HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Adedokun, S. A. Articles by Applegate, T. J. Search for Related Content PubMed PubMed Citation Articles by Adedokun, S. A. Articles by Applegate, T. J.

Endogenous Amino Acid Flow in Broiler Chicks Is Affected by the Age of Birds and Method of Estimation¹

S. A. Adedokun^*, C. M. Parsons, M. S. Lilburn, O. Adeola^* and T. J. Applegate^*,2

^* Department of Animal Sciences, Purdue University, 915 W. State Street, West Lafayette, IN 47907-2054; Department of Animal Sciences, University of Illinois, Urbana-Champaign 61801; and Department of Animal Sciences, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691

² Corresponding author: applegt{at}purdue.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The determination of ileal endogenous amino acid (IEAA) and total amino acid (TAA) concentrations is critical to standardizing amino acid digestibility coefficients. The IEAA and TAA flow in broiler chicks was measured on d 5, 15, and 21 at 2 geographical locations. Chicks were fed a nitrogen-free diet (NFD) or graded levels of casein (50, 100, or 150 g/kg of diet), which were assumed to be highly digestible. The IEAA and TAA flow (mg/kg of DM intake) on d 5 in chicks fed the NFD was higher (P < 0.05; Met = 154, Thr = 539) than values for d 15 (Met = 51, Thr = 274) and d 21 (Met = 50, Thr = 274). A comparison of the regression and NFD analyses showed higher (P < 0.05) flows on d 5 for those birds fed the NFD diet, with no differences between methods on d 15 and 21, with the exception of Lys, Met, and Glu (d 21). At all 3 ages evaluated, there was a linear increase (P < 0.05) in IEAA and TAA flows with increasing level of dietary casein. There was a decrease in IEAA and TAA flows that stabilized between d 15 and 21. The result suggests that the NFD and the regression method will give similar results on d 15 and 21. These observations also suggest that the relatively low apparent amino acid digestibility coefficients for broiler chicks at a younger age may be the result of a significantly high level of endogenous amino acid flow.

Key Words: casein • chick • endogenous amino acid flow • nitrogen-free diet • regression

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
To formulate commercial diets for chickens most accurately, it is important to have accurate estimates of nutrient availability from common feed ingredients. With respect to protein or amino acid digestibility, digesta samples taken from the terminal ileum minimize some of the biological variability associated with the excretion of the protein by-products (e.g., urine nitrogen) into the terminal portion of the gastrointestinal tract and avoids the effects of microbial fermentation in the hindgut (Lemme et al., 2004). Apparent ileal digestibility of proteins is widely accepted and used, but it does not accurately reflect the true digestibility of various feed ingredients because it does not account for basal and diet-specific endogenous amino acid flows. The basal endogenous nutrient flow of amino acids and nitrogen contributes to apparent digestibility values and arises from various sources, including salivary and digestive secretions (enzymes), sloughed epithelial cells, and intestinal microbes (Nyachoti et al., 1997). In addition, up to 25% of total daily protein synthesis in pig is secreted into the gastrointestinal tract (Simon et al., 1983). In addition to the basal endogenous amino acid flow, diet-specific endogenous contributions have also been identified. Although the latter is difficult to measure, the basal endogenous amino acid flow has been measured by using different methods (Ravindran and Bryden, 1999; Lemme et al., 2004), and these values can be used to correct apparent amino acid digestibility coefficients to obtain standardized ileal digestibility coefficients.

The basal endogenous amino acid flows have been estimated in broilers, layers, and roosters (Ravindran and Hendriks, 2004) at different ages (6, 15, 21, 28, 80 wk) by using different techniques (Ravindran et al., 2004). In most cases, there were significant effects of age on ileal endogenous amino acid (IEAA) flow. The different methods of basal endogenous amino acid and nitrogen estimation have included nitrogen-free diets (NFD; Sibbald, 1987), sources of highly digestible protein (HDP), the homoarginine method by using the guanidination reaction, enzyme-hydrolyzed casein (Moughan et al., 1990), and the regression method. Given the fact that the nutrient composition of broiler diets is similar for approximately 2 to 3 wk of age as practiced in North America, it is important to evaluate the effects of age and the method of estimation on the quantity of IEAA and total amino acid (TAA) flow in the terminal ileum. Likewise, digestibility estimates are usually determined at different sites (i.e., laboratories), so it is also important to estimate the variability in IEAA flow attributable to laboratory.

