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The Effect of a Commercial Enzyme Preparation on Apparent Metabolizable Energy, the True Ileal Amino Acid Digestibility, and Endogenous Ileal Lysine Losses in Broiler Chickens

S. M. Rutherfurd^*, T. K. Chung and P. J. Moughan

^* Institute of Food, Nutrition and Human Health; Riddet Centre, Massey University, Palmerston North, New Zealand; and DSM Nutritional Products Asia Pacific Pte Ltd., Singapore

¹ Corresponding author: S.M.Rutherfurd{at}massey.ac.nz

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The effect of a commercial enzyme preparation containing xylanase, -amylase, and ß-glucanase on dietary AME content and the apparent and true ileal amino acid digestibility of a corn-soy broiler diet and endogenous ileal lysine flow was determined. Two predominantly corn-soy diets also containing wheat bran and canola meal were formulated; one diet contained no added enzymes, whereas the other was supplemented with -amylase, ß-glucanase, and xylanase. Titanium dioxide was included as an indigestible marker. The diets were given to broiler chickens, and AME and true ileal amino acid digestibility were determined. Portions of the 2 test diets were guanidinated and fed to similar aged broiler chickens and endogenous lysine flows determined. The chickens appeared healthy throughout the study, and the mean bird weights at the time of slaughter were not significantly different (P < 0.05) among any of the treatment groups. Dietary AME content was significantly (P < 0.05) higher for the enzyme-supplemented corn-soy diet (2,829 kcal/kg) compared with its unsupplemented control diet (2,766 kcal/kg). True ileal amino acid digestibility was significantly (P < 0.05) higher for all amino acids investigated. The increase ranged from 4% for arginine and glutamic acid to 12% for cystine. There was no significant difference in endogenous ileal lysine flow between broilers fed the unsupplemented diet and those fed the enzyme-supplemented diet. Overall, enzyme supplementation with an enzyme blend containing -amylase, ß-glucanase, and xylanase increased the AME content of a corn-soy broiler diet as well as apparent and true ileal amino acid digestibility for all amino acids, but had no effect on endogenous ileal lysine flow.

Key Words: lysine • digestibility • broiler • endogenous • corn-soy

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Some poultry diets, particularly those containing wheat or barley, contain significant quantities of non-starch polysaccharides (NSP), which act as antinutritional factors by increasing digesta viscosity leading to a decreased digestibility of starch, protein, and fat (Choct and Annison, 1990). Xylanase and ß-glucanase are enzymes that degrade NSP (Choct et al., 2004) and have been shown to improve the nutritive value of wheat-and barley-based diets for birds (Cowieson, 2005; Juanpere et al., 2005; Meng et al., 2005) by reducing the antinutritional effects of nonstarch polysaccharides (Preston et al., 2001; Choct et al., 2004).

Nonstarch polysaccharides are not found in high levels in traditional corn-soybean diets, however, and as a result, much less work has been conducted to investigate the effects of xylanase and ß-glucanase on corn-soybean broiler diets. Corn (Zea mays) is a commonly used ingredient in commercial poultry diets, and the nutritional value of corn can be quite variable depending on the content of starch, oil, protein, and anti-nutritional factors such as phytate, resistant starches, and enzyme inhibitors (Cowieson, 2005). Resistant starch can result from heat processing (Brown, 1996), starch granule structure (Tester et al., 2004), or interaction with other nutrients (Brown, 1996), and its presence can have a significant impact on the AME content of processed feedstuffs. Exogenous enzyme blends containing various combinations of amylases, proteases, xylanases, glucanase, cellulase, mannanase, and pectinase have been added to corn-soy poultry diets and found to improve bird performance (Zanella et al., 1999; Yu and Chung, 2004; Cowieson and Adeola, 2005), AME (Meng and Slominski, 2005; Saleh et al., 2005), and more specifically ileal protein digestibility (Zanella et al., 1999; Cowieson and Adeola, 2005; Meng and Slominski, 2005; Saleh et al., 2005) and ileal amino acid digestibility for some amino acids (Zanella et al., 1999). In contrast, other workers have shown no effect of supplementation of some enzyme preparations on AME (Scheideler et al., 2005) ileal digestible energy and nitrogen (Cowieson and Adeola, 2005) and protein, starch (Meng and Slominski, 2005), and fat digestibility (Zanella et al., 1999). The mechanism by which dietary AME content increases with enzyme supplementation is not clear. Zanella et al. (1999) and Meng and Slominski (2005) found no improvement in starch (the primary energy source) digestion after enzyme supplementation with a variety of enzymes. Slight increases in fat and NSP digestibility have been observed (Zanella et al., 1999; Meng and Slominski, 2005; Saleh et al., 2005), but whether protein digestibility is improved is not clear. Zanella et al. (1999) included a protease in their enzyme blend, which may have assisted in the digestion of dietary proteins. However, it is also possible that an increased starch and nonstarch polysaccharide digestion resulting from amylase and xylanase inclusion reduces endogenous protein losses in the gut of the broiler chicken.

