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ANIMAL NUTRITION |
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* Department of Animal Sciences, Purdue University, West Lafayette, IN;
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Virginia-Maryland Regional College of Veterinary Medicine; and
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Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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Key Words: phytase • amino acid • pig • phosphorus • calcium • digestibility
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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; Cromwell et al., 1993; Kornegay and Qian, 1996), resulting in an increased digestibility of P and a decreased P excretion. The phytate molecule, being extremely anionic, has been shown to bind several cations including Ca, Zn, Cu, and Mn (Maga, 1982; Reddy et al., 1982; Morris, 1986). It has also been shown that the anionic phosphate groups of phytate possess the ability to bind proteins (Prattley et al., 1982) and AA, having the greatest affinity for the basic AA, Lys, Arg, and His (Reddy et al., 1982).
Officer and Batterham (1992) demonstrated an increased ileal digestibility of CP and some AA by 7 to 12% when microbial phytase was added to the diet. Mroz et al. (1991) and Khan and Cole (1993) also showed an increase in ileal CP digestibility by 12.8 and 3.5%, respectively, with the addition of dietary phytase. However, Nasi (1990) and Kemme and Jongbloed (1993,b,c) found no effect of adding microbial phytase on total tract protein digestibility. Kemme et al. (1995) reported an increase in ileal digestibility of AA, and Jongbloed et al. (1995) and Christensen and Nielsen (1995) demonstrated an increase in apparent total tract digestibility (ATTD) of N. However, Lantzsch and Drochner (1995) showed no improvement in N digestibility when microbial phytase was added to the diets of breeding sows. More recently, Johnston et al. (2004) reported a 4.4% increase in average AA digestibility, when 500 U/kg of phytase were added and Ca and P levels were each reduced by 0.10 percentage units. However, in a review, Adeola and Sands (2003) caution against the use of an equivalency value of microbial phytase from AA due to inconsistencies in the literature.
The objectives of this study were to evaluate the efficacy of microbial phytase for improving the apparent ileal digestibilities (AID) and the ATTD of AA, N, Ca, P, DM, and energy and to calculate equivalency values of microbial phytase for AA.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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Ten crossbred barrows were fitted with steered ileo-cecal valve cannulas, as described by Radcliffe et al. (2005), at an average BW of 33.2 ± 1.5 kg and used to study the effects of added phytase on mineral and AA digestibilities. Pigs were individually housed in metabolism pens (1.2 x 1.2 m) and were given a minimum of 2 wk to recover from surgery before the start of the experiment, at which time the average initial BW was 48.3 ± 2.3 kg. Water was supplied ad libitum, and feed was supplied at a level of 9% of metabolic BW (BW0.75) per day in 2 equal daily feedings (0600 and 1800).
Dietary Treatments
All diets were corn-soybean meal-based and formulated to contain adequate concentrations of all nutrients except CP and AA (NRC, 1998). Diets 1, 2, and 3 were formulated to contain 12.0, 11.1, and 10.2% CP, respectively (Table 1). Diets 4 and 5 had the same ingredient composition as diet 3 plus 250 or 500 U of added phytase (Natuphos, BASF Corp., Mt. Olive, NJ) per kilogram of diet, respectively. Crude protein was lowered in diets 2, 3, 4, and 5 by adding a mixture of cornstarch and dextrose (1:1 on a wt:wt basis) to diet 1 in place of corn and soybean meal. The addition of the cornstarch-dextrose mixture decreased the protein concentration but allowed the relative proportion of individual AA within the diet to remain the same. A Cr-premix was added to all diets at a level of 0.20% (provided 0.033% Cr from chromic oxide) as an indigestible marker so that apparent digestion coefficients could be calculated.
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). Pigs were weighed every 2 wk before the start of the next period. These weights were used to adjust feeding levels in the subsequent period.
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. Urine was collected in buckets from drop pans under each pen. A 25% HCl solution was added to each bucket to maintain a urine pH < 5. Total urine and fecal samples were collected twice per day during the collection periods and frozen at –20°C for subsequent laboratory analysis.