In the current study, we estimated IEAA and TAA flow in chicks at different ages (d 5, 15, and 21) by using 3 different methods (NFD, HDP, regression). The same study was conducted in 2 geographical locations to determine the variability attributable to age by experimental location and diet by location. Our hypothesis was that IEAA and TAA flow were not age and geographical location dependent. We assumed in this study that casein, at a low dietary concentration, is completely digested and that amino acids are completely absorbed.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Diet Formulation
Four semipurified diets were formulated to contain graded levels of casein. Casein, which was the only source of protein in the diet, was added at 0, 50, 100, or 150 g/ kg of diet. The composition of all experimental diets and the analyzed amino acid concentrations are reported in Tables 1 and 2. A positive control diet that met or exceeded the NRC (1994) recommendations was also made. This was the diet that chicks were fed before they were placed on the 4 experimental diets. For the calculation of IEAA and TAA flow, chromic oxide was added to all treatment diets at 3 g/kg of diet as an indigestible marker. All the diets were formulated and made from the same batch at a single location.

For the entire period of the study, chicks were raised in battery cages (Alternative Design Manufacturing and Supply Inc., Siloam Springs, AR) in an environmentally controlled room with 24 h of light. Room temperatures for the first, second, and third week were 35, 30, and 25°C, respectively. Feed and water were provided ad libitum. All animal care procedures were approved by the Purdue University and University of Illinois Animal Care and Use Committees.

Sampling and Ileal Digesta Processing
On d 5, 15, and 21, after the birds were euthanized by CO₂ asphyxiation, digesta from the ileal region between Meckel’s diverticulum to 5 mm proximal to the ileocecal junction was flushed with distilled water. For those birds sampled on d 5, a 50-mL syringe was used for flushing, whereas a wash bottle was used on d 15 and 21. The ileal digesta from all birds within a cage were pooled, frozen, and stored at –40°C, freeze-dried, and pulverized using a mortar and pestle.

Chemical Analysis
Dry matter content was determined on ground diets and ileal digesta by drying the samples at 100°C for 24 h. For amino acid analyses, samples were hydrolyzed in 6 N HCl for 24 h at 110°C under a nitrogen atmosphere. Sulfur-containing amino acids (Met and Cys) were acid-hydrolyzed after performic acid oxidation. For Trp analysis, samples were hydrolyzed by using barium hydroxide. The amino acids in the hydrolysate were determined by HPLC after postcolumn derivatization (AOAC, 2000; method 982.30 E [a, b, c]). Amino acid concentrations were not corrected for any incomplete recovery resulting from hydrolysis. Chromium was determined by the inductively coupled plasma atomic emission spectroscopy method (AOAC, 2000; method 990.08) following nitric and perchloric acid wet ash digestion.

Calculations
Ileal endogenous amino acid and TAA flows were calculated as milligrams of amino acid or TAA flow per kilogram of DM intake (DMI) by using the formula proposed by Moughan et al. (1992):

Statistical Analysis
Data were analyzed by using the GLM procedure of SAS (SAS Inst. Inc., Cary, NC). Orthogonal polynomial contrasts (linear) were used to compare the treatment means. Values for the regression method were obtained by regressing IEAA flow against dietary casein levels. Differences between treatment means were separated by using the Tukey adjustment where F-ratios indicated significance. A comparison of the IEAA and TAA flows between the NFD and the regression methods was made by calculating the SE of difference of means as outlined in Samuels and Witmer (1999). The probabilities of the t-values were determined by using the t-test of SAS.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Birds in the 2 geographical locations were in a good health condition throughout the duration of the study. Mortality at both locations was 2% or less for the entire experimental period. Five-day average feed intakes per bird (mean ± SEM) for birds sampled on d 5, 15, and 21 were 29 ± 0.8, 119 ± 3.6, and 163 ± 5.3 g for the NFD; 29 ± 0.8, 114 ± 3.9, and 147 ± 5.3 g for the 50 g of casein/kg of diet; 29 ± 0.8, 115 ± 3.6, and 166 ± 5.8 g for the 100 g of casein/kg of diet; and 34 ± 0.8, 132 ± 3.6, and 193 ± 5.3 g for the 150 g of casein/kg of diet. Mean BW (gain or loss) of birds on the NFD were 2 ± 0.5, –6 ± 1.6, and –19 ± 4.4 g for d 5, 15, and 21, respectively. When the diet containing 50 g of casein was fed, mean weight gain was 8 ± 0.5, 6 ± 1.7, and –8 ± 4 g for birds sampled on d 5, 15, and 21, respectively. The corresponding BW gain for birds fed diets containing 100 or 150 g of casein/kg of diet was 13 ± 0.5, 18 ± 1.6, and 22 ± 4.8 or 18 ± 0.5, 39 ± 1.6, and 39 ± 4.4 g, respectively, for birds sampled on d 5, 15, and 21.

