The greenhouse effect: What consequences for seasonality of pasture production?
A computer model was used to predict changes in pasture growth rate (PGR) and annual herbage production (AHP) which might result from increased temperatures and CO2 levels, and changes in rainfall expected to occur over the next sixty years due to the greenhouse effect. The model (GROW) evaluates soil moisture, soil fertility and temperature limitations to PGR under regular 2- or 4-weekly cutting management. Two scenarios (1.5°C and 3.5°C increases in air temperature, accompanied by a 10% increase and 5% decrease in rainfall, respectively) were modelled for Palmerston North. The effect of changes in temperature, rainfall, CO2 and pasture composition on seasonal PGR and AHP are graphed individually and discussed. Using as a baseline long-term average temperature and rainfall for Palmerston North, with a nominal Olsen 'P' of 20, the model predicted an AHP of 15.3 t DM ha-1. As a result of changes in temperature and rainfall AHP is predicted to increase to 17.3 tDM ha-1 and 18.2 t DM ha-1 for scenarios 1 and 2 respectively. With higher CO2 AHP may increase further to 21.2 t DM ha-1 and 22.2 t DM ha-1 respectively. The model predicts a potentially large change in the seasonality of PGR. Summer PGR is little changed from present levels but there are large increases in late autumn, winter and early spring PGR. Farm management practices are likely to change in order to match animal feed demand with changed PGR.