Low production of cool-season forages in summer limits the size and profitability of cattle operations in northern, temperate regions. Warm-season grasses (C4 photosynthesis pathway) can be integrated into a cool-season (CS) grazing system to supplement summer forage deficits. Producers can plant either annual or perennial warm-season (WS) grass pastures for this purpose. This study was designed to evaluate productivity of annual and perennial WS grass pastures integrated within a CS forage grazing system. The study is being conducted at the University of Illinois Orr Center Beef Unit located in Baylis, IL. In May 2004, three replicated pastures were planted with a perennial WS mixture of Eastern gamagrass, big bluestem and little bluestem. Annual WS pastures were sown with a sorghum x sudangrass hybrid (BMR) in June 2005. Using rotational stocking, beef cow-calf groups grazed adjacent CS pastures until mid-summer when they were moved to WS pastures. Forage yield, nutritive value, and cattle performance were evaluated in 2005. Dry weather in June slowed establishment of annual WS pastures. After rainfall in late June, sorghum grew rapidly and forage mass greatly exceeded perennial WS pastures - 10.3 ± 0.42 Mt/ha vs. 3.2 ± 0.42 Mt/ha. Forage nutritive value, as indexed by percentage ADF, was higher in sorghum pastures (66 ± 2.1) compared with perennial WS pasture (77 ± 0.41). Overall, both WS pastures worked well to provide supplemental forage during summer. Perennial WS pastures were ready to graze earlier in summer, which could be advantageous during a dry spring. For cattle, the higher productivity and forage nutritive value of annual WS pastures resulted in marginally higher (P = 0.13 to 0.16) daily weight gain over perennial WS pasture. Preliminary results suggest annual WS pastures may benefit cattle performance more than perennial WS pasture when integrated within a CS grazing system.