The combustion method for neutral detergent insoluble protein (NDIP) is used to measure the amount of indigestible protein bound within the fiber portion of animal feed, particularly relevant for assessing heat-damaged or low-quality proteins. In this method, the feed sample is treated with an acid detergent solution to isolate the neutral detergent fiber (NDF), which includes cellulose, lignin, and associated protein. A portion of the NDF residue is analyzed using combustion to determine its nitrogen content, and the total nitrogen content is multiplied by a standard conversion factor to determine crude protein.