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METABOLISM AND NUTRITION |
Institute of Food, Nutrition and Human Health, Massey University, Private Bag 11 222, Palmerston North, New Zealand
1 Corresponding author: v.ravindran{at}massey.ac.nz
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: particle size • feed form • digestive tract • digesta parameter • broiler
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The anticipated ban on the use of antibiotic growth promoters in animal feeds has encouraged nutritionists to explore alternative feed management strategies to improve the health and digestive efficiency of broiler chickens. Promotion of the development of the gizzard is one such nutritional strategy, which can be achieved by manipulating feed particle size (Nir et al., 1995; Engberg et al., 2002). A well-developed gizzard is associated with improvement in gut motility (Ferket, 2000) and may prevent pathogenic bacteria from entering the small intestine (Bjerrum et al., 2005), thus reducing the risk of coccidiosis and other enteric diseases (Cumming, 1994; Engberg et al., 2002, 2004; Bjerrum et al., 2005). Part of the beneficial effects of feed particle size may arise from their influence on intestinal morphometry, but there are no published data on this aspect. Data on the effect of feed particle size on the particle size distribution of digesta in poultry are also scanty. Lentle (2005) has speculated that greater proportions of coarser particles in the diet result in greater numbers of coarser particles transiting the gizzard, which may increase the permeability of digesta to enzymes and improve digestive efficiency.
The feeding of a coarser grain size is sound from the viewpoint of energy expenditure. The grinding of whole grain for broiler feeds constitutes the second greatest energy expenditure after pelleting (Reece et al., 1985). Increasing the screen size in the hammer mill from 4.76 to 6.35 mm has been reported to reduce the total energy consumption by 27% (Reece et al., 1986a). However, coarse grinding is said to reduce pellet quality, although there is no objective evidence for this claim.
In any study examining the effects of feed particle size in broiler diets, the physical form of the diet (mash vs. pellets) should also be considered. Evidence suggests that the large-grain particle size influences broiler performance to a greater extent when birds are fed mash diets than when fed pelleted or crumbled diets (Nir and Ptichi, 2001; Amerah et al., 2007). The objectives of the present study were to compare the effects of particle size and feed form on the performance, energy utilization, digestive tract development, intestinal villus morphology, and duodenal and colonic digesta particle size spectra of broilers fed wheat-based diets.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Diets
Whole wheat was ground in a hammer mill (Bisley’s Farm Machinery, Auckland, New Zealand) to pass through a screen size of 3 or 7 mm for medium and coarse grades, respectively. The particle size spectrum of each grade was subsequently characterized by dry sieving by using the method described by Baker and Herrman (2002). In brief, a representative ground sample (100 g) was passed through a geometric series of sieve sizes, and the amount of sample retained on each sieve was determined. The geometric mean diameter and geometric SD were then determined (Baker and Herrman, 2002). The geometric mean diameters of wheat ground through 3- and 7-mm screen sizes were determined to be 0.839 and 1.164 mm, respectively, with corresponding geometric SD values of 1.69 and 1.54.
Two broiler starter diets (Table 1
), each based on one of the milled wheat grades plus soybean meal, were formulated to meet or exceed the NRC (1994) recommendations for major nutrients for broiler starters. Half of each formulated diet was fed in mash form and the other half was cold pelleted with a pellet mill (Orbit 15, Richard Size Limited Engineers, Kingston-upon-Hull, UK) capable of manufacturing 180 kg of feed/h and equipped with a die ring (3-mm holes and 35-mm thickness). Conditioning time of the mash was 90 s to a temperature of approximately 70°C. Each of the 4 dietary treatments was fed ad libitum to 6 pens, each housing 8 birds. Water was freely available throughout the trial.
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AME Determination
The feed intake and total excreta output of each pen were measured from d 17 to 20 posthatching. The pooled excreta from each pen were mixed into slurry in a blender for 5 min and subsampled for determination of DM, gross energy (GE), and nitrogen content. Each subsample was dried, ground to pass through a 0.5-mm sieve, and stored in airtight plastic containers at –4°C pending analysis.
The DM content, total nitrogen, and GE were determined in requisite samples of diets and excreta. Dry matter content was determined by using standard procedures (methods 930.15, 925.10; AOAC, 1990). Nitrogen in the diets and excreta was determined by combustion (method 968.06; AOAC, 1990) with a CNS-200 carbon, nitrogen, and sulfur autoanalyzer (Leco Corporation, St. Joseph, MI). Gross energy was determined by adiabatic bomb calorimetry (Gallenkamp Autobomb, London, UK) standardized with benzoic acid. Apparent ME values were calculated by using the following formula, with appropriate corrections made for differences in DM content:
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Nitrogen-corrected AME values were determined by correction for zero nitrogen retention by simple multiplication with 8.73 kcal/g of nitrogen retained in the body as described by Hill and Anderson (1958).