Ileal endogenous amino acid and TAA flow in the terminal ileum of broiler chicks on d 5, 15, and 21 are shown in Tables 3, 4, and 5. There was no geographical location by diet interaction for any amino acid or TAA on d 5 (Table 3). A significant interaction between geographical location and diet for some of the amino acids was observed on d 15 and 21 with diets containing the 4 levels of casein (Tables 4 and 5). Ileal endogenous amino acid flow increased linearly with increasing levels of casein at all ages (P < 0.05). When 0 (NFD), 50, or 100 g of casein/ kg of diet was fed (d 5), the following IEAA were found, as a percentage of the amount from the 150 g of casein/ kg of diet: Met 39, 70, and 84; Thr, 50, 73, and 82; and TAA 41, 71, and 82.

There was a significant interaction between geographical location and diet on d 21, with the exceptions of Arg, Leu, Lys, Met, Phe, Cys, and Tyr (Table 5). There was a similar trend in IEAA and TAA flow at both geographical locations (data not shown), with a linear increase (P < 0.05) from 0 to 150 g of casein/kg of diet.

The IEAA and TAA flow when NFD was fed, as well as the comparison of flows between the NFD and the regression methods are reported in Table 6. There were no significant interactions between age and geographical location for chicks fed the NFD, so the data from the 2 geographical locations were pooled. Ileal endogenous amino acid flow on d 5 was greater (P < 0.05) than on d 15 and 21. There was no significant difference in IEAA and TAA flow between d 15 and 21. The amino acids with the greatest flow were Glu, Asp, Leu, and Thr. The IEAA flows on d 21 for Met, Thr, and TAA were, respectively, 32, 51, and 46% of the respective flow on d 5. A comparison of IEAA and TAA flows determined with the NFD and the regression methods showed that on d 5, flows were greater (P < 0.05) for the regression method. On d 15 and 21, however, both methods produced similar results, except for Lys, Met, and Glu, where the estimates were higher on d 21 for the NFD method (Table 6).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

A number of studies have evaluated IEAA flow in chickens by using different approaches, different classes of chickens, and chickens at different ages (Ravindran and Hendriks, 2004). The uniqueness of the data herein is that IEAA flow was determined at relatively young ages (d 5, 15, and 21) by using 3 different methods (NFD, feeding of HDP, and regression). Nyachoti et al. (1997) reported that each of these methods is based on certain assumptions with its own set of limitations, and we hoped that data generated from the same populations of birds at similar ages by using the 3 different methods would enable us to compare results based on the different assumptions on which they are based.

The IEAA and TAA flows were highest on d 5 whether the determination was made by NFD, HDP, or the regression method. Ileal EAA flows increased with increasing level of dietary casein at the 3 ages evaluated in this study. The amino acids with the greatest flow were Glu, Asp, Leu, Thr, Pro, and Ser. This result may have originated from mucin produced in the gastrointestinal tract. However, the effect of a high level of Glu in casein cannot be ruled out. The relatively low levels of Met and His in the ileal digesta could be attributed to the fact that these amino acids, particularly Met, are absorbed in the greatest proportions in the gastrointestinal tract (Webb, 1990). The high levels of Glu and Asp in the flow could reflect the importance of these amino acids, especially Glu, in gastrointestinal tract metabolism. Because of the nature of the diet (NFD, HDP), the sources of endogenous secretion could be mucoproteins, sloughed cells, or various digestive secretions (enzymes). These results suggest that at a younger age (d 5), the contribution of endogenous amino acids to total ileal digesta amino acids is higher (approximately 2 times) than at d 15 and 21. The endogenous secretions, however, remained stable between d 15 and 21. By using the data from the amino acids with the highest flow, one can argue that when an NFD is fed, the makeup of endogenous amino acids is largely from mucoproteins, which have been reported to be high in Glu, Asp, Ser, Thr (Lemme et al., 2004), and Pro (Ravindran and Hendriks, 2004). Hence, the difference observed in the flow of different amino acids with increasing age could be attributed to a decreased rate of mucin secretion with age, an increased rate of digestion and absorption of endogenous proteins (Nasset, 1972; Ravindran and Bryden, 1999), or both. The relative amount of endogenous amino acid flow will be largely a function of the major source of endogenous secretion into the gut at any given point in time.

