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METABOLISM AND NUTRITION |
* Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand; Danisco Animal Nutrition, Marlborough, Wiltshire SN8 1XN, United Kingdom; and Faculty of Veterinary Science, The University of Sydney, Camden, New South Wales 2570, Australia
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: broilers • dietary electrolyte balance • excreta quality • microbial phytase • nutrient utilization
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The inclusion of microbial phytase in poultry diets, as reviewed by Selle and Ravindran (2007), is now an increasingly common practice. Although originally introduced to reduce P excretion in pigs and poultry and the P load on the environment (Ravindran et al., 1995), several recent studies have shown that exogenous phytase also enhances protein and energy utilization (Ravindran et al., 1999; Selle et al., 2000; Cowieson et al., 2006a,b). However, the extent to which microbial phytase improves available energy and amino acid digestibility is variable, and the factors responsible for the observed variability in nutrient utilization have not been completely identified. Factors that may contribute to this variability are complex and include, inter alia, concentrations and sources of dietary phytate, dietary concentrations of Ca, nonphytate P, and amino acids, and the level and type of added phytase and, in phytase amino acid digestibility assays, dietary marker selection (Ravindran et al., 1999, 2000; Selle et al., 2006; Selle and Ravindran, 2007). This is complicated by the fact that the underlying mechanisms whereby phytate and phytase influence protein and energy utilization are not well understood.
However, Cowieson et al. (2004) recently demonstrated that phytate and phytase alter secretion patterns of Na in the gastrointestinal tract of broiler chickens. In this study, aqueous phytic acid increased Na excretion from 39 to 155 mg/bird per 48 h, whereas phytase reduced Na excretion from 155 to 86 mg/bird per 48 h. Moreover, similar patterns were subsequently demonstrated at the ileal level (Ravindran et al., 2006). It is therefore probable that added phytase modifies the effective DEB of practical diets. By definition, Na affects the DEB level, and the fact that phytate induces the transfer of Na into the gut lumen implies that phytate is effectively influencing the DEB of intestinal digesta. Consequently, it follows that phytate may compromise the efficiency of Na-dependent transport systems and the intestinal uptake of amino acids and glucose by this depletion of Na, which is counteracted by phytase. Therefore, it has been proposed that DEB may influence the magnitude of amino acid responses to exogenous phytase (Selle and Ravindran, 2007; Selle et al., 2007).
The importance of DEB in relation to excreta moisture and litter quality is well recognized, as discussed by Francesch and Brufau (2004). In addition, there is empirical field evidence that the inclusion of microbial phytase in broiler diets may be associated with wet litter problems (Debicki-Garnier and Hruby, 2003; Pos et al., 2003).
The aim of the present study was to examine the possible interaction between DEB and microbial phytase in broiler starter diets. The effects on the performance, AME, and apparent ileal digestibility (AID) of amino acids, and apparent ileal availability and total tract retention of selected minerals were determined. The possible interactive effects of DEB and phytase on excreta quality were also examined.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Diets
The study was conducted as a 2 x 4 factorial arrangement of treatments to evaluate the effects of 4 DEB levels (150, 225, 300, and 375 mEq/kg of diet) on responses to supplemental phytase [Phyzyme XP, Danisco Animal Nutrition, Marlborough, UK; 500 phytase units (FTU)/ kg of diet] in corn-, soybean meal-, and canola meal-based broiler starter diets. One FTU is defined as the quantity of enzyme that releases 1 µmol of inorganic P/min from 0.00015 mol/L of sodium phytate at pH 5.5 and 37°C in a buffer solution containing acetic acid, sodium acetate, and calcium chloride (Engelen et al., 1994).
The diets were based on corn, soybean meal, and canola meal and were formulated to meet or exceed the requirements for nutrients for broiler starters, except Ca, total P, and nonphytate P (Table 1
). The nonphytate P level was maintained at 1.5 g/kg below the current NRC (1994) recommendations. The Ca:total P ratio was maintained at a ratio of 1.3:1. The DEB was manipulated by the use of ammonium chloride and sodium bicarbonate (NaHCO3). Titanium oxide (3 g/kg) was added to all diets as an indigestible dietary marker.
