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Effect of phytase supplementation to a low- and a high-phytate diet for growing pigs on the digestibilities of crude protein, amino acids, and energy^1,2,3

S. F. Liao^*, A. K. Kies, W. C. Sauer^*,4, Y. C. Zhang^*, M. Cervantes and J. M. He^*

^* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2P5, Canada; and DSM Food Specialties, 2600 MA Delft, The Netherlands; and and Instituto de Ciencias Agrícolas, Universidad Autónoma de Baja California, Mexicali, México 21100

	Abstract

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Supplementation of microbial phytase usually improves the digestibility and utilization of phosphorus in feedstuffs of plant origin. The effect of phytase supplementation on the digestibilities of AA also has been examined, but the results have been inconsistent. This study was carried out to determine the effect of phytase (Natuphos) supplementation, at a rate of 2,000 phytase units/kg, to two basal diets on the apparent ileal digestibilities (AID) of GE, CP, and AA, and on the apparent total-tract digestibilities (ATTD) of CP and GE. The basal diets contained 18% CP and were formulated (as-fed basis) to contain either a low (0.22%) or high content (0.48%) of phytate P. The high-phytate diet contained 20% rice bran, which is a rich source of phytate and has low intrinsic phytase activity. Eight barrows (average initial BW = 40.6 kg), fitted with a simple T-cannula at the distal ileum, were fed the four diets according to a replicated 4 x 4 Latin square design. The pigs were fed twice daily at 0800 and 2000, equal amounts each meal, at a rate of 2.4 times the daily maintenance requirement for ME. Each experimental period comprised 14 d. Ileal digesta were collected from 0800 to 2000 on d 12, 13, and 14. Feces were collected from 0800 on d 8 until 0800 on d 12. Chromic oxide was used as the digestibility marker. The AID of GE, CP, and AA and the ATTD of CP and GE were less in the high- than in the low-phytate diet (P < 0.01). With the exception of glutamic acid, phytase supplementation did not affect (P > 0.10) the AID of CP and AA. There was no effect (P > 0.05) of phytase on the ATTD of CP and GE. These results show that if a response occurs to phytase supplementation, it is independent of the dietary phytate content.

Key Words: Amino Acids • Crude Protein • Ileal Digestibility • Phytase • Phytate • Swine

	Introduction

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It is well recognized that supplementation of microbial phytase to swine diets improves the absorption and utilization of P in feed ingredients of plant origin (Simons et al., 1990; Cromwell et al., 1993; Lei et al., 1993a,b). Some studies have shown that supplementation of phytase also has a positive effect on the apparent ileal digestibilities (AID) of AA in swine (Officer and Batterham, 1992a; Mroz et al., 1994; Kemme et al., 1999) and poultry (Ravindran et al., 2001). Other studies, however, have shown no effect of phytase supplementation on the AID of AA (Traylor et al., 2001; Liao et al., 2004).

It was hypothesized that a response in the AID of AA to phytase supplementation depends on the phytate content and the activity of intrinsic phytase in the diet. In other words, a positive response in the AID of CP and AA to phytase supplementation is more likely to occur when the content of phytate is high and the intrinsic phytase activity is low than when the content of phytate is low and the intrinsic phytase activity is high in the diet.

The aforementioned hypothesis was tested in this study. Two diets, relatively high and low in phytate P, were formulated. The high-phytate diet was created by formulating a diet that contained 20% rice bran. Rice bran has a high phytate content and a low activity of intrinsic phytase (Liao et al., 2002).

The objective of this study was to investigate the effect of phytase supplementation to the high- and to the low-phytate diets on the AID of CP, AA, and GE, and the apparent fecal digestibilities (ATTD) of CP and GE in growing pigs.

	Experimental Procedures

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Dietary Treatments
Two basal diets, with the same ME content, were formulated to contain a relatively high and low concentration of phytate P (Table 1). The high-phytate diet contained 20% rice bran, which is a rich source of phytate P. To each basal diet, Aspergillus niger phytase (Natuphos, DSM Food Specialties, Delft, The Netherlands) was supplemented at a rate of 2,000 U of phytase activity (FTU)/kg (as-fed basis). The recommended level for growing pigs is 500 FTU/kg (as-fed basis). One FTU is defined as the quantity of enzyme that liberates 1 µmol of ortho-phosphate per minute from 5.1 mM Na-phytate at pH 5.5 and at 37°C (Engelen et al., 2001). The diets were supplemented with inorganic P to meet the NRC (1998) standards for available P, which is 0.23% for growing pigs. Canola oil was included in the diets to increase the content of ME according to NRC (1998) standards. Vitamins, minerals, and L-lysine·HCl were supplemented to ful-fill NRC (1998) standards. Chromic oxide (0.25%) was included in the diet as the digestibility marker.

