J APPL POULT RES 2008. 17:331-339. doi:10.3382/japr.2007-00111
© 2008 Poultry Science Association
Effects of a New Recombinant Phytase on the Performance and Mineral Utilization of Broilers Fed Phosphorus-Deficient Diets
J. P. Zhou*,
Z. B. Yang*,1,
W. R. Yang*,
X. Y. Wang
,
S. Z. Jiang* and
G. G. Zhang*
* Department of Animal Sciences and Technology, and College of Life Sciences, Shandong Agricultural University, Tai-an, Shandong, P. R. China, 271018
1 Corresponding author: yangzb{at}sdau.edu.cn
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SUMMARY
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An experiment was conducted with Arbor Acres broiler chickens that were fed 3 experimental diets—a control diet containing an adequate level of available phosphorus (AP) and 2 diets that were deficient in AP but supplemented with phytase at a level of either 500 or 750 phytase units/kg—to assess the effects of a novel microbial phytase supplement in broilers fed AP-deficient diets on growth performance and mineral utilization. Similar average daily gain, feed intake, and feed efficiency (P > 0.05) were obtained among broilers fed different diets. Compared with broilers fed the control diet, broilers fed diets with phytase had greater (P < 0.05) retention of Ca, P, and Zn. Moreover, the levels of Cu, Zn, Mg, and Mn in the tibia bone at 28 d of age, and Zn and Mn at 42 d of age in birds fed diets with phytase exceeded (P < 0.05) those of birds fed the control diet. Supplementation of phytase increased Zn and Mg contents in the plasma at 42 d of age. Birds responded similarly to phytase supplemented at a level of 500 or 750 phytase units/kg in terms of growth performance, mineral retention, and mineral content in the serum and bone. Therefore, with the supplementation of this novel phytase, it is possible to reduce the dietary levels of P and other minerals to below the recommended levels of the Feeding Standard of Chicken in P. R. China (ZB B 43005-86).
Key Words: phytase • broiler • growth performance • mineral utilization
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DESCRIPTION OF PROBLEM
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Plants are the main source of dietary P. However, approximately two-thirds of the total P (TP) in plants is in the form of phytate [1, 2] and is therefore unavailable or is poorly utilized by monogastric animals because of the lack of phytase activity in the digestive tract of these animals. Phytic acid in plants also complexes with various nutrients, such as protein, starch, and minerals, which negatively affect the utilization of these nutrients by monogastric animals [3]. Addition of phytase to the diets of poultry has been shown to increase the bioavailability of phytate P [4] as well as to improve the utilization of other nutrients that are bound to plant phytate [5, 6]. However, the efficacies of these improvements have been inconsistent among the reports, largely because of differences in dietary composition, especially P and Ca contents, and the types of phytase supplemented. For a given phytase source, the equivalence of phytase to dietary available P (AP) was shown to be influenced by dietary Ca:TP ratios and the level of AP, and supplementation of phytase was more effective in a diet with a low concentration of AP [7]. Therefore, it would be highly desirable to supplement the low-AP diet with phytase with a high efficiency in releasing phytate P. Although postpelleting application systems can avoid thermostability problems at high pelleting temperatures, inherently heat-stable phytase, which can withstand steam-pelleting, offers several advantages in practice [8]. Recently, we have developed a new recombinant phytase product, and thermostability studies have shown that this recombinant phytase retained 70% activity after exposure to 90°C for 5 min, and retained 65% activity after 30 min [9]. However, its effects on bird performance and nutrient metabolism have not been assessed. The objectives of this study were to evaluate the effects of this novel phytase product on the growth performance and dietary mineral utilization of broilers fed diets that were deficient in P and to test the hypothesis that dietary AP could be reduced to below the level recommended by the Feeding Standard of Chicken (ZB B 43005-86) of P. R. China [10] by supplementing phytase, without a negative effect on bird performance.