Diet and fecal samples were dried in a forced air oven at 60°C to a constant weight, and ileal samples were lyophilized to a constant weight. Dried diet, fecal, and ileal digesta samples were ground (Foss Tecator Cyclotec Mill, Hoeganaes, Sweden) to pass through a 1-mm screen. Diets were analyzed for Ca, P, and Cr following nitric-perchloric acid (5:3, vol/vol) wet digestion. Procedures were validated using the National Institute of Standards and Technology spinach leaf standard (SRM 1570a) and a feed standard that had been validated by multiple external laboratories. Total P concentrations were assayed photometrically using the vanadomolybdate procedure (AOAC, 1990), and Ca and Cr were determined with an atomic absorption spectrophotometer (model 5100 PC, Perkin Elmer, Norwalk, CT) using the manufacturer’s recommended procedure. For Ca analysis, lanthanum oxide was used as a matrix modifier. Samples were analyzed for DM content using standard methods (AOAC, 1990).
Fecal, urine, and feed samples were digested with sulfuric acid, and N concentrations were assayed using the Kjeldahl procedure (AOAC, 1990). Total CP was calculated as N x 6.25. The AA composition of the diets and digesta were determined at the University of Missouri—Columbia Agriculture Experiment Station Chemical Laboratory. Samples for AA analysis were prepared using a 24-h hydrolysis in 6 N HCl at 110°C under a N atmosphere. For Met and Cys, performic acid oxidation occurred before acid hydrolysis. Amino acids in hydrolysate were determined by HPLC after postcolumn derivatization [Beckman 6300 Amino Acid Analyzer, Beckman Coulter Inc., Fullerton, CA; AOAC, 2000; 982.30 E (a, b, c)].
Dietary phytase activity was determined using a modification of the method described by Simons et al. (1990). A unit of phytase activity was defined as the quantity of enzyme that liberated 1 µmol of inorganic P/min from 1.5 mmol/L of sodium phytate at pH 5.5 and 37°C. Gross energy content of the dried feed and feces was determined using an automated bomb calorimeter (Parr, model 1271, Parr Instrument Company, Moline, IL).
The apparent digestibilities of Ca, P, N, energy, and AA were calculated using the indicator/marker method as described by Adeola (2001).
Statistical Analysis
Data were analyzed using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC). Pig served as the experimental unit. The model included pig(square), period, treatment, and carryover effect. Linear contrasts were used to test the effect of increasing phytase concentrations or decreasing protein concentrations and to test the differences between treatment means. Effects were considered not significant at P 
0.10.
Linear functions were derived for CP concentrations (diets 1 through 3) and for phytase concentrations (diets 3 through 5) with the linear model: Y = a + bX, in which Y = the response measurements; X = 0, 7.5, or 15% CP reduction of the 12% CP diet or 0, 250, or 500 U/kg of phytase added to the reduced CP diet (diet 3). The linear equation for phytase was solved for 500 U/kg of phytase, and the product was set equal to the protein equation and solved. The product was subtracted from 15% (maximum reduction) to give the protein reduction equivalent for 500 U of phytase per kilogram of feed.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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The analyzed dietary composition was similar to the calculated composition except for phytase activity, which was lower than calculated (176 U/kg vs. 250 U/kg and 366 U/kg vs. 500 U/kg). Crude protein concentrations, as estimated by N concentration in the diet, were 11.8, 11.0, and 10.1% for diets formulated to contain 12.0, 11.1, and 10.2% CP, respectively (Table 2). Assayed Ca and P (0.47% Ca, 0.40% P) concentrations (Table 1
) were also in good agreement with calculated values (0.44% Ca, 0.40% P).
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Pigs gained an average of 580 g/d throughout the experiment with an average G:F of 0.287 (data not shown). There were no effects of dietary CP concentration or phytase supplementation on ADG or feed efficiency, but the effect of phytase approached a linear trend for ADG (P = 0.12) and tended to linearly improve G:F (P < 0.08). Because pigs were on each treatment for 2 wk and were limit fed, performance differences caused by treatment were not expected.
