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Poult Sci 2008. 87:706-712. doi:10.3382/ps.2007-00330
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METABOLISM AND NUTRITION

Comparison of Various Methods for Endogenous Ileal Amino Acid Flow Determination in Broiler Chickens

A. Golian*, W. Guenter{dagger}, D. Hoehler{ddagger}, H. Jahanian* and C. M. Nyachoti{dagger},1

* Department of Animal Sciences, Ferdowsi University of Mashhad, Mashhad, Iran 1163-91775; {dagger} Department of Animal Sciences, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2; and {ddagger} Evonik Degussa Corporation, Kennesaw, GA 30144-3694

1 Corresponding author: martin_nyachoti{at}umanitoba.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The purpose of this study was to compare estimates of ileal endogenous amino acid (IEAA) losses determined in broiler chicks with a nitrogen-free diet (NFD), diets containing highly digestible proteins as intact casein or enzymatically hydrolyzed casein (EHC), and the regression method (RM). Male Ross 308 broiler chicks were fed a commercial starter diet from d 1 to 15 of age and the following test diets from d 15 to 21: NFD and diets containing 5, 10, or 15% casein or EHC as the sole protein source. All diets contained chromic oxide as a digestibility marker. Each diet was assigned to 6 replicate cages, with 10 birds each. On d 21, birds were euthanized to sample ileal digesta. Feeding increasing levels of EHC or casein linearly (P < 0.0001) increased IEAA flow. Nonlinearity (P < 0.05) was detected for Ile, Val, Glu, and Ser with EHC and for Met with casein. Compared with the NFD method, IEAA values extrapolated to zero EHC or casein intake were similar for all amino acids except Ile (200 vs. 321 mg/kg of DM intake), Val (270 vs. 341 mg/ kg of DM intake), Ala (217 vs. 262 mg/kg of DM intake), and Ser (343 vs. 577 mg/kg of DM intake), whose flows were higher with EHC (P < 0.05). The present results show that IEAA values determined with the NFD, EHC, and casein diets, at different inclusion levels, were different for most amino acids and that, for all amino acids, the values obtained with NFD and RM involving feeding graded levels of casein or EHC were comparable. Thus, using IEAA values obtained with either the NFD or the RM to calculate standardized ileal amino acid digestibility coefficients will give similar values.

Key Words: chick • highly digestible protein • ileal endogenous amino acid flow • nitrogen-free diet • regression


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The nutritional value of feedstuffs is largely determined by their content of available nutrients, in particular amino acids (AA). Ileal digestibility coefficients have been used to show the availability of AA in broiler chickens. A major aspect of ileal AA digestibility (IAAD) determination is the influence of endogenous AA excretion measured at the terminal ileum. The digestive tract secretes large quantities of enzymes and other nitrogenous compounds into the gut lumen (Fauconneau and Michel, 1970; Alpers, 1987). The proportions of this endogenous nitrogen secretion that are directly related to the amount of DM intake (DMI) are referred to as the nonspecific or basal losses, whereas the additional losses that are dependent on the composition of the test material are referred to as specific losses (Butts et al., 1993; Donkoh and Moughan, 1994). Part of this endogenous protein is digested and reabsorbed (Buraczewski, 1980), and the remaining protein flows to the terminal ileum with the undigested feed materials and affects the measurements of the apparent IAAD (AIAAD) coefficients. The true IAAD (TIAAD) coefficients of feedstuffs can be obtained through correction of AIAAD for the actual (i.e., ingredient-specific losses) ileal endogenous amino acid (IEAA) flow. The TIAAD coefficients take into account the basal and specific IEAA losses, and the latter depends on the composition of the feed ingredient. One way to include specific losses is to feed the animal with a graded level of the test material and measure the slope of the regression line that corresponds to TIAAD (Rodehutscord et al., 2004; Kluth et al., 2005; Kluth and Rodehutscord, 2006). The problem with this method is the calculation of TIAAD for different levels of a test material, which becomes expensive and time consuming. This assay method may also be criticized on the basis that the estimation of actual IEAA is constrained by linear functions, which may not be a true description of the real physiological functions (Donkoh et al., 1995).