The objective of the present work was to test a commercial enzyme preparation containing xylanase, -amylase, and ß-glucanase for an effect on dietary AME, apparent and true ileal amino acid digestibility and endogenous ileal lysine flow for broiler chickens given a corn-soy diet containing wheat bran and canola meal.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Diet Formulation
Two corn-soybean meal diets containing wheat bran and canola meal were prepared, one containing no supplementary enzymes (corn-soy) and the other containing 0.4 g of Ronozyme A (CT) and 0.1 g of Ronozyme WX (CT) added per kilogram of diet (corn-soy + enzyme). Ronozyme A (CT) consisted of 200 kilo-Novo -amylase units and 350 fungal ß-glucanase units/g of enzyme concentrate, and Ronozyme WX (CT) consisted of 1,000 fungal xylanase units/g of enzyme concentrate. Both enzyme preparations were obtained from Novozymes A/S, Bagavaerd, Denmark. Titanium dioxide (BDH Laboratory Supplies, Poole, UK) was included in each diet as an indigestible marker. The ingredient composition of the experimental diets is shown in Table 1 and the nutrient composition in Table 2.

Enzymes were added to the guanidinated corn-soy diet to give guanidinated corn-soy diet with 0.4 g of Ronozyme A (CT) and 0.1 g of Ronozyme WX (CT) added per kilogram of diet and an unmodified guanidinated corn-soy diet.

Determination of Apparent Metabolizable Energy
All experimental procedures were approved by the Massey University Animal Ethics Committee. One-day-old male (Ross 308) broiler chickens were obtained from a local hatchery and raised in battery brooders. From d 1 to 20 the birds received a standard corn-soy commercial starter diet. On d 21, 60 birds were randomly allocated to the 2 unguanidinated diets, one being the corn-soy control diet and the other being the corn-soy diet supplemented with 0.5% enzyme (-amylase, ß-glucanase, and xylanase), such that there were 30 birds per diet. The birds were group-housed in wire cages with constant light/dark at a temperature of 23 ± 2°C. Birds were randomly allocated such that there were 5 birds per cage and 6 cages (the experimental unit) per treatment. Water was available at all times. The experimental period lasted 8 d in total during which time total excreta were collected to determine the AME and ileal digesta were collected to determine ileal amino acid digestibility. The first 3 d of the trial served as an acclimatization period, and on d 4 the birds were weighed. Excreta were collected quantitatively for the next 4 d and food intakes measured. Excreta were pooled for birds within each cage and freeze dried and stored at –20°C prior to analysis for gross energy.

Determination of True Ileal Amino Acid Digestibility
On d 7 of the AME trial, the birds were fed ad libitum until 1600 h. To maximize food intake on d 8, the birds were fasted from 1600 h on d 7 until 0800 h on d 8 when the feeding regimen for that day recommenced. There was approximately 2 h from when the food was made available to the birds to when the slaughter of the birds commenced.

The broilers were killed by a lethal injection of sodium pentobarbitone (Pentabarb 300, Chemstock Animal Health Ltd., Christchurch, New Zealand), after which the body cavity was opened and the ileal section between the terminal ileum and the diverticulum was dissected out (Ravindran et al., 1999). Digesta were gently flushed out with distilled water, collected, and pooled from all birds within each cage before being freeze-dried ready for analysis for amino acids and titanium dioxide.

