Ten prairie plant species were grown in monoculture and mixtures of two, four, six, and all ten species for four years at a study site near Blacksburg, VA (USA) to examine relationships between species richness of perennial prairie plant mixtures and forage yield. Mixtures were highly productive, exceeding 16t (1t=1000kg)ha −1 after four years with no fertilization or irrigation. Forage yield was affected by sown species richness only in years 1 and 4 when most mixture treatments yielded more than monoculture plots. The majority of multi-species plots (71%) exhibited a positive biodiversity effect where mixtures yielded more than respective monocultures. The data also suggested a strengthening of biodiversity effects with time (more multispecies plots with relative yield totals >1 and a positive net biodiversity effect). Mixtures that were more productive than the best performing species grown in monoculture (transgressive overyielding) increased from 25% in year 1 to 54% by year 4. Plots sown with ten species produced the most consistently high and stable yields and also effectively suppressed weeds. Livestock producers in cool-season regions could improve forage productivity with minimal external inputs by adding pastures sown with moderately diverse (n=4–10 species) prairie plant mixtures.