AN INTEGRATED MODEL FOR GROWTH AND NUTRITIONAL-VALUE OF TIMOTHY

A dynamic model was formulated to simulate the above-ground dry matter growth (DM), and concentrations of crude protein (CP) and metabolizable energy (ME) in stands of timothy (Phleum pratense L.). DM growth was estimated in relation to intercepted radiation, air temperature, soil water status and tissue-N concentration. CP was estimated in relation to plant uptake of N from soil mineralization and applied fertilizer, and the dilution of N during plant growth. ME was estimated in relation to plant development. The modelling approach was validated on a set of data from Sweden (63-degrees-45'N, 20-degrees-17' E) in which a stand of timothy, supplied with three levels of N, was monitored over three seasons. The model simutaneously fitted DM, CP and ME by weighting the observations with the inverse of the variances of the observations. The model fitted the data with RMS errors for DM: 52 g m-2, CP: 1.6 %-units and ME: 0.39 MJ kg DM-1. The sensitivity of the model to parameters and initial conditions is discussed in relation to method of parameter estimation. The utility of the model is discussed in relation to tactical management of leys and planning of animal-feeding systems.