Apparent Total Tract Digestibilities
The Ca and P ATTD were not affected by CP concentration in the diet (Table 3). However, the addition of microbial phytase to the low CP diet increased linearly (P < 0.001) the ATTD of P. Calcium ATTD was numerically improved by the addition of phytase to the diet, but the effect was not significant (P < 0.13). Lowering the concentration of CP in the diet resulted in a linear increase (P < 0.001) in DM ATTD. Adding phytase to the low CP diet did not affect DM ATTD. Energy ATTD was not affected by phytase addition, but it increased with the lower CP diets (P < 0.005).
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Due to the restricted feed intake (9% of BW0.75/d) and no feed refusals, daily N intake decreased (P < 0.001) as the concentration of CP was reduced in the diet (Table 3). Daily urinary and fecal N excretion were not different for pigs fed all diets. Similarly, N retention and N ATTD, calculated as a percentage of N intake, were not affected by dietary treatment. However, because dietary CP concentrations were not the same across all diets, it is important to evaluate N retention and ATTD as the amount retained per kilogram of feed intake. When this is done, increasing the concentration of CP in the diet increased linearly (P < 0.004) the amount of N retained per kilogram of diet by 4.0 g (calculated from Table 3).
Apparent Ileal Digestibility of Ca, P, and DM
Dietary CP concentration had no effect on the AID of P or DM (Table 4). Calcium AID tended (P < 0.1) to decrease linearly as the concentration of CP in the diet was decreased. This response was primarily due to a greater Ca AID in the diet containing 12.0% CP, compared with the diets containing 11.1 or 10.2% CP. The addition of microbial phytase to the low CP diet linearly improved the AID of Ca (P < 0.004) and P (P < 0.001) but had no effect on DM AID.
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Lowering the concentration of dietary CP resulted in a linear reduction (P < 0.05) in the AID of all AA analyzed except Pro (Table 4). On average, AA AID was decreased 4.7 percentage units as the CP concentration of the diet was decreased from 12.0 to 10.2%. Decreases in the AID of AA ranged from 2.9 (Leu and Glu) to 7.4 (Val) percentage units. Overall, the AID of CP calculated from N concentration (N x 6.25) decreased (P < 0.001) from 73.8 to 72.0 to 66.7% as the concentration of dietary CP was decreased from 12.0 to 11.1 to 10.2%, respectively. When phytase was added to the low CP diet, the AID of all AA numerically improved. However, this was only significant for Gly (P < 0.04), Ala (P < 0.08), Val (P < 0.04), Ile (P < 0.05), Thr (P < 0.06), TSAA (P < 0.06), Asp (P < 0.04), Glu (P < 0.05), Phe (P < 0.08), Lys (P < 0.06), and Arg (P < 0.06). Apparent ileal digestibility of Gly (P < 0.04), Ala (P < 0.08), Val (P < 0.03), Ile (P < 0.05), Thr (P < 0.06), Cys (P < 0.06), Asp (P < 0.04), Glu (P < 0.05), Phe (P < 0.08), Lys (P < 0.06), and Arg (P < 0.06) were improved an average of 2.9 percentage units when 500 U of phytase were added per kilogram of diet. Improvements in AID ranged from 2.2 (Phe) to 5.8 (Gly) percentage units when 500 U of phytase were added per kilogram of diet. Crude protein AID tended to be increased linearly (P = 0.07) from 66.7 to 69.3 to 70.1% as the concentration of added phytase in the diet was increased from 0 to 250 to 500 U/kg, respectively.
Because CP concentrations differed among some of the diets, it is not only important to evaluate AA digestibility as a percentage of AA intake but also as the amount of each individual AA absorbed per kilogram of diet. This was achieved by taking the digestion coefficient and multiplying it by the concentration of that particular AA in the diet (grams of AA per kilogram of feed DM). This is important because it allows for the development of a protein response surface for increasing CP concentrations in the diet. For example, Lys AID decreased 8.4% (79.4 vs. 72.7%) when the CP concentration was decreased from 12.0 to 10.2%. However, because this decreased Lys AID occurred in a diet containing less total Lys, it actually resulted in a 20.3% decrease in Lys AID expressed as the amount of Lys digested per kilogram of diet. The results of this are shown in Table 5. Decreasing the concentration of dietary CP resulted in a decreased AID of all AA measured (P < 0.001) when reported as the amount of each AA digested per kilogram of feed intake. The effects of phytase addition were the same as those reported for AID as a percentage because CP concentrations were essentially the same in all diets with added phytase.
