Livestock unit calculation: a method based on energy needs to refine the study of livestock farming systems

Abstract

Originally based on the ingestion of forage by herbivores and by extension on their energy requirements, the concept of a livestock unit (LU) is widely used, without always being precisely defined, to study livestock farming systems. However, the calculation of the LU value of a given animal can be considered quite rough since it does not account for its size, production level, or rearing conditions on which its energy and thus feed requirements depend. We propose an LU calculation adapted to the bovine, ovine and caprine species based on the net energy needs of the animals using the IPCC (Intergovernmental Panel on Climate Change) equations and parameters. Due to a sensitivity analysis of these parameters, we confirm that the weight of the animals and their production level are determinants of their net energy requirements. By integrating these two parameters, we propose two types of equations that differ in their level of precision and in the availability of the required data. Based on an equivalence of 29 000 MJ for an LU represented by a 600 kg dairy cow producing 3 000 kg of milk per year with 4% fat, we kept the notion of a reference animal. The proposal allows us to account for, through the size of the animals, their level of production and their genotype, the variability of the animals’ feed requirements. The LU coefficients calculated in this way can differ significantly from historical coefficients and thus modify the conclusions of comparisons of farming systems between themselves or over time.