Standardized amino acid digestibility determination in poultry
The need to formulate poultry diets on a digestible amino acid basis rather than on total amino acid basis is increasingly becoming relevant. To achieve this, standardized amino acid digestibility (SAAD) values of various feed ingredients that are used in poultry feeds need to be determined. In view of this, correction of apparent digestibility values for basal endogenous amino acid flows is critical. One of the factors militating against the adoption of SAAD values is the lack of consistency of results due to several factors including the method of endogenous amino acid (EAA) flow estimation. To answer these questions, we designed a series of studies to evaluate the effects of age of bird, species, strain, and method (nitrogen-free diet, NFD; highly digestible protein, HDP; and the regression method) of EAA flow determination on EAA flow. The results from these studies showed that bird age, species, and experimental method all influenced EAA flow. Within a particular species, ileal endogenous total amino acid flow (TAA) on d 21 was 45% (broiler chicks, NFD method) and 36% (turkey poults, NFD method) of endogenous TAA acid flow on d 5, but there was no difference in EAA flow between d 15 and 21. When comparison was made between species (broiler chicks and turkey poults), endogenous TAA flow in broiler chicks was 46% (d 5, NFD method) and 60% (d 21, NFD method) of respective flows in turkey poults, however, no species effect was seen on d 15 and 21. Increasing level of dietary casein resulted in linear increases in EAA flow. Endogenous amino acid flow for the NFD and the regression methods on d 21 were not different (d 21) but differences were seen between the two methods on d 5. The NFD and HDP were used to standardize apparent digestibility values for four meat and bone meal (MBM) samples from different sources and five plant source feed ingredients. For most of the feed ingredients, no difference was observed between the two methods of standardization within a particular age. The age effects noted with apparent digestibility values disappeared after standardization for most of the amino acids. In a subsequent experiment, the effects of standardization on three strains (21-d old broiler chicks, laying hens, and cecectomized roosters) of poultry fed six feed ingredients were evaluated. Additionally, the effects of method of standardization (fasted, NFD, and HDP) within cecectomized roosters using a precision-feeding method were also determined. Results from these studies showed that SAAD in broiler chicks, laying hens, and cecectomized roosters, especially between broiler chicks and cecectomized roosters are not always similar. The SAAD values for each poultry strain vary with feed ingredients and the particular amino acid. Within the precision-fed cecectomized roosters, standardization method did not affect SAAD values for the two sources of distillers dried grain with solubles and canola meal. However, SAAD values for corn, soybean meal, and MBM were higher for the NFD and HDP standardization methods than the fasted rooster method. The final experiment was designed to evaluate the role of dietary treatments and age of turkey poults on crude mucin, sialic acid, intestinal morphometric measurements, on EAA flow, and mucin mRNA expression level. Results from this study showed age and diet effects on crude mucin and sialic acid level in ileal digesta. All the morphometric measurement evaluated in this study increased with age. Mucin mRNA expression level in mucosa taken from mid ileum was not different in poults fed the HDP and corn-soybean meal (C-SBM) based diets. However, the level of mucin mRNA in poults fed C-SBM based diet was higher than in poults fed the NFD. Despite previous criticisms of NFD method of endogenous amino acid determination, it was the most consistent of all the methods evaluated in these studies.^
Todd J. Applegate, Purdue University.
Agriculture, Animal Culture and Nutrition