Amino Acid Digestibility Assay Research Articles

Digestible indispensable amino acid score (DIAAS): 10 years on.

The objective of the review is to revisit the findings of the 2011 Food and Agriculture Organization of the United Nations (FAO) Expert Consultation on Dietary Protein Quality Evaluation in Human Nutrition, and to report on progress on uptake of the findings. It is evident that since 2011 there has been a concerted research effort to enhance an understanding of the protein quality of foods. The validity of the growing pig ileal protein digestibility assay has been confirmed and numerous studies reported using the growing pig as a model to give true ileal amino acid digestibility values for foods as consumed by humans. This has allowed for the determination of digestible indispensable amino acid scores (DIAAS) for a range of foods. A new non-invasive true ileal amino acid digestibility assay in humans which can be applied in different physiological states, called the dual-isotope assay, has been developed and applied to determine the DIAAS values of foods. It is concluded that DIAAS is currently the most accurate score for routinely assessing the protein quality rating of single source proteins. In the future, the accuracy of DIAAS can be enhanced by improved information on: the ideal dietary amino acid balance including the ideal dispensable to indispensable amino acid ratio; dietary indispensable amino acid requirements; effects of processing on ileal amino acid digestibility and lysine bioavailability. There is a need to develop rapid, inexpensive in vitro digestibility assays. Conceptual issues relating DIAAS to food regulatory claims, and to holistic indices of food nutritional and health status are discussed. The first recommendation of the 2011 Consultation regarding treating each indispensable amino acid as an individual nutrient has received little attention. Consideration should be given to providing food label information on the digestible contents of specific indispensable amino acids.

Enhancing the nutritive value of canola meal for broiler chickens through enzymatic modifications.

Enzymatic modification of canola meal (CM) is a potential method to enhance its nutritional value as it can depolymerize nonstarch polysaccharides (NSP) and mitigate its potential antinutritive properties. Based on the previous studies, pectinase A (PA), pectinase B (PB), xylanase B (XB), and invertase (Inv) were used for the enzymatic modifications. The highest NSP depolymerization ratio was obtained when 4 g/kg of each PA, PB, and XB, and 0.2 g/kg of Inv were used during 48 h incubation at 40 °C. In the current study, changes in pH, simple sugars, sucrose, oligosaccharides, and NSP contents during the enzymatic modification (CM+E) of CM were measured and compared to Control (CM) without enzymes addition or with the addition of bacteriostat sodium azide (CM+E+NaN3). The results showed that spontaneous fermentation occurred during incubation. After incubation, the pH of the slurry decreased, lactic acid was produced, phytate disappeared, and the concentration of simple sugars decreased substantially. The NSP of the slurry was progressively depolymerized by the enzyme blend. The chemical composition and nutritive value of enzymatically-modified CM (ECM) were evaluated. Ross 308 broilers were randomly assigned to 18 cages of six birds each for the standardized ileal amino acid digestibility (SIAAD) and nitrogen-corrected apparent metabolizable energy (AMEn) assay. A corn/soybean meal-based basal diet formulated to meet Ross 308 breeder recommendations and two test diets contained 70% of the basal diet and 30% of CM or ECM, respectively, were fed to Ross 308 from 13 to 17 d of age. No difference was observed between SIAAD of CM and ECM. The AMEn value of ECM was 2118.0 kcal/kg on a dry matter basis which was 30.9% greater (P < 0.05) than the CM.

Effect of extrusion processing of soybean meal on ileal amino acid digestibility and growth performance of broiler chicks

