MODELING GROWTH AND COMPETITION OF A MULTI-SPECIES PASTURE SYSTEM

Abstract

Pastures are often a mixture of different plant species. The growth patterns of these mixtures are determined by theinteractions and competition among the coexisting species, as well as their response and interactions with the environment.Grassland management for economic and environmental sustainability should be based on the integrated view of the pasturesystem and the understanding of the processes involved. Computer models are ideal to study such complex systems. Most modelingefforts have focused on binary combinations of an agronomic crop and a major weed species in agricultural systems. In thisresearch, the GRAzing SImulation Model (GRASIM) was extended to account for growth and interspecies competition amonga mixture of plant species, including grasses, legumes, and weeds. In the multispecies GRASIM, a user-specified number ofspecies grow separately on a daily time step, competing for light, soil water, and nitrogen. Forage experiments were conductedon a naturalized pasture at the Pennsylvania State University Beef Research Farm (University Park, Pa.). The pasture was dividedand rotationally stocked at two intensities (high and low). Forage biomass data from 1998 and 1999 were used to developand evaluate the multispecies GRASIM model. The multispecies GRASIM reasonably simulated the growth dynamics of multiplespecies on two grazing treatments across two seasons, despite the high variability of the pasture. The model-simulated and observedbiomass data for the dominant species across the two treatments and the two growing seasons gave regression coefficientsof determination (R2) ranging from 0.25 to 0.98. Model limitations and directions for future efforts are outlined and discussed.The multispecies GRASIM can be used to help evaluate coexisting species interaction and their response to the environmentand provide insight into the complex mixed-species pasture ecosystem.