The aim of the current study was to assess the effect of a non-protein nitrogen product, Optigen II, on dry matter and nitrogen digestibility using a rumen in sacco method. The Control (CON) diet consisted of soybean meal (SBM), compound feed and grass haylage, while the Optigen II (OPT) diet replaced one-third of the SBM with an isonitrogenous quantity of OPT thereby, reducing the SBM content from 15 to 10% of the total dietary dry matter (DM). This translates to a cow consuming 20 kg of DM being provided 150 g OPT. The components of each diet were weighed and combined (5 g DM in total) in 10 × 20 cm Dacron bags. The Dacron bags were then incubated in the rumens of 2 different fistulated cows and triplicate bags removed after 2, 5, 8, 16, 24 or 48 h. Fermentation was stopped by dipping the bags in cold water, after which the bags were cleaned and freeze-dried. After each incubation interval, the bags were weighed to determine the residual DM present followed by determination of total nitrogen concentration of the DM residue. NDF content was determined at 0 and 24 h. At each time point, DM, N, and NDF digestibility were compared between CON and OPT diets by means of a t-test. After 48 h incubation, about 70% of the diet DM had disappeared from the bags, with diet having no effect on the rate of DM digestibility. Approximately 80% of dietary N disappeared from the bags after 48 h, with no differences in digestibility observed between the 2 diets. NDF digestibility, analyzed at 0 and 24 h, did not differ between the diets, with 85 and 83% of the initial NDF disappearing from the CON and OPT diets, respectively. Overall, this in sacco study found no differences in terms of DM, N, and NDF digestibility between the CON and OPT treatments.