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Articles

Microbial Phytase Does Not Improve Protein–Amino Acid Utilization in Soybean Meal Fed to Young Chickens¹

Department of Animal Sciences and Division of Nutrition Sciences, University of Illinois, Urbana, Illinois 61801

²To whom correspondence should be addressed at University of Illinois, Department of Animal Sciences, 290 Animal Sciences Laboratory, 1207 West Gregory Drive, Urbana, IL 61801. E-mail: d-baker1{at}uiuc.edu

	ABSTRACT

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Three growth trials were conducted with young chicks to evaluate crude protein (CP) utilization in soybean meal (SBM) as affected by dietary addition of microbial phytase. In assay 1, chicks were fed two CP-deficient (50 or 150 g CP/kg) levels of dehulled SBM, and each SBM level was then supplemented with equimolar amounts of cystine or methionine (Met) or with 1200 U phytase/kg. At 50 g CP/kg, cystine or Met supplementation improved (P < 0.05) measures of growth performance, but when 150 g CP/kg from SBM was fed, only Met addition improved (P < 0.05) weight gain, food efficiency and protein efficiency ratio (PER). Thus, Cys was more limiting than Met in the diet that contained 50 g CP/kg, but Met was clearly first-limiting in the diet that contained 150 g CP/kg. Phytase supplementation did not improve (P > 0.10) chick performance at either level of CP. Chicks in assay 2 were fed 100 g CP/kg furnished by SBM, casein or corn gluten meal in the absence and presence of 1200 U phytase/kg. Weight gain, gain/food and PER values were greater (P < 0.05) in chicks fed SBM than in those fed casein, and greater (P < 0.05) in chicks fed casein than in those fed corn gluten meal. Phytase supplementation had no effect (P > 0.10) on any measure of chick performance, regardless of the protein source fed. In assay 3, three deficient levels of CP (50, 100 and 150 g/kg) from SBM were fed in the absence and presence of 1200 U dietary phytase/kg. Weight gain, food efficiency and protein accretion increased linearly (P < 0.05) as a function of protein intake, but phytase supplementation had no effect (P > 0.10) on slopes of the body weight and protein accretion curves. Likewise, phytase addition did not affect (P > 0.10) measures of protein utilization, i.e., weight gain/protein intake and protein gain/protein intake at any of the CP levels that were fed. Because sulfur amino acids are the growth-limiting factors when protein-deficient levels of SBM are fed to young chicks, we conclude that dietary addition of phytase does not improve sulfur amino acid utilization in SBM.

KEY WORDS: • dietary phytase • soybean meal • methionine • cystine • chicks • protein utilization

	INTRODUCTION

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The efficacy of supplemental microbial phytase for improving phosphorus (P)³ , and specifically phytate-P, utilization has been investigated extensively, and phytase has been shown to consistently improve the utilization of not only phytate-P but also phytate-bound Ca, Zn and Mn as well (1 2 3 4 5) . Whether supplemental dietary phytase improves copper or iron utilization, however, is not clear (6 7 8) . It has been suggested that the phytate complex that exists in plant-based foods and feeds may also bind dietary proteins, lipids and carbohydrates (9 10 11 12 13 14 15 16) and inhibit proteolytic enzymes in the gut (14 ,17) . Phytate binding of energy-furnishing nutrients would have the potential of reducing both protein and energy utilization. Microbial phytase derived from Aspergillus niger is an accepted feed additive that is used extensively in commercial swine and poultry diets, and there is considerable interest in using phytase in pet foods and human foods. It is therefore important to establish whether efficacy exists for phytase beyond that of rendering essential mineral elements more bioavailable.

Commercially available phytase products, although very effective in releasing a portion of the bound P, Ca, Zn and Mn in plant-derived foods and feeds, is not a perfect solution for improving the bioavailability of these nutrients. Thus, the phytase enzyme is heat labile, which makes heat-based food and feed processing a concern, and it is rather expensive when added to swine and poultry diets. Hence, if protein utilization were to be improved by phytase addition to a diet, the cost effectiveness of the enzyme would be enhanced substantially.