Comparison between the NFD and the regression methods showed that the amino acid flow estimate for the regression method (d 5) was higher. However, with age there was no difference in IEAA flow for most of the amino acids.

An increase in IEAA flow resulting from increased dietary protein agrees with findings in growing rats (Darragh et al., 1990; Moughan and Rutherfurd, 1990) and pigs (Butts et al., 1993). The effects of negative nitrogen balance on IEAA flow in pigs were reported previously (De Lange et al., 1989; Butts et al., 1993), and these studies concluded that negative whole-body nitrogen balance did not result in lower endogenous Lys losses (and amino acids in general) when an NFD was fed along with a parenteral infusion of balanced amino acids or saline. Pancreatic secretion has been reported to have been stimulated by the presence of peptides in the gastrointestinal tract (Brannon, 1990). This is accomplished by stimulating the gastrointestinal tract to increase the secretion of protein into the gastrointestinal tract or by inhibiting the digestion and absorption of endogenous protein(s) along the length of the gut.

When the IEAA flow from diets containing the 4 levels of casein was compared on d 5, increasing the level of dietary casein in the diets resulted in increased IEAA and TAA flow. This observation supports the fact that increasing dietary protein will increase IEAA and TAA secretion, as reported in pigs (Butts et al., 1993) and in rats (Darragh et al., 1990). However, this increased flow, especially when 150 g of casein/kg of diet was fed, will be difficult to attribute completely to endogenous origin. The response was the same for all levels of casein investigated in this study, with the order of abundance of amino acids at the terminal ileum, with Glu > Asp > Leu, with the least abundant being Trp < Met. On d 15, the trend observed for the NFD was Glu > Asp > Thr, whereas it was Glu >Asp > Ser for the diets containing 50, 100, and 150 g of casein/kg. Although the trend was similar to what was observed on d 5, there was a significant reduction in IEAA and TAA flow from d 5 to 15 (approximately 50% reduction). The IEAA flow decreased from d 5 to 15, after which the flow remained relatively constant between d 15 and 21. The lowest contribution to amino acid flow was from Met and His. These observations are in agreement with what was reported for pigs (Taverner et al., 1981) and chickens (Siriwan et al., 1994). Because the amino acids with high flow in this study are the predominant amino acids in mucin proteins (Ravindran and Bryden, 1999), one could infer that the contribution of mucin to the endogenous amino acid pool in this study was significant (Lien et al., 1997). The IEAA flow determined with the regression method was higher when compared with the NFD method on d 5; however, values on d 15 and 21 for the 2 methods were not different. The CV between the 2 methods for Lys, Thr, Met, Glu, Asp, and TAA were between 12 and 27% on d 5, 1 and 18% on d 15, and 8 and 23% on d 21.

This study confirmed previous reports in growing pigs (Hodgkinson et al., 2000) that the presence and levels of dietary protein or amino acids will increase IEAA flow and that this effect is dose dependent. The data generated from the 3 methods used in estimating IEAA flow in this study point to the fact that a high level of casein (100 g/ kg of diet and above) resulted in increased IEAA flow. When apparent digestibility values of the casein diets were standardized by using the NFD (data not shown), the standardized ileal amino acid digestibility values were not different for the 3 levels of casein. This means that the increase in IEAA flow with increased casein level was not a result of decreased digestibility, but a result of increased secretion of endogenous amino acid. It is important to note that the NFD method of standardization did not correct for the increased stimulatory effect of protein or amino acids in the casein diet, which is expected to increase with increased dietary peptide concentration (Hodgkinson et al., 2000). This is supported by the fact that the values obtained when the regression line was extrapolated to zero protein-amino acid intake were closer to values obtained when NFD or 50 g of casein/ kg of diet was fed than for when higher casein levels were fed.

