The aim of the study was to investigate the possibility of using Near Infrared Reflectance Spectroscopy (NIRS) for determination of crude protein content in residues after incubation in the rumen. Samples of whole-crop grain silages (oats and triticale) of differing maturities (heading, milk, and milk-dough stages of growth) were incubated in the rumen of cannulated bulls for 2, 4, 8, 16, 24, 48, and 72 h. After incubation the bags were rinsed, washed and dried (45 °C, 48 h) and N content was determined chemically in the residues. The NIRS measurements were carried out on dried and ground (1 mm) silage samples. Samples were scanned in duplicate runs (a second run after rotation of the cup) over the range of 1445–2348 nm with a 19 filter-spectrophotometer InfraAlyser 450R (Technicon). Spectral data were recorded as log 1/R (R-reflectance) with IDAS PC software. Calibration equations were calculated with 3, 6 and 9 wavelengths (filters) for three sets: all samples (2 cereals x 3 stages of maturity x 7 incubation times x 3 animals; n = 126), A-subset (n = 72) for samples incubated for 2–16 h, B-subset for samples incubated for 24–72 h (n = 54). Based on a calibration for all samples (n = 126), crude protein content could be accurately predicted with R equal to 0.95, 0.96 and 0.97 and SEC equal to 0.46, 0.41 and 0.38, when 3, 6 or 9 wavelengths were considered in the equation, respectively. The precision of prediction increased as the number of wavelengths in the calibration equation increased.