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South African Journal of Animal Science

On-line version ISSN 2221-4062
Print version ISSN 0375-1589

S. Afr. j. anim. sci. vol.53 n.6 Pretoria  2023 

Effects of feed form (pellet or mash), corn particle size, and Bacillus-based probiotic supplementation on performance traits and digestive tract health of broiler chickens



I. Yousefian AstanehI; M. ChamaniI,; S.N. MousaviII; A.A. SadeghiI; M. Amin AfsharI

IDepartment of Animal Science, Science and Research Branch, Islamic Azad University, Tehran, Iran
IIDepartment of Animal Science, Varamin-Pishva Branch, Islamic Azad University, Varamin, Iran




A total of 720 Ross 308, male broiler chicks were used to study the effect of feed form, corn particle size, and probiotic supplementation as a completely randomized block design with 2*2*2 factorial arrangement (eight treatments and five replications). The factors included the type of feed (pellet or mash), the average size of corn particles (590 and 1220 urn), and the addition or absence of a probiotic (DIPro) to the diet. At the end of the experiment, relative weights (% of BW) of abdominal fat and gizzard were calculated as a percentage of live weight. Using pellets increased the body weight gain (BWG), feed intake (FI), and improved feed conversion ratio (FCR). Feeding coarse corn particles substantially reduced the body weight gain and increased the feed conversion from 1 -10, 25-42, and 1 -42 days of age. Feed intake reduced from 1 -42 d and 42-25 d by consuming coarse corn particles. The use of probiotics increased feed intake and body weight gain from 25-42 and 1 -42 d. The pelleted diet increased gizzard and proventriculus pH, whereas coarse corn particles reduced pH. The pelleted feed decreased the relative weight of the gizzard and increased the abdominal fat percentage. Coarsely ground corn increased the abdominal fat and gizzard percentage. Probiotic supplementation improved feed intake, body weight gain, and feed conversion from 1-42 d. Pelleted feed plus probiotics and a 590-um particle size produced the best performance in broilers.

Keywords: digestive tract; feed form; growth performance, particle size; probiotics




Over the past years, nutritionists have been seeking the best feed formulation to improve growth performance in livestock and poultry (Chewning et al., 2012). In recent years, feed particle size and feed form have been identified as important factors in poultry performance, gut development, and health (Svihus, 2011; Mateos et al., 2012). It is believed that a reduction in feed particle size increases the influence of enzymes; pelleting increases the contact surface, but may have a negative effect on the development of the gastrointestinal tract, especially the gizzard (Kheravii et al., 2017). The status of the gizzard is important in increasing feed efficiency and improves the health of the gastrointestinal tract. Coarse feed particles lead to a reduction in digesta passage rate, an increase in pancreatic secretions, and thus increase the digestibility of substances, such as nitrogen and phosphorus (Abdollahi et al., 2013).

Like particle size, the feed form (mash or pellet) also affects the performance of broiler chickens. Many studies have shown that the use of pelleted feed reduces the development of the gastrointestinal tract, including the development of the gizzard, pancreas, and small intestine (Abdollahi et al., 2011 ; Aguzey et al., 2018). Various studies have evaluated the effect of pelleting on feed availability, but no positive effect of pelleting on protein availability has been found (Abdollahi et al., 2013). Huang et al. (2006) studied the effects of feed particle size and feed form (mash and pellet) on the intestinal Salmonella typhimurium population. They found that pelleted diets increased the death of S. typhimurium in the gizzard and decreased the death of S. typhimurium in the caeca. The gizzard weight of birds fed pelleted diets was lower than mash diets, and gizzard pH was higher in pelleted diets. Pelleted diets can increase feed consumption and subsequently cause undeveloped gizzards. Undeveloped gizzards reduce the digestibility of nutrients, increase the pH of gizzard contents, and possibly increase the incidence of necrotic enteritis. Researchers and consumers are focused on poultry rearing without the use of antibiotic growth promoters and are looking for effective and non-antibiotic alternatives to control diseases such as necrotic enteritis. In addition to appropriate feed form and ingredient particle size, other suggested strategies for this improvement in poultry performance include proper vaccination programs, coccidiosis control, use of suitable probiotics, prebiotics, organic acids, plant extracts, or essential oils.

