Once the growing time until slaughter is set, biophysical allocation factors are not influenced over time, unlike those of economic allocation, which is highly influenced by price variability.

This study provides a generic and robust biophysical allocation method for estimating environmental burdens of co-products, in accordance with ISO allocation rules. The method can be considered an original contribution to international debates on allocation methods applied to livestock products in LCA. In this paper, it is applied to cattle-related product, but it is generic and the principles can be adapted to any kind of livestock species. It should be considered and discussed by stakeholders in livestock production industries.