Animal Trial Procedures
Eight Genex F₂ barrows (Large White x Landrace) with an average initial BW of 40.6 kg were obtained from the University of Alberta Swine Research and Technology Center. The barrows were housed individually in stainless steel metabolic crates (height = 85 cm; length = 140 cm; width = 65 cm), and the temperature was maintained between 20 and 22°C. After a 14-d adjustment period to the crates, each barrow was fitted with a simple T-cannula at the distal ileum, approximately 5 cm from the ileocecal sphincter. The preparation of the cannulas was previously described by Sauer et al. (1983) and modified by De Lange et al. (1989). The surgical procedure was adapted from the procedure described by Sauer et al. (1983). A detailed description of pre- and postoperative care was previously given by Li et al. (1993).

Following a 7-d recuperation period after surgery, the barrows were fed the four experimental diets (Table 1) according to a repeated 4 x 4 Latin square design (n = 8). Each experimental period comprised 14 d. The diets were fed to the pigs at a rate of 2.4 times the maintenance requirement for ME (i.e., 100 kcal/BW_kg^0.75), based on the average BW of the pigs, which was determined at the initiation of each experimental period. An equal amount of feed was offered twice daily at 0800 and 2000.

The experimental proposal, surgical procedures, and procedures for care and treatment of the pigs were reviewed and approved by the Faculty of Agriculture, Forestry and Home Economics Animal Care Committee of the University of Alberta in accordance with the guidelines of the CCAC (1993).

Sample Collection and Chemical Analyses
Samples of the feed ingredients were taken after the ingredients were ground to pass a 2-mm mesh screen. Samples of the diets were taken during the time the meal allowances were prepared. A continuous collection of feces was initiated at 0800 on d 8 of each experimental period for a period of 96 consecutive hours. Feces were frozen at –28°C immediately after collection. Ileal digesta were continuously collected into a soft plastic tube (length = 20 cm; i.d. = 4 cm), from 0800 to 2000 on d 12, 13, and 14. Before collection, 8 mL of 10% (vol/vol) formic acid solution was placed into each tube. The tube was attached to the barrel of the cannula with a rubber band and replaced as soon as it was nearly filled with digesta. Digesta were frozen at –28°C immediately after collection. Detailed procedures for collection of ileal digesta were previously described by Li et al. (1993).

Before analyses, feces were pooled and a subsample was retained for each pig for each experimental period; the same procedures were followed for digesta. Feces were dried in a forced-air oven at 60°C to a constant weight. Digesta were freeze-dried. The dried samples of feces and digesta, and samples of ingredients and diets were ground to pass a 0.5-mm mesh screen in a Thomas-Wiley Laboratory Mill (Arthur H. Thomas Co., Philadelphia, PA).

Dry matter was determined according to AOAC (2000) Method 930.15. Gross energy was determined with an AC-300 Leco automatic calorimeter (Leco Corp., St. Joseph, MI). Nitrogen was determined with a Leco FP-428 nitrogen determinator. The P contents of the diets were determined spectrophotometrically by the molybdovanadate procedure (Method No. 965.17; AOAC, 2000). The NDF content of the diets was determined by the method of Goering and Van Soest (1970). The phytate P in the diets was determined according to the procedure described by Haug and Lantzsch (1983). The intrinsic phytase activities in the diets were determined by a colorimetric enzymatic procedure (Engelen et al., 1994, 2001). Chromic oxide was determined according to the procedure of Fenton and Fenton (1979). Analyses of ingredients and diets were carried out in triplicate and analyses of feces and digesta in duplicate.

For AA analyses, approximately 0.1 g of finely ground digesta (<0.1 mm) was weighed into a screw-capped test tube, mixed with 3 mL of 6 N HCl, and hydrolyzed for 24 h in an oven at 110°C. The hydrolyzed samples were mixed with the internal standard, DL-amino-n-butyric acid, and centrifuged at 1,110 x g for 15 min at 4°C. The supernatant fraction of the sample was analyzed using a Varian 5000 HPLC system with a reverse-phase column and a Varian Fluorichrom detector (Varian Canada, Inc., Mississauga, ON), as outlined by Jones and Gilligan (1983). The AA were derivatized with an o-phthaldialdehyde reagent solution. The mobile phase consisted of two solvents (A and B) with a flow rate of 1.1 mL/min. Solvent A contained 0.1 M sodium acetate (pH 7.2) and methanol and tetrahydrofuran in a ratio of 18:1. Solvent B was pure methanol. Peaks were recorded and integrated using the Ezchrom chromatography data system (Version 4.2; Shimadzu Scientific Instruments Inc., Columbia, MD), as described in detail by Sedgwick et al. (1991). The sulfur-containing AA, proline, and tryptophan were not determined.

Digestibility Calculation and Statistical Analyses
The AID of GE, CP, and AA and the ATTD of GE and CP were calculated by using the following equation:

where D_D is the AID or ATTD of CP, AA, or GE in the assay diet (%); A_F is the concentration of CP, AA, or GE in ileal digesta or feces (%); I_D is the chromic oxide concentration in the assay diet (%); A_D is the concentration of CP, AA, or GE in the assay diet (%); and I_F is the chromic oxide concentration in ileal digesta or feces (%).