Bacillus-based probiotics are resistant to heat and widely resistant to pelleting temperature or low digestive pH, so that most of the swallowed bacteria remain intact in the small intestine (Lee et al., 2010; Harrington et al., 2015). These qualitative parameters have increased the tendency to use Bacillus species in the poultry industry. Although Bacillus-based probiotics are known for their high temperature resistance, there is no consistent information on their ability to survive at conventional pelleting temperatures used in the feed manufacturing industry. The interaction of Bacillus supplementation at different temperatures on the performance and immune response of broiler chickens is not known; there is a possibility of physical and chemical changes caused by different thermal conditions (Amerah et al., 2013). Although there are several studies on the effects of feed form and feed particle size on broiler performance, none of them has investigated the use of probiotics in diets of different feed form and particle size.


Material and Methods

To conduct this experiment, 720 male, Ross 308 broilers with initial weights of 45 ± 1.43 g were used in a 2×2×2 factorial arrangement as a completely randomized block design with eight treatments and five replications. The factors studied in this experiment included the feed form (pellet and mash), the type of screen size (3 and 8 mm; the geometric mean of 3-mm screen size was 590 μm and for the 8-mm screen size was 1220 μm) and the addition or absence of a probiotic (250 g DiPro probiotic/ton feed). The probiotic (commercial name, DiPro) was made by Takgene, Iran, and contained equal ratios of B. subtilis and B. licheniformis (each 6 x 1012 cfu/ kg). The treatments were:

1- mash feed with 3-m mill screen size, without probiotic

2- mash feed with 3-mm mill screen size, probiotic supplementation

3- mash feed with 8-mm mill screen size, without probiotic

4- mash feed with 8-mm mill screen size, probiotic supplementation

5- pelleted feed with 3-mm mill screen size, without probiotic

6- pelleted feed with 3-mm mill screen size, probiotic supplementation

7- pelleted feed with 8-mm mill screen size, without probiotic

8- pelleted feed with 8-mm mill screen size, probiotic supplementation.

Diets were formulated based on corn and soybean, according to Ross 308 nutritional recommendations (Aviagen, 2014) (Table 1). Performance traits, including BWG, FI, and FCR, were recorded from 1-10, 11-24, 25-42, and 1-42 days of age. Feed intake and feed conversion ratio data were corrected based on mortality and weight. At the end of experiment (42 d of age), one bird was selected randomly from each pen, weighed, and then slaughtered according to the regulations approved by the Animal Protection Committee of the Iranian Institute for Animal Research (04-02-2012; Protocol 10938-5-16-17). Relative weights (% of BW) of bursa, spleen, breast, thighs, and abdominal fat were calculated.

Immediately after slaughter, the digesta contents of the gizzard, proventriculus, and various parts of the intestine were taken and centrifuged at 13000 × g at 14.4-15.5 °C for 10 min to homogenize (Boros et al., 1998). The pH values of samples were measured by putting the electrode of a digital pH meter (Testo205 model, Germany) into samples for 3 min. After each measurement, the electrode was washed before the next measurement was carried out (Morgan et al., 2014).

The data from this experiment were analysed using SAS 9.1 software (2004). Treatment means were compared using Duncan's multiple range test at a significance level of P <0.05.



In all periods, pelleted feed increased body weight gain compared to mash (P <0.05) (Table 2). In all periods, coarse feed particles reduced body weight gain in comparison to fine particles, except from 11-24 d (P <0.05). Probiotic supplementation increased body weight gain from 25-42 and 1-42 d (P <0.05). There was a triple interaction on body weight gain in all periods (P <0.05). The dual effect of feed form × feed particle size was marked from 1-10 d and feed form × probiotic supplementation from 11-24 d (P <0.05).

In all periods, pelleting increased feed intake compared to mash (P <0.05) (Table 3). Increasing feed particle size reduced feed intake from 25-42 d and 1 -42 d (P <0.05). The probiotic supplementation increased feed intake from 1-10, 11-24, 25-42, and 1-42 d (P <0.05). The interaction of feed form × feed particle size on feed intake was considerable from 11 -25 d (P <0.05). The interaction of feed form × probiotic supplementation on feed intake was substantial from 1 -10, 11 -24, and 1 -42 d (P <0.05). In all periods, pelleting reduced the feed conversion ratio (P <0.05). In all periods, the coarse feed particles increased the feed conversion ratio (P <0.05). The addition of probiotics to feed caused a reduction in feed conversion ratio, except from 11 -24 d (P <0.05). The interaction of feed form × feed particle size on feed conversion ratio was substantial from 1-10 d (P <0.05). The interaction of feed form × probiotic supplementation on feed conversion ratio was substantial from 1 -10 and 11 -24 d (P <0.05). The interaction of feed particle size × probiotic supplementation on feed conversion ratio was substantial from 1-10 d (P <0.05). The triple interaction on feed conversion ratio was substantial in all periods (P <0.05). From 1-10 d, the interaction of mash feed and 8-mm screen size at both probiotic levels increased feed conversion ratio (P <0.05). From 11-24 and 1-42 d, the interaction of mash feed at 8-mm screen size and without probiotics increased the feed conversion ratio (P <0.05).


