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METABOLISM AND NUTRITION |
* Avian Science Research Centre, Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, Scotland; and Syngenta Animal Nutrition Inc., Chestnut House, Beckhampton, Marlborough, Wiltshire SN8 1QJ, UK
1 Corresponding author: vasil.pirgozliev{at}sac.ac.uk
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: phytase • chicken • turkey • performance • endogenous excretion
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Phytases (myo-inositol hexaphosphate phosphohydrolases) are enzymes that can hydrolyze the ester bonds between the phosphate groups and the inositol ring in phytates, increasing the dietary available P (Irving and Cosgrove, 1974; Cowieson et al., 2004a,b). Initially, poultry diets were supplemented with exogenous phytase to reduce the inclusion of inorganic P and allow better utilization of phytate-P (Nelson et al., 1971). However, later studies demonstrated that the benefits of using dietary phytases are not restricted to improving mineral retention, but also may improve performance, energy, and amino acid availability (Ravindran et al., 1999; Selle et al., 2000; Newkirk and Classen, 2001; Ravindran et al., 2001; Rutherfurd et al., 2002; Pirgozliev et al., 2005; Cowieson et al., 2006a,b).
Recent research (Cowieson et al., 2004b; Pirgozliev et al., 2005) has also demonstrated that supplementing diets with phytase significantly reduces the endogenous secretions, measured as sialic acid (SA), from the GIT of broiler chickens. Sialic acid is a generic term for a family of acidic monosaccharides found at the terminal ends of sugar chains attached to cell surfaces and to soluble glycoproteins (Angata and Varki, 2002). An increased concentration of SA is often associated with health problems such as cellular senescence, bacterial infections (e.g., Campylobacter), certain pathological conditions, and osmotic fragility. The most widely distributed SA is N-acetylneuraminic acid, which is believed to be a biosynthetic precursor of all other SA molecules. Sialic acid is associated with the gastrointestinal mucin (Montagne et al., 2000), so SA excretion can be used as an indicator of mucin losses from the GIT of experimental animals (Larsen et al., 1993). It has been hypothesized that reduced GIT secretion—and thus improved gut health in the presence of phytase—is one mechanism involved in the mode of action of dietary phytases (Cowieson et al., 2004b; Pirgozliev et al., 2005). Although a significant number of studies have shown the effect of phytase on most individual farmed poultry species, information comparing different species directly is lacking. Dietary phytase is known to improve nutrient availability and bird performance; however, it is not clear whether different species follow the same pattern of response. Rodehutscord and Dieckmann (2005) found that chickens, turkeys, ducks, and quail have different capacities to utilize plant P because of their different intestinal absorption characteristics and differences in endogenous P losses from the GIT among species. Phytate hydrolysis in the intestines also depends on the phytase activity of the feed ingredients (Eeckhout and De Paepe, 1994), intestinal pH (Simon and Igbasan, 2002), microflora (Kerr et al., 2000), and endogenous phytase activity (Maenz and Classen, 1998; Applegate et al., 2003), suggesting that feeding the same phytase concentrations can provoke different responses among species. The responses of different poultry species to phytases in the diet are commercially very important for optimizing the use of phytases for the individual species.
The objective of this experiment was to directly compare the responses of young broiler chickens and turkeys to different dietary phytase concentrations. Performance, AME, SA excretions, and ileal villus morphology of 21-d-old broiler chickens and turkeys were determined.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The study was approved by The Animal Experimental Committee of SAC.
Diet Formulations
Eight cornsoy-based diets were prepared and were supplemented with an evolved Escherichia coli-derived phytase. Diets were manufactured to be nutritionally adequate for each of the species but were designed to have a relatively low P content (Table 1
). The basal diets were supplemented with exogenous phytase (Quantum 2500 D, EC 3.1.3.26
[EC]
; Syngenta Biotechnology Inc., Research Triangle Park, NC) 250, 500, or 2500 phytase units (FTU; phytase activity, units per kilogram of diet), respectively. The enzyme was added to the diets in powder form by serial dilution, and all diets were fed as mash. All diets were supplied with 5 g/kg of TiO2 as a marker.
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Nine hundred sixty 1-d-old birds (480 female Ross 308 broiler chickens; Grampian Ltd., Whitburn, UK; and 480 female BUT 6 turkeys; Europolt, Manchester, UK) were obtained from 2 commercial hatcheries. The birds were allocated to 64 floor pens (193 x 126 cm floor area) from 0 to 21 d of age. Fifteen birds were placed in each pen after the initial weight was recorded. Birds were allocated on a stratified weight basis (minimizing the differences in weights of birds within a block). The study was conducted in a split-plot design, because all pens were allocated to 6 rooms in the same house, 3 for turkeys and 3 for chickens. Each dietary treatment was replicated 8 times. Feed and water were available ad libitum throughout the growing experiment. The experimental rooms were equipped with a positive pressure ventilation system to meet commercial recommendations. During the study, the temperature was initially 33 ° C and was gradually reduced to 20 ° C after turkeys were 20 d of age. The RH was maintained between 50 and 70%. The light regimen was 23 h of light and 1 h of dark. The average growth performance for each floor pen was also determined at 21 d of age.