The standardized IAAD (SIAAD) method, which involves correcting AIAAD coefficients for basal IEAA losses, has been suggested as a means to overcome the technical problems with determining the actual IEAA losses and yet obtain digestibility coefficients that are adequate for practical feed formulation (Ravindran and Bryden, 1999; Stein et al., 2001; Lemme et al., 2004). Indeed, estimates of SIAAD coefficients have been shown to have a very small variability with the assay condition (Donkoh and Moughan, 1994; Ravindran and Bryden, 1999; Stein et al., 2001). However, different methods such as a nitrogen-free diet (NFD; Muramatsu, 1990; Stein et al., 2001) or diets containing casein, enzymatically hydrolyzed casein (EHC), or crystalline AA (Darragh et al., 1990; Moughan and Rutherfurd, 1990; Butts et al., 1991) and the regression method (Fan et al., 1995) have been used and may produce different basal IEAA values and lead to different SIAAD coefficients of a test material. The purpose of this experiment was to compare the IEAA estimates in broiler chicks fed NFD, EHC, or casein diets, the regression method, or both based on feeding graded levels of highly digestible protein (HDP).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
The protocol for this experiment was reviewed and approved by the University of Manitoba Animal Care Committee, and chicks were cared for according to the Canadian Council on Animal Care (1993) guidelines.

Experimental Diets

The 7 test diets used were an NFD and diets containing 5, 10, and 15% casein or EHC as the sole source of protein. The composition of all experimental diets and the analyzed AA concentrations are shown in Tables 1Go and 2Go. A standard commercial starter diet containing 21% CP and 3,100 kcal/kg was fed to chicks to 15 d of age before they were placed on the experimental diets. Chromic oxide was included in the experimental diets at 3 g/kg as a digestibility marker and was used to calculate IEAA flow values.


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  		Table 1. Composition of experimental diets (g/kg, as-fed basis)
 

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  		Table 2. Analyzed dietary composition (g/kg of diet) of nitrogen-free diet (NFD), and diets containing graded levels of casein or enzymatically hydrolyzed casein (EHC) on a DM basis
 
Chicks, Housing, and Feeding

A total of 420 one-day-old male chicks (Ross 308, Aviagen, Carlton Hatchery, Grunthal, Manitoba, Canada) were randomly divided into 42 groups of 10 chicks each and assigned to 42 cages in battery brooders. On d 15, all groups were reweighed to ensure similar BW among all groups. Six groups of chicks were randomly assigned to each of the 7 test diets from 15 to 21 d of age. Fresh water and feed were available to all chicks for ad libitum intake throughout the experiment. The chicks were kept in battery cages in an environmentally controlled room with 24 h of florescent light. Room temperature was maintained at 32, 28, and 24°C for wk 1, 2, and 3, respectively.

Sampling and Ileal Digesta Processing

All chicks were euthanized by cervical dislocation and ileal digesta (from Meckel’s diverticulum to 3 cm proximal to the ileo-cecal junction) from all chicks within each group were gently and quickly squeezed out, pooled into a bag, and placed on ice. The time to collect the digesta sample was within approximately 1 min after a bird was killed. Digesta samples were transferred to a –20°C freezer within 1 h and freeze-dried the next day.

Chemical Analysis

Dietary DM was determined according to the method of AOAC (1990). Diets and freeze-dried digesta samples were finely ground in a coffee grinder (CBG5 Smart Grind, Applica Consumer Products Inc., Shelton, CT), and CP (N x 6.25) was determined by using a N analyzer (model CNS-2000, Leco Corp., St. Joseph, MI). Chromium was determined according to the procedure described by Williams et al. (1962). The AA compositions of the diet and digesta samples were determined by Degussa AG (Hanau-Wolfang, Germany). Dietary AA contents were determined by using ion-exchange chromatography with postcolumn derivatization with ninhydrin (Llames and Fontaine, 1994). Amino acids were oxidized with performic acid, which was neutralized with sodium metabi-sulfite (Commission Directive, 1998). Samples were quantified with the internal standard method by measuring the absorption of reaction products with ninhydrin at 570 nm. For Trp determination, samples were hydrolyzed with barium oxide. Chromium and AA analysis were performed in duplicate and singly, respectively.

Calculations

The IEAA flows, expressed as milligrams per kilogram of DMI, were calculated by using the following equation (Moughan et al. 1992):