Of all the levels of casein evaluated in this study, the NFD method was the most consistent (across geographical locations) because there was no significant interaction between geographical location and age for any of the amino acids. Despite the fact that the interaction between geographical location and age was significant for the 50, 100, or 150 g of casein diets, the mean values for IEAA flow across all the amino acids were similar on d 5 and 21. For example, the endogenous Met flows (mg/kg DMI) on d 21 when 4 levels of casein were fed were as follows: NFD, 44 (geographical location 1) vs. 55 (geographical location 2); 50 g of casein, 91 (geographical location 1) vs. 109 (geographical location 2); 100 g of casein, 146 (geographical location 1) vs. 153 (geographical location 2); 150 g of casein, 208 (geographical location 1) vs. 282 (geographical location 2); with a SD of 27.4 (geographical location 1) and 35.5 (geographical location 2).

The results of this study also showed a lack of interaction between geographical location and diet when an NFD was fed, and there was also no geographical location by diet interaction on d 5 when all 4 levels of casein were fed. On d 15 and 21, however, there were significant interactions between geographical location and diet for some of the amino acids. The observed interaction could be a result of the differences in the SD, which was approximately twice as high in geographical location 1 as in geographical location 2 for some of the amino acids. This observation is difficult to explain.

In conclusion, the data suggest that it may be important to reevaluate the basis on which the apparent digestibility method is often considered to be the standard and preferable way of evaluating amino acid digestibility of feed ingredients. The relatively higher levels of amino acids of endogenous origin on d 5 may be responsible for the effects of age on amino acid digestibility estimates during the first 3 wk. We believe, based on the apparent digestibility values, that nutrient digestibility and absorption at early ages is not as efficient as at later ages. However, the question is whether such a conclusion will still be valid after correction for the contribution by basal endogenous amino acid to the ileal digesta amino acid pool. In addition, the IEAA and TAA flow obtained by the regression method is similar to values obtained by the NFD (d 15 and d 21) method relative to the HDP method. In addition to this, the results from this study suggest that at a level of 100 g of casein/kg of diet, the assumption that all amino acids in the digesta are of endogenous origin may not hold. Finally, results from the NFD method were the most consistent at all ages evaluated in this study. However, at older ages (d 15 and 21) both the NFD and regression methods gave similar results. To improve consistency across laboratories, we recommend that there should be a standard NFD and a uniform method of sample collection.

	ACKNOWLEDGMENTS

 
Partial funding for this project was supplied by US Poultry and Egg Association (Tucker, GA), Fats and Proteins Research Foundation (Alexandria, VA), Degussa Corporation (Kennesaw, GA), ADM Inc. (Decatur, IL), Novus International Inc. (St. Louis, MO), and Ajinomoto Heartland LLC (Chicago, IL).

	FOOTNOTES

 
¹ Journal Paper No. 2006-17946 of the Purdue University Agricultural Research Programs.

Received for publication February 26, 2007. Accepted for publication August 13, 2007.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
AOAC. 2000. Official Methods of Analysis. 17th ed. Assoc. Offic. Anal. Chem., Arlington, VA.

Brannon, P. M. 1990. Adaptation of the exocrine pancreas to diet. Annu. Rev. Nutr. 10:85–105.[CrossRef][ISI][Medline]

Butts, C. A., P. J. Moughan, W. C. Smith, and D. H. Carr. 1993. Endogenous lysine and other amino acid flows at the terminal ileum of the growing pig (20 kg body weight): The effect of protein free, synthetic amino acid, peptide and protein alimentation. J. Sci. Food Agric. 61:31–40.[CrossRef][ISI]

Darragh, A. J., P. J. Moughan, and W. C. Smith. 1990. The effects of amino acid and peptide alimentation on the determination of endogenous amino acid flow at the terminal ileum of rat. J. Sci. Food Agric. 51:47–56.[CrossRef][ISI]

De Lange, C. F. M., W. C. Sauer, and W. Souffrant. 1989. The effect of protein status of the pig on the recovery and amino acid composition of endogenous protein in digesta collected from the distal ileum. J. Anim. Sci. 67:755–762.[Abstract/Free Full Text]

