The selection for rapid growth has resulted in appetite indirectly becoming an important criterion in the selection of modern broiler strains (Applegate, 2012). Besides ensuring an adequate and balanced nutrient intake, feed intake (FI) has been suggested as the single-most important factor determining the growth rate of broilers (Ferket and Gernat, 2006). Higher FI increases weight gain (WG) and consequently reduces the proportion of energy used for bird’s maintenance in relation to gain and improves feed efficiency (Svihus et al., 2004). Feed processing is now a common practice in poultry feed production worldwide and, includes single or multiple manipulation of feed ingredients or complete feed before offering it to the bird. Today, most poultry feeds are manufactured by employing a combination of physical grinding with hammer or roller mills with hydrothermal processing including either pelleting, expansion, or extrusion. Despite its impact on feed texture and nutritional value, less attention has been paid to the impact of feed processing variables, such as particle size and physical feed form on FI, growth performance and intestinal function of broilers. Performance of broilers is influenced by feed particle size and, the effects vary with age of birds and the grain type being fed. Moreover, dietary nutrient density and macro-structural characteristics of pellets such as pellet durability, hardness, length and diameter, have the ability to influence pelleting efficiency and therefore growth performance.
Broilers have a requirement for a certain degree of physical structure in their feed to meet their innate feeding behaviour (Ferket and Gernat, 2006). In recent years, the use of dietary structural components, such as coarse particles in poultry diets have attracted considerable attention due to their effects on the development and functionality of the upper digestive tract (Svihus et al., 2002). However, pelleting process further grinds the large feed particles and minimises the differences in particle size distribution. In broilers fed pelleted diets, made of finely ground ingredients, the foregut, especially the gizzard, is no longer functional in terms of mixing, grinding and reverse peristalsis contractions (Cumming, 1994; Duke, 1994). The current review focuses on the effect of physiological consequences of particle size and feed form on broiler performance and the development of gastrointestinal tract (GIT), in general, and the gizzard, in particular.