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Linear regression was used to develop protein and phytase response equations for AA AID as a percentage (Table 6) and as the amount digested per kilogram of diet (Table 7). By setting the phytase equation equal to the protein equation and solving for 500 U of added phytase per kilogram of diet, phytase equivalency values for individual AA were calculated. The protein equations estimate AA digestibility based on a percentage unit reduction in dietary CP content. Diet 1 was formulated to contain 12% CP. Diet 2 was formulated to contain 11.1% CP, which represents a 7.5% decrease in CP content relative to diet 1, and diet 3 was formulated to contain 10.2% CP, which represents a 15.0% decrease in CP content relative to diet 1. Therefore, the protein response equations describe the response of each AA as CP is decreased by 0 to 15%. Therefore, when the phytase response equation is set equal to the protein response equation and solved for a given amount of phytase, the CP value obtained is equal to the decrease in CP. To obtain the percentage reduction in CP allowed when phytase is added, this value must be subtracted from 15%. For example, the protein and phytase response equations for Asp are Y = 80.54 –0.386X1 and Y = 74.99 + 0.0051X2, respectively, in which Y = the AID of Asp, X1 = reduction in CP, and X2 = added phytase (U/kg). By setting these equations equal to each other and solving for X1, the following equation is derived: X1 = (74.99 + 0.0051X2 –80.54)/–0.386. If this equation is solved for 500 U of phytase activity per kilogram of diet (X2), then it is determined that X1 = 7.77%. Therefore, pigs fed 500 U/kg would perform comparably with pigs fed a diet in which the CP had been decreased by 7.77%, or if 7.77% is subtracted from 15.0%, then it is determined that 500 U/kg of phytase can replace 7.23% of the Asp in the diet. To adjust this number to an absolute amount instead of a relative amount, the replacement coefficient (7.23%) was multiplied by the amount of Asp in the diet (10.4 g/kg in the 10.2% CP diet) to arrive at the equivalency value of 0.075 g/kg for 500 U/kg of phytase. Based on these calculations, 500 U of phytase per kilogram of diet would replace 3.39 to 10.72% of the AA in the diet. Converting these to absolute equivalency values, 500 U of phytase per kilogram of diet can replace 0.011 to 0.188 percentage units of individual AA. The effects of phytase on CP digestibility as a whole were evaluated based on N concentration (N x 6.25) or based on the sum of all AA in the diet. When estimating CP based on N concentration, 500 U of phytase per kilogram of diet would replace 5.20% of the CP or 0.525 percentage units of CP. When estimating total protein using the sum of all AA in the diet, 500 U of phytase per kilogram of diet could replace 8.20% of the total protein or 0.910 percentage units of protein.
