11.4.3.3 Pastoral and rangeland farming

In western, southern and higher-altitude areas of New Zealand, higher temperatures, a longer growing season, higher CO₂ concentrations and less frost are very likely to increase annual pasture production by 10 to 20% by 2030, although gains may decline thereafter (MfE, 2001). In eastern New Zealand and Northland, pasture productivity is likely to decline by 2030 due to increased drought frequency (see Section 11.3.1). Sub-tropical pastoral species with lower feed quality such as Paspalum are likely to spread southwards, reducing productivity (Clark et al., 2001), particularly in the Waikato district. The range and incidence of many pests and diseases are likely to increase. Drought and water security problems are likely to make irrigated agriculture vulnerable, e.g., intensive dairying in Canterbury (Jenkins, 2006).

In Australia, a rise in CO₂ concentration is likely to increase pasture growth, particularly in water-limited environments (Ghannoum et al., 2000; Stokes and Ash, 2006; see also Section 5.4). However, if rainfall is reduced by 10%, this CO₂ benefit is likely to be offset (Howden et al., 1999d; Crimp et al., 2002). A 20% reduction in rainfall is likely to reduce pasture productivity by an average of 15% and liveweight gain in cattle by 12%, substantially increasing variability in stocking rates and reducing farm income (Crimp et al., 2002). Elevated concentrations of CO₂ significantly decrease leaf nitrogen content and increase non-structural carbohydrate, but cause little change in digestibility (Lilley et al., 2001). In farming systems with high nitrogen forage (e.g., temperate pastures), these effects are likely to increase energy availability, nitrogen processing in the rumen and productivity. In contrast, where nitrogen is deficient (e.g., rangelands), higher temperatures are likely to exacerbate existing problems by decreasing non-structural carbohydrate concentrations and digestibility, particularly in tropical C₄ grasses (see Section 5.4.3). Doubled CO₂ concentrations and warming are likely to result in only limited changes in the distributions of native C₃ and C₄ grasses (Howden et al., 1999b).

Climatic changes are likely to increase major land-degradation problems such as erosion and salinisation (see Section 11.4.3.1). They are also likely to increase the potential distribution and abundance of exotic weeds, e.g., Acacia nilotica and Cryptostegia grandiflora (Kriticos et al., 2003a, b) and native woody species, e.g., A. aneura (Moore et al., 2001). This is likely to increase competition with pasture grasses, reducing livestock productivity. However, the same CO₂ and climate changes are likely to provide increased opportunities for woody weed control through increased burning opportunities (Howden et al., 2001b). A warming of 2.5°C is likely to lead to a 15 to 60% reduction in rabbit populations in some areas via the impact on biological control agents, e.g., myxomatosis and rabbit haemorrhagic disease virus (Scanlan et al., 2006).

Heat stress already affects livestock in many Australian regions, reducing production and reproductive performance and enhancing mortality (see Section 5.4.3). Increased thermal stress on animals is very likely (Howden et al., 1999a). In contrast, less cold-stress is likely to reduce lamb mortality in both countries. Impacts of the cattle tick (Boophilus microplus) on the Australian beef industry are likely to increase and move southwards (White et al., 2003). If breakdown of quarantine occurs, losses in live-weight gain from tick infestation are projected to increase 30% in 2030 and 120% in 2100 (in the absence of adaptation). The net present value of future tick losses is estimated as 21% of farm cash income in Queensland, the state currently most severely affected.