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One-day-old male broiler (Ross 308) chicks were obtained from a commercial hatchery. The chicks were individually weighed and assigned to 48 cages (8 birds per cage) in battery brooders so that the average weight per cage was similar. Each of the 8 dietary treatments was then randomly assigned to 6 cages. On d 12, the birds were transferred to grower cages and maintained in these cages until the termination of the trial on d 21. The brooders and grower cages were housed in an environmentally controlled room. The birds received constant fluorescent illumination. The temperature was maintained at 31°C on d 1 and then gradually reduced to 22°C by 21 d of age. The treatment diets were offered for ad libitum consumption. Water was freely available throughout the trial period.
Body weights and feed intake were recorded weekly. Mortality was recorded daily. Any bird that died was weighed and the weight was used to adjust the feed conversion ratio. Feed conversion ratios were calculated by dividing total feed intake by weight gain of live plus dead birds. On d 21, representative samples of freshly voided excreta were obtained for DM determination and also scored for wetness on a scale of 1 to 5 (with 1 representing normally formed excreta and 5 representing very watery excreta).
Collection and Processing of Samples
From d 17 to 20 posthatching, feed intake and total excreta output were measured quantitatively per pen over 4 consecutive days for the determination of AME. Daily excreta collections were pooled and mixed in a blender. Representative subsamples were then obtained and lyophilized. Dried excreta samples were ground to pass through a 0.5-mm sieve and stored in airtight plastic containers at –4°C until chemical analyses were performed. Samples of diets and excreta were analyzed for DM, gross energy, N, and selected minerals (Ca, P, K, Na, and Cl).
On d 21, all birds were killed by intracardial injection of sodium pentobarbitone and the small intestine was immediately exposed. Contents of the lower ileum were expressed by gentle flushing with distilled water into plastic containers. The ileum was defined as that portion of the small intestine extending from the Meckel’s diverticulum to a point approximately 40 mm proximal to the ileo-cecal junction. The ileum was divided into 2 halves and the digesta was collected from the lower half toward the ileo-cecal junction (Ravindran et al., 2005). Ileal digesta from birds within a pen were pooled, resulting in 6 samples per dietary treatment. The digesta samples were frozen immediately after collection and subsequently lyophilized. Dried excreta and ileal digesta samples were ground to pass though a 0.5-mm sieve and stored at –4°C before chemical analysis. Samples of diets and ileal digesta were assayed for DM, N, amino acids, gross energy, selected minerals (Ca, P, K, Na, and Cl), and Ti.
Chemical Analysis
Dry matter determination was carried out according to standard procedures (AOAC International, 2005; method 930.15). Gross energy was determined by using an adiabatic bomb calorimeter (Gallenkamp, London, UK) standardized with benzoic acid. Nitrogen content was determined by the combustion method (AOAC International, 2005; method 968.06) with a CNS-2000 C, N, and S analyzer (Leco Corporation, St. Joseph, MI). The samples were wet acid digested with a nitric and perchloric acid mixture, and concentrations of minerals were determined at specific wavelengths for each element (Ca, 393.3; P, 185.9; Na, 589.5; and K, 766.4 nm) by inductively coupled plasma-optical emission spectroscopy with a Thermo Jarrell Ash IRIS instrument (Thermo Jarrell Ash Corporation, Franklin, MA). The instrument was calibrated against standards (Junsei Chemical Co. Ltd., Tokyo, Japan) of known concentration. Chloride concentrations were determined by using the potentiometric titration method (Harris, 2002). This method uses the precipitation of silver chloride when chloride anion is combined with silver cation, and the measurement of voltage change as the titration proceeds and ions are being consumed. Titanium content was measured on a UV spectrophotometer following the method of Short et al. (1996).
