Growth and nutritive value of pastures under climate extremes

The main aim of this thesis was to investigate the impacts of future climate extreme scenarios, including increased temperatures and severe droughts, on the nutritive value of key pasture grassland species. My research involved the evaluation of changes in forage nutritional composition and digestibility under contrasting climate regimes. In addition, I looked at possible explanations for the observed changes in forage nutritive value associated with plant growth and morphological traits, including productivity and structural allocation (e.g. dead material and leaf:stem biomass ratios). In order to improve the understanding of climate change impacts on forage nutritive value of future pasture systems, I conducted a series of experiments (short- and long-term) at the Pastures and Climate Extremes experimental field facility and a glasshouse facility at the Hawkesbury Campus of Western Sydney University, Richmond, New South Wales, Australia. I predicted that both forage production and nutritive value would be adversely impacted by extreme scenarios of warming and drought. I further predicted that the combination of these scenarios would have a more pronounced negative impact on forage than the single climate treatments. I found that the impacts on pasture production and forage nutritive value were greater under severe drought than under warming, and their combination resulted in effects that were not always greater than those associated with warming or drought treatments on their own. In addition, I observed that climate extremes strongly impact pasture productivity, resulting in low forage production and increased dead biomass, with the magnitude varying by species. The effects of climate treatments on forage nutritive value (including nutritional composition and digestibility) were mixed, including both marginal increases and declines and null effects. The outcomes of this thesis show the importance of exploring species-specific nature in response to seasonal droughts and elevated temperatures to better understand climate change impacts on pasture systems and inform future planting decisions and breeding programmes. Further, this thesis provides insights for grazing industries in Australia and abroad about the potential impacts of extreme events on pasture management and animal production