A practical methodology for defining and applying animal units to Australia’s livestock industries

Animal units are a standard metric used for calculating, combining, comparing and managing the financial, biological, and other effects of different herbivore species and classes. The concept has evolved over many decades because of the need for more uniform and accurate property valuations, enterprise analyses, and stocking rate calculations and assessment of grazing pressure. Historically, Australia has used a confusing range of animal-units names and definitions. Units with the same name were often based on standard animals that differed in liveweight (LW), age, sex, reproductive state, and level of activity. In addition, units with the same name were based on different attributes, including LW, metabolic LW (LW0.75), metabolisable energy (ME) demand, and forage demand. This paper addresses these inconsistencies and describes an animal-unit methodology that accommodates multiple grazing species and animal classes. The methodology provides common units of measurement for a range of applications and across grazing industries. Two animal units are proposed: (1) the animal equivalent (AE); and (2) the dry sheep equivalent (DSE). The measurable attribute for these animal units is ME demand, calculated using the Nutrient Requirements of Domesticated Ruminants (NRDR) equations and assumptions. Equivalence is derived by estimating the ME demand of animals according to sex, age, LW, and production level using the same equations and expressing the result relative to a standard animal unit. The methodology also enables estimation of forage intake through intake constants of 7.5, 8.0, or 8.5 kg dry matter (DM)/AE.day for cattle in regions of high-, moderate- or low-productivity, respectively, and 0.96 kg DM/DSE.day for sheep and goats. This approach improves consistency and comparability in grazing management and analysis.