Amino acids were determined by hydrolyzing the samples with 6 N HCl (containing phenol) for 24 h at 110 ± 2°C in glass tubes sealed under vacuum. Amino acids were detected on a Waters ion-exchange HPLC system, and the chromatograms were integrated by using dedicated software (Millenium, version 3.05.01, Waters, Millipore, Milford, MA) with the amino acids identified and quantified by using a standard amino acid mixture (product no. A2908, Sigma, St. Louis, MO). The HPLC system consisted of an ion-exchange column, two 510 pumps, a Waters 715 ultraWISP sample processor, a column heater, a postcolumn reaction coil heater, a ninhydrin pump, and a dual-wavelength detector. Amino acids were eluted by a gradient of pH 3.3 sodium citrate eluent to pH 9.8 sodium borate eluent at a flow rate of 0.4 mL/min and a column temperature of 60°C. Cysteine and Met were analyzed as cysteic acid and Met sulfone, respectively, by oxidation with performic acid for 16 h at 0°C and neutralization with hydrobromic acid before hydrolysis.
Calculations
The AME values of the diets were calculated by using the following formula, with appropriate corrections for differences in moisture content:
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Nitrogen-corrected AME values were determined by correction for zero N retention by simple multiplication with 8.73 kcal/g of N retained in the body as described by Hill and Anderson (1958).
The AID of N and amino acids, and the apparent ileal availability of minerals were calculated with the following formula by using the Ti marker ratio in the diet and ileal digesta: apparent ileal digestibility/availability of nutrients =
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where (NT/Ti)d is the ratio of nutrient and Ti in the diet, and (NT/Ti)i is the ratio of nutrient and Ti in ileal digesta.
Statistical Analysis
Two-way ANOVA was used to determine the main effects (DEB and phytase) and their interaction by using the GLM procedure of SAS (SAS Institute, 1997), with cage as the experimental unit. Differences were considered significant at P < 0.05, although P-values between 0.06 and 0.10 are mentioned in the text if the data suggested a trend. When a significant F-test was detected, means were separated by using the least significant difference test (Snedecor and Cochran, 1967).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. The concentrations of electrolyte minerals (Na, K, and Cl) in the drinking water could influence and alter the DEB of ingesta, and for this reason, the concentrations in the water were analyzed. The results showed that the concentrations in the water were too low to have any impact on the DEB.
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Mortality was not influenced by DEB or supplemental phytase. Mortality during the trial was low (1.8%; data not shown). The effects of treatments on the performance of broilers are summarized in Table 4. The main effect of DEB was significant (P < 0.05) for weight gains and feed per gain. Feed intake was unaffected (P > 0.05) by increasing DEB levels. Increasing the DEB values from 150 to 300 mEq/kg had no effect on weight gains and feed per gain, but the gains were lowered and the feed per gain was increased at 375 mEq/kg. Supplemental phytase improved (P < 0.05) the weight gains and feed intake at all DEB levels, as shown by the lack of a DEB x phytase interaction (P > 0.05). Feed per gain was lowered (P < 0.05) by phytase addition. However, a tendency for a DEB x phytase interaction (P = 0.06) was also observed, indicating that the responses to phytase supplementation may be affected by DEB level. The responses in feed per gain were greater at the lowest DEB level, and phytase addition had no effect on feed per gain at the highest DEB level.
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). The excreta DM were greater at 150 and 225 mEq/kg and lowered at the higher levels. Excreta DM was lowest at 375 mEq/kg. Supplemental phytase had no effect (P > 0.05) on excreta scores and excreta DM.
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). Increasing DEB levels had no effect on these criteria to 300 mEq/kg. At 375 mEq/kg, N retention, AMEn, and N digestibility were found to be lowered. Phytase addition improved the AMEn values, but the improvements were observed only at the first 3 DEB levels, and there was no response at 375 mEq/kg. The improvements in AMEn with 500 U/kg of phytase addition in 150, 225, and 275 mEq/kg of DEB were 53, 60, and 38 kcal/kg of DM, respectively. Similarly, for the AID of N, despite significant phytase effects, phytase responses were noted only at the first 3 DEB levels, and added phytase had no effect at a DEB level of 375 mEq/kg. However, the DEB x phytase interaction was not significant (P > 0.05) for both AMEn and the AID of N.
