The effect of future climate scenarios on the balance between productivity and greenhouse gas emissions from sheep grazing systems
Introduction
Australia's grazing industries may need to adapt to maintain productivity under climatic change (Howden et al., 2008). Historically, the climate in southern Australia is highly variable and pasture systems have had to adapt to be resilient to the inter-annual variability (Chapman et al., 2009, Cullen et al., 2009). Typically, sheep grazing systems in the area are of low resource input and are heavily reliant on pasture as an affordable source of nutrients. Any nitrogen (N) inputs largely come from legume fixation, with supplementary feed being used to fill gaps in pasture supply. Howden et al. (2008) proposed that climate and atmospheric changes, as well as soil type and pasture species, are likely to further impact on the quality, quantity and reliability of forage production in the region. Given the uncertainties associated with future climate projections, estimates are for average daily temperatures to rise by up to 5 °C by 2070 in some regions of southern Australia and annual rainfall to change by −30 to +5% (CSIRO and BoM, 2007). The resilience of rainfed pastures is dependent on the current climate and future changes in temperature, rainfall and atmospheric carbon dioxide (CO2) (Cullen et al., 2009). The rise in temperature and increased atmospheric CO2 concentration should increase leaf photosynthetic potential, decrease plant N content and reduce stomatal conductance (Long et al., 2004). The increase in daily temperatures and elevated atmospheric CO2 concentration should make conditions more favourable for C4 grass species, which are more heat tolerant, cold sensitive and deeper rooting than C3 species; however, C4 grasses are generally less digestible than C3 grasses due to their relatively high fibre content (Howden et al., 2008). There is increasing interest in using tropical C4 grasses such as kikuyu (Pennisetum clandestinum) across southern Australia (Evergraze, 2008), which may have an impact on productivity and GHG emissions.
The main GHG emitted by livestock systems include methane (CH4), nitrous oxide (N2O) and CO2. Sources of CO2 emitted from livestock systems are mainly associated with fertiliser production, processing activities, electricity and fossil fuel use from the manufacturing and transport of agricultural inputs and products (Biswas et al., 2010, West and Marland, 2002). System CH4 emissions are produced by microorganisms called methanogens as a by-product of anaerobic fermentation via enteric or manure sources (McDonald et al., 1995). Nitrogen inputs, such as manure, can contribute to N2O being directly emitted by denitrification of nitrate or indirectly by denitrification of nitrogen lost in leaching, runoff or NH3 volatilisation (de Klein and Eckard, 2008). In Australia, agriculture contributes 16% (88 Mt CO2-eq.) of total emissions reported in the National Inventory (DCCEE, 2009). Agriculture is the dominant source of both CH4 and N2O emissions nationally, which account for 59% and 86% respectively of the net national emissions of these GHGs (DCCEE, 2009).
The loss of nutrients that contribute to GHG emissions from a system can be evaluated by assessing the CO2-eq. emissions associated with production. The amount of CO2-eq. emissions are often expressed per unit product and/or per unit area (i.e. emissions intensity, Guinée et al., 2002). There are few published studies that have investigated the influence of geographic location on the relationship between animal productivity and associated greenhouse gas emission intensity in a changing climate.
The objectives of the present study were to quantify the net effects on sheep grazing systems in southern Australia of future climate scenarios on (1) total annual pasture intake, supplementary feed intake and lamb live weight at weaning per hectare, and (2) the net CO2-eq. emissions/ha and per kg lamb live weight at weaning for sites with different climatic zones, soil types and pastures using a whole farm system model.
Section snippets
Sites simulated
The productivity and CO2-eq. emissions from sheep grazing systems were modelled at four sites in southern Australia (Table 1). The four sites studied were chosen to represent a range of climatic zones and soil types in the region. Regionally specific dryland (i.e. rainfed) perennial pasture systems were modelled at each site characteristic of the region, which was based on sites used in the Sustainable Grazing Systems (SGS) Program. The SGS Program, which investigated various aspects of
Feed intake and productivity
Sheep at Albany on the kikuyu/subclover pasture had on average a higher annual total feed intake compared to the other sites studied across climate scenarios (averaging 0.45–0.47 t DM yr−1 compared to 0.41–0.44 t DM yr−1 respectively; Table 3). Due to the management of supplementary feed in the simulations, all the grazing systems maintained the daily live weight of a ewe between 55 and 64 kg over a year, depending on level of feed intake. Fig. 1 shows that the average annual pasture intake of a ewe
Discussion
The simulation results presented indicate that pasture systems that can continue to be productive in warmer and drier climates have the potential to maintain lamb production and this should be linked to lower GHG emissions intensity per ha and per unit of lamb production (Fig. 3, Fig. 4, Fig. 5). This is illustrated by a comparison of the Albany and Dookie sites. At Albany the kikuyu based pasture increased pasture intake in future climate scenarios, which maintained productivity of lamb live
Implications
If the future brings warmer conditions, then this study showed that some pasture systems in southern Australia may have to adapt both pasture species and the grazing system to remain productive. The C3-species grazing systems may benefit from the inclusion of a pasture species that is more tolerant of warmer and drier conditions in the future to help maintain pasture supply, which may not necessarily mean higher emissions intensities per ha or per unit product. As expected, higher rainfall
Conflict of interest statement
There is no conflict of interest associated with this work.
Acknowledgements
This work was supported by funding from the Dairy Australia, the Meat and Livestock Australia and the Australian Government Department of Agriculture, Fisheries and Forestry under its Australia's Farming Future Climate Change Research Program.
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