Table 4


Table 5 shows the main and interaction effects of feed form, feed particle size, and probiotic supplementation on the pH of different parts of gastrointestinal tract at the end of experiment. Pelleting the diet caused an increase in gizzard pH (P <0.05) but did not affect the pH of other parts of the gastrointestinal tract (P >0.05). The addition of probiotics did not affect the pH of different parts of the gastrointestinal tract (P >0.05). The interaction between screen size and probiotic level and a triple effect on gizzard pH was important (P <0.05). Triple interaction effects on the pH of gizzard and proventriculus were substantial (P <0.05). The use of mash feed ground with 8-mm screen size, with and without probiotic, reduced gizzard pH compared to other feeds, except the pelleted feed with ground corn of 8-mm screen size, with and without probiotic supplementation (P <0.05).

Using pelleted feed compared to mash caused to a reduction in the percentage of gizzard and an increase in the percentage of abdominal fat (P <0.05). The coarse feed particles increased gizzard and abdominal fat percentage (P <0.05). However, probiotic supplementation did not have any effect on these parameters (P >0.05).


Table 6



Birds fed pelleted feed compared to mash had higher average weight, body weight gain, feed intake, and a better feed conversion ratio in different periods. In agreement with these findings, Amerah & Ravindran (2008) reported that birds fed pelleted diets had better feed conversion than those fed mash diets. Rezaeipour & Gazani (2014) also reported that feed form had an effect on broiler feed intake and body weight in all experimental periods. Several studies investigated the positive effects of pelleted feeds on performance. Using pelleted feeds can increase the feed intake and body weight gain in broiler chicks (Aguzey et al., 2018). Abdollahi et al. (2011) reported that the use of pellets in the starter period (1-21 days) increased the feed intake of broiler chickens by 14%. The improvement in feed conversion ratio as a result of pelleted feed is possibly due to decreasing feed wastage (Jensen, 2000). When using pellets, broiler chickens spend less time consuming feed, which reduces the maintenance energy requirement (Abdollahi et al., 2018; Aguzey et al., 2018). The effect of using pellets on increasing the net energy of the diet is probably due to this. Zang et al. (2009) showed that pelleted diets increased daily body weight gain, feed intake, and improved feed efficiency during the starter, grower, and total phases. In their study, regardless of the age, pelleted diets improved the apparent metabolizable energy and apparent crude protein and organic matter digestibility. In the pelleting process, the particle size of the feed becomes smaller and more uniform as it passes through the steam and then undergoes pressure in the die holes (Svihus et al., 2004; Abdollahi et al., 2018; Aguzey et al., 2018). Some studies have found that particle size in pelleted diets has no effect on the performance of broiler chickens, which is related to the uniformity of feed particles during pelleting process (Svihus et al., 2004; Peron et al., 2005). In agreement with the results of the current study, Benedetti et al. (2011) reported that the pelleted diets could increase feed intake and body weight gain in broiler chickens. In the current study, the birds fed diets containing coarse particles (1220 μm geometric mean diameter) compared to diets containing finely ground corn (590 μm geometric mean diameter) had lower body weight gain from 1-10 and 25-42 days of age, whereas feed intake was reduced using coarse corn particles from 25-42 days of age. Coarse corn particles increased the feed conversion ratio from 1-10 and 25-42 d, which is consistent with Lott et al. (1992). They stated that increasing the geometric mean of feed particles from 716 to 1196 μm in crumble diets led to a reduction in daily body weight gain and an increase in feed conversion ratio. In agreement with the present study, Chuiung et al. (2012) showed that broiler chickens fed a diet with a mean particle size of 300 μm had higher daily weights and better feed conversion than those fed a diet with a particle size of 600 μm. Singh et al. (2014) observed that increasing the pre-pelleting inclusion of whole corn in diets caused a decrease in body weight gain.