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. The main effect of DEB was significant (P < 0.05) only for the availability of Na and Cl, whereas added phytase influenced (P < 0.05) the availability of all minerals. Increasing DEB levels increased the availability of Na, with the availability coefficients changing from negative to positive at levels above 300 mEq/kg. The availability of Cl was observed to be lowered with increasing DEB levels. Supplemental phytase improved the availability of all determined minerals. The interaction between DEB and phytase was significant (P < 0.05) only for P. The availability of P was increased at all DEB levels, but the increment was greater at the 300 mEq/kg level.
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. Retentions of Ca and P were influenced (P < 0.05) by DEB, phytase, and the DEB x phytase interaction. Phytase addition improved Ca retention at DEB levels of 150 and 225 mEq/kg but had no effect at the higher DEB levels. Phytase improved P retention at all DEB levels, but the magnitude of response was varied at different DEB levels. Total tract retention of Na was influenced (P < 0.05) by DEB, with retention being lowered at DEB levels higher than 225 mEq/kg and becoming negative at 375 mEq/kg. Dietary electrolyte balance had no effect (P > 0.05) on K retention. The retention of Cl increased (P < 0.05) with increasing DEB levels. Phytase addition had no effect (P > 0.05) on the retention of Na and Cl.
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). In general, the AID of amino acids were unaffected when the DEB level was increased from 150 to 225 mEq/kg of diet, but decreased at the 300 and 375 mEq/kg levels. The average amino acid digestibility decreased by 2.6 and 4.4 percentage units when the DEB was increased from 225 to 300 and 375 mEq/kg of diet, respectively. Phytase addition improved (P < 0.05) AID of 13 of the 17 amino acids assessed. The exceptions were the digestibility of Ala (P = 0.06) and Phe (P = 0.08), which tended to be improved, whereas those of Tyr and Met were unaffected by added phytase. Although phytase effects were significant, the improvements in average AID of amino acids were noted only at DEB levels of 150 and 225 mEq/kg, and there was no response at higher DEB levels. However, the DEB x phytase interaction was determined to be not significant (P > 0.05).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In addition, DEB significantly influenced the AID of 13 of the 17 amino acids assessed. There are, however, few published reports of the effects of DEB on amino acid digestibility in pigs and poultry. In poultry, Balnave and Oliva (1991), using NaHCO3 to increase the DEB of broiler diets from approximately 180 to 380 mEq/kg, found that this manipulation improved the AID of cystine and His but had only negligible effects on other amino acids. In pigs, conflicting findings have been reported. In grower pigs, Patience et al. (1986) investigated the effects of increasing DEB in the basal diet from 127 mEq/kg to 349 and 466 with NaHCO3 and to 486 mEq/kg with KHCO3 addition. Increasing DEB did not significantly influence the AID of Lys and Trp, which were the only amino acids assessed. Haydon and West (1990) increased the DEB in diets for grower pigs from –50 to 400 mEq/kg, mainly by NaHCO3 addition. This increase in DEB enhanced the AID of 16 amino acids by an average of 6.1% (0.755 vs. 0.800). In contrast, Blank et al. (1999) increased the DEB from 225 to 640 mEq/kg, via the addition of 30 g/kg of NaHCO3, in diets offered to weanling pigs. These authors reported that this increase in DEB reduced ileal digestibility of amino acids by an average of 5.9% (0.780 vs. 0.734), a finding that is in general agreement with that of the present study. Collectively, these reports indicate that DEB has the capacity to influence the digestibility of amino acids in pigs and poultry, but the findings are inconsistent.
In broiler starter diets containing 3.0 g/kg of nonphytate P, supplemental phytase significantly improved growth performance, AMEn, ileal digestibility and total tract retention of N, ileal availability of Ca, P, Na, K, and Cl, and total tract retention of Ca, P, and K. Although the overall outcomes of phytase amino acid digestibility assays in broilers have been ambiguous (Selle et al., 2006), phytase addition significantly increased the ileal digestibility of 13 of the 17 amino acids assessed in the present study. The positive effects of phytase on energy and protein utilization observed in our study are consistent with previous studies reporting improvements in AME (Namkung and Leeson, 1999; Camden et al., 2001; Ravindran et al., 2006) and ileal amino acid digestibility (Kornegay et al., 1999; Camden et al., 2001; Ravindran et al., 2006) in broilers. In contrast, in other studies, amino acid responses to phytase in birds fed corn-soybean meal diets have been negligible (Dilger et al., 2004; Onyango et al., 2005). Some of the factors that may contribute to the reported variable responses have been discussed by Selle and Ravindran (2007), and these include, inter alia, ingredient type, dietary Ca and nonphytate P levels, inclusion level and source of phytase, source and concentrations of phytate in the diet, and the choice of inert marker used in digestibility assays.