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. By setting these protein and phytase response equations equal to one another and solving for 500 U of phytase per kilogram of diet, it was determined that 500 U of phytase per kilogram of diet can replace from –1.97 to 4.53% of individual AA in the diet. Converting this to an absolute amount, it was determined that 500 U/kg of phytase was equivalent to –0.006 to 0.055 percentage units of individual AA. It should be noted that His was the only AA that provided a negative response, and that His AID, as measured by the amount digested per kilogram of diet, was not significantly affected by the addition of phytase. In addition, linear regression of phytase concentration against His AID provided a relatively poor fit (r2 = 0.47). Based on solving the response equations of phytase and protein for CP AID as estimated by N concentration (N x 6.25), it was determined that 500 U of phytase per kilogram of diet could replace 1.49% of the dietary CP or 0.150 percentage units. When the phytase equivalency value of total dietary protein estimated as the sum of all AA was determined, it was found that 500 U of phytase per kilogram of diet could replace 3.01% of the total dietary protein or 0.334 percentage units. |
DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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; Ravindran et al., 1994), which is unavailable to the pig. The addition of microbial phytase to swine diets hydrolyzes some of this phytate P, making it available for absorption by the pig, thus increasing P digestibility (Simons et al., 1990; Jongbloed et al., 1992; Radcliffe and Kornegay, 1998). Kornegay et al. (1998) estimated that 500 U of phytase fed per kilogram of diet was capable of replacing 0.98 g of P from inorganic P sources. In the current study, the AID of P was increased from 56.5 to 66.1%, and the ATTD of P was increased from 46.3 to 57.6% when 500 U of microbial phytase was added per kilogram of diet. This 11.3 percentage unit increase in P ATTD represents an increase of 0.45 g of P digested per kilogram of diet. By dividing 0.45 g of P by the estimate of P bioavailability from inorganic P derived by Kornegay et al. (1998) of 76.7%, it can be concluded that based on the results of this study, 500 U of phytase per kilogram of diet can replace 0.59 g of P from inorganic P sources. This value is lower than many reported in the literature (Jongbloed et al., 1992; Kornegay et al., 1998; Radcliffe and Kornegay, 1998), where it has been reported that 500 U of phytase fed per kilogram of diet can replace approximately 1 g of P from inorganic P. These differences can be explained by the fact that pigs in the current study were growing and finishing animals in which the P requirement is lower, whereas many of the equivalency values reported in the literature are from nursery pigs. In addition, some inorganic P was included in the basal diet, and animals were limit fed. All of these factors contribute to a decreased equivalency value estimate due to their effects on decreasing phytase efficacy, increasing the efficiency of P digestion, and a lowered P requirement of older, heavier weight animals.
In addition to the effect of phytase on P digestibility, phytase has been shown to improve Ca (Radcliffe, 1997) and AA digestibilities (Officer and Batterham, 1992; Kemme et al., 1995; Zhang and Kornegay, 1999). The mechanism through which phytase causes an increase in Ca and AA digestibilities is related to the negative charges carried on each phosphate group of phytate. At a low to neutral pH, the anionic phosphate groups of phytic acid possess the ability to bind AA or proteins (Cosgrove, 1980; Prattley et al., 1982; Thompson, 1986), having the greatest affinity for the basic AA: Lys, Arg, and His (Reddy et al., 1982). In this study, the low protein basal diet was formulated to contain 10.2% CP and 0.78% phytate. If it is assumed that the average AA has a molecular weight of 110, then this diet contained 9,273 mmoles (1,020,000/110) of AA. Likewise, if the amount of phytate in the diet is divided by the molecular weight of phytate, then this diet contained 12.0 mmoles (7800/648) of phytate. Therefore, for every individual AA bound by each molecule of phytate, CP digestibility should decrease by approximately 0.13% (12.0/9,273*100).
The addition of 500 U of phytase per kilogram of diet to the low CP diet resulted in an increase in CP AID from 66.7 to 70.1%. This represents a 3.4 percentage unit increase or a 5.1% increase in CP digestibility. By multiplying 5.1% times the amount of CP in the diet (10.2%) it is determined that 500 U of phytase per kilogram of diet can replace 0.52 percentage units of CP. Estimating the phytase equivalency value for CP in this manner assumes that dietary CP is 100% available, which is not the case. Therefore, in this study, multiple concentrations of CP (10.2, 11.1, and 12.0) and multiple concentrations of phytase (0, 250, and 500 U/kg) were fed so that response equations for CP and phytase could be developed. Equivalency values were then derived by setting the protein response equation equal to the phytase response equation and solving for a set amount of phytase. Based on protein and phytase response equations for CP AID (%), 500 U of phytase per kilogram of diet can replace 5.20% of the CP in the diet or 0.530 percentage units of CP. This value is in good agreement with those previously reported (Khan and Cole, 1993; Mroz et al., 1994; Kemme et al., 1999). Officer and Batterham (1992) fed semisynthetic diets to pigs weighing approximately 40 kg of BW and observed a 12 percentage unit increase in CP AID when 1,000 U of phytase were added per kilogram of diet. By multiplying this increase by the amount of CP in the diet (15.0%), 1,000 U of phytase per kilogram of feed released 1.8 percentage units of dietary protein. Mroz et al. (1994), feeding a 17.0% CP diet to pigs from 45 to 110 kg, found a more moderate increase in CP AID of 2.5 percentage units or 3.5%. This translates to 800 U of phytase per kilogram of diet being equivalent to 0.595 percentage units of dietary protein.
