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Accurate estimation of ileal endogenous amino acid (EAA) losses is important when formulating diets on a standardized ileal digestible amino acid basis. In addition to the undigested and unabsorbed amino acids of dietary origin, amino acids of endogenous origin, which can be basal or diet specific, are found in digesta. The improvement in the techniques used in amino acid analysis as well as a shift from sampling excreta to ileal digesta has resulted in more accurate amino acid digestibility coefficients. Despite this, however, it is important to determine the amino acids in the digesta that are of endogenous origin. Although the need for standardization and its associated advantages is still subject to debate, it is important to evaluate how values from various methodologies compare. Several methods have been used to estimate ileal EAA flow. The classical methods, including the regression method, the use of nitrogen-free diet (NFD), and the fasted cecectomized rooster method, are the most widely used. The criticisms with the last 2 methods are that birds are not in a normal physiological state and the ileal EAA flow is, therefore, underestimated. Different methods have resulted in different endogenous flow estimates, with the NFD method having the lowest values when compared with flows from the regression and highly digestible protein methods. In addition to the influence of methods on ileal EAA flows, the influence of the age of the birds on flow is important. Data on EAA losses are copious in the literature; however, variation in data across and within laboratories calls for investigation of factors contributing to the variation. This review compares results from different methods and examines the issue of repeatability and consistency of EAA losses data from different laboratories. Finally, composition of an NFD for estimating EAA losses is proposed.

1. This study was conducted to determine and compare endogenous amino acid (EAA) flow in broilers, laying hens, and caecectomised roosters fed on either a nitrogen-free diet (NFD) or a highly digestible protein diet (HDP). 2. Also, the EAA flows using three methods (fasted, NFD, or HDP) in precision-fed caecectomised roosters were compared. 3. EAA flow did not differ between broilers and laying hens on either the NFD or HDP diets. 4. Endogenous amino acid flow in caecectomised roosters was 3.5-12-fold higher than for broilers and laying hens for both methods (NFD and HDP) of estimation. 5. When caecectomised roosters were fasted or precision-fed (NFD or HDP), there was no difference in EAA flow between the NFD- or HDP-fed birds. 6. However, flows from fasted roosters were lower than from roosters receiving either the NFD or HDP diet. 7. Results from this study showed that EAA flow is method-dependent. Total amino acid flow from fasted birds was 31% less than when a NFD was fed to caecectomised roosters.

1. This study determined the effect of bird type (broilers, laying hens, or caecectomised roosters) on amino acid digestibility of feedstuffs from 5 plant sources and one animal source. 2. The standardised amino acid digestibility (SAAD) were obtained by correcting apparent ileal amino acid digestibility (AIAAD) values for basal ileal endogenous amino acid (EAA) flow obtained by feeding a N-free diet (NFD) to broilers and laying hens or from fasted EAA flow from caecectomised roosters. 3. The apparent total amino acid (TAA) digestibilities did not differ between broilers and roosters for three of the 6 feed ingredients. 4. Broilers had higher apparent total amino acid (TAA) digestibility than laying hens and roosters when fed on the maize diet (canola meal, maize, and soybean meal). 5. The apparent TAA digestibilities were similar across bird types for the dark distillers' dried grain with solubles, but the apparent lysine digestibility was much lower in the caecectomised roosters (15%) than the broilers (49%) and laying hens (43%). 6. The standardised TAA digestibility values in roosters were higher than in broilers for three of the 6 feed ingredients (canola meal, soybean meal, or meat and bone meal). 7. There were no differences between broilers and roosters, however, in the standardised TAA digestibility values for maize, dark and light DDGS. 8. The standardised TAA digestibility values for laying hens were lower for maize, higher for meat and bone meal, but no different for the remaining ingredients when compared with broilers. 9. The results from this study showed that both the apparent and standardised amino acid digestibility values in caecectomised roosters, laying hens, and broilers ingredients are similar for some, but not all, feed ingredients. 10. Nutritionists should, therefore, be cautious about using digestibility coefficients obtained by different methodologies as values may differ.

