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Abstract
The pasture growth module AgPasture was integrated into the APSIM (Agricultural Production System Simulator) simulation model, allowing pasture-based systems to be modelled in combination with other land uses at farm scale or within land use change studies. The model's predictions of pasture growth were evaluated against 32 pasture growth datasets from a diverse range of soil types and climatic zones across New Zealand. The pasture herbage accumulation simulated by the model closely matched actual measurements over varying intervals. Both predicted and measured pasture growth rate demonstrated the same seasonal pattern, including mean growth rate and inter-annual variation across measurement years. Predicted and measured annual average net herbage accumulation (NHA) on a dryland pasture was similar over 37 observation years (mean, 6.83 and 7.27 t DM/ha respectively; coefficient of variation, 29% and 27% respectively) and highly correlated (R 2 = 0.838, P < 0.0001; relative root mean squared deviation (RMSD) = 16%). The model's prediction of annual average NHA of all simulated pastures, spanning a wide range of pasture environments, also matched the measurement data well (R 2 = 0.777, P < 0.0001; relative RMSD = 21%). However, discrepancies between simulated and observed values occurred in some seasons and at some sites. Analysis of these discrepancies identified areas where the model could be improved by incorporating more accurate descriptions of the effects of plant development and grazing, soil temperature and the interactive effects of high temperature and soil moisture dynamics.
Acknowledgements
This work was funded by the New Zealand Ministry of Science and Innovation as a part of the programme ‘Rural futures: Building adaptive management capability to deliver sustainable pastoral farm systems (C10X0809)’. The authors thank Brendan Cullen, Craig Beverly, Ian Johnson and Neil Huth for assistance and comments in the early stage of model development and integration, Ray Moss and Mike Sprosen for information on pastures in the two detailed study sites, and Warren King, Ronaldo Vibart and two anonymous referees for comments on an early version of the manuscript.
