Technical Abstract: Planned species diversity provides the primary product in agricultural systems. For grazinglands, this is palatable and nutritious forage biomass. Associated diversity contributes many of the additional ecosystem functions needed for sustainable agriculture. Understanding the functional role of associated diversity may contribute to increased farm profitability and ecosystem sustainability by reducing the need for external inputs. Plant community composition is somewhat amenable to modification by selective addition or suppression of species, but may be constrained by site factors beyond the manager's control (e.g. climate, landscape position). Our objective was to identify site factors that constrain species composition, so that this information can be used to develop objective criteria for species selection in managing grazed systems. Inventory data on vegetation, soils and landscape attributes were collected from 130 pastures on 44 farms from 1998-2005. Stepwise regression was used to identify the variables related to a series of descriptors of plant community composition, including richness and cover of all species, of various functional groups, and of species of particular interest. Few common patterns emerged. Geographic location (latitude and/or longitude) was significant in most cases. Slope and elevation were significant for some descriptors, but aspect was rarely relevant. Soil texture was consistently important, but soil organic matter and nutrient status were not significant in most cases. The difficulty of identifying strong predictor variables suggests that management plays a very strong role in determining the plant community composition of grazinglands.