In more recent work, Kemme et al. (1999) reported a 1.6 percentage unit increase or a 2.2% increase in CP AID when 900 U of phytase were added per kilogram of diet to a 13% CP diet fed to pigs from 37 to 95 kg of BW. This equates to 900 U of phytase being equivalent to 0.29 percentage units of dietary protein. Zhang and Kornegay (1999) fed diets identical to those used in this study in a grow-finish trial and a metabolism trial. They estimated that 500 U of phytase added per kilogram of diet to a 10% CP diet could replace 1.01 percentage units of CP. Johnston et al. (2004) observed an improvement in AA AID when phytase was added to the diet and dietary Ca and P concentrations were reduced but observed no effect of phytase on N AID. Variations in phytase equivalency values for CP reported in recent reports (reviewed by Adeola and Sands, 2003) occur because of differences in the basal CP content of the diet, differences in the concentration of phytate P and inorganic P in the diet, differences in the Ca:P ratio, and differences in the grain feedstuffs used in the basal diets. In addition, with the exception of the study by Zhang and Kornegay (1999), only one concentration of phytase and one concentration of CP without added phytase were fed. Therefore, equivalency values can only be estimated for that one concentration of phytase addition, and the bioavailability of CP from the diet cannot be factored into the equation.
When estimating the phytase equivalency value for CP based on response equations generated by feeding multiple concentrations of protein and multiple concentrations of phytase to a low CP diet, it may be more correct to estimate equivalency values based on protein response curves for the amount of protein digested per kilogram of diet instead of the percentage of dietary intake digested.
To date, no research has estimated the equivalency value of microbial phytase for CP based on digested CP (grams per kilogram of intake). However, several studies designed to estimate the P equivalency value of phytase have reported more accurate equivalency estimates using digested P (grams per kilogram of intake) rather than P digestibility as a percentage of P intake (Jongbloed et al., 1996; Radcliffe and Kornegay, 1998). Estimating equivalency values based on digestibility as a percentage accounts only for changes in P digestibility and ignores that the concentration of P in the diet may have also changed. Therefore, it is possible to have no change in P digestibility, whereas the amount of P digested (grams per kilogram) may have increased as a result of a greater concentration of dietary P. As a result, equivalency estimates based on digestibility as a percentage may undervalue inorganic P sources, resulting in high equivalency estimates. In the current study, the estimate of the CP equivalency of 500 U of phytase per kilogram of diet decreased from 0.525 percentage units to 0.150 percentage units when the amount of CP digested per kilogram of diet was used instead of CP digestibility as a percentage of protein intake. Using the 0.150 percentage unit increase and assuming that the average AA has a molecular weight of 110, approximately 136 (15,000/110) mmoles of AA are being released by phytase. Because there are approximately 12 mmoles of phytate per kilogram of diet, an average of 11.3 AA are being released from each phytate molecule by phytase (136/12). If the equivalency value of 0.525 percentage units is used that was generated from CP digestibility data as a percentage of CP intake, then phytase is releasing an average of 39.8 AA (52,500/110/12) per phytate molecule. This number does not seem realistic. In fact it is questionable whether phytate could bind and phytase could release an average of 11.3 AA per phytate molecule, particularly considering that maximal phytate P hydrolysis is only about 60% in vivo (estimated from Kornegay et al., 1998).