The present experiments were conducted to investigate the efficacy of extrusion of inadequately processed soybean meal (SBM) on amino acid (AA) digestibility and performance of broiler chicks. In Exp. 1, 180 day-old Ross broiler chicks were randomly distributed between 6 pen replicates (15 chicks each) of 2 dietary treatments; diets based on SBM or extruded SBM (ESBM) as the main protein sources. Based on the results of Exp. 1 and AA digestibility assay, Exp. 2 was designed using 216 day-old Ross broiler chicks (6 replicates of 18 chicks each), in which dietary SBM content was replaced by 86% using ESBM. This replacement caused about a 9% decrease in dietary crude protein (CP) level. Results showed that coefficients of CP and AA digestibility were greater (P < 0.05) for ESBM than those of SBM. Extrusion processing of SBM increased digestibility coefficients of Lys, Thr, Cys, Leu, Phe, Tyr, Pro, Ser, and Gly. Except during the starter period, dietary inclusion of ESBM increased (P < 0.05) average daily feed intake (ADFI) in Exp. 1 and caused increases (P < 0.01) in average daily weight gains (ADWG) throughout the trial period. Also, using ESBM improved the (P < 0.01) feed conversion ratio (FCR) during starter and finisher periods. In Exp. 2, reducing dietary CP level using ESBM decreased both ADFI (P < 0.05) and ADWG (P < 0.01) in the starter period. During grower, finisher, and entire trial periods, however, ADFI and ADWG were not influenced by dietary treatments. Dietary inclusion of ESBM improved (P < 0.05) FCR value in the finisher period. Although carcass yield was not affected by dietary treatments, reducing dietary CP level resulted in an increase (P < 0.05) in abdominal fat percentage. Moreover, a decrease in dietary CP level reduced (P = 0.08) breast yield. The present findings indicate that extrusion of SBM could improve its nutritive value for broiler chicks. Using ESBM, one can reduce dietary CP level by about 9% without any detrimental effect on performance parameters of broiler chicks.

Evaluation of the pepsin digestibility assay for predicting amino acid digestibility of meat and bone meals

Sixteen meat and bone meal (MBM) samples were obtained and selected from various company plants to provide a wide range in pepsin nitrogen digestibility values. Pepsin digestibility was determined using either 0.02 or 0.002% pepsin. Amino acid (AA) digestibility of the 16 MBM samples was then determined using a precision-fed cecectomized rooster assay. The 0.02% pepsin digestibility values were numerically higher than the 0.002% pepsin values. The values varied from 77 to 93% for 0.02% pepsin and from 67 to 91% for 0.002% pepsin. The rooster AA digestibility results showed a wide range of values among MBM samples mostly due to the 4 samples having lowest and highest AA digestibility. A precision-fed broiler chick ileal AA digestibility assay confirmed that there were large differences in AA digestibility among the MBM samples having the lowest and highest rooster digestibility values. Correlation analyses between pepsin and AA digestibility values showed that the correlation values (r) were highly significant (P < 0.0001) for all AA when all 16 MBM samples were included in the analysis. However, when the MBM samples with the 2 lowest and the 2 highest rooster digestibility values were not included in the correlation analyses, the correlation coefficient values (r) were generally very low and not significant (P > 0.05). The results indicated that the pepsin nitrogen digestibility assay is only useful for detecting large differences in AA digestibility among MBM. There also was no advantage for using 0.02 versus 0.002% pepsin.

Nutrient analysis, apparent metabolisable energy and ileal amino acid digestibility of full fat soybean for broilers

Nutrient composition, ileal amino acid digestibility and apparent metabolisable energy (AME) of four full-fat soybean (FFSB) samples, collected from commercial feed mills in Southeast Asia, were evaluated using laboratory analyses and animal studies. The AME assay was conducted with broilers using the classical total excreta collection between days 21 and 29 posthatch. A maize-soy basal diet was formulated and test diets, each containing different FFSB samples, were developed by replacing (w/w) 300g/kg of the basal diet with FFSB. The AME of FFSB was calculated based on the difference between AME values of the basal and test diets. The ileal amino acid digestibility assay was conducted with 29-day-old broilers using assay diets containing FFSB as the only source of protein. Digesta from lower ileum was collected on day 34. The ratios between the titanium and amino acids in the diet and digesta were used to calculate the digestibility. The crude protein and fat contents of the four FFSB samples ranged from 351 to 399g/kg and from 177 to 192g/kg, respectively. The AME contents varied widely between the four samples, ranging from 12.62 to 15.46MJ/kg. The standardised ileal digestibility coefficient of protein ranged from 0.763 to 0.821. Variations in the digestibility of amino acids and digestible amino acid contents generally followed that observed for protein digestibility. In conclusion, FFSB contained more fat and AME than soybean meal, but less digestible contents of protein and amino acids due to the diluting effect of fat. Overall, these data show that properly processed FFSB is a valuable feedstuff and offers an attractive method of adding fat to poultry diets.