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MATERIALS AND METHODS
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Enzyme
The phytase product used in this study was derived from Aspergillus niger phytase (phyA) and was produced in the Laboratory of Life Sciences (Shandong Agricultural University, Tai-an, P. R. China). A phyA gene from a high extracellular phytase-producing A. niger species was cloned and overexpressed in Pichia pastoris GS115 by using the secretive expression vector pPICZaA. After cultivation, the active phytase was secreted as a predominantly extracellular protein. The activity of the expressed phytase in fermented broth was 30,000-fold greater than that of native phytase, with a specific activity of 503 U/mg [9]. One unit of phytase is defined as 1 µmol of inorganic P/min from sodium phytate at pH 5.5 at 37°C.
Experimental Design and Diets
A 48-d experiment was conducted with Arbor Acres (AA) broilers [11] fed 2 phase diets: d 1 to 28 (starter) and d 29 to 48 (finisher). The experiment consisted of 3 treatments for each period: 1) a control diet with sufficient AP (0.45% for the starter diet and 0.43% for the finisher diet) and a diet deficient in AP (0.30% for the starter diet and 0.28% for the finisher diet) but supplemented with phytase at the level of 500 or 750 FTU/kg of DM. Calcium levels in the diets were set at a 1.2:1 ratio of Ca:TP. Diets were formulated to meet the requirements for other nutrients recommended for AA broiler chickens by the Feed Standard of Chicken (ZB B 43005-86; Table 1
) [10]. The ingredients were chosen to meet the desired dietary TP levels, and the dietary nutrient levels were calculated based on the Feed Standard of Chicken (ZB B 43005-86) [10]. After mixing, the diets were pelleted at 90°C for 15 s.
Bird Feeding and Management
A total of 900 AA broiler chickens (1 d of age, both sexes) were obtained from a commercial hatchery. The birds were randomly allocated to 9 pens in an environmentally controlled room (100 birds/pen) and weighed on a pen basis. Light was provided continuously for 24 h with overhead incandescent lighting of 10 lx, and the room temperature was controlled and gradually reduced from 33°C in the first week to 24°C at 48 d. At the beginning of the experiment, birds in 9 pens were randomly assigned to 1 of the 3 dietary treatments [3 pens (replicates) per treatment] and were provided with the starter diet corresponding to the assigned treatment. Feed was provided ad libitum, and the birds had free access to water. Mortalities and health status were observed and recorded daily throughout the entire experimental period. All birds used in this study were cared for in accordance with local ethical guidelines.
Measurements
Growth Performance
Starter diets were fed to d 28 and then changed to finisher diets until the end of the experiment on d 48. On d 28 and 48, the chicks and feed residues were weighed for determination of average daily gain, average daily feed intake, and FCR in each period.
Determination of Mineral Retention
On d 48 of the experiment, 3 chicks with similar BW were selected from each pen, transferred in individual wire cages equipped with water troughs, and housed in the cages for 3 d. The birds were provided with the experimental finisher diet corresponding to what was fed in the pen. After the chicks were housed in the cages, plastic bottle caps were attached around the vents by suture under local anesthesia (local injection of 0.5% Procaine; 2.0 mL/bird) so that a bottle could be attached for excreta collection. This was followed by a 24-h period of fasting (only water was accessible). The suture was conducted by a veterinarian in accordance with the standard guidelines. The birds were then force-fed 40 g of an experimental diet with force-feeding tubes according to the method described by Bilgili et al. [12], and a bottle was attached to the cap to collect all the excreta for the following 24 h. The chicks had free access to water during this 24-h period. At the end of the collection period, the excreta of each bird was oven-dried at 65°C for 48 h, weighed, ground to pass a 1.0-mm screen, and stored in a sealed container for determination of Ca, P, Mn, Zn, and Cu.
Mineral Concentration in Blood Plasma and in Bone Ash
At d 28 and 42 of the experiment, 3 birds were randomly selected from each pen, and a blood sample from each broiler was drawn by puncture of the wing vein. Plasma was obtained by centrifugation (1,360 x g, 10 min, 4°C) of the blood sample and was used for subsequent determination of concentrations of Ca, P, Mg, Zn, and Cu. After blood samples were collected, the birds were killed by cervical dislocation and exsanguination, and the left tibias were immediately removed and cleaned. As described by Brenes et al. [13], the tibias were subsequently dried at 105°C for 12 h, extracted with ether, dried again, and weighed. The dry, fat-free bones were ashed at 550°C, and the ashes were weighed and used to determine concentrations of Ca, P, Mg, Zn, Mn, and Cu.