Therefore, additional actions of phytase on AA digestibilities need to be considered. The accuracy of these predictions needs to be evaluated including the limitations. Amino acid digestibilities reported in this paper are referred to as AID because they do not account for endogenous AA losses. The major problem with not accounting for endogenous AA losses is that the protein response curves generated in this study may provide somewhat misleading results. Endogenous protein losses can be characterized as diet independent losses and diet dependent losses. The diet independent endogenous losses will remain constant regardless of diet type and CP concentration of the diet. As a result, in this study, pigs fed the lower CP diet had a greater proportion of diet independent endogenous AA loss in their ileal digesta compared with pigs fed the greater protein diets. As a result, CP and AA AID of these pigs seemed lower than for pigs fed the greater CP diets. Some of this observed effect could be real, but a proportion of it is due to the greater concentration of endogenous CP loss collected in the ileal digesta. Therefore, ideally, AID should be adjusted to true ileal digestibilities by accounting for endogenous protein loss. However, estimating endogenous protein loss is very difficult, and all of the techniques available have major limitations (Nyachoti et al., 1997). The net effect of adjusting for endogenous AA losses would be that the y intercept of the protein response curve would be greater and the slope would be lessened. The effect on estimates of CP and AA equivalency values of phytase would depend on the y intercept and slope of the phytase response curve; therefore, estimates could either decrease or increase.
Another possible mode of action through which phytate may negatively impact AA and CP digestibility is through binding and inhibition of proteolytic enzymes. Mroz et al. (1991) reported an increase in trypsin activity in duodenal digesta when phytase was added to the diet. Deshpande and Cheryan (1984), using an in vitro system, found an inhibition of 
-amylase activity when phytate was added to the solution. If phytate in the diet inhibits digestive enzyme activity, it would help explain many of the reported equivalency values of phytase for AA and protein, which appear to be too large to be caused by the release of AA from phytate alone.
Officer and Batterham (1992) observed a 7 to 12% increase in AA digestibilities for several AA when 1,000 U of phytase were added per kilogram of diet. However, the only AA significantly improved by phytase addition was Lys. Similarly, Mroz et al. (1994) found a beneficial effect of adding phytase on all AA except Thr, but only the effect on Met was significant. Kemme et al. (1999) reported significant improvements in the range of 1 to 2 percentage units for most AA measured. More recently, Zhang and Kornegay (1999) reported improvements in AA AID ranging from 7.8 to 15.0%. These estimates were based on response curves for protein and phytase using AA AID as a percentage of AA intake. Johnston et al. (2004) determined the AID of Lys, Ile, Leu, Phe, His, Arg, Val, Thr, and Trp as influenced by dietary Ca and available P concentrations and microbial phytase inclusion. The overall effect of phytase only tended (P < 0.10) to improve Ile and Leu AID but had no effect on any of the other amino acids measured. However, lowering the concentration of dietary Ca and P and adding 500 U/kg of microbial phytase resulted in an improvement (P < 0.05) in the AID of all AA measured, except His and Trp, which tended (P = 0.10) to be improved. In the current studies, estimates of the improvement in AA AID ranged from 3.39 to 10.72% with an average improvement of 6.94%. However, when response curves for protein and phytase were based on the amount of each AA digested per kilogram of feed intake, improvements in AID were decreased. With the exception of His (1.97% decrease), all AA AID were improved from 0.59 to 4.53%, with an average of 2.3%. When improvements in CP digestibility due to the addition of 500 U of phytase per kilogram of diet were estimated as the sum of all AA digestibilities, CP AID was improved by 3.01%. This translates into a phytase equivalency value of 0.334 percentage units of CP for 500 U of phytase per kilogram of diet.
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IMPLICATIONS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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Footnotes |
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1 Corresponding author: jradclif{at}purdue.edu
Received for publication February 27, 2005. Accepted for publication November 16, 2005.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONIMPLICATIONSLITERATURE CITED |
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-amylase activity. J. Food Sci. 49:516–519.This article has been cited by other articles:
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T. A. Woyengo, J. S. Sands, W. Guenter, and C. M. Nyachoti Nutrient digestibility and performance responses of growing pigs fed phytase- and xylanase-supplemented wheat-based diets J Anim Sci, April 1, 2008; 86(4): 848 - 857. [Abstract] [Full Text] [PDF]
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