Nutritional evaluation of new reduced oligosaccharide soybean meal in poultry

The nutritional values of a novel reduced-oligosaccharide soybean meal (SBM-RO) and conventional SBM (SBM-CV) were evaluated and compared in 4 experiments. The first experiment was a TMEn assay with conventional roosters. The second experiment was a precision-fed cecectomized rooster assay that was conducted to determine TMEn and amino acid (AA) digestibility. The third experiment was a standardized ileal AA digestibility assay, in which broiler chicks were fed semi-purified diets containing 20% protein (from only the test ingredient) for 17 to 21 d of age and ileal digesta were collected on d 21. The fourth experiment was a growth performance trial (7 to 20 d of age) where broiler chicks were fed corn-SBM diets (adequate in all AA) containing 38.84% SBM-RO or SBM-CV. The protein content (100% DM basis) of the SBM-CV and SBM-RO was 51.9 and 54.8%, respectively. The gross energy of the 2 SBM was similar. The TMEn values in both conventional roosters and cecectomized roosters were significantly higher (P < 0.05) for SBM-RO than for SBM-CV (difference was approximately 200 kcal/kg of DM). Amino acid digestibility in cecectomized roosters was not different between SBM-CV and SBM-RO, with the exception of Trp, Ala, Asp, and Cys (SBM-RO > SBM-CV, P < 0.05). No significant differences between the SBM were found for AA digestibility in the standardized ileal AA digestibility assay. In the growth performance trial (experiment 4), the corn-SBM diet containing SBM-RO yielded significantly higher feed efficiency than the diet containing SBM-CV (P < 0.001). The results indicated that the SBM-RO contains higher ME than the SBM-CV and that digestibility of AA in SBM-RO is similar to SBM-CV.

A comparison of digestible amino acid databases: Relationship between amino acid concentration and digestibility

SUMMARY Databases compiling the amino acid (AA) digestibility values of 20 common poultry feed ingredients from 2 commercial companies, Evonik Degussa (Burr Ridge, IL; chick assays) and Ajinomoto Heartland (Chicago, IL; rooster assays), were used to investigate the influence of AA concentration on observed digestibility values. Casual observation of the rooster assay database suggested that AA digestibility may be related to AA concentration. Ideally, digestibility should be a property of the feed ingredient and should not be related to the concentration at which an ingredient is fed. If the AA concentration during an assay has a large effect on its digestibility, then the results may not be valid at different levels of feed ingredients or when the AA concentration in the test situation is different from actual feeding levels in a diet. Differences in digestibility values in the Evonik Degussa and Ajinomoto Heartland databases were compared. Multiple regression techniques were then used to test the hypothesis that the results of the AA digestibility assays were related to the AA concentration in the feed ingredient being tested. Total AA concentration and digestible AA averaged 6 and 14% higher in the rooster database than in the chick database, respectively. Differences in assay type were quantitative because slopes of digestible AA = f(total AA concentration) were positive for 19 of 20 ingredients in each database. However, when all data were included in 1 model, the influence of concentration on digestibility was very unlikely to have been due to chance (P < 0.0006), as evidenced by the interaction of concentration × assay type. Removing the lowest (Trp) or highest (Ile) AA was not critical to the conclusions. The effect of AA concentration on its digestibility was quantitatively the same for each AA (P < 0.50) except Cys (P < 0.05). We confirmed, based on data from the 3 selected articles, that this relationship was not due only to the different samples in the different databases. Therefore, to best predict the digestibility of any AA in any ingredient, its intended concentration in the diet should be known.