The chick bioassays described herein were designed to determine whether a generous level (1200 U/kg diet) (the recommended dietary concentration of phytase used in commercial poultry and pig production ranges from 300 to 600 U/kg diet) of supplemental phytase (Natuphos; BASF Corp, Parsippany, NJ) would improve protein utilization in soybean meal (SBM). SBM is the most common oilseed protein source used in animal diets, and soybean concentrates, flours and isolates are used extensively in human foods as well. Soy products contain 60–70% of their P as phytate-P (18 ,19) . Work with chicks and pigs has clearly demonstrated that the protein in soy products is first limiting in sulfur amino acids (SAA) and second limiting in threonine (20 21 22 23 24) . Hence, in the protein quality studies reported here wherein SBM was fed as the sole source of dietary protein, any improvement in protein digestion or utilization mediated by supplemental phytase should be caused by improved digestibility, absorption or postabsorptive utilization of SAA.

	MATERIALS AND METHODS

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General procedures.

Housing and handling procedures for all assays were in accord with policies of the University of Illinois Committee on Laboratory Animal Care. Female crossbred chicks (New Hampshire male x Columbian Plymouth Rock female) from the University of Illinois Poultry Farm were used. The chicks were fed a nutritionally adequate corn-SBM diet (25) for the first 7 d posthatching. After being deprived of food overnight, the chicks were weighed, and chicks within a narrow weight range were selected. The selected chicks were then wing-banded and randomly assigned to pens, after which pens were randomly assigned to dietary treatments. Chicks were housed in thermostatically controlled starter batteries with raised wire floors in an environmentally controlled building. Both diets and water were provided for ad libitum consumption, and uniform light was provided on a 24-h basis.

The microbial phytase product (Natuphos) was obtained from BASF Corporation (Parsippany, NJ). The phytase premix was assayed and found to contain 805 U of phytase activity/g, with 1 U of phytase activity defined as the quantity of enzyme required to produce 1 µmol of inorganic P/min from 5.1 mmol of sodium phytate/L at pH 5.5 and a water bath temperature of 37°C.

Dehulled SBM was evaluated in all three assays, and corn gluten meal (ADM Corp., Decatur, IL) and casein (Erie Casein, Erie, IL) were also evaluated in assay 2. The protein sources used were subjected to kjeldahl nitrogen analysis. Analytical values for total and digestible methionine (Met), cystine, SAA, lysine and arginine are shown in Table 1 , and the digestible levels of these amino acids (g/kg CP) are compared with the ideal (i.e., required) levels of these same amino acids on the basis of the work of Baker and Han (26) .

The objective of this assay was to determine the order of Met or cystine limitation and the effect of microbial phytase on SAA utilization in SBM when fed to provide severely or marginally deficient levels of crude protein (CP) for chicks. A total of 128 chicks with an average initial weight of 99 g were fed either 50 or 150 g CP/kg furnished by SBM, and each CP level was either unsupplemented or supplemented with an equimolar amount of cystine or DL-Met or with 1200 U of microbial phytase/kg. Supplemental levels of cystine were selected to remain at a constant percentage of CP and were added at 0.3 and 0.9 g/kg of diet for the diets containing 50 and 150 g CP/kg, respectively. Because Met is 81.4% efficient (wt/wt) in furnishing cysteine via transsulfuration (27) , isosulfurous levels of supplemental Met (0.37 and 1.11 g/kg for diets containing 50 and 150 g CP/kg, respectively) were selected for addition. The SBM used in the assay was analyzed to contain 466 g CP/kg (28) . The assay was conducted using a cornstarch/dextrose basal diet (Table 2 ), with SBM, cystine, Met and phytase being added at the expense of cornstarch. With the exception of protein, all other nutrients met or exceeded NRC (25) nutrient recommendations. The basal diet contained 9.4 g Ca/kg and 3.9 g available P/kg, and levels of limestone and dicalcium phosphate were adjusted so that all treatment diets would contain 10 g Ca/kg and 4.5 g available P/kg, thereby meeting recommended requirements for these nutrients (25) . Experimental diets were fed to four replicate pens of four chicks per pen during a 12-d assay period (8–20 d posthatching). After the 12-d experimental period, body weight of individual chicks and group food intakes were recorded, after which weight gain, food intake, food efficiency and protein efficiency ratio (PER; g weight gain/g CP intake) were calculated for each replicate pen and then for each dietary treatment.

The same basal diet and the same SBM as that used for assay 1 was used, and in addition a phytate-containing source of corn protein (corn gluten meal, 629 g CP/kg) and a phytate-free protein source (casein, 875 g CP/kg) were evaluated. The 3 x 2 factorial arrangement of treatments consisted of three protein sources (all fed to provide exactly 100 g CP/kg) and two levels of phytase (none and 1200 U/kg). Four pens of four chicks were given each dietary treatment for 9 d (8–17 d posthatching).

