The production of perennial ryegrass and kikuyu pastures in south-eastern Australia under warmer and drier future climate scenarios

Abstract

Grazed pastures in south-eastern Australia are typically based on temperate (C3) species, such as perennial ryegrass (Lolium perenne). With predictions for warmer and drier future climates there has been growing interest in the performance of more heat tolerant and deep rooted subtropical (C4) pasture species such as kikuyu (Pennisetum clandestinum). This study uses an existing pasture model to investigate the production of kikuyu grass compared to the commonly used perennial ryegrass in four regions of southeastern Australia under historical and future climate scenarios. Four sites were chosen to represent a range of climatic zones and soil types within south-eastern Australia: Dookie in northern Victoria, Hamilton in south-western Victoria, Ellinbank in west Gippsland and Elliott in north-western Tasmania. The biophysical SGS Pasture Model was used to estimate pasture production (t of dry matter (DM)/ha), metabolisable energy (ME) content and total ME yield (GJ/ha). The ME yield of a kikuyu/subterranean clover (Trifolium subterranean) pasture and a perennial ryegrass/subterranean clover pasture were modelled for a range of temperature and rainfall scenarios projected for southern Australia over the next 60 years. To represent the range of climate scenarios, the baseline climate from the years 1971 to 2000 was adjusted by a: • 0, 1, 2, 3 and 4°C increase in average daily temperature (with a corresponding 380, 435, 535, 640 and 750 parts per million atmospheric carbon dioxide concentration); and • -30, -20, -10, 0 and +10% changes in annual rainfall. This created a matrix of 25 climate scenarios. Sheep (wethers maintaining a liveweight of 45kg) were used to maintain the pasture herbage mass at 2.0 t DM/ha by adjusting their stocking rate. Nitrogen was supplied to the dryland grazing system so that it was non-limiting to plant growth. This study showed that across sites less rainfall reduced the ME yield for both species. The kikuyu ME yield responded more strongly to an increase in temperature, up to 4°C, whereas the annual ME yield of perennial ryegrass began to plateau or decline at 3 to 4°C warming. Given future projected climatic changes, this study suggests that it would not be beneficial for kikuyu grass to be the main source of ME per hectare at each site compared to the average ME yield provided by perennial ryegrass, across levels of rainfall studied, until temperatures were to increase by 3, 1, 2, 3°C at Dookie, Ellinbank, Elliott and Hamilton, respectively. However, across sites, kikuyu grass could provide a useful source of ME during the months of December to February compared to perennial ryegrass.