CROPGRO-Perennial Forage model parameterization for simulating Piatã palisade grass growth in monoculture and in a silvopastoral system

Silvopastoral systems are important to intensify pasture production and mitigate climate change effects. However, very few studies have been performed to adapt crop models to simulate these systems. The aim of this study was to parameterize the CROPGRO-Perennial Forage model for estimating Piatã palisade grass growth in monoculture and in a silvopastoral system with eucalyptus. To generate the dataset required for that, two field experiments were carried out. The first experiment was conducted between February 2011 and July 2012 with Piatã palisade grass under irrigated and rainfed conditions and cut-and-carry management. The second experiment was conducted from December 2014 to January 2016, in a single pasture of Piatã palisade grass and in a silvopastoral system with the same forage species and eucalypt trees. The trees were arranged in single rows, in East-West orientation, with 15 m between rows and 2 m between trees in the rows. This experiment was conducted under grazing management and rainfed conditions, during 11 growth cycles, with the pasture variables being assessed at four distances from the North row of eucalypt (0.00 m, 3.75 m, 7.50 m and 11.25 m), in the silvopastoral system, and in the single pasture. CROPGRO testing for the silvopastoral system considered only the competition for solar radiation since the model does not allow simulating belowground competition. For this, measured data of incoming solar radiation at each position were used as input for the simulations. The model was able to simulate forage live mass in the single pastures (R2 from 0.72 to up to 0.89, Agreement index from 0.88 to up to 0.95, and Nash-Sutcliffe efficiency between 0.65 and 0.85), and in the silvopastoral system (R2 from 0.76 to up to 0.89, Agreement index between 0.93 and 0.96, and Nash-Sutcliffe efficiency between 0.71 and 0.86). Despite these results, improvements should be performed in the model for simulating the effect of flowering on pasture growth, changes on pasture canopy caused by animals trampling, and competition for resources in silvopastoral systems.