The hypothesis of this study was that DEB influences responses to phytase supplementation for amino acid digestibility, because phytate and phytase alter patterns of Na secretion into the gut lumen, which effectively influence the DEB. However, significant DEB x phytase treatment interactions in amino acid digestibility and AMEn responses were lacking in the present study. It is relevant that the interaction for feed efficiency approached significance (P = 0.06). At a DEB of 150 mEq/kg, phytase improved feed efficiency by 5.1% (1.455 vs. 1.381), but at a DEB of 375 mEq/kg, phytase depressed feed efficiency by 0.9% (1.476 vs. 1.489). This tendency for an interaction does suggest that phytase is likely to be more effective at a low DEB. It is also noteworthy that, although the main effects of phytase on AMEn and AID of amino acids were significant, close examination of the data clearly showed that there were no phytase responses at the highest DEB level. It is evident that improvements in AMEn and ileal amino acid digestibility were observed only at DEB levels of 150 to 300 mEq/kg, and there was no effect at a DEB level of 375 mEq/kg. Therefore, although the hypothesis tested was not conclusively established, the observed pattern suggested that phytase is less likely to enhance the ileal digestibility of amino acids in diets with high DEB.
In the present study, DEB was essentially manipulated by the addition of NaHCO3. Consequently, it is not clear whether the treatment effects and interactive trends observed are due primarily to the variations in DEB or to differences in dietary concentration of Na per se. Because the second proposition may also be valid, a consideration of the effects of dietary Na concentrations on amino acid digestibilities is merited.
Sodium is known to play a critical role in the intestinal uptake of amino acids because of its involvement in Na-dependent transport systems and Na+,K+-adenosine tri-phosphatase (ATPase) activity, or the "sodium pump" (Glynn, 1993). Sklan and Noy (2000) fed broilers diets containing either 0 or 3.7 g/kg of NaCl and found that Na+,K+-ATPase activity was strongly depressed in birds offered the low-Na diet, with only minimal uptake of nutrients in all intestinal segments. The researchers concluded that Na plays a central role in the uptake of nutrients that would be compromised by low levels of dietary Na. In addition, Gal-Garber et al. (2003) experimented with maize-soybean meal broiler diets containing 0, 2.2, and 5.5 g/kg of NaCl and suggested that the determination of expression and activity of Na+,K+ATPase would permit clarification of the role of Na in intestinal uptake of nutrients. Therefore, it is relevant that dietary phytate has been shown to significantly depress Na availability at the ileal level, evidently by stimulating the movement of Na into the intestinal lumen. Increasing the phytate level from 2.8 to 3.8 g/kg of phytate P has been shown to reduce the ileal availability coefficient of Na from –0.237 to –0.379, and this was counteracted by 1,000 FTU/ kg of phytase, which increased the Na availability coefficient from –0.515 to –0.177 (Ravindran et al., 2006). However, the mechanism whereby phytate draws Na into the gut lumen is not clear, although it seems possible that phytate stimulates NaHCO3 secretion to buffer gut pH. Irrespective of the mechanism whereby phytate increases gut Na levels, it does appear that phytase has a Na "sparing" effect in the small intestine of broilers.
As noted earlier, it seems reasonable to assume that phytase facilitated both amino acid and Na small intestinal uptakes in diets with lesser DEB values, which was negated at the highest DEB level. In association with this, phytase increased ileal availability of Na from an average of –0.236 to –0.080 at the lower DEB levels of 150 and 225 mEq/kg, whereas at the highest DEB level of 375 mEq/kg, the impact of phytase on ileal Na availability was less pronounced (0.455 vs. 0.490). This association suggests that there may be a tangible relationship between phytase-induced increases in the digestion or absorption of Na and amino acids. If so, this would be consistent with the proposal that phytase may spare Na by reducing sodium bicarbonate secretion in the duodenum (Cowieson et al., 2006a).