Protein–phytate interactions in pig and poultry nutrition: a reappraisal

Protein-phytate interactions are fundamental to the detrimental impact of phytate on protein/amino acid availability. The inclusion of exogenous phytase in pig and poultry diets degrades phytate to more innocuous esters and attenuates these negative influences. The objective of the present review is to reappraise the underlying mechanisms of these interactions and reassess their implications in pig and poultry nutrition. Protein digestion appears to be impeded by phytate in the following manner. Binary protein-phytate complexes are formed at pH levels less than the isoelectric point of proteins and complexed proteins are refractory to pepsin digestion. Once the protein isoelectric points are exceeded binary complexes dissociate; however, the isoelectric point of proteins in cereal grains may be sufficiently high to permit these complexes to persist in the small intestine. Ternary protein-phytate complexes are formed at pH levels above the isoelectric point of proteins where a cationic bridge links the protein and phytate moieties. The molecular weights of protein and polypeptides in small-intestinal digesta may be sufficient to allow phytate to bind nutritionally important amounts of protein in ternary complexes. Thus binary and ternary complexes may impede protein digestion and amino acid absorption in the small intestine. Alternatively, phytate may interact with protein indirectly. Myo-inositol hexaphosphate possesses six phosphate anionic moieties (HPO(4)(2-)) that have strong kosmotropic effects and can stabilise proteins by interacting with the surrounding water medium. Phytate increases mucin secretions into the gut, which increases endogenous amino acid flows as the protein component of mucin remains largely undigested. Phytate promotes the transition of Na(+) into the small-intestinal lumen and this suggests that phytate may interfere with glucose and amino acid absorption by compromising Na(+)-dependent transport systems and the activity of the Na pump (Na(+)-K(+)-ATPase). Starch digestion may be depressed by phytate interacting with proteins that are closely associated with starch in the endosperm of cereal grains. While elucidation is required, the impacts of dietary phytate and exogenous phytase on the site, rate and synchrony of glucose and amino acid intestinal uptakes may be of importance to efficient protein deposition. Somewhat paradoxically, the responses to phytase in the majority of amino acid digestibility assays in pigs and poultry are equivocal. A brief consideration of the probable reasons for these inconclusive outcomes is included in this reappraisal.

Comparison of amino acid digestibility of feedstuffs determined with the precision-fed cecectomized rooster assay and the standardized ileal amino acid digestibility assay

The objective of this study was to evaluate and compare amino acid digestibility of several feedstuffs using 2 commonly accepted methods: the precision-fed cecectomized rooster assay (PFR) and the standardized ileal amino acid assay (SIAAD). Six corn, 6 corn distillers dried grains with or without solubles (DDGS/DDG), one wet distillers grains, one condensed solubles, 2 meat and bone meal (MBM) and a poultry byproduct meal were evaluated. Due to insufficient amounts, the wet distillers grains and condensed solubles were only evaluated in roosters. Standardized amino acid digestibility varied among the feed ingredients and among samples of the same ingredient for both methods. For corn, there were generally no differences in amino acid digestibility between the 2 methods. When differences did occur, there was no consistent pattern among the individual amino acids and methods. Standardized amino acid digestibility was not different between the 2 methods for 4 of the DDG samples; however, the PFR yielded higher digestibility values for a high protein DDG and a conventionally processed DDGS. The PFR yielded higher amino acid digestibility values than the SIAAD for several amino acids in 1 MBM and the poultry byproduct meal, but it yielded lower digestibility values for the other MBM. Overall, there were no consistent differences between methods for amino acid digestibility values. In conclusion, the PFR and SIAAD methods are acceptable for determining amino acid digestibility. However, these procedures do not always yield similar results for all feedstuffs evaluated. Thus, further studies are needed to understand the underlying causes in this variability.

