About 90% of dairying land is sown in pasture grazing species and 2% is used to grow grain and seed crops. There are a broad range of impacts pathways - direct and in-direct - for pasture.
Direct impacts are those where there are relatively straight-forward linkages between climate and impacts, where as indirect impacts are those that feedback in whole dairy system or ecosystem functions.
The direct impacts are driven from:
These climatic drivers impact on:
The indirect impacts and system impacts are in response to drivers from;
interactions with increased CO2
changes to the composition of pastures
changes in forage quality
increases in the impacts from pests and diseases
Photosynthesis, in C3 grasses, increases as temperature rises up to 25 C. The optimum temperature range for growth of cool temperate C3 grass species ranges from 20 - 25 C. For warm temperate grasses the upper threshold is between 25C and between 29 and 35 C.
Plant growth will reduce when temperatures rise above the optimum range, and this is manifest through smaller leave sizes, reduced tiller emergence, reduced root initiation and partial death of the root system.
Warmer temperatures throughout the year are likely to extend the growing season and reduce the likelihood of frost damage .
Increased temperature alone are unlikely to substantially reduce plant growth over the next century.
In winter, increasing temperatures will likely increase growth as temperatures move closer to optimal range, and there is reduced frost damage.
Water availability impacts
A lack of water is the major factor that limits plant growth because the plant cells need to be adequately hydrated to enable photosynthesis to happen. Photosynthesis normally recovers rapidly but the effects on metabolism can persist. C4 plants are equally or even more sensitive to water stress.
Plant growth reductions can been seen by reduced leaf appearance, and extension rates and increased tiller and plant mortality.
Waterlogging can also reduce photosynthesis, but the impact is dependant on the extent and duration of the waterlogging event.
Growth in waterlogged plants can also be reduced, with reductions in both leaf and root biomass.
Impacts of changing carbon dioxide concentrations on pasture
Based on studies of plants grown in CO2 concentrations ranging from 350 to 700pm, photosynthesis will increase however increases in pasture yields occur within optimum temperature ranges. Plants grown in elevated CO2 also make more efficient use of nitrogen and water. Legumes tend to benefit from increased CO2 more than non-fixing species like the grasses.
Overall, pasture yields are expected to go up but the interactions between environmental factors and the nutrients are not straight forward. Notwithstanding deliberately sown mixtures of pasture species, the mix of species in an established pasture is likely to change and this change might include the invasion of weedy species . The nutritive value of pasture plants will change and the balance of protein, fibre, and carbohydrate and fats component of the plants will be different with higher atmospheric CO2 concentrations. Overall the shift towards low feed value tropical pasture species and weeds is likely to have a negative effect on pasture quality.
Impacts of Pests and Disease
Under altered climate patterns the host-disease or pest interactions will be different. Some of the factors expected to change are the rate of pest and disease development and the host resistance. Warmer temperatures may increase the prevalence and geographical spread of plant diseases and pasture pests in New Zealand. Changes to soil moisture and humidity complicate how these changes play out. For example, drought stressed pastures which are also invaded by pests are more likely to die out and are exposed to an invasion by weeds. Also warm moisture tropical conditions are ideal for fungal diseases and the toxins associated with fungus pose animal health issues.