The effects of DEB and dietary Na concentrations on ileal availability and total tract retention of Na are noteworthy. Increasing DEB levels significantly increased ileal availability of Na but significantly reduced Na retention on a total tract basis. This clearly demonstrates a fundamental alteration to the patterns of Na secretion and absorption in the small intestine and hindgut of poultry in response to DEB. However, the paradoxical aspect is that increasing the DEB beyond 225 mEq/kg increased ileal Na digestibility but depressed amino acid digestibility. Oviedo-Rondon et al. (2001) concluded that the Na requirement for starter broilers offered corn-soybean meal diets is 2.8 g/kg, which indicates that the Na concentration of 5.2 g/kg in the 375 mEq/kg DEB diets was excessive. It is possible that this excess of dietary Na triggered its absorption from the small intestine as a homeostatic mechanism that was independent of nutrient uptakes.
Weight gain and feed efficiency of broiler chicks were unaffected by increasing DEB from 150 to 300 mEq/kg, but were markedly reduced at 375 mEq/kg. Feed intake, however, was not influenced by DEB, indicating that the poor performance at 375 mEq/kg is mediated by changes in nutrient absorption and metabolism, which is consistent with the lower AMEn and amino acid digestibilities observed at the highest DEB level. These data suggest that the broiler chickens are able to adjust their body acid-base balance over a relatively wide range of DEB. Similar results have been reported by Mongin and Sauveur (1977), Johnson and Karunajeewa (1985), and Borges et al. (2003b).
The ileal availabilities of Ca and P were unaffected by DEB levels but, as expected, were improved by phytase addition. These findings confirm the efficacy of the microbial phytase used in releasing phytate-bound P and Ca. In general, the improvements in ileal P availability coefficients determined with added phytase were comparable to those reported by Dilger et al. (2004) and Ravindran et al. (2006). It is instructive that the liberation of P from phytate was not reduced by DEB. This finding suggests that phytase maintained its ability to dephosphorylate phytate even at 375 mEq/kg, presenting further evidence that DEB may influence nutrient absorption patterns rather than hydrolysis.
Increasing DEB levels resulted in reductions in excreta DM and in poorer excreta scores. This effect is likely due to increasing dietary Na levels, consistent with previous reports (Murakami et al., 1997; Oviedo-Rondon et al., 2001; Borges et al., 2003a). Excess Na is known to elicit various physiological responses in the body, including increased water consumption, excretion of excess Na via the urine, and lowered kidney glomerular filtration ratio, culminating in increased moisture content of excreta voided (Freeman, 1983). Reduction in the apparent total tract retention of Na with increasing DEB levels is indicative of excess Na excretion and consistent with poor excreta quality. Anecdotally, there is the suggestion that increased dietary K levels, with the transition from animal to plant protein sources, may be the cause of wet-litter problems that have been associated with phytase supplementation. In the present study, however, supplemental phytase had no effect on excreta moisture or excreta scores.
In summary, the present data demonstrate that the adverse effects of high DEB levels on bird performance are related to compromised energy and protein utilization. The influence of microbial phytase on ileal amino acid digestibility in poultry remains a controversial issue, and the factors contributing to the variable phytase responses remain to be elucidated (Selle and Ravindran, 2007). The present data also suggest that microbial phytase is more likely to enhance amino acid digestibilities in a context of low DEB. It follows that this may be a contributing factor to the inconsistent phytase responses in amino acid digestibility assays reported in the literature. The present data also indicate that the impact of phytate and phytase-induced movements of Na into the small intestinal lumen may influence the absorption of dietary amino acids and the reabsorption of endogenous amino acids (Cowieson et al., 2004; Cowieson and Ravindran, 2007) because of the involvement of Na in intestinal uptakes of amino acids and other nutrients. In practical terms, because phytase has been shown to improve ileal Na availability, it may be beneficial to assign Na matrix values to phytase to capture economic savings and to increase the magnitude of responses following phytase supplementation.
Received for publication June 14, 2007. Accepted for publication December 21, 2007.
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