Wire-meshed metabolic cages equipped with trays were used for excreta collection. At d 21, 3 birds from each floor pen were placed in a metabolic cage for 1 h and the excreta were collected from the trays.
DM Digestibility and ME Determination
The analytical methods used in this work were described elsewhere and are summarized briefly here (Cowieson et al., 2006a,b). Excreta were freeze-dried, milled (0.75 mm mesh), analyzed for Ti in feed, and analyzed by inductively coupled plasma spectrometry (Optima 4300 DV dual-view ICP-OE spectrometer; PerkinElmer, Beaconsfield, UK) after sulfuric acid digestion. The gross energy (GE) of feed and excreta was determined using a bomb calorimeter (Parr-1755; Parr Instrument Company, Moline, IL). The DM digestibility coefficient (DMD) was calculated using the following equation:
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where Tiexcreta and Tifeed are the concentrations of Ti in the excreta and diet, respectively. Apparent ME calculations were based on DMD results:
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SA Determination
The concentration of fecal SA was determined by the periodate-resorcinol method as described by Jourdian et al. (1971). The method involves conversion of free and glycosidically bound SA to chromogenic substances by treatment with periodic acid followed by resorcinol. The color of the samples was stabilized by 2-methyl-propan-2-ol, and after centrifugation the absorbance of the supernatant was determined spectrophotometrically at 630 nm (Spectronic 301; Milton Roy Company, Ivyland, PA). This procedure detects total, free, and glycosidically bound N-acetyl neuraminic (sialic) acid.
Ileal Villus Morphometry
On d 21, 1 bird from 4 pens of each treatment was killed by cervical dislocation. Approximately 4 cm of the middle part of the ileum was sampled and stored for 2 wk in 10% formalin-buffered saline. The samples were embedded in paraffin wax, sectioned at approximately 5 µm, and 4 gut segments were fixed in each slide. Morphometric measurements were determined on 20 villi for each slide (stereo binocular microscope, Olympus, Tokyo, Japan; CCD camera and monitor, JVC, Tokyo, Japan; image analysis software, Bioscan Optimas 3.01 for Windows, Bioscan Inc., Edmonds, WA). The length of the villus was represented by the distance from the crypt opening to the tip on the right side of the villus, as explained by Sigleo et al. (1984). Villus thickness was measured at a point one-third from the villus tip.
Statistical Analysis
Statistical analyses were performed using the Genstat VII statistical software package (IACR Rothamstead, Hertfordshire, UK). Comparisons among performance, AME, DMD, SA, and ileal morphometry were performed by ANOVA (following a split-plot design). A regression analysis was used to test the relationships between AME of the feed, BW, dietary enzyme concentration, and excreted SA.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The average weights of the chickens and turkeys at 21 d of age were 670 and 466 g, respectively. Chickens clearly grew faster (P < 0.001; Table 2) and consumed more (P < 0.05) than turkeys throughout the study period.
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). Birds fed enzyme-supplemented diets had better performance (P < 0.001). Average differences between birds fed 2,500-FTU diets and control birds were about 17 and 19% for feed intake and weight gain, respectively. There were positive linear relationships between added dietary enzyme concentrations and feed intake (r2 = 0.40; P < 0.001) and between enzyme concentrations and weight gain (r2 = 0.88; P < 0.001; Table 3). No interaction between species and enzyme supplementation was detected. Chickens were more efficient (P < 0.05) in utilizing the feed compared with turkeys. However, turkeys fed the 500-FTU diet had a 15% higher (P < 0.05) feed efficiency (FE) compared with turkeys fed the control diet. It is interesting that supplementing chick diets with phytase did not change the FE, whereas the FE for turkeys was improved (7.6% overall).
