METHODS FOR EXPLORING MANAGEMENT OPTIONS TO REDUCE GREENHOUSE-GAS EMISSIONS FROM TROPICAL GRAZING SYSTEMS

Increasing atmospheric concentrations of 'greenhouse gases' are expect ed to result in global climatic changes over the next decades. Means o f evaluating and reducing greenhouse gas emissions are being sought. I n this study an existing simulation model of a tropical savanna woodla nd grazing system was adapted to account for greenhouse gas emissions. This approach may be able to be used in identifying ways to assess an d limit emissions from other rangeland, agricultural and natural ecosy stems. GRASSMAN, an agricultural decision-support model, was modified to include sources, sinks and storages of greenhouse gases in the trop ical and sub-tropical savanna woodlands of northern Australia. The mod ified model was then used to predict the changes in emissions and prod uctivity resulting from changes in stock and burning management in a h ypothetical grazing system in tropical northeastern Queensland. The se nsitivity of these results to different Global Warming Potentials (GWP s) and emission definitions was then tested. Management options to red uce greenhouse gas emissions from the tropical grazing system investig ated were highly sensitive to the GWPs used, and to the emission defin ition adopted. A recommendation to reduce emissions by changing burnin g management would be to reduce fire frequency if both direct and indi rect GWPS of CO2, CH4, N2O, CO and NO are used in evaluating emissions , but to increase fire frequency if only direct GWPS of CO2, CH4 and N 2O are used. The ability to reduce greenhouse gas emissions from these systems by reducing stocking rates was also sensitive to the GWPs use d. In heavily grazed systems, the relatively small reductions in stock ing rate needed to reduce emissions significantly should also reduce t he degradation of soils and vegetation, thereby improving the sustaina bility of these enterprises. The simulation studies indicate that it i s possible to alter management to maximise beef cattle production per unit greenhouse gases of per unit methane emitted, but that this is al so dependent upon the emission definition used. High ratios of livewei ght gain per unit net greenhouse gas emission were found in a broadly defined band covering the entire range of stocking rates likely to be used. In contrast, high values of liveweight gain per unit 'anthropoge nic' greenhouse gas emission were found only at very low stocking rate s that are unlikely to be economically viable. These results suggest t hat policy initiatives to reduce greenhouse gas emissions from tropica l grazing systems should be evaluated cautiously until the GWPs have b een further developed and the implications of emission definitions mor e rigorously determined.