Corn, sorghum, wheat and barley are the most used cereals in the preparation of feed for the livestock, poultry, swine, and fish industry. Roller and hammer mills are the two types of processing equipment generally used to grind grains into smaller particle sizes.[23][24] Milling cereal grains by mechanical action involves several forces like compression, shearing, crushing, cutting, friction and collision. The particle size of the ground cereal is very important in the animal feed production; smaller particle sizes increase the number of particles and the surface area per unit volume which increase access to digestive enzymes.[25] Other benefits are increased ease of handling and easier mixing of ingredients.[23] The average particle size is given as geometric mean diameter (GMD), expressed in mm or microns (µm) and the range of variation is described by geometric standard deviation (GSD), with a larger GSD representing lower uniformity.[26] According to Lucas (2004), GMD and GSD are accurate descriptors of particle size distribution when the particle size distribution is expressed as log data, and are distributed log normally.