A 3 × 3 factorial experiment was conducted to evaluate IVDMD of Kentucky 31 tall fescue (KY-31) and Tifton 85 bermudagrass (T-85) with or without supplementation. The main effects included relative forage quality (RFQ) and type of supplement. Each forage type was harvested at different intervals during the yr of 2013 and 2014 to represent grazing and hay production scenarios in the state of Georgia. After harvested, the nutritive values of forages were assessed via near-infrared spectroscopy, and samples were stratified by RFQ. Forage samples were selected to represent high (HIG), medium (AVG), or poor (LOW) RFQ for KY-31 (171, 111, and 78, respectively) and T-85 (142, 101, and 80, respectively). Supplemental treatments included: 1) commercial molasses-urea based supplement (MUR) or 2) corn gluten feed (CGF) to provide the equivalent of 0.36 kg of supplemental CP; or 3) no supplement (CON). The forage-supplement mixtures were subjected to 48-h in vitro digestion to evaluate the digestibility characteristics. Two in vitro incubations were conducted and bottles (n = 140 in each incubation) were considered the experimental units. Across forage type, IVDMD increased as the incubation time was extended, and it was greatest (P < 0.001) when samples were incubated for 48 h. The same response was observed for NDF and ADF digestibilities. No supplement × RFQ interaction was observed for KY-31 (P = 0.55). Across RFQ for KY-31, 48-h IVDMD increased (P = 0.008) with the addition of LIQ and CGF compared with CON (49.9, 49.6, and 47.7%, respectively). Across supplementation, IVDMD of KY-31was greatest (P < 0.001) for HIG and AVG compared with LOW (62.5, 50.2, and 34.5%, respectively). A supplement × RFQ interaction was observed for T-85 (P < 0.003). In vitro dry matter disappearance was lowest for LOW × CON compared with all other supplement × RFQ combinations. These results suggest that forage quality is the main factor affecting IVDMD. However, the addition of LIQ and CGF can increase IVDMD of KY-31 and low RFQ T-85.