Formula

Statistical Analysis

Data were analyzed by using the GLM procedure of SAS (SAS Institute, 1990). Orthogonal polynomial contrasts in linear regression were used to compare the means of different inclusion levels. The LSD was used to separate the differences between treatment means when the F-ratio was significant. Regression of IEAA flows on protein levels of casein or EHC were determined, and the extrapolation of the regression lines to zero for the 2 protein sources and their slopes were compared. Nonlinearity in the regression equation was evaluated.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The average of IEAA flow in chicks fed graded levels of EHC, casein, or NFD diets are shown in Tables 3Go and 4Go, respectively. A linear (P < 0.0001) increase in the IEAA values with increasing levels of EHC or casein were observed for all AA. Nonlinearity (P < 0.05) was detected for Ile, Val, Glu, and Ser with EHC and for Met with casein. In the present study, the endogenous AA with the larger flow at the terminal ileum of chicks fed the EHC diet were Glu, Asp, Pro, Ser, and Thr, whereas the AA with the lowest quantity were Met and Cys (Table 3Go). The order of the larger ileal endogenous flow of AA in casein-fed birds were Glu, Ser, Asp, Thr, and Pro, and the lowest were similar to those of EHC-fed birds (Table 4Go). The sum of these 5 AA nearly doubled as the level of casein or EHC increased from 5 to 15% in the diet (Tables 3Go and 4Go), and their proportions relative to total IEAA were 58, 62, and 64% when chicks were fed diets containing 5, 10, and 15% HDP, respectively. The linear increase in the flow of endogenous AA is probably related to the presence and levels of dietary EHC, casein, or their substrates in the gastrointestinal tract, which may negatively affect the breakdown and reabsorption of endogenous protein (Snook and Meyer, 1964), thus increasing the quantity of endogenous AA at the terminal ileum. Darragh et al. (1990) assumed that the EHC may be absorbed completely in the small intestine of rats and that IEAA values from animals fed this diet may be used to calculate the TIAAD coefficients. Butts et al. (1993) reported that a higher AA excretion at the terminal ileum of pigs is related to the presence of dietary protein or peptides in the gut. Adedokun et al. (2007a,b) reported a large increase in IEAA flow in broiler chicks and turkey poults fed graded levels of casein and suggested that chicks and poults responded to stimulatory effects of dietary protein by either increasing endogenous secretions or by increasing enterocyte turnover. Similarly, Moughan and Rutherfurd (1990) found that the IEAA values measured in casein-fed rats was higher than those fed a NFD, which is in agreement with our data from chicks fed either the EHC or casein diet at all inclusion levels. The higher level of IEAA values in EHC- or casein-fed birds reflects the influence of highly digestible proteins on ileal endogenous secretions. The results of our study with chicks as well as data from pigs (Butts et al., 1993) and growing rats (Darragh et al., 1990; Moughan and Rutherfurd, 1990; Donkoh et al., 1995) revealed that the measurement of endogenous AA flow at the terminal ileum of animals fed these highly digestible proteins may vary with the dietary protein level and is different from the basal endogenous AA losses (with the NFD) that may be used to standardize (Stein et al., 2001) the AIAAD of feed ingredients. It is thus obvious that using the IEAA data from casein- or EHC-fed birds to calculate the SIAAD coefficients may produce different values than those obtained from the NFD-fed birds. Although, feeding the NFD is the most used method to determine ileal endogenous losses, this approach has been criticized as being nonphysiological (Low 1980) and may lead to a lower rate of body protein synthesis (Moughan and Rutherfurd, 1990; Muramatsu, 1990; Butts et al., 1991), which in turn may affect the estimates of IEAA flow. It is suggested that the protein-depleted state per se does not affect basal IEAA flow (de Lange et al., 1989), and it appears that the dietary protein in the gut directly stimulates endogenous protein synthesis and secretions. Darragh et al. (1990) reported a similar estimate for IEAA flow in rats fed either crystalline AA (assumed balanced AA diet) or NFD and suggested that the negative body nitrogen retention that occurred in NFD-fed rats may not influence or lower the IEAA losses per se. Therefore, it seems that the proportion of endogenous secretions such as mucoproteins, enzymes, and desquamated cells is dependent on the level of protein in the diet. Lemme et al. (2004) reported that the large quantity of AA in mucoproteins were Glu and Asp in broilers, whereas Lien et al. (1997) isolated the ileal mucin of pigs and found high quantities of Thr, Pro, and Ser. Our results indicate that the AA in larger proportion of total IEAA (mucoproteins, enzymes, and desquamated cells) in birds fed the NFD or HDP diets were similar to those found previously in the mucoproteins of birds (Lemme et al., 2004) and growing pigs (Lien et al., 1997). The low proportions of the ileal endogenous Met and cystine flow in birds fed any of the diets in our study indicated that these AA are either highly absorbed or are secreted into the gastrointestinal tract at low levels.