Chemical Analyses
All chemical analyses were performed in duplicate. Before analysis, feed and excreta samples were ground to pass a 1.0-mm screen and were ashed as described above for bone samples. The Ca, Mg, Zn, Mn, and Cu contents in the ash were determined with an atomic absorption spectrophotometer (SP9-400, PYE, Cambridge, UK). Phosphorus was analyzed by the vanadate colorimetric method with an ultraviolet spectrophotometer (7200, Unic, Shanghai, China). Analyses of Ca, P, Mg, Zn, and Cu in plasma samples were performed with a full-automation biochemistry analyzer (T600-020, RiLi, Tokyo, Japan).
Calculation and Statistical Analysis
Retention of Ca, P, Mn, Zn, or Cu was calculated by using the following equation:
where E1 is the amount (mg) of the element (Ca, P, Mn, Zn, or Cu) in 40 g of diet that was force-fed to each bird and E2 is the amount (mg) of the corresponding element (Ca, P, Mn, Zn, or Cu) in excreta collected during the 24-h period after force-feeding. Percentage of ash was calculated on the basis of dried, fat-free bone, and mineral contents were calculated as the percentage of ash.
Data were analyzed statistically by 1-way ANOVA with PROC MIXED [14]. Values obtained from individual replicate pens were used as the units for statistical analysis. Differences among the 3 treatments were determined by least squares means with PDIFF and were adjusted with a Tukey test [14]. In all analyses, significance was declared at P < 0.05.
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RESULTS AND DISCUSSION
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Growth Performance
The growth performance of birds in different treatments is summarized in Table 2. Birds that consumed AP-deficient diets supplemented with phytase had average daily gain, average daily feed intake, and FCR similar (P > 0.05) to that of birds consuming a normal diet with adequate AP in either phase or during the entire experimental period. The growth performance was also similar (P > 0.05) between the 2 groups of birds fed diets that were supplemented with different levels of phytase. No mortalities were observed, and all chicks appeared to be healthy throughout the study.
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Table 2. Growth performance of chicks fed a normal diet or low available P (AP) and Ca diets supplemented with recombinant phytase at a level of 500 or 750 FTU1/kg of DM
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This study demonstrated that broiler chicks fed AP-deficient diets supplemented with microbial phytase at 500 FTU/kg of DM or above had growth performance similar to that of broilers fed a diet with adequate AP. Reduced growth performance has been reported for chicks fed low-AP diets [15, 16]. Therefore, the results of the current study suggest that phytase supplementation of diets that were deficient in AP maintained the performance of birds at a level similar to that of birds fed a normal diet with an adequate AP content. Other studies with different sources of microbial phytase have also shown a similar trend [16–18]. Supplemental phytase acts on the phosphate groups associated with the inositol ring of the phytic acid backbone and liberates phytate-bound P, making more P available to the birds. Subsequently, birds consumed an amount of AP similar to that of birds fed the normal-AP diet. This was supported by a substantial improvement in P retention associated with phytase supplementation and similar P contents in the tibia ash of all treatment groups in the present study.
Mineral Retention
Compared with chicks that consumed a diet with adequate AP, chicks fed the AP-deficient diets supplemented with phytase had greater (P < 0.05) retention of Ca, P, and Zn (Table 3). The largest improvements in mineral retention with phytase supplementation of diets deficient in AP at levels of 500 and 750 FTU/kg were observed for P, which were, respectively, 51.7 and 61.7% greater than that of the control birds. This was followed by Zn (26 and 30%) and Ca (9 and 10%). However, there was no difference (P > 0.05) in the retention of P, Zn, or Ca between the 2 levels of phytase supplementation. Retention of Cu and Mn was similar (P > 0.05) among the 3 groups of birds.
