The kinetic behavior of the dry matter (DM), crude protein (CP) and neutral detergent fiber (NDF) of the elephant-grass Mineiro variety was estimated through in situ incubation of the samples from this forage harvested at 61, 82, 103, 124 and 145 days after planting. The kinetic parameters were estimated by using a compartmental model of digestion based on sequential first-order kinetics for the lag and digestion events. The potentially degradable and undegradable fractions, the duration of the discrete lag time of the NDF and the apparent ruminal digestibility of the DM, CP and NDF were affected by the forage cutting age. The rise in the NDF undegradable fraction was associated with the increase in the mean retention time (MRT) of forage particles, resulting in a higher rumen fill effect (RFE) of this nutrient. The forage cutting age did not affect the discrete lag time of the DM and CP nor the first-order rate constants for digestion and lag of the DM, CP and NDF. The apparent ruminal digestibility of the DM and NDF was higher when the plants achieved approximately 146 and 152 cm in height. The MRT and the RFE increased linearly with the cutting height of the plants. The limiting factor exercised by the undegradable fraction upon voluntary intake, by assuming an inverse relationship between feed intake and MRT, could explain the lower performance of ruminants fed on this forage when later cuts are employed.