Determination of Endogenous Ileal Lysine Flow
Twenty-four birds were also selected on d 21 and randomly allocated to the 2 unguanidinated corn-soy diets, unsupplemented or supplemented with enzymes, such that there were 12 birds per diet. The birds were housed as described above except that there were 2 birds per cage and 6 cages (the experimental unit) per treatment. Water was available at all times. The experimental period lasted 8 d in total. The birds were fasted overnight on the penultimate day as described above. On the final day of the trial, the birds were intubated with 18–20 g of guanidinated diet such that the birds fed the unsupplemented diet during the first 7 d of the trial received guanidinated unsupplemented diet and birds fed the enzyme-supplemented diet on the first 7 d received the guanidinated enzyme-supplemented diet. Approximately 2 h after intubation the birds were killed and ileal digesta sampled as described above. Digesta were pooled for pairs of birds within each cage and freeze-dried and stored at –20°C prior to analysis for amino acids, including homoarginine and titanium dioxide.

Chemical Analysis
Gross energy was determined by bomb calorimetry using a Leco AC-350 Automatic Calorimeter (Leco Corporation, St. Joseph, MI).

Amino acid content was determined in duplicate 5 mg of diet and digesta samples using a Waters ion-exchange HPLC system, utilizing postcolumn ninhydrin derivatization and detection at 570 nm (440 nm for proline), following hydrolysis in 6 M glass-distilled HCl containing 0.1% phenol for 24 h at 110 ± 2°C in evacuated sealed tubes. Samples for cystine and methionine were analyzed in the same manner as described above except that they underwent prior oxidation by incubating in 30% H₂O₂: 88% formic acid (1:9) for 16 h at 4 ± 2°C (Moore, 1963). Tryptophan was not determined. Where appropriate, the weight of each amino acid was calculated using free amino acid molecular weights. Norleucine was used as an internal standard.

Titanium was determined based on the method of Short et al. (1996). Essentially samples were ashed before being digested in 60% (vol/vol) sulfuric acid. The mixture was then incubated with 30% H₂O₂ and the absorbance read at 405 nm.

Data and Statistical Analysis
Apparent energy metabolizability and apparent amino acid digestibility were calculated based on the equations of Cowieson et al. (2003). True ileal amino acid digestibility and endogenous ileal lysine flow were calculated based on the equations of Moughan and Rutherfurd (1990) where endogenous amino acid flows were those reported by Rutherfurd et al. (2004). The true ileal digestibility of lysine was calculated based on homoarginine digestibility and assuming that homoarginine is absorbed to the same extent as lysine.

A linear model with diet as a fixed effect was fitted to the data (GLM procedure, SAS, 1999). Differences between dietary treatments were tested using LSD where appropriate.

	RESULTS

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The chickens receiving the corn-soy diet appeared healthy throughout the study. The mean bird weights at the time of slaughter were not significantly different (P < 0.05) among any of the treatment groups.

The Effect of Enzyme Supplementation on the Apparent Metabolizable Energy of a Corn-Soy Broiler Diet
The apparent energy metabolizability and the AME content of the corn-soy diet fed to broiler chickens with or without enzyme supplementation are shown in Table 3. Apparent energy metabolizability for the corn-soy diet supplemented with enzyme was not significantly different (P = 0.095) from its unsupplemented counterpart. The AME content of the enzyme supplemented corn-soy diet was significantly (P < 0.05) higher than that for the unsupplemented corn-soy diet, although the actual difference was small (2.3%).

True ileal amino acid digestibility was calculated from the apparent amino acid digestibility data corrected using endogenous amino acid flows determined previously and reported by Rutherfurd et al. (2004; Table 5). As observed with the apparent amino acid digestibility data, true ileal amino acid digestibility was significantly (P < 0.05) higher with enzyme supplementation for all amino acids. For the unsupplemented corn-soy diet, true amino acid digestibility ranged from 76% for cystine to 100% for methionine. After enzyme supplementation, true amino acid digestibility ranged from 86% for cystine to 102% for methionine. Overall, when the corn-soy diet was supplemented with enzymes, the mean increase in true ileal amino acid digestibility across all amino acids, except cystine, was 6%. The increase in true ileal digestibility after enzyme supplementation ranged from 2% for methionine to 8% for threonine, serine, and glycine.