Assay 3.

Slope-ratio methodology was used in this assay to assess the protein quality of SBM as affected by microbial phytase. Six experimental diets consisting of three CP-deficient levels of SBM (50, 100 and 150 g/kg) and two levels of phytase (none and 1200 U/kg) were fed to a total of 192 chicks, consisting of eight replicate pens of four chicks per pen. The experimental feeding period was 13 d (8–21 d posthatching), and the average initial weight at d 8 was 94 g. The CP levels provided by SBM were selected to represent three amino acid–deficient levels of dietary protein, all of which were first limiting in SAA (23) . The assay was conducted with the same source of SBM that was used in assays 1 and 2, and SBM and/or phytase was added to the cornstarch/dextrose basal diet (Table 2) as a replacement for cornstarch. With the exception of protein, all other nutrients met or exceeded nutrient recommendations (25) . A purified mineral mix (Table 2) was used in this assay, and this resulted in both Ca (12 g/kg) and available P (7.2 g/kg) exceeding recommendations (25) for these nutrients.

After the 13-d experimental period, body weight of individual chicks and group food intakes were recorded, after which weight gain, food intake, CP intake and food efficiency (gain/food) and PER were calculated for each replicate pen and dietary treatment. Sixteen representative chicks at assay initiation together with all chicks in four replicate pens per treatment at assay termination were killed with CO₂ gas. They were subsequently frozen at -28°C and then freeze-dried to determine percent body water. Whole body protein was then estimated based on whole body water as described by Velu et al. (29) , after which protein accretion values were calculated. Multiple linear regression analysis was performed, with weight gain and protein accretion being regressed on CP intake for the three CP levels fed in the absence or presence of 1200 U of phytase/kg. This facilitated the use of slope-ratio methodology to evaluate SBM protein utilization as affected by phytase supplementation.

Statistical analysis.

Analysis of variance was performed on pen means data using the General Linear Models (GLM) procedure of SAS (30) appropriate for completely randomized designs. Means separation in assay 1 was carried out using the LSD pairwise multiple-comparison procedure of SAS with set at 0.05. The factorial treatment arrangements in assays 2 and 3 were analyzed using single df orthogonal comparisons. In assay 3, weight gain and whole body protein accretion were regressed on CP intake, and independent linear regression equations were generated for the three CP levels furnished by SBM fed in the presence or absence of microbial phytase using the GLM procedure of SAS (30) . The intercepts for the equations generated for the three CP levels fed in the absence and presence of phytase were not different (P > 0.10). Therefore, multiple-linear regression analysis consisting of two straight lines with a common intercept was performed using the GLM procedure of SAS (30) . Increasing CP levels supplied by SBM fed in the presence or absence of microbial phytase were then analyzed using slope-ratio methodology.

	RESULTS

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Assay 1.

Chicks fed diets with 50 g CP/kg from SBM gained slower and less efficiently (P < 0.05) than those fed diets with 150 g CP/kg; PER values, however, were greater (P < 0.05) for chicks fed the lower CP diets (Table 3 ). At 50 g CP/kg, equimolar Met or cystine supplementation elicited a performance response, with Met addition improving (P < 0.05) all growth and efficiency criteria, and cystine improving (P < 0.05) both food efficiency and PER. In contrast, when 150 g CP/kg from SBM was fed, Met but not cystine improved (P < 0.05) all measures of growth performance and efficiency. Phytase supplementation did not improve weight gain at either level of CP, and it actually decreased (P < 0.05) gain/food ratio. At the lower level of CP, PER was also decreased in chicks fed the diet containing added phytase.

When fed to provide 100 kg CP/kg diet, chicks fed SBM gained faster (P < 0.05) and had higher (P < 0.05) gain/food and PER values than did chicks fed either casein or corn gluten meal (Table 4 ). Likewise, all performance criteria were greater (P < 0.05) for chicks fed casein than for those fed corn gluten meal. Phytase supplementation did not elicit a response (P > 0.10) in any measure of chick performance, and there were no phytase x protein source interactions.

