Structural architecture of pelleted broiler diets: A comprehensive narrative review of key factors for an optimized macro- and microstructure

Most broiler diets are pelleted, resulting in a pellet macrostructure with a specific particle size distribution, as well as a distinct microstructure, which forms the internal matrix of the pellet and determines the particle distribution once the pellets are dissolved in the anterior digestive tract. Despite the importance of macrostructure to facilitate feed intake and microstructure to facilitate digestive function and nutrient digestibility, there is a paucity of scientific deliberations which can be used to optimize micro- and macrostructure through changes to grinding and pelleting. Due to the grinding effect of the pellet mill and an often rather small pellet diameter, it is difficult to achieve a sufficiently coarse microstructure, even when major ingredients are coarsely ground. Although more targeted research is needed, aiming for 40% particles > 1 mm after wet sieving, of which half should be > 2 mm, seems appropriate. Feeding behavior is key to understanding macrostructure, as it has been established that birds target and pick one particle approximately once per second during a feeding session. Although the maximum size of particles at different ages is not clear, research has shown that birds are able to consume larger particles than often presented to them under current industrial practices. Given that birds pick one particle at a time, increasing pellet diameter above the commonly used 3 to 4 mm could potentially facilitate a higher feed intake. Pellets may disintegrate before reaching the feeder. Recent research indicates that although a high level of fines reduces feed intake, broilers readily consume fines, and appear to tolerate a larger proportion of fines than currently assumed, particularly in the starter phase. A major challenge when interpreting research is that there is a lack of standardization of how structure is reported. It is suggested that a more thorough description of the grinding and pelleting process is implemented in scientific reports on diet structure. A more detailed description of particle size distribution both through dry sieving of the pelleted diet and through wet sieving of the dissolved pellet should also be included.