Modeling the Biomass and Harvest Index Dynamics of Timothy

Residual stubble, following harvest or the grazing of perennial grasses, is an important element of the C cycle in agricultural systems that has not been considered in most growth models. The stubble fraction from aboveground biomass can be quantified by predicting the harvest index (HI) of forage grasses, i.e., the amount harvested or grazed as a proportion of the aboveground biomass. A HI simulation module, using new functions to describe shoot apex height and plant weight density, was developed and integrated into the timothy (Phleum pratense L.) growth model CATIMO. The model was calibrated and validated using data from two timothy cultivars and different N fertilizer rates from five experiments conducted in eastern Canada. The root mean square errors (RMSEs) between simulated and measured values for HI (0.11), shoot apex height (7 cm), stubble biomass (25 g dry matter m(-2)), aboveground biomass (68 g dry matter m(-2)), and harvestable biomass (76 g dry matter m(-2)) were acceptable with normalized RMSEs (13-41%) compared with the coefficients of variation of measured data (13-30%). The HI also has implications for determining forage nutritive value. This improved CATIMO model provides a framework to explore options for optimizing yield and nutritive value and to quantify the stubble biomass of timothy within the context of C sequestration.