Improvement of time-driven models of lamina cocksfoot digestibility by a process-based model to take account of plant N nutrition and defoliation
Résumé
For management purposes, models of lamina digestibility (DW) that are thermal time driven (Tsum) fail to account for plant nitrogen status (0 < Ni < 1)and defoliation intensity (residual sheath height: Sr). The objective of this paper was to enrich them using a functional processed-based model, which assumes that the decline in DW 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 Sr) for two spring and two summer regrowth was done to analyse DW of laminae (youngest fully expanded) and tiller (whole green lamina) levels. For laminae, we show that both processes contribute to explain the decline in DW at a rate that depends on Ni but noton Sr. For tillers, consistent results were found justifying the use of a simple model: DW = f(+Ni; )Ni • Tsum; )Sr). 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 DW. We used both models to create applications for reasoning yield in function of herbage mass in complement than in function of time.