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 metabolizab le energy (ME) in stands of timothy (Phleum pratense L.). DM growth wa s estimated in relation to intercepted radiation, air temperature, soi l water status and tissue-N concentration. CP was estimated in relatio n to plant uptake of N from soil mineralization and applied fertilizer , and the dilution of N during plant growth. ME was estimated in relat ion to plant development. The modelling approach was validated on a se t of data from Sweden (63-degrees-45'N, 20-degrees-17' E) in which a s tand of timothy, supplied with three levels of N, was monitored over t hree seasons. The model simutaneously fitted DM, CP and ME by weightin g the observations with the inverse of the variances of the observatio ns. 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 par ameters and initial conditions is discussed in relation to method of p arameter estimation. The utility of the model is discussed in relation to tactical management of leys and planning of animal-feeding systems .