Consequences of calcium interactions with phytate and phytase for poultry and pigs

Despite increasing practical experience and cascades of scientific reports on exogenous microbial phytases, several issues associated with their use remain unresolved because of the ambiguous and, at times, conflicting data that has been generated. One possible cause of these inconsistent outcomes is dietary calcium (Ca) levels, which are mainly derived from limestone. Thus the purpose of this review is to examine Ca interactions with dietary phytate and phytases, particularly exogenous, microbial phytases, and their consequences for poultry and pigs. The polyanionic phytate molecule has a tremendous capacity to chelate cations and form insoluble Ca–phytate complexes, which are refractory to phytase activity. Thus Ca–phytate complex formation along the gastrointestinal tract, where one phytate (IP 6) molecule binds up to five Ca atoms, assumes importance and approximately one third of dietary Ca may be bound to phytate in digesta. Consequently, phytate limits the availability of both P and Ca as a result of insoluble Ca–phytate complex formation, the extent of which is driven by gut pH and molar ratios of the two components. It is accepted that Ca–phytate complexes are mainly formed in the small intestine where they have a substantial negative influence on the efficacy of mucosal phytase. However, exogenous phytases are mainly active in more proximal segments of the gut and lower pH levels, so their efficacy should not be influenced by Ca–phytate complexes in the small intestine. There is, however, data to indicate that Ca and phytate interactions occur under acidic conditions with the formation of soluble and insoluble Ca–phytate species, which could negatively impact on exogenous phytase efficacy. Also, Ca will tend to elevate gut pH because of limestone's very high acid binding capacity, which will favour Ca–phytate interactions and may influence the activity of exogenous phytases depending on their pH activity spectrum. The de novo formation of binary protein–phytate complexes that are refractory to pepsin hydrolysis may be fundamental to the negative impact of phytate on the digestibility of protein/amino acids. However, high dietary Ca levels may disrupt protein–phytate complex formation by interacting with both phytate and protein even at acidic pH levels, thereby influencing the outcomes of phytase amino acid digestibility assays. Finally, it is increasingly necessary to define the Ca and nonphytate-P requirements of pigs and poultry offered phytase-supplemented diets.

Phytate-degrading enzymes in pig nutrition

Phytate, the mixed salt of phytic acid ( myo-inositol hexaphosphate), derived from plant-sourced feed ingredients is invariably present in practical diets for pigs. Typically, swine diets contain in the order of 3.0 g kg − 1 phytate-bound phosphorus (phytate-P) but phytate concentrations are subject to variation. Importantly, phytate-P is only partially utilised by pigs because they do not generate sufficient endogenous phytase activity. Phytate-degrading enzymes, via step-wise dephosphorylation of phytate, have the capacity to liberate phytate-P, thus enhancing P absorption and reducing P excretion, which are both nutritionally and ecologically beneficial consequences. The commercial introduction of microbial phytases in 1991 has greatly magnified the interest in the roles of phytate and phytase in pig nutrition. The capacity of microbial phytases to enhance growth performance of pigs offered diets with inadequate P levels is well documented. However, in some instances, phytase has been shown to improve performance of pigs offered P adequate diets thus phytase-induced improvements in growth performance should not be attributed entirely to increased P availability. This raises the possibility that phytase is increasing the utilisation of nutrients other than P. These so-called ‘extra-phosphoric’ effects of phytase remain controversial, particularly in relation to protein and amino acid availability. There are conflicting opinions that are reflected in the inconsistent outcomes of studies to determine the effect of phytase on ileal digestibility of amino acids and protein utilisation in pigs. In phytase amino acid digestibility assays, it seems likely the choice of chromic oxide as the dietary marker has contributed to these ambiguous results, which may be further complicated when ileal digesta samples are taken from cannulated pigs fed on a restricted, twice-daily basis. In order to resolve this critical issue, there is an urgent need to assess the impacts of selection of dietary markers, methods of ileal digesta collection and feeding regimen relative on the outcomes of phytase amino acid digestibility assays in pigs. However, inconsistent results from phytase studies in pigs are not confined to amino acid digestibility assays. Arguably, insufficient attention has been paid to dietary substrate levels in relation to phytase inclusion from both scientific and practical standpoints. Phytate analyses are not straightforward and there is a real need to develop more accurate and rapid methods to facilitate phytate determinations. The properties of phytate vary between (and within) feed ingredients where solubility of phytate may be critical; which, in turn, is a function of gut pH in pigs. Contemporary phytases have the capacity to degrade approximately 50% of dietary phytate at the level of the ileum, which may mean higher inclusion rates are warranted. Consequently, there is scope for the development of more effective ‘second-generation’ phytate-degrading feed enzymes and their possible introduction, coupled with a better scientific understanding of relevant fundamental issues, will ensure that phytate-degrading enzymes will contribute to viable and sustainable pig production to an even greater extent in the future.

