Forage composition is an important part of grassland productivity, and can be affected by management and climate. This study described how management (different grazing pressures) and precipitation affected forage composition of 1.6 ha pastures on a southern tallgrass prairie site during 1984-1995. Forage produced (% of total forage) by four groups of plants was defined annually; the four dominant warm-season grasses (‘Big 4’), invasive warm-season grasses (7 species), annual bromes (3 species), and broadleaved forbs (17 species). Temporal changes in composition were examined, and Spearman’s correlations were used to determine if composition was related to recorded annual and quarterly precipitation. Forage composition of all paddocks changed in 1988-89 and persisted till 1995. The ‘Big 4’ grasses declined and invasive warm-season grasses, forbs, and annual bromes increased. These changes coincided with drought periods in 1988-89 that interacted with management to cause pasture differences. Significant (P<0.05) correlations were noted between ‘Big 4’ grasses, forbs, and invasive warm-season grasses, and yearly or ‘seasonal’ (Oct-Sept) precipitation. Other relationships were noted for shorter time intervals. On a quarterly basis, positive correlations were noted between ‘Big 4’ grasses and 1st and 4th quarter precipitation, annual bromes and 2nd quarter rainfalls, and forbs and 3rd quarter rainfalls. Experimental seasonal forecasts were moderately accurate at defining occurrence and timing of higher precipitation levels for the southern Great Plains during the 4th and 1st quarters of the 1997-98 El Nino event. Given the frequent occurrence (8 of 20 years) of El Nino events in the last part of the 20th Century, and continued improvements in forecasting skills, accurate quarterly forecasts could be a useful support tool for the tactical management of native prairie.