The standardized ileal amino acid digestibility (SIAAD) of 5 plant-based ingredients was determined in 5- and 21-d-old broiler chicks and turkey poults. Two diets (N-free diet, NFD, and a highly digestible protein diet 10% casein, HDP) were fed to determine ileal endogenous amino acid flow, and each was used for endogenous amino acid correction for the plant-based ingredients. The plant-based ingredients evaluated were 2 samples of corn distillers dried grains with solubles (light and dark DDGS), canola meal, corn, and soybean meal. Each diet was fed for 5 d to 6 replicate cages containing 30 or 8 birds, which were sampled on d 5 or 21, respectively. For chicks, apparent ileal amino acid digestibility increased with age for all the feed ingredients. After standardization, SIAAD increased with age when chicks were fed DDGS and corn, but not soybean meal or canola meal. For all the feed ingredients evaluated, SIAAD values for both methods of standardization were not different at d 5. However, the HDP method resulted in greater SIAAD values for most of the amino acids in light DDGS, canola meal, and corn (d 21). In poults, the apparent ileal amino acid digestibility values increased with age for all the feed ingredients except the dark DDGS and canola meal. After standardization, there was no effect of age on amino acid digestibility in all the feed ingredients evaluated, with the exception of corn. Within a particular age (d 5 or 21), both methods of standardization resulted in similar amino acid digestibility values except for corn, for which the SIAAD values were greater (NFD method) in 11 amino acids. Results from this study showed that age-appropriate endogenous amino acid standardization is necessary and, in some cases, results in no age-associated differences in amino acid digestibility. With the exception of corn, standardization of amino acid digestibility with ileal endogenous amino acid flow from birds fed an NFD or HDP did not differ for most plant feedstuffs.

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.

The aim of this study was to determine the effect of the standardization method and age on standardized ileal amino acid digestibility (SIAAD) of 4 meat and bone meal (MBM) samples in broiler chicks and turkey poults. The SIAAD were obtained by correcting apparent ileal digestibility (AID) values for basal ileal endogenous amino acid flow obtained from chicks and poults fed a N-free diet or a highly digestible protein diet (HDP; 100 g of casein/kg of diet). Four diets were formulated to contain 20% CP by using MBM as the only source of CP. Each diet was fed to 6 replicate cages of 30 or 8 birds/cage at 5 and 21 d, respectively, for 5 d. With the exception of all-beef MBM, chick AID values on d 21 were higher (P < 0.05) than on d 5. In poults, however, AID was not different between the 2 ages, with the exception of His, which was higher (P < 0.05) at d 21. There was no difference between methods on SIAAD on d 5 or 21 for all 4 MBM samples in either species. There was no effect of the method of standardization on SIAAD at either age for most amino acids. For chicks, however, standardization of the all-beef MBM sample with HDP resulted in higher (P < 0.05) SIAAD values for most of the amino acids (d 5). In poults at d 5, however, the SIAAD for most of the amino acids in the blended MBM were higher (P < 0.05) with HDP correction. The CV in SIAAD in chicks between the 2 methods of standardization was higher on d 5 (8.1%) relative to d 21 (5.0%) for all treatments. In poults, greater variability was observed on d 21 (8.2%) relative to d 5 (6.7%). Results from this study showed that at d 21, SI-AAD with either the N-free or the HDP diet resulted in similar digestibility values.

Ileal endogenous amino acid (IEAA) flow in turkey poults was determined at 2 experimental locations on d 5, 15, and 21 posthatch using 3 methods, namely a N-free diet (NFD), a highly digestible protein (casein), and the regression method, obtained by regressing IEAA flow against dietary casein levels. The diets were semipurified and contained 0, 50, 100, or 150 g of casein/kg of diet as the sole source of dietary protein. Each diet was fed for 5 d to 6 replicate cages of 30 (d 5), 10 (d 15), or 8 (d 21) birds per cage. There was no interaction between locations and age or locations and diet, so the data from both locations were pooled. Ileal endogenous amino acid flow on d 5 (NFD method) was higher (P < 0.05) than on d 15 or 21. Ileal endogenous amino acid flow estimated from the NFD and the regression methods was different on d 5 (P < 0.05), but there were no differences in IEAA flow for most of the amino acids on d 15 and 21. Increasing the level of casein resulted in a linear (P < 0.05) increase in IEAA flow. The amino acids with the lowest flow were Trp and Met, whereas Glu had the greatest flow. The results obtained from this study indicate that data generated across laboratories were repeatable. The results also suggest that about twice as much amino acids of endogenous origin are found in the digesta of poults on d 5 relative to d 21. Also, as poults age, there is a decrease in IEAA flow from d 5 to 15, but flows from d 15 to 21 are not different. These observations suggest that apparent digestibility coefficients for poults on d 5 and d 15 or 21 could be significantly influenced by the level of endogenous amino acids, more so on d 5.

