REVIEW: Methods to measure forage and diet particle size in the dairy cow1

Dairy cows require adequate fiber for proper rumen function and production of milk, fat, and protein. However, cattle not only have a chemical fiber requirement but also a physical fiber requirement. An important consideration regarding forage particle size is the method used to measure particle size distribution. Many systems currently exist to measure particle size, and even more methods exist to use particle size data to calculate physically effective fiber in rations. However, because there is not a standard method for the dairy industry or dairy researchers, several different systems are currently being used, and their data are sometimes used interchangeably though their results may not be comparable. Many of the systems attempting to estimate physically effective fiber are based on the theory that there is a critical size threshold for particles leaving the rumen, and that particles above this threshold are effective because they stimulate chewing to promote particle size reduction and rumen escape. Various methods used to measure and interpret forage and diet particle size in dairy diets will be discussed.