Evaluation of Amino Acid Digestibility Methods

Summary

The use of digestible amino acids (AA) for the formulation of poultry diets has been recognized as being preferable to formulating diets on a total AA basis. Thus, determination of AA digestibility of feed ingredients has become crucial. To date, there is a large body of published work on AA digestibility coefficients for different feed ingredients. However, nearly all of this work has been done using the Precision-fed cecectomzied rooster assay which many believe may not be accurate for young poultry.

Situation

The protein requirement of chickens is in fact a requirement for amino acids (AA). Therefore, primary interest has been placed on determining AA requirements and using that information for feed formulation. However, it is well known that not all AA in feedstuffs are equally digested and available to the animal. Consequently, more attention has recently been given to the determination of AA digestibility of feedstuffs recognizing that it may vary greatly depending upon the feed ingredient. Additionally, digestibility of AA has been recognized as being a sensitive indicator of biological availability. The use of digestible AA for feed formulation has been recognized as being preferable to formulating diets on a total AA basis. Thus, determination of AA digestibility of feed ingredients has become crucial. To date, there is a large body of published work on AA digestibility coefficients for different feed ingredients. A number of different methodologies for determining AA digestibility coefficients have been reported in the literature. Among these methods, the precision-fed cecectomized cockerel assay has been widely used for estimating AA digestibility of feedstuffs, yielding very consistent results. Criticisms of this methodology have arisen from the fact that crop intubation is not a normal feeding pattern, and estimations are carried out in adult, physiologically mature birds that might not reflect the digestive capabilities of younger chickens. As an alternative, Payne et al. (1968) described the ileal digestibility assay, in which AA digestibility is based on analysis of ileal digesta. The main advantage of this method is that the test diets are fed ad libitum, which is a normal feeding pattern, and birds of different ages can be used. For either assay, true rather than apparent AA digestibility is preferred. However, since different methodologies have been proposed for estimation of endogenous AA losses, the assay of choice is still a subject of debate. In order to merge different criteria generated from a variety of studies, the term 'standardized ileal digestibility' has been proposed. Under this concept, the ileal apparent AA digestibility data generated by the chick assay are corrected for the basal endogenous AA secretions determined by the enzymatically-hydrolyzed casein method. While adoption of a standardized assay has clear advantages, the use of the cecectomized cockerel technique is recognized as being far less expensive and labor intensive. Therefore, a comparison of techniques to determine possible discrepancies is then justified. To date, little work has been published comparing the AA digestibility coefficients obtained by the different assays (i.e. chick ileal digestibility vs precision-fed cecectomized rooster assay).

Response

To compare the AA digestibility coefficients obtained by the standardized chick ileal digestibility and the precision-fed cecectomized rooster assays, two experiments were conducted. The six test diets contained corn, wheat, soybean meal, poultry by-product meal, feather meal and fish meal as the sole source of protein. After the birds were allowed to consume the experimental diets the birds were euthanized by CO2 asphyxiation for collection of ileal digesta. True AA digestibilities of the diets from both experiments were also determined using the total fecal collection “precision-fed” cecectomized rooster assay.

Impact

In the present research there was an indication that the AA digestibility coefficients differ depending upon the methodology employed for their estimation. In recent years, it has been recognized that the standardized ileal digestibility assay might be more appropriate for estimation of AA digestibility, although the rooster assay can be more advantageous for routine evaluation of feed ingredients. Thus, the method of choice is still debatable. Data reported herein also suggest that for some feed ingredients, AA digestibility coefficients obtained by the rooster assay might not be applicable to young chicks. Thus, the incorporation of different AA digestibility values in diet formulation for young chickens might become necessary. The use of AA digestibility values from either method would have dramatic effect on diet formulation and cost, thus it is crucial that we determine the best method.

State Issue

Agricultural Profitability and Sustainability

Details

• Year: 2006
• Geographic Scope: International
• County: Clarke
• Program Areas:
  • Agriculture & Natural Resources