Ileal endogenous amino acid (IEAA) and total amino acid (TAA) flow (mg/kg of DM intake) in turkey poults and broiler chicks at 3 ages (5, 15, and 21 d) were compared by feeding a N-free diet (NFD) or graded levels of casein (highly digestible protein, HDP). The semipurified diets contained 0 (NFD), 50, 100, or 150 g of casein/kg of diet as the only source of amino acids. Each diet was fed to 6 replicate cages containing 30 (5 d), 10 (15 d), or 8 (21 d) birds per cage for 5 d prior to the collection of ileal digesta. At d 5, IEAA and TAA flow in poults fed the NFD, 50, and 100 g of casein/kg of diet was higher (P < 0.05) than in chicks. The IEAA flows within and between both species on d 15 and 21 were not different. Similar trends were observed for the HDP diets (50, 100, or 150 g of casein/kg of diet). An interaction (P < 0.05) between species and age was observed for most of the amino acids for all treatments. The results from this study suggest that at d 5, poults have significantly higher concentration of IEAA and TAA output relative to chicks. However, by d 15 and 21, there were no species differences in IEAA or TAA flow. These results also showed that IEAA flow is species and age dependent. The increased IEAA flow observed at d 5 should be taken into consideration when formulating starter diets on a digestible amino acid basis.

Two experiments (Exp.) were conducted to establish amino acid (AA) digestibility coefficients (DC) for broilers given corticosterone (CS)-induced stress using the apparent ileal digestibility assay. For Exp. 1, 192 Ross x Ross 708 male broilers were placed into 16 floor pens (12 birds/pen). For Exp. 2, 120 Ross x Ross 308 male broilers were placed into 10 floor pens (12 birds/pen). Pens contained nipple drinkers, pan feeders, and soft-wood shavings. Both experiments were completely randomized designs using pen as the experimental unit. In both experiments, chicks were given a common starter diet from d 1 to 20. From d 21 to 30, broilers were provided a control diet or the control + 15 mg of CS/kg of diet dissolved in soybean oil (8 and 5 replications/treatment in Exp. 1 and 2, respectively). Diets were based on corn (65.07%) and soybean meal (26.36%) and contained an indigestible marker (chromic oxide 0.3%). Diets were formulated to contain 3,175 kcal of ME, 18.5% CP, 0.79% digestible TSAA, and 1.00% digestible Lys. Stress validation was accomplished by measuring BW gain, feed intake, and liver weight on d 30. Evidence that stress occurred was apparent due to the fact that broilers fed CS had lower BW gain and higher liver weight than those fed control. On d 30, the ileal contents were removed from 3 birds/pen, pooled, dried, and analyzed for AA content. Amino acid DC were calculated using the following equation: DC = 100 -(dietary marker % x ileal AA %) / (ileal marker % x dietary AA %) x 100. Amino acid digestibility did not differ (P > 0.05) between treatments in either experiment. Based on this research, future research should be directed at establishing DC for other nutrients in stressed broilers or the effect of different nutrients on the stress response.

From 0 to 4, 4 to 8, and 8 to 12 wk of age, Nicholas (NIC) and British United Turkeys (BUT) toms were fed one of two series of experimental diets. All toms were fed the same diet, appropriately adjusted for age period, from 12 to 16 and 16 to 24 wk. The first series was formulated to 1984 NRC specifications whereas the second series contained higher concentrations of lysine and total sulfur amino acids (NRC+). The NIC toms were heavier (P < or = .05) at 4, 8, 12, and 16 wk, but there were no strain differences at 20 wk. The NRC+ diets significantly increased BW in both strains at 12 wk, but there were no diet effects at 16 or 20 wk. At 12 wk, BUT toms had significantly more carcass protein and less carcass lipid compared with NIC toms, but there were no significant dietary effects on these variables. In both strains, the NRC+ diets significantly increased the relative weight of the eviscerated carcass and both the absolute and relative weights of the Pectoralis major muscles at 12 wk. At 20 wk, the absolute and relative weight of the Pectoralis minor muscles was significantly heavier in BUT toms and in toms fed the NRC+ diets. The tibia and femur were longer in BUT toms compared with NIC toms. From 1 to 24 wk, weekly BW from 50 toms per strain and treatment combination were used to calculate growth curve parameters according to the Gompertz equation. The slope of the growth curve was significantly less in BUT toms, but the BW and age at the point of infection were increased compared with NIC toms. There were no significant diet effects on any growth curve parameters.