Grassland ecology is concerned primarily with those factors influencing the composition of plant species under grazing conditions, particularly how they relate to sustaining productive plant communities. With the recent trend of livestock operations opting for less capital-intensive production systems, more emphasis has been placed on inexpensive pasture systems that rely on complex species mixtures and tightly coupled nutrient cycling to produce forage. Grazing systems with pastures of a highly diverse botanical composition are perceived to be desirable in terms of persistence, yield stability, and productivity. Historically, research evaluating diversity effects on forage productivity was conducted in clipped experimental plots, with no actual grazing. Although clipped plots provide useful information that enables screening of several treatments, selective grazing by ruminants may impact biodiversity differently. Grazing animals are key drivers of vegetation structure and botanical composition through their selective grazing, trampling, and excretion patterns. A two-year study conducted by USDA-ARS and The Pennsylvania State University reported no differences in dry matter intake or milk production of dairy cows grazing different levels of forage diversity. The cows maintained a high level of milk production and intake on all forage mixtures and showed an ability to adapt to a variety of forage species. Although animal productivity was not impacted, complex mixes produced more forage in a dry year and had reduced weed invasion in both years compared with a simple mix. Other research has shown that grazing ruminants show a diurnal preference for different forages, that ruminants will satiate on a favorite food and switch to another food when given a choice, and that some plants contain secondary compounds that may provide health benefits to grazing ruminants. Further research is needed to define dietary selection and grazing behavior to improve animal and plant health and productivity in pasture systems.