Parameterization of the APSIM model for simulating palisadegrass growth under continuous stocking in monoculture and in a silvopastoral system

Sustainable intensification of livestock production systems has become a global demand. In silvopastoral systems, contrasting shading levels caused by the presence of trees interferes in the productive responses of the pasture under grazing. The environment in which plants develop in a pasture canopy is dynamic and complex due to the nature of soil-plant-animal interactions. These dynamic aspects can be rationalized, evaluated, and explained using mathematical modeling. The objective of the present study was to parameterize the APSIM-Tropical Pasture model to simulate palisadegrass [Brachiaria brizantha (Hochst A Rich) Stapf, syn. Urochloa brizantha] growth under continuous stocking and variable stocking rate, in a full sun system and in contrasting shading conditions of a silvopastoral setting with eucalyptus (Eucalyptus grandis W. Hill ex Maiden × Eucalyptus urophylla S. T. Blake). Trees were planted in triple-row groves, with three groves per 2-ha silvopasture area. The spacing between groves was 30 m, and tree density was 135 ha⁻¹, in East-West orientation. The data used to calibrate and validate the model were collected in a 36-month experiment with full sun and shade treatments of the silvopastoral setting in Sinop, MT, Brazil. Live forage mass estimates had coefficient of determination varying between 0.76 and 0.94, Willmott agreement index ranged from 0.93 to 0.96, and root mean square error between 275 and 610 kg DM ha⁻¹. The APSIM-Tropical Pasture model can simulate Marandu palisadegrass growth under continuous stocking with variable stocking rate, but improvements are needed to better simulate the effect of N fertilizations in different periods on growth. The model can simulate pasture growth under shading levels but our simulations did not consider competition for soil moisture, which should be considered in the future, since it can be an important factor for other silvopastoral designs or microclimatic conditions.