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 reduction of commonly used by- products in poultry feed such as distillers dried grains with solubles (DDGS) and soybean meal (SBM) has also been investigated. Pacheco et al. (2014) evaluated the effect of particle size of SBM (410 or 1025 microns) and DDGS (480 or 745 microns) on live performance. The inclusion of fine SBM (410 microns) improved pellet quality whereas coarse SBM (1025 microns) had positive effect on live performance. Fine DDGS (480 microns) increased FI and body weight without any impact on the FE. However, coarse DDGS (745 microns) in broiler diet increased gizzard weight. Therefore, coarse particle size of DDGS and SBM in broiler diets results in better growth performance. Physical form of feed has significant impact on growth performance of broilers (Dozier et al., 2010). Many studies have reported improvement in broiler performance when fed pelleted feed compared to mash feed (Amerah et al., 2008; Chewning et al., 2012; Mingbin et al., 2015). Use of crumble or pellet form of feed reduces the feed wastage and prevents particle selection. Pelleting process also improves palatability and increases nutrient digestibility (Mingbin et al., 2015). Zang et al. (2009) also reported improvement in FI, WG and FE when broilers were fed pelleted diet. Mingbin et al. (2015) conducted a study to evaluate the effects of feed form (mash and crumble-pellet) and feed particle size (fine, medium and coarse) on performance and GIT development of broilers. Results showed that feed form had greater effect on broiler performance and GIT development than feed particle size. Figure 1 shows effect of feed form and feed particle size on growth performance of broilers.
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).
Particle size also affects the intestinal tract segments otherthan the gizzard, but results are contradictory. Amerah et al. (2007) reported non-significant change in villus height, crypt depth, and epithelial thickness in the duodenum with increase in maize particle size. However, Liu et al. (2006) and Xu et al. (2015) reported positive effect of coarse maize on intestinal morphology. The inclusion of coarse maize reduced the number of mast cells in the duodenum, jejunum, and ileum compared with finely ground maize (Liu et al., 2006). Reduction in mast cell in the small intestine is beneficial. Increase in mast cell numbers reflects Eimeria infection in broilers (Morris et al., 2004). Particle size of ingredients also affects the intestinal microbiota profile. Jacobs et al. (2010) reported change in cecal microbiota profile with use of different corn particle sizes (Table 3). The Lactobacilli population was significantly increased (P < 0.05) when the largest corn particle size of 1,387 μm was included in the diet. Increase in Lactobacilli concentration is considered to be beneficial because it can prevent colonization of pathogens such as E. coli (Engberg et al., 2002). The Bifidobacteria population was significantly decreased with increasing corn particle size from 557 μm to 1,387 μm. However, E. coli population was not affected by corn particle size. Singh et al. (2014) also found that the counts of Lactobacillus and Bifidobacteria species increased and those of Clostridium, Campylobacter and Bacteroides species were decreased with increasing inclusion levels of coarse maize (0 to 600 g/kg). Coarse mash diets can also increase the Lactobacilli population in the ceca and rectum (Engberg et al., 2002). It can be concluded that large particle size not only influence the GIT development but also change the cecal microbial populations. Large particle size increases the Lactobacilli counts and reduces the pathogenic bacteria in the caecum of broilers. Gracia et al. (2016) tested the effect of whole wheat and oat hulls addition in pellet and mash diets on GIT development and Campylobacter jejuni in cecum. Whole wheat and oat hulls in mash diets significantly reduced cecal Campylobacter jejuni colonization at 42 days whereas no clear reduction was observed for pellet diets.
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 (Singh et al., 2014). Whole grain feeding may also influence starch digestive dynamics and provide more gradually or slowly digestible starch. This effect on starch digestion may lead to improvement in energy utilization and FE (Liu et al., 2015). Wu et al. (2004) reported that pre-pellet 20% whole grain addition improved energy utilization. Published studies on whole grain feeding reported contradictory results. Many studies reported beneficial effect of whole grain feeding on broiler performance whereas others reported no advantage or even poorer performance (Singh et al., 2014). The inclusion of 5-15% whole wheat in grower and finisher diets of broilers reduced final body weight, FCR and breast meat yield by 3.8, 3.9 and 5.7%, respectively. Water intake, nitrogen excretion and litter weight were decreased by 5.8, 15.5 and 11.0%, respectively (Facts and Figures, 15177).
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).
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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