Reducing the particle size can increase the effect of enzymes on their substrates (Amerah & Ravindran, 2008; Goodband et al., 2002); performance reduction by increasing the geometric mean of feed particles can be due to the slower effects of enzymes on these particles. According to the results in the current study, the average body weight gain decreased by consuming coarse particles from 1 -11 d but no difference was observed from 11 -24 d; an increase in body weight gain was observed from 25-42 d. This difference can be attributed to the effect of fine particles on increasing starch digestibility and metabolizable energy (Peron et al., 2005). As more energy is needed due to increased maintenance requirements in the final days, increasing the dietary energy by using fine particles can reduce the use of dietary amino acids to provide energy. If the dietary energy is high in the final phase, this can increase body weight gain. In this regard, Kilburn & Edwards (2001) reported that finely ground corn particles increased the metabolizable energy of diets. Zang et al. (2009) reported that reducing the feed particle size increased the metabolizable energy from 19-21 d. They stated that a combination of reducing feed particle size and gelatinization of starch could improve enzymatic digestion, and as a result, they observed improved apparent metabolizable energy, crude protein, and organic matter in their study.

In another study, increasing the size of corn particles (781, 950, 1042, and 2242 μm) improved performance and nitrogen retention in growing chickens (Parsons et al., 2006). However, when the particles were larger than 1042 μm, the apparent metabolizable energy was negatively affected. The researchers concluded that coarse particles could increase gizzard size; increasing the energy requirements for gizzard maintenance can affect broiler performance negatively. This justifies the loss in growth improvement using coarse particles, despite the increase in gizzard size, in the current study. However, if corn particles are very fine or very coarse, the use of nutrients by birds may be reduced (Benedetti et al., 2011). In contrast to our study, Benedetti et al. (2011) showed that increasing the geometric mean of the particle diameter from 460-870 μm resulted in an increase in the feed conversion ratio and a decrease in body weight gain from 1-21 d. Dozier et al. (2006, 2009) investigated the effects of increasing the size of corn particles (1500 GMD) from 80 to 235 g/kg from 18-41 d and 2691 GML μm from 150 to 280 g/kg from 18-56 d and observed that body weight gain, feed intake, and feed conversion ratio were not affected. It seems that the combination of diet, the type of grain (Kilburn & Edwards, 2004), the feed form (Mirghelenj & Golian, 2009), the bird's age, the availability of bedding material, the type of feed, type of birds (Mtei et al., 2019), and further processing of the feed (pelleting or grinding) (Goodblad et al., 2002) determine the effect of the particle size of the diet on growth performance. Some researchers suggest that to achieve good feed quality for broiler chickens, 650-700 μm corn particles are required (Dozier, 2003). Oppong-Sekyere et al. (2012) showed that increasing the geometric diameter of the feed particles from 713-1462 urn resulted in an increase in feed conversion ratio and a decrease in body weight gain and final body weight.

Based on our results, the effect of using a probiotic reduced feed intake and increased body weight gain, which is consistent with the study of Apata (2008). This has been reported to be due to the positive effects of using probiotics on the intestinal morphology (Chichlowski et al., 2007). Competitive exclusion is another mechanism that can be mentioned for the positive effect of probiotics on body weight gain. The harmful microorganisms compete with intestinal villi to absorb nutrients from the intestines, and since probiotics control the population of harmful bacteria, they can increase the nutrient utilization of birds (Ng et al., 2009). Although the exact mode of action for the effects of probiotics is still unknown, some mechanisms, such as the production of antibacterial agents (bacteriocins and bacteria inhibitors), the secretion of non-specific antimicrobials, the induction of antimicrobial compound production (diphenin) by the host, direct enzymatic activities of probiotics in the intestinal lumen, the reduction of luminal pH, inhibition of bacterial adhesion, competition for nutrients, and immunity protection have been proposed (Fooks & Gibson, 2002; Gillor et al., 2008). There is increasing evidence that the same effects may be obtained with dead bacteria or inseparable components of a bacterial cell, such as peptidoglycans or DNA fragments (Oelschlaeger, 2010). In the starter phase, birds fed diets containing probiotics had a lower body weight gain than those fed diets without the probiotic. This result may be due to effects of probiotics on bile salt deconjugation and a reduction in fat digestion in starter diets because of the low production of bile in young birds (Lin, 2014).

In the current study, the coarse feed particles decreased the gizzard and proventriculus pH compared to fine particles. In agreement with the present study, other researchers have shown that the use of coarse feed particles reduces the pH of the gizzard (Engberg et al., 2002; Naderinejad et al., 2016) and proventriculus (Engberg et al., 2002). Naderinejad et al. (2016) also showed that finely-ground ingredients in pelleted diets increased the pH of the gizzard contents. In a study by Engberg et al. (2004), the addition of whole wheat increased gizzard weight and decreased the pH of the gizzard contents.