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The concentration of excreted SA was higher (P < 0.05) in chickens than in turkeys (Table 2), and no relationship was found with enzyme concentrations, but there was a significant species x enzyme interaction (Table 2). For turkeys, the highest phytase activity (2,500 FTU) also apparently tended to increase the SA secretions by turkeys compared with birds fed diets with lower phytase activities. To reduce the effect of variation in DM intake, SA excretion was presented on a metabolic BW basis (W0.75; Table 2). An interaction between species and phytase was also detected for SA excretion per W0.75. It is notable that the loss of SA (W0.75) was higher from turkeys than chickens in the absence of phytase, whereas supplementation with phytase (250 and 500 FTU) reduced (P < 0.05) the excretion of SA in turkeys. A negative linear relationship was observed between AME and SA excretion (r2 = 0.62, P < 0.001; Figure 1). Although there was a trend (P = 0.196) for chickens to have longer villi compared with turkeys, enzyme supplementation did not significantly affect ileal villus morphometry of the 2 species (Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) that both species benefited from dietary phytase, and there was a linear relationship (P < 0.001) between feed intake, weight gain, and phytase activity. However, the patterns of response of chickens and turkeys seemed to differ. Adding phytase to the diets of chickens improved the feed intake by 11.2%, compared with only 6.5% for turkeys (Table 2). Interestingly, on average, turkeys responded somewhat better to phytase with respect to weight gain (13.2%) than did chickens (10.2%). Phytase did not improve the FE in chickens, but on average, turkeys benefited from the enzyme by 7.6% compared with birds fed the control diet. Exogenous phytase increased the dietary AME by 1.4% for chickens and 5.7% for turkeys. Overall, phytase improved the AME intake of chickens by 13.1% and that of turkeys by 9.8%. However, AME intake is a function of feed intake and dietary AME, and the effect of phytase on intake was evidently of greater importance in chickens than in turkeys, where the direct effect of phytase on AME seemed to play a larger role. This may be due to differences in the AME of the diets compared with the AME recommended by breeders. However, it also may be due to differences in the responses of the different species and may indicate that phytase supplementation of turkey diets needs to be considered separately from that of chickens. It should be noted that the AME determined in the diets for chickens was higher than expected and thus may have limited improvements in performance. However, the extent of AME improvement (P = 0.003) upon phytase supplementation in this study is in agreement with previously published results for a corn-soy feed (Pirgozliev et al., 2005; Cowieson et al., 2006b). The quadratic relationship between dietary AME and phytase concentrations (r2 = 0.43; P < 0.001) was similar to the pattern reported by Cowieson et al. (2006b). In the study reported here, birds fed diets with 500 FTU had higher overall AME, which is in accordance with the results of Cowieson et al. (2006b), who reported a maximum AME at 150 to 300 FTU, although phytase concentrations varied between 150 and 24,000 FTU. This pattern of AME response to phytase was explained by the reduced capacity of lower molecular weight inositol phosphate esters to interact with starches and stimulate an increase in endogenous losses (Cowieson et al., 2004a,b). The consequence of this is that most of the improvements in AME associated with phytase addition may be expected to be achieved by the removal of 1 or 2 phosphate groups, with further improvements increasingly unlikely as each subsequent phosphate is cleaved (Cowieson et al., 2006b). Although logical, however, this theory cannot explain the linear relationship between phytase activity and improved bird performance in this study. The theory of enzymatic breakdown of phytate compounds distinguishes between liberation of phytate molecules from complexes with other tissue components and enzymatic cleavage of phosphate residues on the myo-inositol ring (Zyla et al., 2004). The stepwise manner of dephosphorylation of IP6 (Venekamp et al., 1995; Greiner et al., 2000) will lead to a release of different myo-inositol isomers and phosphates. The conditions in the GIT of poultry are not ideal for phytase to function effectively, so the exogenous phytase will not be able to completely dephosphorylate dietary phytates. However, the increased amount of different myo-inositol isomers should be considered. Waagbo et al. (1998) demonstrated that feeding fish inositol-supplemented diets improved their performance. However, dietary myo-inositol did not provide an advantage when P-sufficient diets were fed to broiler chickens (Pearce, 1975; Zyla et al., 2004). Interestingly, when myo-inositol was added to P-deficient diets, it improved the weight gain of the birds (Zyla et al., 2004), suggesting that myo-inositol by itself acts as a growth promoter. The existence of inositol phosphates in biology has been known for over 80 yr (Posternak, 1919), and its biological importance is well documented (Irvine and Schell, 2001; Beemster et al., 2002; Fisher et al., 2002). However, there is a lack of information about the effect of inositol on nutrient availability and performance when fed to poultry. The performance results from this study suggest that increased phytase concentrations linearly increased the dephosphorylation of IP6, providing more myo-inositol isomers of lower relative molecule mass. The linear relationship between dietary enzyme concentrations and weight gain (r2 = 0.88; P < 0.001)—because birds fed 2,500 FTU increased their weight gain by 19% compared with control—may be due to the reduced adverse effects of phytate but also to beneficial effects of inositol esters of lower relative molecule mass. Although the role of inositol in nutrition is not well understood, the results from this study support previous findings (Zyla et al., 2004). To study the mode of action of dietary phytases, more attention should be paid to myo-inositol isomers released after phytase activity.