Digestive Tract and Intestinal Morphology Measurements.
On d 21, 4 birds from each replicate pen, with BW closest to the mean weight of the pen, were killed by cervical dislocation. Two of these birds were used for measurements of the digestive tract and the other 2 birds were used for the microscopic study of intestinal morphology.
The live BW and weight of digestive tract segments from the crop to ceca of each bird were determined by weighing of wet tissue. The length of each intestinal segment was determined with a flexible tape on a glass surface to prevent inadvertent stretching (Lentle et al., 1998). Thus, the length (±0.1 mm) of the duodenum (from the pyloric junction to the distal-most point of insertion of the duodenal mesentery), the length of the jejunum (from the distal-most point of insertion of the duodenal mesentery to the junction with Meckel’s diverticulum), the length of the ileum (from the junction with Meckel’s diverticulum to the ileocecal junction), and the sum of the lengths from the ostium to the tip of each ceca were determined. After division and freeing of each of these components from any adherent mesentery, their full and empty weights (±0.1 g) were determined, along with those of the crop, proventriculus, and gizzard.
For intestinal morphological examinations, sections from the middle of the duodenum and jejunum (about 5 cm in length) were excised and flushed with ice-cold saline and immediately placed in Bouin’s fluid. Samples were transferred into 70% ethanol after 24 h. Each fixed sample was then embedded in wax and sectioned at a thickness of 7 µm, stained with alcian blue and hematoxy-lineosin, and examined by light microscopy. Four intestinal segments were fixed in each slide, and the slides were viewed on a Zeiss Axiophot microscope (Carl Zeiss, Oberkochen, Germany). Visual measurements of villus height and crypt depth were made on 10 villi at 100x and 200x magnifications with imaging software (Image Pro Plus, Version 4.1.0.9, Media Cybernetics, Silver Spring, MD). The variables measured were villus height (the distance from the apex of the villus to the junction of the villus and crypt), crypt depth (the distance from the junction to the basement membrane of the epithelial cell at the bottom of the crypt), and the total extent of the mucosal layer (the distance from the apex of the villus to the basement membrane of the epithelial cell at the bottom of the crypt).
Determination of Particle Size Distribution in the Diets and in Duodenal and Colonic Digesta.
Two more birds per pen were euthanized by intravenous injection of sodium pentobarbitone on d 21. Digesta was removed from the duodenum by simple drainage and from the colon by manual expression. Samples from the 2 birds within each pen were then pooled, giving a total of 6 digesta samples per segment per dietary treatment. The particle size spectra of various samples were determined by wet sieving according to the method described by Lentle et al. (2006). Briefly, weighed samples of the diet and duodenal and colonic digesta samples were each divided into 2 subsamples. One was oven-dried at 80°C in a forced-draft oven for 3 d to determine the DM content, and the other was suspended in 50 mL of water before it was washed through a set of sieves (Endocot, London, UK) sized 2.0, 1.0, 0.5, 0.25, 0.106, and 0.075 mm. The diet samples were suspended in 50 mL of water and left to stand for 30 min prior to sieving to ensure adequate hydration. The contents of each of the sieves were subsequently washed onto dried, preweighed filter papers, dried for 24 h in a forced-draft oven at 80°C for 3 d, and reweighed. The dry weights of particles retained by each sieve and of fines remaining in the bottom pan were expressed as the percentages of total DM recovered.
Pellet Durability.
Pellet durability was determined in a Holmen Pellet Tester (New Holmen Pellet Tester, TekPro Ltd., Norfolk, UK) according to the method described by Svihus et al. (2004). Thus, pellet samples (100 g) were circulated pneumatically through a closed pipe for 30 s before being passed through a 3-mm sieve. The pellet durability index was calculated as the relative proportion, by weight, of pellets retained on the 3-mm sieve.
Data Analysis
The means of pens were used to derive performance data. The performance data were analyzed by 2-way AN-OVA by using PROC GLM (SAS Institute, 1997). Differences were considered to be significant at P < 0.05, and significant differences between means were separated by the least significant difference test.
Gross and microscopic measurements of the digestive tracts of individual birds were first subjected to log transformation to achieve near normal distribution on graphic analysis in Systat (Wilkinson, 1990). Two-way ANOVA was used to identify any interaction between feed form and feed particle size on the microscopic morphometric parameters of gut components. For gross digestive tract measurements, a 3-way ANOVA was used to identify any interaction between feed form, feed particle size, and the various digestive tract components (crop to ceca) so as to understand how feed form and particle size influenced the overall tract development. A similar model was used for analysis of (log-transformed) digesta content, with the digesta content of the various segments of the small intestine being the third factor. In each case, the significance of individual components within the 3 levels was explored by post hoc testing with Bonferroni correction.
