Technical Abstract: Sustainability of forage production in the Northeast USA is negatively affected by environmental and climatic variability. Complex forage mixtures may be better adapted to variable environments and produce more dry matter (DM) than simple mixtures. We evaluated forage production and nutritive value of well adapted and commonly sown species combined in 2-, 3-, 6-, and 9-species mixtures under rotational grazing management. Mixtures containing 6-species produced more forage (9894 kg DM ha-1) compared to 2- (8671 kg DM ha-1) or 3- (8444 kg DM ha-1) species mixtures. However, forage production varied within species richness groups. Forage nutritive value was affected by mixture complexity. The 2-species mixtures had greater crude protein (CP) and less neutral detergent fiber (NDF) content (CP: 241 g kg-1; NDF: 361 g kg-1) than the 3- (CP: 215 g kg-1; NDF: 400 g kg-1) and 6- (CP: 224 g kg-1; NDF: 400 g kg-1) species mixtures but these values were not different from the 9-species mixtures (CP: 232 g kg-1; NDF: 363 g kg-1). Eighty five percent of the variation (r2: 0.85) in CP and NDF were explained by the legume and grass proportion respectively. We conclude that species selection is more important than mixture complexity in achieving high yields and forage quality with forage mixtures.