Improvement of time-driven models of lamina cocksfoot digestibility by a process-based model to take account of plant N nutrition and defoliation

For management purposes, models of lamina digestibility (D-W) that are thermal time driven (T-sum) fail to account for plant nitrogen status (0 < Ni < 1) and defoliation intensity (residual sheath height: S-r). The objective of this paper was to enrich them using a functional processed-based model, which assumes that the decline in D-W over time depends on the metabolic : structural tissue ratio and plant ageing. An experiment combining two N fertilizer rates and two defoliation regimes (differences in S-r) for two spring and two summer regrowth was done to analyse D-W of laminae (youngest fully expanded) and tiller (whole green lamina) levels. For laminae, we show that both processes contribute to explain the decline in D-W at a rate that depends on Ni but not on S-r. For tillers, consistent results were found justifying the use of a simple model: D-W = f(+Ni; -Ni x T-sum; -S-r). Two different databases (from experiment and commercial farms) were used for validation: RMSD varied from 35 to 49 g kg(-1). The process-based model strengthened the empirical model and improved the comprehensiveness of how management practices change D-W. We used both models to create applications for reasoning yield in function of herbage mass in complement than in function of time.