This study examined the relationship between herbage mass and tiller population density in terms of size/density compensation theory, in tall fescue (Festuca arundinacea Schreb.) and prairie grass (Bromus catharticus Vhal) swards. Tall fescue herbage mass and tiller density information was collected from an experiment where two nitrogen levels, two seeding arrangements and two grazing frequencies were evaluated. In prairie grass, the same information was obtained from swards managed with two levels of defoliation frequency and intensity. In tall fescue size density compensation mechanisms were observed when nitrogen nutrition limitation was overcome. Prairie grass showed less phenotypic plasticity than tall fescue and density/compensation mechanisms were less evident.