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  		Table 3. Ileal endogenous amino acid flow (mg/kg of DM intake) in chicks fed 4 levels of enzymatically hydrolyzed casein (EHC) diets
 

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  		Table 4. Ileal endogenous amino acid flow (mg/kg of DM intake) in chicks fed 4 levels of casein diets
 
The IEAA flow values obtained with the NFD and the regression method (RM) are reported in Tables 5Go. The ileal endogenous flows of AA were similar regardless of whether they were obtained by using the NFD or the RM with EHC or casein (P < 0.05) except for Ile, Val, Ala, and Ser, whose values were significantly (P < 0.05) higher with EHC. The slope of the regression lines obtained from the EHC or casein diets were similar (P > 0.05) for all AA except for His, Ile, and Ser. The RM has been used extensively to predict the flow of IEAA, and it is assumed that the values obtained reflect those to be expected when animals are fed protein-containing diets. Other researchers have reported similar results with the NFD and the RM (Donkoh et al., 1995; Fan et al., 1995). When protein-free diets are used, all AA found in the ileal digesta are assumed to be of endogenous origin, whereas when the RM is used with feeding graded levels of protein, the recovery of the AA at the terminal ileum is related to the protein intake (Nyachoti et al., 1997). It is postulated that the increase in AA flow with the increase in dietary protein intake at a constant DMI is entirely related to undigested feed protein if there is no change in the amount of endogenous AA secretion. However, there is evidence that the quantity of endogenous secretion increases in response to protein consumed (Temler et al., 1983; Moughan and Rutherfurd, 1990; Adedokun et al. 2007a,b). Consequently, some, if not all, of the increase in AA flow that occurs with an increase in dietary protein intake is possibly the result of enhanced secretion of endogenous proteins. Compared with the NFD method, the RM appears to provide a better estimate of IEAA (Nyachoti et al., 1997). However, results of the present study in chickens support previous findings that the NFD and the RM provide similar values for ileal endogenous flow of AA in swine (Fan et al., 1995), in rats (Donkoh et al., 1995), and in 15- or 21-d-old broiler chicks and poults (Adedokun et al., 2007a,b). A linear relationship between dietary levels of casein or EHC and endogenous ileal flow of all AA in the present study supports the theory of a stimulatory effect of dietary protein on endogenous protein synthesis and secretion into the gastrointestinal tract. The higher flow of IEAA observed because of feeding increasing levels of casein and EHC as the sole sources of dietary protein presents a greater challenge in deciding the appropriate level of these HDP to be used in generating the estimates of IEAA. The impact of protein as well as fiber and other antinutrients on endogenous AA losses is well established. The IEAA losses in NFD-fed animals are also related to the physical flow of feed DM through the digestive tract and are not influenced by the dietary composition. In addition, it has been reported that the basal IEAA losses in milligrams per kilogram of DMI decrease with an increase in DMI (Butts et al., 1993) or an increase in the live BW of animals (Hess and Séve, 1999), or both. These findings should be carefully considered when determining the basal endogenous losses with the NFD method in broiler chicks.


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  		Table 5. Ileal endogenous amino acid (mg/kg of DM intake) flows in 21-d-old broiler chicks fed a nitrogen-free diet (NFD), and flows from the regression method when the regression line was extrapolated to a 0% casein or enzymatically hydrolyzed casein (EHC) level in the diet
 
An accurate and consistent determination of endogenous AA losses at the terminal ileum or in the feces is required to evaluate the nutritional value of feedstuffs for poultry. The nutritional value of feedstuffs is largely determined by their content of available nutrients, in particular AA. Availability is defined as the proportion of ingested AA that is absorbed from the digestive tract of the animal (Nyachoti et al., 1997). The conventional rooster assay method (fecal method) has been criticized for providing digestibility coefficients of feedstuffs in cecectomized roosters that may not be applicable to young chicks. Different IEAA determination methods have been used in young chicks in the past 3 decades, which have generated different basal or actual values for IEAA flow. These different basal or actual endogenous flow values have been used to convert the AIAAD to SIAAD or TI-AAD coefficients, respectively. This variability in IEAA determination provides variable TIAAD or SIAAD coefficients for feedstuffs, which in turn make them unreliable to be used in broiler chicken feed formulations.

Results of the present study show that the ileal endogenous flow determined with NFD or with protein-containing diets are different for all AA. However, it is not known whether these differences in the IEAA values obtained with NFD or different levels of EHC or casein will lead to significant differences on the SIAAD of grains or protein sources for chickens. For the majority of AA, the estimates of basal ileal endogenous flow determined with either the NFD or the RM method involving graded levels of casein or EHC diets were comparable. Thus, using the basal IEAA estimates obtained with either the NFD or RM method to calculate the SIAAD coefficients will produce similar values. Given that the SIAAD estimates are independent of dietary protein level and composition, using the NFD method, which is relatively simple, may allow feed ingredients to be compared more accurately, thus leading to a greater consistency in feed evaluation.


   ACKNOWLEDGMENTS
 
The authors would like to thank Garry Crow for his assistance in statistical analysis. The authors also would like to thank Harry Muc for his technical assistance. Funding for this study was provided by Evonik Degussa Corporation (Kennesaw, GA) and the Natural Sciences and Engineering Research Council of Canada (Ottawa, Ontario).

Received for publication August 8, 2007. Accepted for publication December 24, 2007.


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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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