Development of a novel bioassay for determining the available lysine contents of foods and feedstuffs

Lysine is an important indispensable amino acid, and describing the lysine content of a food or feedstuff provides useful information about nutritional value. However, when a food or feedstuff is subjected to heating the lysine present can be altered to nutritionally unavailable derivatives. These derivatives can revert back to lysine during the acid hydrolysis step used in amino acid analysis causing an overestimate of the lysine content. There have been many chemical methods developed to determine the reactive (unmodified) lysine content of foods and feedstuffs, but these do not take into account the incomplete absorption of lysine from the small intestine. There are also a number of animal-based assays for determining available lysine (the lysine that can be absorbed in a form that can be used for protein synthesis). The true ileal amino acid digestibility assay is commonly used to determine amino acid availability and is accurate for application to unprocessed foods and feedstuffs but is not accurate for lysine and possibly other amino acids when applied to heat-processed foods or feedstuffs. For such protein sources, assays such as the slope-ratio assay, indicator amino acid oxidation assay and the BIOLYSINEtrade mark assay (true ileal digestible reactive lysine assay) have been developed to determine available lysine. The present paper discusses the efficacy of the BIOLYSINEtrade mark assay as well as other assays for determining available lysine in processed foods and feedstuffs.

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS)—A Concept for Describing Protein Quality in Foods and Food Ingredients: A Critical Review

Protein Digestibility-Corrected Amino Score (PDCAAS) is discussed. PDCAAS is now widely used as a routine assay for protein quality evaluation, replacing the more traditional biological methods [e.g., measurement of the Protein Efficiency Ratio (PER) in rats]. PDCAAS is based on comparison of the essential amino acid content of a test protein with that of a reference essential amino acid pattern and a correction for differences in protein digestibility as determined using a rat assay. Although PDCAAS is a rapid and useful method, it often shows discrepancies when compared to PER values. These discrepancies relate to the following issues: uncertainty about the validity of reference patterns, invalidity of correction for fecal (versus ileal) digestibility, truncation of PDCAAS values to 100%, failure to obtain full biological response after supplementation of the limiting essential amino acid, discrepancies between protein and amino acid digestibility, effects of processing on protein quality, and effects of the presence of antinutritional factors in the matrix containing the protein. Part of the discrepancy between PDCAAS and PER can be overcome by modifications of PDCAAS. This article describes some proposed modifications and puts forward the suggestion that the rat protein fecal digestibility assay be replaced by an in vitro ileal amino acid digestibility assay based on a computer-controlled gastrointestinal model.

Amino acid availability: aspects of chemical analysis and bioassay methodology

It is important to be able to characterise foods and feedstuffs according to their available amino acid contents. This involves being able to determine amino acids chemically and the conduct of bioassays to determine amino acid digestibility and availability. The chemical analysis of amino acids is not straightforward and meticulousness is required to achieve consistent results. In particular and for accuracy, the effect of hydrolysis time needs to be accounted for. Some amino acids (for example, lysine) can undergo chemical modification during the processing and storage of foods, which interferes with amino acid analysis. Furthermore, the modified amino acids may also interfere with the determination of digestibility. A new approach to the determination of available lysine using a modified in vivo digestibility assay is discussed. Research is required into other amino acids susceptible to structural damage. There is recent compelling scientific evidence that bacterial activity in the small intestine of animals and man leads to the synthesis and uptake of dietary essential amino acids. This has implications for the accuracy of the ileal-based amino acid digestibility assay and further research is required to determine the extent of this synthesis, the source of nitrogenous material used for the synthesis and the degree of synthesis net of amino acid catabolism. Although there may be potential shortcomings in digestibility assays based on the determination of amino acids remaining undigested at the terminal ileum, there is abundant evidence in simple-stomached animals and growing evidence in human subjects that faecal-based amino acid digestibility coefficients are misleading. Hindgut microbial metabolism significantly alters the undigested dietary amino acid profile. The ileal amino acid digestibility bioassay is expected to be more accurate than its faecal-based counterpart, but correction of the ileal amino acid flow for amino acids of endogenous origin is necessary. Approaches to correcting for the endogenous component are discussed.

