MODELING GROWTH OF COCKSFOOT (DACTYLIS-GLOMERATA L) AND TALL FESCUE (FESTUCA-ARUNDINACEA SCHREB) AT THE END OF SPRING IN RELATION TO HERBAGE NITROGEN STATUS

The objective was to establish a model of vegetative regrowth of plant matter in a stand of cocksfoot and tall fescue according to their nitrogen status. For three years (1989-1991), at the end of each spring, we studied regrowths in relation to differing supplies of nitrogen (120 kg/ha or low supply or nil). The herbage nitrogen status (IN) was estimated from the nitrogen concentration in the herbage and the above ground dry matter. The accumulated above ground dry matter was related to the amount of photosynthetic active radiation intercepted (PARi). The two parameters of the model, PARi (resulting from incident radiation and LAI) and the radiation use efficiency (RUE) depended on the herbage nitrogen status. For LAI, the parameters a, b, c of the model (LAI = a + b*T + c*T*IN) were not significantly different for the two species, (T = accumulated daily temperatures from the beginning of regrowth). On the other hand, RUE was about 15% higher for tall fescue in relation to a higher net photosynthesis activity. For other regrowths than those used for model calibration, measured above ground biomasses and LAI data were in good agreement with simulated data.