||Increasing species richness of temperate pastures beyond one or two forage species could improve grazing system productivity. An experiment in western Illinois, USA, was initiated in August 2001 to test this idea. The main study objective was to determine how pastures sown with increasing levels of species richness would affect herbage yield and cow-calf performance. Three seed mixtures that contained three, five, or eight forage species were sown into 3- to 6-ha pastures. Mixtures were replicated three times and rotationally stocked with beef cow–calf groups. Herbage mass and accumulation were estimated by a rising plate meter method and weight gain evaluated cow–calf performance. We also evaluated forage nutritive value indices and changes in forage species composition. After pasture establishment in 2001, herbage mass was marginally higher (P = 0.15) in eight-species mixtures (98 g m-2) compared with three-species mixes (43 g m-2). Once grazing started, pasture mix had no effect on herbage responses or stocking rate (P > 0.05). Cow–calf performance was also unaffected by pasture mix, although average daily gain (ADG) was higher in 2003 (P < 0.05). Cow and calf ADG averaged 0.33 and 1.17 kg d-1, respectively, in 2003 compared with 0.05 and 1.01 kg d-1 in other years. Overall, species richness in pastures had minimal effects on forage yield and cattle performance. Grazing management (e.g., stocking rate) and climatic conditions more strongly influence grazing system productivity.