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Table 3. Mineral retention of chicks fed a normal diet or low available P (AP) and Ca diets supplemented with recombinant phytase at a level of 500 or 750 FTU/kg of DM1
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The increased retention of Ca, P, and Zn in broiler chicks by supplementing AP-deficient diets with phytase, as compared with that of broilers fed the normal-AP diet, was consistent with other reports [16, 19, 20]. These increases in Ca and P retention were likely the net result of a diet that was deficient in AP and Ca and the supplementation of phytase. The elements Ca and P are keys to life; therefore, the birds had to utilize them efficiently, because in this study both of these elements were lower than the amounts required for normal growth. Phytase supplementation increased the availability of P and Ca that was either bound to or associated with phytate, which would not have been utilized by the birds in the absence of phytase. The increase in Zn retention might have been due to a greater availability of Zn from the phytate-mineral complex [16], because the diets used in this study contained an adequate level of Zn. An ample number of reports have shown that supplementation of microbial phytase increases Zn retention in monogastric animals [16, 21, 22]. Because bones contain the greatest proportion of minerals in the body, the increased retention of minerals observed in this study suggests that mineralization of the elements in the bone would also be improved by the supplementation of phytase. This was confirmed by the increased proportions in tibia bone ash observed in the present study (Table 4).
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Table 4. Tibia ash and mineral content in the ash of chicks fed a normal diet or low available P (AP) and Ca diets supplemented with recombinant phytase at a level of 500 or 750 FTU/kg of DM1
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Mineral Concentration in Bone Ash
The tibias had similar (P > 0.05) ash contents among the 3 groups of birds at either 28 or 42 d of age, and the same trend was also observed for P content of the ash (Table 4
). At 28 d of age, phytase supplementation of a diet deficient in AP at a level of 750 FTU/kg increased (P < 0.05) concentrations of Cu, Zn, Mg, and Mn in the tibia ash by 11.92, 2.38, 12.82, and 18.74%, respectively, as compared with that of the normal-AP diet. Supplementation of phytase at 500 FTU/kg of DM also increased (P < 0.05) the content of Cu, but not the contents of Zn, Mg, or Mn in the ash. On the contrary, supplementing a diet deficient in AP with phytase at both levels reduced (P < 0.05) the Ca content in the tibia ash of chicks at 28 d of age. There was no difference (P > 0.05) in the contents of Zn, Mg, or Mn between the 2 levels of enzyme supplementation. At 42 d of age, only the contents of Ca, Zn, and Mn were affected (P < 0.05) by the treatments. Compared with the control treatment, the concentration of Zn in bone ash increased by 2.21 (P < 0.05) and 4.86% (P < 0.01), respectively, with phytase supplementation of the AP-deficient diet at levels of 500 and 750 FTU/kg of DM. A similar trend was also found for the effect of treatment on Mn concentration in the ash, but a difference was observed (P < 0.05) only between the control diet and the AP-deficient diet supplemented with phytase at a level of 750 FTU/kg of DM. In contrast to that observed at 28 d of age, Ca concentration in the bone ash was greater for birds supplemented with phytase at a level of 500 FTU/kg of DM (P > 0.05) compared with birds fed the control diet. However, this was not observed with the AP-deficient diet supplemented with 750 FTU/ kg of DM of phytase.
Similar percentages of tibia ash between broiler chicks that were fed an AP-adequate diet and an AP-deficient diet supplemented with phytase suggest that all the groups of broiler chicks in this study had a similar degree of bone mineralization. This is consistent with the growth performance data obtained in this study. Feeding chickens diets deficient in P and Ca quite often resulted in a reduced percentage of tibia ash [16, 23], and phytase addition was reported to reverse this trend [24–26]. All of these reports indicated that supplementation of phytase improved the bone mineralization of broiler chicks fed a reduced-P and reduced-Ca diet. However, some researchers reported no difference in the tibia ash of birds fed diets with graded levels of AP [27, 28]. In this study, the effect of phytase in increasing the percentage of bone ash is consistent with the effect observed in improving mineral retention.
