Climate change in Queensland's grazing lands: II. An assessment of the impact on animal production from native pastures

The 160 million ha of grazing land in Queensland support approximately 10 million beef equivalents (9.8 million cattle and 10.7 million sheep) with treed and cleared native pastures as the major forage source. The complexity of these biophysical systems and their interaction with pasture and stock management, economic and social forces limits our ability to easily calculate the impact of climate change scenarios. We report the application of a systems approach in simulating the flow of plant dry matter and utilisation of forage by animals. Our review of available models highlighted the lack of suitable mechanistic models and the potential role of simple empirical relationships of utilisation and animal production derived from climatic and soil indices. Plausible climate change scenarios were evaluated by using a factorial of rainfall (±10%)3°C temperature increasedoubling CO2 in sensitivity studies at property, regional and State scales. Simulation of beef cattle liveweight gain at three locations in the Queensland black speargrass zone showed that a ±10% change in rainfall was magnified to be a ±15% change in animal production (liveweight gain per ha) depending on location, temperature and CO2 change. Models of 'safe' carrying capacity were developed from property data and expert opinion. Climate change impacts on 'safe' carrying capacity varied considerably across the State depending on whether moisture, temperature or nutrients were the limiting factors. Without the effect of doubling CO2, warmer temperatures and ±10% changes in rainfall resulted in -35 to +70% changes in 'safe' carrying capacity depending on location. With the effect of doubling CO2 included, the changes in 'safe' carrying capacity ranged from -12 to +115% across scenarios and locations. When aggregated to a whole-of-State carrying capacity, the combined effects of warmer temperature, doubling CO2 and ±10% changes in rainfall resulted in 'safe' carrying capacity changes of+3 to +45% depending on rainfall scenario and location. A major finding of the sensitivity study was the potential importance of doubling CO2 in mitigating or amplifying the effects of warmer temperatures and changes in rainfall. Field studies on the impact of CO2 are therefore a high research priority.