The objective of this study was to compare 2 warm-season annual grasses to cool season perennial pasture (CSP) and alfalfa for ruminal nutrient digestibility and nitrogen synthesis in a dual-flow continuous culture rumen fermentation system. Eight dual-flow continuous culture fermentors were used during 2 consecutive 10-d periods. Dietary treatments were (1) alfalfa, (2) CSP, (3) brown midrib sorghum-sudangrass (BMRSS), and (4) teff grass from the organic dairy production system at the University of Minnesota organic dairy in Morris, MN. The CSP comprised orchardgrass, meadow fescue, bromegrass, red clover, and white clover. Treatments were randomly assigned to fermentors with 7 d for diet adaptation and 3 d for data and sample collection. Fermentor samples for pH, NH₃-N, and VFAs were collected on d 8, 9, and 10. Apparent dry matter, organic matter, neutral detergent fiber, and acid detergent fiber digestibility were lower (P < 0.05) for BMRSS, teff, and CSP, (49.8%, 33.3%, 58.4%, and 60.8%, respectively) compared with alfalfa (69.4%, 54.1%, 75.5%, and 75.5%, respectively). The BMRSS, teff grass, and CSP were not different from each other. True dry matter and organic matter digestibility were lower (P < 0.05) for BMRSS, teff, and CSP (65.4% and 47.2%, respectively) compared with alfalfa (85.8% and 69.2%, respectively). Fermentor pH and total volatile fatty acids were not affected by forage. Ammonia N concentrations (mg/dL) were greater (P < 0.05) for alfalfa (22.5) compared with CSP (7.5), BMRSS (7.4), and teff (8.9), respectively. Crude protein degradation was not affected by forage treatment. The flow of NH₃-N was greatest (P < 0.05) for alfalfa (26.5%) compared with the other forages (11.6%), reflecting the greatest NH₃-N concentration. The flow of total N was greatest (P < 0.05) for alfalfa (1.99 g/d), lowest for CSP (1.50 g/d) and BMRSS (1.51 g/d), and intermediate for teff grass (1.70 g/d). Overall, the fermentation of the warm season grasses was similar to the CSP, indicating the potential for successful use of warm season grasses for grazing systems.