	DISCUSSION

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The Effect of Enzyme Supplementation on the Ileal Amino Acid Digestibility of a Corn-Soy Broiler Diet
Enzyme supplementation led to an increase in the apparent and true ileal amino acid digestibility of a corn-soy diet when fed to broiler chickens. The degree of improvement was small for some amino acids, such as methionine, where apparent and true ileal amino acid digestibility increased by 2–3% with enzyme supplementation. However, for some amino acids the increase in digestibility was sizeable. For all the amino acids (apart from methionine and arginine) enzyme supplementation improved apparent ileal amino acid digestibility by more than 5% compared with the unsupplemented corn-soy diet.

The result for cystine was exceptional when compared with the other amino acids with 23 and 12% increases in apparent and true ileal digestibility, respectively, when the corn-soy diet was supplemented with enzymes. This result is difficult to explain in the context of the other amino acids where increases in apparent and true ileal digestibility ranged from 3 to 11% and 2 to 8% respectively. The determined cystine concentration in the enzyme-supplemented diet was 31% higher than that of the unsupplemented diet, and the reason for this is unclear. The amino acid content of the enzyme supplement was also determined, but cystine from the enzyme-supplement contributed only a negligible amount to the total cystine content of the enzyme-supplemented diet.

The calculated apparent ileal digestibility of total nitrogen for the corn-soy diet fed to broilers in the present study was 81%. This value is similar to apparent ileal crude protein digestibility values reported for similar diets with similar aged broilers of 80% (Cowieson and Adeola, 2005, Meng and Slominski, 2005), 82% (Saleh et al. (2005), and 79% (Zanella et al., 1999). Meng and Slominski (2005) also fed their corn-soy diet to broilers after supplementation with xylanase (1,000 U/kg of diet), glucanase (400 U/kg of diet), pectinase (1000 U/kg of diet), cellulase (120 U/kg of diet), mannanase (280 U/kg of diet), and galactanase (180 U/kg of diet) and found that apparent ileal crude protein digestibility was increased by 5.5 to 84%. In the present study, we observed an increase in apparent ileal nitrogen digestibility of 7% with only the addition of xylanase, amylase, and glucanase. Saleh et al. (2005) found a 4% increase in apparent ileal crude protein digestibility with the supplementation of an enzyme blend containing cellulase, hemicellulase, and other unspecified enzymes, although this increase was not statistically significant. Zanella et al. (1999) reported an increase of 5% for the apparent ileal crude protein digestibility in broilers after supplementation of a corn-soy diet with xylanase (0.8 U/kg of diet), amylase (2 U/kg of diet), and proteinase (6 U/kg of diet).

Along with apparent ileal nitrogen digestibility, which has been discussed above, Zanella et al. (1999) also reported broiler apparent ileal digestibility values for individual amino acids for an unsupplemented corn-soy diet and one that had been supplemented with xylanase, amylase, and proteinase. Amino acid digestibility ranged from 74% for glycine to 96.2% for methionine in the unsupplemented diet and 78% for glycine to 97.4% for methionine in the enzyme-supplemented diet. In the present work and if the cystine data are excluded, then similar ranges in digestibilities for the corn-soy diet (72 to 88%) and the enzyme-supplemented corn-soy diet (79 to 92%) were observed to those reported by Zanella et al. (1999), even though the enzyme supplementation differed. In both studies methionine appeared to be the most digestible amino acid in the corn-soy diets with and without enzyme addition.

Endogenous Ileal Lysine Flow in a Corn-Soy Diet Unsupplemented or Supplemented with Enzymes
The conversion of lysine to homoarginine in the corn-soy diet was 90%, which was considered high given the insolubility and complexity of the material being guanidinated. The nitrogen content of the guanidinated corn-soy diet was 25% higher than in its unguanidinated counterpart, most likely due to residual o-methylisourea remaining in the diet after dialysis. The fat content of the guanidinated diet was 2.3% compared with 7.1% for the unguanidinated diet. Some lipid was probably lost during the centrifugation process in which soy oil floated in the supernatant and was unavoidably removed with the supernatant fraction. Methionine was also lower (by 6%) in the guanidinated diet, likely due to synthetic methionine being lost during the dialysis process. Additional fat, methionine, and water were thus added to the guanidinated corn-soy diet to make it more equivalent to its unguanidinated counterpart.

