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

Summary

​The adult equivalent (AE) system describes and quantifies the grazing pressure of pasture by ruminants. The AE rank of an animal is calculated as the ratio of its metabolisable energy (ME) requirements relative to that of a 'standard animal'. Previous research has indicated that the Australian feeding standards considerably over-estimated the energy requirements of cattle consuming tropical forages in northern Australia.

In this project, modifications were made to the equations of the feeding standards to improve predictions of forage dry matter (DM) and ME intake by northern cattle. However, these changes were not tested with cattle in temperate regions, or sheep in any region.

Simulations using cattle growth data from northern Australia showed that the estimate of AE score was similar using the modified or unmodified equations, providing they were used systematically.

Objectives

The objectives of this study were to:

• revise estimates of the energy requirements of the standard animal and use this for estimating intakes of cattle in northern Australia
• standardise calculations of AE ratings of cattle for more consistent use on a national scale
• demonstrate how the revised AE standard can be applied to practical grazing situations involving multiple species.

Key findings

• The project identified and documented wide variations in the current definition of an AE, which has led to inconsistencies in application.
• Modifications were made to the equations in the Australian feeding standards for application to cattle in northern Australia, which provided more consistency between predicted and observed forage intakes and improved the estimates of energy requirement.
• Using the revised equations, the energy demand of the standard animal (2.25 year-old 450 kg Bos taurus steer walking 7 km/day with zero weight change) was determined to be 64.3 MJ ME/day, which is lower than the previous estimate of 72.6 MJ ME/day. This resulted in correspondingly lower estimates of intake for northern cattle.
• On a national application of the AE system, simulations showed that the unmodified feeding standard equations calculated similar results to the modified equations. Therefore, the unmodified version of the feeding standards can be used to determine the AE rank, as long as the same equations and assumptions are used for all calculations.

Benefits to industry

Using the current version of the feeding standards as the default for deriving AE rank allows the improved intake estimates for northern Australia to be integrated into an animal unit methodology that applies to both sheep and cattle across Australia. It also accommodates any future changes to the feeding standards, and the incorporation of other grazing animals over time, and provides a standardised, scientifically-sound national animal unit methodology.

The revised intake estimates for AEs in northern Australia can improve research outcomes and communication between researchers, service providers and producers. For example, by updating information in EDGE network material and software programs like Stocktake Plus and BreedCow Dynama. The main benefits to producers are better precision in forage budgeting and determining long-term carrying capacity in paddocks (or over properties) by improved estimations of intake in northern Australia.

MLA action

• The results of this project have been distributed to the wider industry to encourage adoption of the outcomes by researchers and service providers across Australia.
• MLA is also working toward updating the relevant extension material and training packages (including Nutrition EDGE and Grazing Land Management).

Future research

• Simple energy and intake requirement tables should be developed both for AE and DSE in Northern Australia.
• The EDGEnetwork packages and Stocktake Plus should be revised to include the new animal unit methodology.