We tested recently published linear equations for predicting developmental morphology of warm-season perennial grasses on three cultivars of switchgrass (Panicum virgatum L.). Mean stage count (MSC, based on the Nebraska system) of 'Cave-in-Rock' and 'Kanlow' switchgrass was measured at Ames, IA, and MSC of Cave-in-Rock and 'Alamo' switchgrass was measured at Stephenville, TX, during 1995. Measured MSC was compared with MSC estimated from day of the year (DOY) or growing degree day (GDD; base of 10 degrees C) equations. The DOY equation more closely estimated MSC for switchgrass grown at Ames than at Stephenville. The GDD equation did not work well for any cultivar at either location. The equations did not work as well in Texas, probably because of genotypic interactions with daylength and climate for developmental morphology. These results indicate that the DOY equation may be useful with some varieties of switchgrass in the central Great Plains.