Hodgkinson, S. M., P. J. Moughan, G. W. Reynolds, and K. A. C. James. 2000. The effects of dietary peptide concentration on endogenous ileal amino acid loss in the growing pig. Br. J. Nutr. 83:421–430.[ISI][Medline]

Lemme, A., V. Ravindran, and W. L. Bryden. 2004. Ileal digestibility of amino acid in feed ingredients for broilers. World’s Poult. Sci. J. 60:423–437.[CrossRef][ISI]

Lien, K. A., W. A. Sauer, and M. Fenton. 1997. Mucin output in ileal digesta of pigs fed a protein-free diet. Z. Ernahrungswiss. 36:182–190.[CrossRef][ISI][Medline]

Moughan, P. J., A. J. Darragh, W. C. Smith, and C. A. Butts. 1990. Perchloric acid trichloroacetic acids as precipitants of protein in endogenous ileal digesta from the rat. J. Sci. Food Agric. 52:13–21.[CrossRef][ISI]

Moughan, P. J., and S. M. Rutherfurd. 1990. Endogenous flow of total lysine and other amino acids at the distal ileum of the protein- and peptide-fed rat. The chemical labeling of gelatin protein by transformation of lysine to homoarginine. J. Sci. Food Agric. 52:187–192.

Moughan, P. J., G. Schuttert, and M. Leenaars. 1992. Endogenous amino acid flow in the stomach and small intestine of the young growing rat. J. Sci. Food Agric. 60:437–442.[CrossRef][ISI]

Nasset, E. S. 1972. Amino acid homeostasis in the gut lumen and its nutritional significance. World Rev. Nutr. Diet. 14:134–153.[Medline]

NRC. 1994. Nutrient Requirements for Poultry. 9th ed. Natl. Acad. Press, Washington, DC.

Nyachoti, C. M., C. F. M. de Lange, B. W. McBride, and H. Schulze. 1997. Significance of endogenous gut nitrogen losses in the nutrition of growing pigs: A review. Can. J. Anim. Sci. 77:149–163.

Ravindran, V., and W. L. Bryden. 1999. Amino acid availability in poultry—In vitro and in vivo measurements. Aust. J. Agric. Res. 50:889–908.[CrossRef][ISI]

Ravindran, V., and W. H. Hendriks. 2004. Endogenous amino acid flows at the terminal ileum of broilers, layers and adult roosters. Anim. Sci. 79:265–271.

Ravindran, V., L. I. Hew, G. Ravindran, and W. L. Bryden. 2004. Endogenous amino acid flow in the ileum: Quantification using three techniques. Br. J. Nutr. 92:217–223.[CrossRef][ISI][Medline]

Samuels, M., and J. A. Witmer. 1999. Statistics for the Life Sciences. 2nd ed. Prentice-Hall, Upper Saddle River, NJ.

Sibbald, I. R. 1987. Estimation of bioavailable amino acids in feed-stuffs for poultry and pigs: A review with emphasis on balance experiments. Can. J. Anim Sci. 67:221–230.

Simon, O., T. Zebrowska, H. Bergner, and R. Munchmeyer. 1983. Investigations of the pancreatic and stomach secretion in pigs by means of continuous infusion of ¹⁴C-amino acids. Arch. Anim. Nutr. 33:9–12.[ISI]

Siriwan, P., W. L. Bryden, and E. F. Annison. 1994. Use of guanidinated dietary protein to measure losses of endogenous amino acids in poultry. Br. J. Nutr. 71:515–529.[CrossRef][ISI][Medline]

Taverner, M. R., I. D. Hume, and D. J. Farrell. 1981. Availability to pigs of amino acids in cereal grains. 1. Endogenous levels of amino acids in ileal digesta and feces of pigs given cereal diets. Br. J. Nutr. 46:149–158.[CrossRef][ISI][Medline]

Webb, K. E. 1990. Intestinal absorption of protein hydrolysis products: A review. J. Anim. Sci. 68:3011–3022.[Abstract]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Adedokun, S. A. Articles by Applegate, T. J. Search for Related Content PubMed PubMed Citation Articles by Adedokun, S. A. Articles by Applegate, T. J.