Modeling Net Herbage Accumulation of an Orchardgrass Sward

Abstract

In order to simulate the effect of a wide range of defoliation regimes and levels of N supply on the net herbage accumulation of an orchardgrass (Dactylis glomerata L.) sward, we developed a model based on growth and senescence submodels. Four experiments (spring and summer regrowths), each comprising four treatments (two N rates × two amounts of residual biomass), were carried out to assess the effect of management on leaf growth, senescence, and radiation use efficiency (RUE). The growth submodel, based on RUE, depended on herbage N status and the development stage during stem elongation in spring. Increases in RUE during stem elongation depended on the percentage of reproductive tillers in the swards, which was a function of the N fertilizer rate. Leaf elongation and senescence rates at the tiller level and the mass per unit leaf area of green and senescent leaves were used to parameterize the senescence submodel. The amount of dead material depended mainly on the residual biomass at the beginning of regrowth. Complete calibration of the model was done using experiments and data from the literature. Other sets of experiments (one on orchardgrass and two others on permanent grasslands) were used for its validation. Root mean square errors for herbage yield were comprised between 11 and 55 g m−2. The chosen model structure allows the growth of other grass species to be easily simulated, knowing their leaf life spans. However, this is a sink‐driven model that is unable to assess tiller mortality and its effect on herbage accumulation rate.