The effective degradability of dry matter (DM), crude protein (CP) and amino acids (AA), and the intestinal effective digestibility (IED) of DM and CP of a green forage corn (GC) and its silage (EC) were determined on freeze-dried samples using three rumen and duodenum cannulated wethers. Two rumen incubations with duplicate bags were performed for each feed. Rumen degradation was determined on one series of bags from each incubation. The other series was freeze-dried and used to determine IED using mobile nylon bags. Microbial contamination of rumen incubated residues (determined with 15N techniques) fitted exponential functions, which showed a greater microbial contribution in EC than in GC in the undegradable DM (18.6% vs. 13.5%) and CP (81.7% vs. 69.4%). Degradability was calculated considering the particle rumen outflow rate (k(p): 0.056/h) of the EC (EDp) or additionally the rate of comminution and mixing (kr: 0.130/h) of these particles (EDep). Ensiling increased EDp (9.33%, p < 0.01) or EDcp (5.30%, p = 0.062) of DM and was associated with losses of nitrogen and with large changes in the AA profile. It is necessary to correct the microbial contamination, because it represents 32.0% (GC) and 42.5% (EC) of the undegraded CP when using k(p) and k(c). Ensiling caused higher degradabilities for some AA as well as large differences in the changes due to the rumen fermentation on the AA profile. However, it had only limited effects on the undegraded protein profile. Ensiling also reduced the IED of DM (23.3% vs. 14.6%; p = 0.057). In conclusion, results do not show losses of nutritive value by ensiling corn cut at vitreous grain stage.