Regardless of phytase supplementation, weight gain, protein accretion, food efficiency (weight gain/food intake) and protein efficiency (weight gain/CP intake and CP accretion/CP intake) responded linearly (P < 0.01) as a function of CP intake for chicks fed graded levels SBM (Table 5 ). Regressing weight gain (g) on CP intake (g) for diets 1–3 (X₁) and diets 4–6 (X₂) resulted in the following common-intercept multiple-linear regression equation: Y = 22.46 ± 2.73 + 3.05 ± 0.06X₁ + 3.09 ± 0.06X₂ (R² = 0.99). Likewise, regressing whole body protein accretion (g) on CP intake (g) produced the following common-intercept multiple-linear regression equation: Y = -4.60 ± 0.40 + 0.55 ± 0.01X₁ + 0.56 ± 0.01X₂ (R² = 0.99). Phytase supplementation did not affect (P > 0.10) the slope of any of the accretion curves. Moreover, phytase supplementation did not affect (P > 0.10) food efficiency or PER values at any of the dietary CP levels fed.

	DISCUSSION

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The results of all three chick bioassays indicated that dietary phytase addition does not improve the utilization of protein provided by SBM. In our growth assays wherein SBM was fed as the sole source of protein to provide 50, 100 or 150 g CP/kg, SAA were clearly the first limiting amino acids. More specifically, Met was first limiting when 150 g CP/kg was fed (probably, also, when 100 g CP/kg was fed), but cystine was first limiting when only 50 g CP/kg was fed (Table 3) . Thus, at the low growth rates occurring when 50 g CP/kg was fed, the maintenance component (high cystine requirement) contributed more significantly to the total requirement than that occurring when higher CP levels were fed. Clearly, if a diet responds to either Met or cystine, one must conclude that cystine is more limiting than Met, because cystine can supply only cystine per se, whereas Met can provide both cystine (via transsulfuration) and Met. Our results, therefore, with SBM first limiting in cystine (Table 3) or first limiting in Met (Tables 3 4 5) indicate that phytase addition to the diet does not improve the utilization of either Met or cystine.

The data in Table 1 together with the results of assay 2 suggested that the growth limiting factor (at 100 g CP/kg) in casein was digestible arginine, and that in corn gluten meal was digestible lysine. Earlier work in our laboratory (36) had shown lysine to be the first limiting amino acid in corn gluten meal and that phytase did not improve protein utilization in this protein source. That chick performance and PER were so markedly superior for SBM relative to casein, when the SAA deficiency of SBM and the arginine deficiency of casein were essentially equal (i.e., 70% of ideal levels in both cases), suggests that the high lysine/arginine ratio in casein exacerbated the existing arginine deficiency (37 38 39 40) .

It has been suggested that the level of dietary available P may influence the efficacy of phytase for improving protein digestibility or utilization (41) . Our first two protein quality bioassays (Tables 3 and 4) were made with dietary available P being present at its minimal requirement in all diets (25) , whereas assay 3 (Table 5) was carried out under conditions where available P was in excess. In comparing PER values across assays, there was no indication that the excess P level used in assay 3 depressed protein utilization. Moreover, there have been several instances where phytase-mediated nitrogen and/or amino acid digestibility responses (albeit small) have been reported in chicks or poults fed P-adequate diets similar to those fed in assays 1 and 2 herein (31 32 33 ,41 ,42) . It seems clear that if protein or amino acid digestibility is improved by phytase supplementation, protein utilization as measured by growth-assay methodology should also be improved. That it is not improved as shown herein suggests that the milieu of changes in rates or quantities of absorbed nutrients manifested by phytase somehow result in a neutral rather than a positive effect on chick performance. Clearly, if available P, Ca, Zn or Mn is deficient in a phytate-containing diet, phytase will elicit a positive growth response (2 ,43 44 45) , but if any one (or all) of these mineral deficiencies is combined with protein or amino acid deficiency, we predict that the expected growth response due to phytase would be attributable solely to improved utilization of the deficient mineral element(s) and not to improved utilization of the deficient amino acid.

Soy products are being used increasingly in foods for humans, and it is well established that the P, Ca, Zn, Mn and Fe in these products are not well utilized, presumably due to phytate binding. Thus, phytase supplementation of soy-based foods is of interest to the food industry. Our research herein with chicks together with similar work in pigs (46) suggests that were phytase to be used as an additive in soy-based foods for humans, protein utilization would not be improved.

	FOOTNOTES

 
¹ Supported by the Illinois Council of Food and Agricultural Research.

³ Abbreviations used: CP, crude protein; Met, methionine; P, phosphorus; PER, protein efficiency ratio; SBM, soybean meal; SAA, sulfur amino acids.

Manuscript received December 20, 2000. Initial review completed February 12, 2001. Revision accepted March 9, 2001.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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