A pasture production model for use in a whole farm simulator

Modular simulation models of crop and animal enterprises can be used in concert to simulate a particular whole farm operation and thus provide management information for individual farmers and farm advisers. This paper reports the development of a pasture module for such a whole farm simulator designed to study irrigation management in New Zealand. The model (CANPAS) describes the production of perennial ryegrass/white clover pastures under either dryland or irrigated conditions. Seasonal growth studies and the principles of ecological physiology form the theoretical base of CANPAS. Whenever possible, parts of other models were also used. With time steps of one day, CANPAS predicts the net above-ground yield of live and dead herbage, herbage digestibility, leaf area index and soil water supply. Harvested yields are also predicted for simulated grazing or cutting managements, assuming part of the above-ground yield of live and dead herbage is left as stubble. Field measurements of harvested pasture yields from the Canterbury Plains of New Zealand were used to validate the model. Validation procedure included the use of a simple set of statistical tests. The final version (CANPAS III) gave an overall r² of 0·83, but poorer performance was found under some test managements. Discussion covers the general aspects of this kind of modelling as well as the specific details of the pasture module.