The rat as a model animal for the growing pig in determining ileal amino acid digestibility in soya and milk proteins.

The growing pig is an established model animal for humans to determine protein digestibility to the end of the small intestine. An ileal amino acid digestibility assay based on the laboratory rat offers an alternative. True ileal amino acid digestibility in high quality refined protein sources was determined and compared between the laboratory rat and the growing pig. Both species of animals were fed their daily ration in nine separate meals. Ileal digesta were collected at slaughter and chromic oxide was used as an indigestible marker. For several of the amino acids examined, there were no significant (p < 0.05) differences between the two species for true ileal amino acid digestibility, but for aspartic acid, serine, cysteine, methionine, tyrosine, phenylalanine and histidine, significant (p < 0.05) interspecies differences were found. The overall average amino acid digestibility, for all amino acids and all diets, was 95.8% for the pig and 96.1% for the rat. Cysteine digestibility was highly variable between individual animals fed the same diet. The extent of amino acid digestion and absorption appears to be broadly similar across the two species, but some important differences in digestibility were recorded.

Ileal amino acid digestibility assay for the growing meat chicken--comparison of ileal and excreta amino acid digestibility in the chicken

1. The apparent and true amino acid digestibilities in sorghum, wheat, soyabean meal, meat-and-bone meal, fish meal and blood meal for growing meat chickens were determined using an assay based on the collection of digesta from the terminal ileum and comparison was made with digestibility values determined using an excreta-based assay. 2. Five-week-old meat chickens were given maize-soyabean meal basal diet or mixtures of the basal diet and test diets containing the 6 ingredients as the sole source of dietary protein (50:50 on weight basis). Apparent amino acid digestibility values of assay diets at ileal and excreta levels were calculated using chromic oxide as the indigestible marker. True digestibility values were calculated using endogenous outputs determined by feeding a protein-free diet. Amino acid digestibilities of the ingredients were calculated by difference. 3. The site of measurement had no influence on endogenous amino acid output, the exceptions being aspartic acid and glutamic acid. The output of these two amino acids was higher in the excreta. 4. Significant differences were found between ileal and excreta-based digestibility of certain amino acids in some ingredients, with excreta values being usually higher than the ileal values, indicating a net catabolism of amino acids in the large intestine. The degree of net amino acid disappearance was found to be variable among amino acids and ingredients. In general, threonine had the lowest digestibility at the ileal level and, compared with other amino acids, the highest degredation during passage through the hindgut. 5. The results showed that digestibility determination based on excreta collection will overestimate the uptake for some amino acids in some feeds. The degree of overestimation was often considerable, ranging from 8.9% (apparent digestibility of threonine in soyabean meal) to 56% (apparent digestibility of aspartic acid in wheat). It is concluded that digestibility values measured at the terminal ileum provide a more reliable measure of amino acid availability than those measured in the excreta.

Amino acid availability in poultry—in vitro and in vivo measurements

Methodology to evaluate the protein quality or amino acid availability in feed ingredients for poultry using in vitro (enzymic, chemical, or microbiological assays), indirect in vivo (plasma amino acid assays), or direct in vivo (growth or digestibility assays) measurements has been reviewed. The specific applications and limitations of these methods are examined. In vitro assays are useful in providing information on heat damage in selected protein sources under defined conditions, and on relative ranking of different samples, but they cannot form the basis of practical feed formulations. While growth assays remain the only direct means of confirming nutritional relevance of values obtained by other procedures, in vivo digestibility assays appear to be most useful, at present, to estimate amino acid availability. Amino acid digestibility assays in poultry should be based on the analysis of digesta from the terminal ileum rather than excreta, because of the variable and modifying effects of hindgut microflora. Techniques used to estimate endogenous amino acid losses in poultry are discussed. The needs for correction of endogenous losses in amino acid digestibility calculations and the relative merits of apparent and true digestible amino acid systems are still being debated. It is, however, clear that both digestible amino systems are superior to the total amino acid system currently employed to formulate practical diets. Digestible amino acid values are likely to form the basis of poultry feed formulations in the future. In particular, there is an urgent need for more precise information on the variation in digestible amino acid contents of locally grown ingredients and on the factors causing this variation (e.g. variety, location, season, agronomic practices, processing, etc.).