There was no effect of enzyme (-amylase, ß-glucanase, and xylanase) supplementation on the endogenous ileal lysine flow determined in broiler chickens fed the corn-soy diet. The determined endogenous ileal lysine flows for the guanidinated corn-soy diets [1,700 mg/kg of DM intake (DMI)] were high compared with some other published values. Other workers have reported endogenous ileal lysine flows in broiler chickens fed a protein-free diet of 235 mg/kg of DMI (Cremers et al., 2001), 209 mg/kg of DMI (Ravindran et al., 2004), and 240 mg/kg of DMI (Siriwan et al., 1994). Enzymatically hydrolyzed casein (EHC) based diets have also been used to determine endogenous lysine flows (Cremers et al., 2001; Ravindran et al., 2004), but in these cases the flows did not agree well across studies and ranged from 266 mg/kg for a diet that contained 11% EHC (Cremers et al., 2001) to 1,048 mg/kg of DMI for a diet that contained approximately 19% EHC (Ravindran et al., 2004).

Broiler chickens have been fed a diet containing guanidinated casein as the sole protein source to determine endogenous ileal lysine flows (Siriwan et al., 1994; Ravindran et al., 2004). In these cases endogenous lysine flows ranged from 560 mg/kg of DMI for a diet containing 26% guanidinated casein (Siriwan et al., 1994) to 783 mg/kg of DMI for a diet containing 19% guanidinated casein (Ravindran et al., 2004). When guanidinated casein diets have been used, the endogenous ileal lysine flow measurements appear to be highly variable between studies. The diet used by Ravindran et al. (2004) contained only 73% of the amount of casein in the diet used by Siriwan et al. (1994), yet Ravindran et al. (2004) reported endogenous lysine flows 1.4 times higher than those reported by Siriwan et al. (1994). Chung and Baker (1992) also determined endogenous amino acids for cecectomized cockerels (2,350 g of body weight) intubated with a protein-free diet or a casein-based diet. They found an endogenous lysine loss of 1,026 mg/kg of DMI for the birds fed the protein-free diet and 1,577 mg/kg of DMI for the birds given the casein based diet, where casein was assumed to be completely digested and absorbed. The endogenous ileal lysine flows determined in the present study were high compared with those observed for most of the other studies, but were similar to those observed by Chung and Baker (1992). The higher values reported here may reflect the influence of antinutritional factors and plant fiber present in the corn-soy diet (Zanella et al., 1999; Cowieson, 2005). Endogenous amino acid flows are also dependent on the protein level in the diet (Hodgkinson et al., 2000; Cremers et al., 2001), and the guanidinated diets used in this study contained approximately 23% protein, a higher level than found in most other published studies.

The finding that enzyme (-amylase, ß-glucanase, and xylanase) supplementation does not influence ileal endogenous amino acid loss is novel and suggests that reported effects of feed enzymes on amino acid digestibility are effects on the actual breakdown of protein and absorption of the amino acids rather than indirect effects mediated by changes in endogenous protein.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The enzyme blend used in this study (-amylase, ß-glucanase, and xylanase) increased the dietary AME content of a corn-soy diet containing wheat bran and canola meal. Enzyme supplementation also improved apparent and true ileal amino acid digestibility. This improvement was not by way of reducing the endogenous ileal amino acid flows because enzyme supplementation had no effect on endogenous ileal lysine flow. It is important to recognize that the corn-soy diet used here contained appreciable amounts of wheat bran and canola meal that would have supplied NSP and potentially antinutritional factors. The extent to which the wheat bran and canola meal contributed to nutrient digestibility or were affected by the addition of feed enzymes has not been quantified in this study.

	ACKNOWLEDGMENTS

 
We would like to acknowledge financial support for this study from DSM Nutritional Products Asia Pacific Pte Ltd., Singapore.

Received for publication October 9, 2006. Accepted for publication December 17, 2006.

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