The effects of treatments on particle size distribution in the duodenal and colonic digesta were compared by discriminant analysis with Systat (Wilkinson, 1990). Pellet durability data were compared by 1-way ANOVA with PROC GLM (SAS Institute, 1997).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) obtained by wet sieving showed that pelleting reduced the relative proportion of particles by >1 mm and increased the proportion of fine particles by <0.075 mm in coarse feeds so that the particle size distributions of the 2 feeds were nearly identical.
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). In pelleted diets, however, particle size had no effect on the performance parameters, as indicated by the feed form x particle size interaction (P < 0.05 to 0.001). Pelleting of the diet increased (P < 0.05) the weight gain and feed intake and improved feed:gain compared with those obtained from birds fed mash diets. No differences (P > 0.05) were observed between the particle size treatments within pelleted diets for any of the performance parameters.
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). However, there was no effect (P > 0.05) of feed particle size on AMEn or a significant interaction of feed form with particle size. Digesta transit time did not vary (P > 0.05) with any of the dietary treatments.
Digestive Tract Measurements
The effects of feed form and particle size on the relative tissue weights, digesta contents, and relative lengths of the various intestinal components are shown in Table 3. The relative empty weights of all components were greater (P < 0.05) in birds fed mash diets than those fed pelleted diets. Although there was no significant main effect of diet particle size on the wet tissue weight of gut components, there was an interaction (P < 0.05) between particle size and feed form, because of weight of all gut components was higher in birds fed medium-particle mash than in those fed coarse-particle mash. There was also an interaction between feed form and the weights of the various gut segments (P < 0.05), because the birds fed mash feed had a relatively heavier gizzard and ceca and birds fed pelleted form had a heavier jejunum.
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An interaction (P < 0.05) between feed form and particle size was noted for the relative length of gut components. All gut components were shorter in birds fed coarse-sized particles compared with those fed medium-sized particles in mash form, but there was no difference in gut length between those fed coarse-sized particles and those fed medium-sized particles in pelleted form. The relative length of all gut components was shorter (P < 0.05) in birds fed pelleted diets. Similarly, all gut components were shorter (P < 0.05) in birds given feeds containing coarse particles. Although there were overall differences (P < 0.05) in the relative lengths of the gut components with feed form and particle size and there were differences (P < 0.05) in the lengths of individual components, there was no interaction (P > 0.05) among the lengths of the individual components, either with feed form or with particle size. Thus, although the form and particle size of the feed influenced the overall length of all gut components, it did not change the pattern of their individual lengths.
Intestinal Morphology
The morphometry of the duodenum and jejunum is shown in Table 4. In both segments, birds fed pelleted diets had a greater (P < 0.05) mucosal extent and had greater villus height and crypt depth measurements compared with those fed mash diets. The main effect of particle size and the 2-way interactions were not significant (P > 0.05) for these morphological parameters.
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). When particle size treatments within feed forms were combined and mash and pelleted diets were compared, discriminant analysis of the particle size distribution of duodenal and colonic digesta showed a successful distinction (P < 0.01) between pelleted and mash diets (Table 6). Thus, the duodenal digesta of the birds fed mash diets had a lower (P < 0.01) proportion of particles of <0.5 mm and a higher proportion of particles of >1 mm. Similarly, there were differences (P < 0.0001) between the colonic digesta of birds fed pelleted diets and those fed mash diets, with birds fed mash diets having lower proportions of particles of >0.25 mm, 0.5 mm, and fines (<0.075 mm) than those fed pelleted diets.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The present data showed that the relative gizzard weight and contents were lower in birds fed pelleted feeds than in those fed mash feeds. Similar findings have been reported by Nir et al. (1995). These results may suggest that pelleting decreased the grinding requirement of the gizzard so that its function was reduced to that of transit.
It is noteworthy that AMEn did not vary with particle size either in mash or pelleted diets, a finding that is similar to that of Svihus et al. (2004) for pelleted diets. In contrast, Peron et al. (2005) reported that AME was lower with pelleted diets made from coarsely ground wheat compared with that made from finely ground wheat.
Nitrogen-corrected AME of birds that were fed pelleted diets was reduced compared with that of birds fed mash diets. It has been hypothesized that the higher intake of pelleted wheat-based diets increases the starch load in the gut and lowers the digestibility of starch (Svihus and Hetland, 2001). This hypothesis was supported by our finding that the extent of the mucosal layer of the duodenum and jejunum was increased in birds fed pelleted diets, which could be viewed as indicating a general increase in the digestive and absorptive capacity of the proximal small intestine in response to the greater flow of nutrients.