An ileal amino acid digestibility assay for the growing meat chicken-effect of feeding method and digesta collection procedures

An ileal amino acid digestibility assay for the growing meat chicken-effect of feeding method and digesta collection procedures

Ileal amino acid digestibility assay for the growing meat chicken—effect of the imposition of a fasting period and the nature of the test diet

1. The objective of this study was to investigate the effects of cold‐pelleting, the length of the fasting period before feeding of the test diet and the nature of die test diet on apparent ileal nitrogen (N) digestibility in the broiler chicken. 2. Four‐week‐old broiler chickens were given a pelleted or non‐pelleted maize/soyabean meal (basal) diet. The birds were starved for 24 h, given a single test meal (25 g) by intubation and killed 4 h after the start of feeding by administration of a barbiturate, to allow sampling of ileal digesta (terminal 15 cm). Cold‐pelleting did not affect apparent ileal N digestibility. 3. Four‐week‐old broiler chickens were fasted for 12 or 24 h and then received a test meal (1 h free access) of either a pelleted soyabean meal or a pelleted meat‐and‐bone meal diet or were continuously fed on one of the two diets. The imposition of a fast did not affect apparent ileal N digestibility. However, a 24 h fasting procedure was preferred, as the between animal variation for apparent ileal N digestibility was lower than for the 12 h fast or for continuous feeding. 4. Four‐week‐old broiler chickens were given either semi‐synthetic starch‐based diets containing maize, wheat bran, meat‐and‐bone meal or fish meal as the sole sources of protein or each of these diets in combination with the basal diet (50:50 on a dry matter basis). With the exception of the maize diet, the apparent ileal N digestibility values calculated by correcting for the digestibility of the basal dietary component were significantly lower than when digestibility was determined directly using a diet in which the respective proteins were the sole protein source. This implies that interactions between the dietary ingredients influence estimates of apparent ileal N digestibility.

Development of an Heal amino acid digestibility assay for the growing chicken—effects of time after feeding and site of sampling

1. The study aimed to establish the optimum time after ingestion and optimum sampling site for the development of an ileal amino acid digestibility assay for broiler chickens. 2. To establish the optimal sampling time, 4‐week‐old broiler chickens were given one of 6 protein sources (meat‐and‐bone, soyabean, cottonseed, fish, maize and wheat meals) as the sole source of protein in a test diet. The diets contained chromic oxide as an indigestible marker. The birds were starved for 24 h, fed and subsequently killed for sampling of ileal digesta (terminal 15 cm) at 2, 3, 4, 5 and 6 h after the start of feeding. 3. For the soyabean, fish, wheat and maize meal diets, sampling time had no significant effect on apparent ileal nitrogen digestibility, whereas for the meat‐and‐bone and cottonseed meal diets there was a significant quadratic effect of sampling time. The amount of digesta collected was maximised and the mean apparent ileal nitrogen digestibility had the lowest variation around the 4 h sampling time. 4. To establish the optimum sampling site, 4‐week‐old chickens were given either a meat‐and‐bone, a soyabean or a wheat bran meal‐based diet. The birds were killed 4 h after the start of feeding, and digesta were sampled from 0.10, 0.15, 0.20 or 0.25 cm of terminal ileum. 5. There was no significant effect of sampling site on the apparent ileal digestibility of dietary nitrogen. The terminal 15 cm of ileum was considered a preferred site for sampling ileal digesta from broiler chickens.