Supplementation of phytase to a diet deficient in AP had a greater effect on bone mineralization at the age of 28 d than at the age of 42 d, as illustrated by the data in Table 4. This was likely because birds were at a more vigorous growth stage at 28 d of age than at 42 d of age. The lower Ca content in tibia ash of chicks fed a diet deficient in AP and Ca with supplementation of phytase compared with birds fed the normal diet at 28 d of age, but not at 42 d of age, suggests that 0.67% Ca in the diet before the age of 28 d would still be lower than the amount required, even though the supplemented phytase improved its utilization.
Mineral Concentration in Plasma
Concentrations of minerals in the plasma were less affected by treatment than was bone ash (Table 5). Differences (P < 0.05) in mineral concentrations in plasma were observed only for Zn and Mg at 42 d of age. Concentration of Zn was greater (P < 0.05) in the plasma of chicks fed the AP-deficient diet supplemented with 750 FTU phytase/kg of DM than that in chicks fed the control diet. On the other hand, concentration of Mg in the plasma was greater (P < 0.05) for chicks fed the AP-deficient diet supplemented with 500 FTU phytase/kg of DM compared with that in chicks fed the control diet.
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Table 5. Mineral concentration (µmol/mL) in the plasma of chicks fed a normal diet or low available P (AP) and Ca diets supplemented with recombinant phytase at a level of 500 or 750 FTU/kg of DM1
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Serum levels of Ca and P are a result of the homeostatic regulation of Ca and P, and are the most important indicator of the bird’s nutritional status of Ca. Viveros et al. [16] found that decreasing the dietary AP level increased the level of Ca but reduced the level of P in the plasma of broilers at 6 wk of age. A similar concentration of minerals (including P and Ca) in the plasma of chicks in this study indicated that birds fed the AP-deficient diet supplemented with phytase had a mineral nutritional status similar to that of birds fed the AP-adequate diet. This is consistent with the report of Catalá-Gregori et al. [29]. Increased plasma concentrations of Zn and Mg in birds fed the AP-deficient diet supplemented with phytase might be due to the improved retention of these minerals associated with phytase supplementation. Although the retention of Mg was not measured in the present study, Viveros et al. [16] did report increased retention of Mg by supplementing the diets with phytase in the form of a low level of AP.
It is often difficult to define the optimal level of phytase supplementation because this involves levels of both dietary phytate P and non-phytate P and the specific activity of the enzyme product used. Moreover, the physiological status of the bird affects its response to phytase. Therefore, the optimal level of phytase supplementation varies as the enzyme source and feeding system change. In the present study, the similar results between the 2 levels of phytase supplementation for most of the measured parameters indicate that the level of supplementation was more economically beneficial at 500 than at 750 FTU/kg of DM for this particular phytase source under the conditions specified in this study. At this supplementation level, dietary AP level could be reduced to 67 or 65% (depending on the growth stage) of the recommended value without any negative effect on bird performance. This reduction in the use of AP, which is usually from chemical compounds in diet formulations attributable to the supplementation of phytase, would represent not only a reduction in the feed cost, but also, and more importantly, in a significant reduction in P excretion from the poultry industry to the environment. However, further research is needed to determine the interaction of the dietary mineral content and the rate of phytase supplementation so that their optimal combination could be used in gaining the most benefit from phytase supplementation.
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CONCLUSIONS AND APPLICATIONS
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- Broiler chickens fed a low-AP diet (0.30% from d 1 to 28, and 0.28% from d 29 to 48) supplemented with phytase (500 and 750 FTU/kg) had growth performance similar to that of broilers fed the normal-AP diet (0.45% for d 1 to 28, and 0.43% for d 29 to 48).
- Mineral utilization was improved for broilers fed the low-AP diet supplemented with phytase as compared with mineral utilization of broilers fed the normal-AP diet.
- Increasing the phytase level to 750 FTU/ kg offered further improvement only in bone Zn content, as compared with phytase supplementation at a level of 500 FTU/kg.
- Dietary AP could be reduced to two-thirds of the recommended level by using phytase.
- Further research is needed to determine the interaction between the dietary mineral content and phytase supplementation.
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ACKNOWLEDGMENTS
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This study was supported by a grant from the China Natural Science Foundation (Beijing, China; no. 30471261).
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REFERENCES AND NOTES
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SUMMARY
DESCRIPTION OF PROBLEM