Based on the following linear model, the digestibility values were subjected to statistical analysis using the GLM procedure of SAS.

where Y_ijk is a digestibility value; µ is the overall mean of the digestibility values; T_i is the fixed effect of dietary treatments and i = 1, 2, 3, 4; P_j is the random effect of experimental periods and j = 1, 2, 3, 4; A_k is the random effect of animals and k = 1, 2, 3, 4, 5, 6, 7, 8; and _ijk is the residual experimental error with N (0, ²). The effect of diet, phytase and the effect of the interaction between phytase and diet were tested by orthogonal contrasts (Hicks, 1973). Probability levels of P 0.05 were defined as significant differences.

	Results

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Diet Analysis and Animal Health
The chemical compositions of the experimental diets are presented in Table 2. The analyzed values of CP and AA in the diets were close to the values calculated based on the analyzed values in the ingredients. As was expected, because rice bran has a relatively high content of NDF, P, and ash (Kaufmann, 2003), the contents of these were higher in the high- than in the low-phytate diet. As was intended, the content of phytate P was higher in the high-phytate (0.48%) than in the low-phytate diet (0.22%). To ensure a possible maximum response to phytase supplementation, phytase was supplemented at a rate of 2,000 FTU/kg of diet rather than at 500 FTU/kg of diet, which is the recommended rate for growing pigs (Jongbloed et al., 2000).

Apparent Ileal Digestibilities of Energy, Crude Protein, and Amino Acids
The AID of GE (P < 0.001), CP (P < 0.001), and AA (P < 0.05), except glycine, were greater in the low-than in the high-phytate diets, regardless of phytase supplementation (Table 3). The supplementation of phytase did not affect the AID of GE (P = 0.153), CP (P = 0.063), or AA (P > 0.10), with the exception of glutamic acid (P = 0.047). In addition, there was no effect of interaction between diet and phytase for the AID of GE (P = 0.236), CP (P = 0.576), and AA (P > 0.369).

	Discussion

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As was previously discussed by Liao et al. (2005), phytase supplementation to diets for pigs does not consistently improve the AID of AA. As was pointed out by Kemme et al. (1999), there are many factors that should be considered, including the type and solubility of protein, in situ pH, the concentrations of dietary minerals and three-way interactions between phytate, protein/AA, and proteolytic enzymes in the digestive tract. In some studies, there has been no effect of phytase supplementation on the AID of AA (Traylor et al., 2001; Rice, 2002; Sands, 2002). In other studies, phytase supplementation affected the AID of some of the AA (Officer and Batterham, 1992; Mroz et al., 1994; Kemme et al., 1999).

As was suggested by Liao et al. (2002), the inconsistent responses in the AID of CP and AA from phytase supplementation may be attributed, in part, to the content of phytate and the activity of intrinsic phytase in the diet. The content of phytate P and the activity of intrinsic phytase in the high-phytate diet were 0.48% and 136 FTU/kg, respectively. In the low-phytate diet, these were 0.22% and 115 FTU/kg, respectively.

Supplementation of phytase to both the high- and low-phytate diet did not affect the AID of CP, AA, and GE (Table 3). Supplementation of phytase to both the high- and low-phytate diet did not affect the AID of CP, AA (with the exception of glutamic acid), and GE (Table 3). There were no interactions between phytase supplementation and diet, suggesting that the response in the AID of CP and AA to phytase supplementation is independent of the phytate P content of the diet. As there was no effect of phytase supplementation on the AID of the measured AA, it is unlikely that there would be an effect on the AID of AA that were not measured. These results agree with those reported by Sands (2002), in studies in which phytase also was supplemented to a high- or a low-phytate P diet.

The AID of GE, CP, and AA and the ATTD of GE and CP were less in the high- than in the low-phytate diet (Tables 3 and 4). The lower AID of GE, CP, and AA are a direct result of the inclusion of 20% rice bran in the high phytate diet. As was shown by Kaufmann (2003), the AID of GE and CP (and also of AA) are relatively low. The AID of GE and CP ranged from 60.5 to 65.8% and from 38.3 to 67.3%, respectively, in four samples of rice bran.

In conclusion, supplementation of phytase to either a high- or low-phytate P diet did not affect the AID of CP, the indispensable AA and GE or the ATTD of CP and GE. Further, these studies show that if a response occurs to phytase supplementation, it is independent of the dietary phytate content.

	Implications

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These results suggest that a possible response in the apparent ileal digestibility of crude protein and amino acids to phytase supplementation is independent of the dietary phytate phosphorus content. Thus, dietary phytate phosphorus content does not need to be the focus of future research designed to evaluate factors that are responsible for possible improvements in protein and amino acid digestibilities with phytase supplementation.

	Footnotes

 
¹ Presented in part at the 2004 Banff Pork Seminar, Banff, AB, Canada.

² Financial support was provided by DSM Food Specialties, Delft, The Netherlands, the Alberta Livestock Industry Development Fund Ltd., and the Alberta Agric. Res. Inst.

³ The authors thank K. Sauer and R. Engelbert for technical assistance.

⁴ Correspondence: 410, Agriculture and Forestry Center (phone: 780-492-7659; fax: 780-492-4265; e-mail: willem.sauer{at}ualberta.ca).

Received for publication August 16, 2004. Accepted for publication June 7, 2005.

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