In this study, no relationship was found between dietary AME and performance. It is not unusual that AME values do not adequately predict the nutritive quality of poultry feeds. Rose and Bedford (1995) and Ravindran et al. (1999) also did not observe a correlation between dietary AME and performance when broilers were fed plain or enzyme-supplemented wheat-based diets. The main criticism of AME is that it does not account for intermediate metabolism (Chudy, 2000). The lack of relationship between bird performance and dietary AME in this study supports the previous finding that although convenient, ME is not the best way to evaluate the nutritive quality of poultry feed (De Groote, 1974; Hoffmann and Schiemann, 1980; Emmans, 1994; Pirgozliev and Rose, 1999). Better performance of the birds fed phytase suggests that these birds can utilize more energy and protein from the feed; however, this cannot be detected by dietary AME, which suggests that intermediary metabolism is affected by phytases, thereby increasing dietary available (net) energy. This hypothesis is also supported by the average increase of the metabolizability coefficients of GE by 1.4 and 2.1% for broilers and turkeys, respectively, in comparison with a 13% improvement in weight gain for both species.
The results from ileal villus morphometry were similar to previous work (Langhout et al., 1999; Pirgozliev et al., 2001). Although not significantly different (P > 0.05), we noted that birds fed 500 FTU had a slightly increased villus width compared with all other birds. In support of this observation, Koutsos et al. (2005) also reported that when diets containing 600 FTU were fed to laying hens, there was an increase in the duodenal villus width. Perhaps the phytase influences the type of microflora within the GIT (Steer and Gibson, 2002), and thus their adherence and any damage that they may cause to the GIT.
The results for SA were in the ranges reported elsewhere (Cowieson et al., 2004b; Pirgozliev et al., 2005). Although no direct correlation was detected between SA excretion and supplemented phytase activities, there was a significant negative relationship between dietary AME (r2 = 0.62; P < 0.001) and excreted SA per kilogram of metabolic BW. The negative relationship with dietary AME in this study suggests that phytase supplementation improves gut health. Mucins are rich in nitrogen and amino acids, so increased mucin production is nutritionally very expensive and inevitably leads to increased endogenous loss (Nyachoti et al., 1997) while increasing the energy for maintenance. It also should be mentioned that the untreated phytate in the control diets is likely to reduce the release of dietary minerals, starch, and amino acids within the GIT and leads to an unbalanced nutrient supply in the intestinal lumen. Cowieson et al. (2004b) demonstrated that phytic acid, a major component of dietary phytate, irritates the GIT and increases the excretion of endogenous amino acids, minerals, and SA. Phytates may irritate the gut wall directly or by enhancing the growth of intestinal microflora, causing inflammation and provoking further immune response and increased production of cytokines (McKay and Baird, 1999). However, activation of the immune system of the bird (e.g., increased production of antibodies in response of invading agents) is another energy-demanding process (Klasing, 1998; Klasing and Calvert, 1999; Eraud et al., 2005). It has also been suggested that a strong immune response will increase the risk of tissue damage (Svensson et al., 1998), increase the production of free radicals (Finkel and Holbrook, 2000), and cause further deleterious effects on the organism. Karadas et al. (2005, 2006) reported that broilers fed phytase-supplemented diets had reduced hepatic oxidative stress and increased antioxidant status compared with birds fed nonsupplemented diets low in P. It may be that the combination of an irritant and an unbalanced supply of nutrients will compromise the gut health of the birds and may help to explain the negative relationship (P < 0.001) between AME and excreted mucin, measured as SA in this study. However, it is difficult to explain the increased excretion of SA in turkeys fed 2,500 FTU although this tendency was also observed in chickens; it may be a function of feed intake and thereby confound the interpretation. Even though the mechanism is not clear, dietary phytases may be capable of reducing the immune response and production of free radicals in broiler chickens.
It can be concluded that both chickens and turkeys can tolerate phytase activities much higher than 1,000 FTU and that these have further beneficial effects compared with lower phytase activities. The results of this study show that dietary phytase influences secretions from the GIT and that these secretions are species and phytase activity dependent, which may alter gut health. However, to increase our knowledge regarding the mode of action of dietary phytases, further research on lower molecular mass myo-inositol isomers, immune response, and antioxidant stress should be completed. Furthermore, the work reported here supports the hypothesis that supplementation of turkey diets needs to be considered independently from supplementation of chicken diets so that optimal responses can be obtained.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received for publication June 29, 2006. Accepted for publication November 18, 2006.
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