No prior studies have compared the in situ degradation of fresh and ensiled Guinea grass cultivars grown in the microclimate of Visayas State University, Baybay City, Leyte, the Philippines. This study evaluated the in situ degradation characteristics of two Guinea grass cultivars—Local Guinea grass (LG) and Mombasa grass (MG)—harvested at 30 and 45 days. Six test diets were prepared: fresh LG (FLG), fresh MG (FMG), and ensiled LG and MG harvested at 30 and 45 days (LG30, LG45, MG30, and MG45, respectively). Three rumen-fistulated Brahman heifers (180 ± 10 kg bodyweight) were dewormed, pre-conditioned, and incubated with the test diets using the sequential addition method at 0, 24, 48, and 72 hours. At 0 and 24 hours, all treatments showed a comparable degradation of DM and ADF (p>0.05). The NDF degradation of all treatments was comparable only at 0 hours. At 24 hours, MG30 showed significantly greater NDF degradation compared to most other treatments (p<0.01). At 48 and 72 hours, MG30 showed a significantly greater degradation of DM, NDF, and ADF compared to most other treatments (p<0.01). MG30 showed a significantly greater effective degradability of DM, NDF, and ADF compared to most other treatments (p<0.01). These findings indicate that MG30 has superior in situ degradability characteristics, making it a promising forage option for ruminant diets.