It has been reported that the consumption of coarse particles by broilers can cause gizzard development. The gizzard development can result in longer retention times and decreasing gizzard and proventriculus pH. Improvements in the size of the gizzard and a marked reduction in pH did not improve the performance traits in the current trial, and are acceptable when not affecting feed intake. Using coarse corn particles (1145 |m) compared to fine particles (271 |m) caused an improvement in feed conversion ratio, feed intake, body weight gain, and gizzard size and a reduction in gizzard pH (Xu et al., 2017). Parsons et al. (2006) reported that the inclusion of coarsely ground corn in the feed improved the nitrogen and lysine retention in relation to the reduction in gizzard pH. In the current study, the reduction in gizzard pH had no impact on performance. This could be because the pancreas produces a large amount of bicarbonate and alkaline materials to alkalize the intestinal environment and creates an optimal environment for enzyme activity, resulting in an electrolyte imbalance in the blood. The production of hydrochloric acid in the proventriculus requires a large amount of Cl-, which can also contribute to the electrolyte imbalance in the blood and a loss in performance. More coarse particles substantially increase the gizzard percentage and decreased gizzard pH, which is consistent with other researchers (Abdollahi & Ravindran, 2013). The feed particles do not exit the gizzard until reaching a certain size, thus a longer retention time of coarse particles can stimulate proventriculus acid secretion, which in turn can make the gizzard more acidic in these birds (Nir et al., 1994; Svihus et al., 2004). They also found that the physical properties of the feed affected the intestinal pH, Enterobacter lactose, and Clostridium perfringens.

In the present study, the gizzard percentage was reduced by pelleting the diet; this concurs with Huang et al. (2006) and Rezaeipour & Gazani (2014), who reported that the pelleting process reduced gizzard and proventriculus weight. Hamedani et al. (2015) found that the use of pelleted feed compared to mash feed reduced the size of gizzard. In this regard, we can say that the main cause of the increase in weight and size of the gizzard is the increase in contractions of the gizzard (Röhe et al., 2014). According to Svihus (2004), pelleting can reduce particle size; small particle sizes are retained less in the gizzard, resulting in an under-developed gizzard.

In the present study, the use of pelleted feed increased the percentage of abdominal fat, which is consistent with Salari et al. (2006). According to Massuquetto et al. (2019), pelleted diets can increase feed intake, and subsequently, abdominal fat in poultry. Increasing the fat percentage is probably due to the increased metabolizable energy supply as a result of the pelleting process. Studies have been conducted to improve the use of dietary energy during pelleting (Yang et al., 2010) and this excess energy is probably stored as fat. The effect of increasing the corn particle size on increasing the percentage of abdominal fat in the present study concurs with the results of other researchers (Jones and Taylor, 2001; Nahas and Lefrancois, 2001; Parsons et al., 2006). However, in contrast to this study, Wu & Ravindran (2004) reported that a coarse size had no effect on abdominal fat; Amerah & Ravindran (2008) observed a reduction in abdominal fat percentage. These differences may be due to the level of dietary energy provided. The effect of increasing particle size was shown to increase the gizzard percentage in the present study, which is consistent with other studies (Svihus et al., 2010). Parsons et al. (2006) evaluated the effects of fine (781 μm), small (950 μm), medium (1042 μm), and large (1109 μm) corn particles on the performance of broiler chickens from 3-6 weeks of age. They found that feeding larger corn particles substantially reduced the feed efficiency compared to other sizes. By increasing corn particle size, the weight of gizzard (as a percentage of body weight) was increased. The consumption of coarse feed particles, in addition to gizzard weight, also increased pancreas weight, which indicates an increase in the activity and secretion of the enzymes by the pancreas (Williams et al., 2008).



The particle size and feed form are very important in the production process. According to the results of this experiment, the use of probiotics in feed has a positive effect on bird performance. Although the coarse feed particles reduced the pH and increased the gizzard size, it is thought that feed intake was reduced due to a decrease in the passage rate of feed from the gastrointestinal tract, leading to a reduction in performance traits. In this experiment, although the gastrointestinal condition in bords fed a particle size of 1220 μm was better than that at 590 μm, the reduced feed intake in this treatment caused the best performance of the birds to be achieved by using 590 μm particle size. Considering that the use of pelleted feed resulted in increased carcass fat, a reduction in the maintenance requirement of bird can be expected, which concurs with the presented references. The energy level in pelleted feed can therefore be reduced compared to mash feed.



We certify that there is no conflict of interest with any financial organization regarding the material discussed in the manuscript.



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Submitted 23 December 2018
Accepted 22 November 2019
Published 23 November 2023



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