Re-defining the animal unit equivalence (AE) for grazing ruminants and its application for determining forage intake, with particular relevance to the northern Australian grazing industries

Abstract

The adult equivalent (AE) system describes and quantifies, in commonly recognised units, the grazing pressure imposed on the pasture by foraging ruminants. The AE rank assigned to an animal is determined as the ratio of its (metabolisable) energy (ME) requirements relative to that of a ‘standard animal’, where ME requirements are usually determined using the feeding standards. Previous research has indicated that the Australian feeding standards (NRDR 2007) considerably over-estimated the energy requirements of cattle consuming tropical forages in northern Australia. In the current project, modifications were made to the equations of the feeding standards which improved predictions of ME, and so also forage dry matter (DM) intake by cattle. However, these changes were not tested with cattle in temperate regions, or sheep in any region meaning that two systems, one using modified and the other unmodified equations, were required to accommodate this regional demarcation. Simulations carried out using cattle growth data from northern Australia showed that the estimate of AE score was similar using either system providing they were used systematically. Furthermore, it was found that the forage intake predicted by direct calculation using the modified system could be closely matched by calculating an AE rank using the unmodified system and then multiplying this rank by an intake constant. The optimum intake constant fluctuated with regional variations in animal productivity. This agreement between predictions suggested that, for most circumstances, the existing (unmodified) feeding standards could be used across regional boundaries. Recommendations are made for application of the revised animal unit systems to practical grazing scenarios and forms the basis of revising current EDGE (NutritionEDGE) material.