The reduction in the relative length of the intestine of birds fed pelleted diets can be less readily explained, because it does not appear to optimize digestion. The fact that this lengthwise reduction is of a general nature and not limited to specific gut compartments indicates that it may result from a general metabolic effect rather than from redundancy of the digestive action of a particular gut compartment. This raises the question of whether there can only be concerted lengthwise growth of the various components of the avian digestive tract and that responses to changing nutrient availability are limited to increases in the extent of the mucosal layer. Such a mechanism may account for the reduction of AMEn because, given limited radial mixing in the small intestine (Lentle, 2005), a high intake of feeds of high nutritional density would allow the nutrient flow to outstrip local absorptive capacity.
With regard to the feeding of mash diets, an improvement of the feed:gain ratio in birds fed coarsely ground mash over those fed medium-ground mash has also been reported in a number of other studies and is generally attributed to the preference for larger sized particles (Proudfoot and Hulan, 1989; Nir et al., 1995) and the lowering of viscosity of the liquid phase of digesta (Yasar, 2003). The current study showed that the gut components of birds fed coarse mash were relatively shorter than those of birds fed medium-ground mash, but that this was accompanied by a significant relative increase in feed intake. This suggests that, at some stage of development, gut component length may vary inversely with intake, whereas the weight and the extent of the mucosal layer of individual gut components varies with nutrient flow.
In the current study, we examined the effect of grinding by the gizzard on the relative proportions of particles of different sizes in duodenal digesta. Birds fed mash diets had a significantly larger mass of gizzard, but a higher proportion of large particles (1 to 2 mm) was found in the duodenal digesta in birds fed mash diets than in those fed pelleted diets. These results indicate that the larger gizzard size was not sufficient to reduce the size of all particles uniformly, contrary to the suggestion of Hetland et al. (2004). It is noteworthy that the relative proportion of fines did not differ between the diets and was not a major basis of discrimination in the analysis. This difference may account for the conclusions of Hetland et al. (2004) because their data were based only on finer particles (<0.88 mm). These data suggest that the gizzard is highly efficient in grinding large particles, but some large particles still escape grinding, as we reported previously (Lentle et al., 2006). In previous studies with whole cereals, Svihus et al. (1997) found that moderate inclusion of whole cereals (which was accompanied by a larger gizzard) did not affect the proportion of particles smaller than 0.100 mm in duodenal digesta.
It is generally believed that there is an inverse relationship between particle size and pellet durability (Angulo et al., 1996). Smaller particles produce more durable pellets because they have more contact points with each other owing to a larger surface area-to-unit volume ratio (Behnke, 2001). However, in the current study, wheat particle size had no effect on pellet durability. In contrast, Reece et al. (1986b) found that pellets made from coarse corn particles gave significantly more durable pellets than those made from fine particles. On the other hand, Svihus et al. (2004) showed numerical improvements in pellet durability when made from fine particles, which was attributed to higher starch gelatinization in the diets made with finely ground wheat. The contradictory results in the literature concerning the effect of particle size on pellet durability may be explained by confounding factors such as the protein and oil contents of the diet (Briggs et al., 1999) and the hardness of the grain (Carre et al., 2005).
In the current study, we found that the pelleting process further reduced the size of coarse particles, thus minimizing the differences in the particle size distribution of coarse and medium grindings. During the pelleting process, the feed is passed through steam, which softens the feed particles before they are pressed through the die by the rolls in the pellet press, causing an additional grinding effect (Engberg et al., 2002; Svihus et al., 2004). This may explain the lack of differences in performance between birds fed pelleted diets based on medium and coarse grinds. In addition, in previous research conducted with pelleted diets, no effect of particle size on broiler performance has been reported (Reece et al., 1986a,b; Proudfoot and Hulan, 1989; Svihus et al., 2004; Peron et al., 2005).
In summary, the data showed that feed form had a greater influence on the different measured parameters than did the particle size. Pelleting reduced the relative proportion of larger particles and increased that of smaller particles in the diet. The improvements in feed:gain in pelleted diets were accompanied by a decrease in the length of all components of the digestive tract. Conversely, the extent of the mucosal layer increased in both the duodenum and jejunum. The feeding of coarsely ground wheat in mash diets improved intake, weight gain, and feed:gain compared with when medium-ground wheat was fed. Pellet durability was not affected by wheat particle size. It would therefore appear that energy savings could be achieved by coarsely grinding wheat grain with no adverse effect on broiler performance.
Received for publication May 30, 2007. Accepted for publication August 27, 2007.
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  A. M. Amerah, V. Ravindran, R. G. Lentle, and D. G. Thomas Influence of Feed Particle Size on the Performance, Energy Utilization, Digestive Tract Development, and Digesta Parameters of Broiler Starters Fed Wheat- and Corn-Based Diets Poult. Sci., November 1, 2008; 87(11): 2320 - 2328. [Abstract] [Full Text] [PDF]
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– – medium; —
— coarse).