Diagnosing the weather and climate features that influence pasture growth in Northern Australia

Northern Australian agricultural industries are vulnerable to a highly variable climate. Year-to-year fluctuations in rainfall strongly affect pasture growth and increase the challenge of matching forage supply with stock numbers. A case study of climate drivers of pasture growth is conducted for the extensively-grazed cattle enterprises in the semi-arid tropics of north-eastern Australia. By assessing past climate, we explore which aspects of climate variability drive variations in pasture growth to inform effective developments in grazing management tools. Insights gained from these investigations could be important to the understanding of how we might integrate dynamical climate models into the decision making process. Currently, dynamical climate models are limited in their ability to provide skilful 6 month forecasts of total rainfall at the paddock scale. In their favour, we show that variables other than total rainfall are more important in forecasting pasture growth. Predicting total rainfall is dominated by the need to effectively predict the extreme events. We demonstrate that these extreme events do not significantly influence pasture growth as much of the rain received during extreme rainfall events is lost as run-off. A forecast would ideally be able to predict the number of rain days that fall when the soil is dry; incorporate the moisture content of soil at the beginning of the season; determine the likelihood of an early break to the wet season resulting in late winter and/or spring rainfall; and establish the interaction of El Niño Southern Oscillation with other climate modes. We stress however, that potential pasture growth is only one aspect of complex farm management decisions. Optimal decision making must also incorporate factors such as grazing management, cattle condition and longer time horizons as part of a whole of farm approach. Keyword: ENSO dynamical seasonal climate models pasture cattl