Australian Rangeland Society

Corresponding author – Ian McLean, Agribusiness consultant.  Email: ian@bushagri.com.au

 

The adult equivalent (AE) system describes and quantifies, in commonly recognised units, the grazing pressure imposed on the pasture by foraging ruminants.

Historically, particularly for cattle in northern Australia, the application of AE’s was not consistent and the resulting estimates of dry matter intake were over estimated. This made matching stocking rate to carrying capacity, a fundamental tenet of grazing management, difficult in practise. Recent work undertaken for MLA by Stu McLennan, Ian McLean and Col Paton has sought to address this.

Their report on this work starts with a thorough, and interesting, history of the evolution and application of animal units in Australia and around the world. The definition and determination of Adult Equivalents is clearly detailed, as are dry sheep equivalents (DSE), and the common application of both measures is discussed.

Modifications were made to the equations of the feeding standards (Nutrient Requirements for Domesticated Ruminants, CSIRO 2007) which improved predictions of ME, and so also forage dry matter (DM) intake by cattle consuming tropical forages in northern Australia. 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 to determine animal units across regional boundaries.

The report ‘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’, is available on the MLA website at https://www.mla.com.au/download/finalreports?itemId=4305.