Grinding, mixing and pelletization are the key processes in poultry feed production. These processes have direct effect on feed quality, feed intake (FI) and poultry performance. Grinding is most typically associated with the size reduction of cereal grains. It is extensively studied that particle size and grinding of raw materials have major impact on mixing and pellet quality. Particle size also plays important role on the development of gastrointestinal tract (GIT) and growth performance of broilers. Generally, reduction in particle size leads to higher ingredient surface area to interact withdigestive acids and enzymes which ultimately improves digestibility. The practice of feeding whole grains to broilers along with balanced concentrate is increasing acceptance in certain regions, including Europe, Canada, Australia and New Zealand. The main driver of this practice is the potential of reducing feed cost by eliminating the grinding step. It has also positive effects on poultry health and welfare. This review summarizes the results of different trials on effect of particle size and whole grain feeding on growth performance and gut health of broilers.
Effect of particle size and feed form on growth performance
Importance of particle size in poultry diets has been recognized because of its benefits associated with gizzard development and improvement in growth performance (Abdollahi et al., 2018). Various studies over the last few decades demonstrated that the particle size has positive impact on the FI of broilers fed mash diets. The Fl in broilers fed mash diets can be increased by increasing particle size and the effect varied with the age of birds and type of grain (Table 1). Attawang et al. (2014) recommends that corn particle size of around 805 microns is enough for younger birds (<28 days). However, with advancement of age larger corn particle size is likely required. The FI of crumbled or pelleted diets was not affected by particle size of maize or wheat (Table 2). Weight gain (WG) and feed efficiency (FE) improved with increase in particle size of grains either in mash or pelleted diets, but the results are contradictory (Tables 1 and 2). Auttawong et al. (2013) tested two dietary levels of coarse corn (0 or 35%) on broilerperformance and reported improvement in FE with coarse corn (1080 microns) with ad libitum feeding over restricted feeding. Similarly, Xu et al. (2015) reported improved zootechnical parameters and digestive functions of broiler birds when crumbled-pelleted diets contained 50% coarse corn. The authors reported improvement in body weight (130 g) and feed conversion ratio (12 points) in 42 days old broilers. This correlates well with the increased gizzard weight (2.75 mg/g body weight), increased digesta retention time (0.78 h) and increased apparent ileal digestibility of energy (8.2%) and nitrogen (12.4%) in broilers fed diets with 50% coarse corn compared to those fed without coarse corn.
Effect of particle size on gut development and health
Feed particle size influences the GIT development to a greater extent when the broilers are fed mash diets compared to pelleted diets. Reducing particle size property of pelleting process may results in suboptimal gizzard development and changes in the morphology and microbiota profile of intestinal tract (Zaefarian et al., 2016). Large particle size supports gizzard functions and gut health development in broilers (Svihus et al., 2004; Choct, 2009). Naderinejad et al. (2016) also found that coarse grinding of maize in pelleted diets had positive effect on gizzard development and functionality which is beneficial for nutrient utilization and growth performance. The gizzard has good ability to grind the feed to a consistent particle size (Hetland et al., 2004). A well-developed gizzard improves grinding activity and gut motility (Ferket, 2000). It increases cholecystokinin release which stimulates the secretion of pancreatic enzymes and gastro-duodenal reflexes (Duke, 1982; Svihus et al., 2004). Coarse particles reduce the digesta rate in gizzard and lower the pH of gizzard (Nir et al., 1994). Low pH of gizzard may increase pepsin activity (Gabriel et al., 2003) and protein efficiency. It also reduces the risk of coccidiosis (Cumming, 1994) and feed-borne pathogens (Engberg et al., 2002).
Effect of whole grain feeding on growth performance and gut health
The primary aims of feeding whole grains in broilers is to reduce feed cost and to improve digestive functions (Singh et al., 2014). It has also good impact on gut health by encouraging the colonization of beneficial bacterial and reduces the incidence of coccidiosis (Cumming, 1989). Engberg et al. (2004) reported that whole wheat feeding can reduce intestinal numbers of Clostridium perfringens which is important for prevention of necrotic enteritis. Whole grain feeding practice meets consumer demands for a natural feeding system and good for animal welfare (Gabriel et al., 2008). Whole wheat feeding under free choice feeding system increased weights and length of the segments of small intestine (Singh et al., 2015). Fernandes et al. (2013) also reported increased in small intestine weight with 50 or 100% of whole sorghum grain in the broiler diet. Cecum length was also significantly increased when birds were fed diets contained whole sorghum grain. Intestine is the biggest immune organ inside the bird body which contributes further to better health and immune response. It has been reported that whole grain feeding approach helps in preventing the enlargement of proventriculus and atrophy of gizzard which are common problems with pelleted diets (Singh et al., 2014). Whole wheat given under free choice feeding increased the relative gizzard weight, irrespective of mash or pellet form of feed
Whole grain can be incorporated in poultry feed either pre or post steam pelleting and offered as either intact pellets or as a whole grain pelleted concentrate blend (Moss et al., 2018). In New Zealand, whole wheat is usually incorporated into broiler diet prior to steam pelleting whereas in Australia, whole wheat is added post pelleting. Effect of pre- and post-pellet inclusions of whole wheat in broiler diet on growth performance is presented in Figure 2 (Truong et al. 2017). The post-pellet inclusion of whole wheat in broiler diets had greater impacts compared to pre- pellet inclusions. Relative to ground grain control diet, post- pellet whole wheat inclusion increased relative gizzard weight, reduced gizzard digesta pH and reduced incidence of dilated proventriculus. The FE was significantly improved in all whole wheat included diets (pre- and post-pellet).
Conclusion
Particle size of grains and by-products used in broiler feed affects growth performance. Particle size has positive impact on the development of GIT mainly the gizzard development and functionality. Large particle size supports gizzard functions and gut health development in broilers. Impact of particle size is clearer for mash diets compared topellet/crumble diets. Use of large particle size of grains increases the Lactobacilli counts and reduces the population of pathogenic bacteria. Whole grain feeding improves digestive functions and encourages the colonization of beneficial bacteria in gut of broilers.
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M. Ali, P. Krishnan and G. Channarayapatna, Evonik (SEA) Pte Ltd, Singapore