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Nitrogen correction of true metabolizable energy values for poultry feed ingredients

Dillard, R., N. Dale and A. Davis

The apparent metabolizable energy (AME) and true metabolizable energy (TME) assays of poultry feed ingredients have traditionally been nitrogen (N) corrected to yield AMEn or TMEn. By correcting to zero N retention in the AMEn bioassay all feed ingredient results are directly comparable to one another. Roosters for the TMEn bioassay are fasted for 24 hours prior to precision feeding of the test ingredient which places them in a negative N balance. The N correction for the TMEn assay was utilized to avoid an overestimation of the endogenous energy correction. In the original published research conducted over 40 years ago, roosters remained in a negative N balance when fed test ingredients and the N correction resulted in TMEn values that were 1 to 14% lower than their corresponding TME values. In the current research N retention, TME and TMEn were determined on over 700 ingredient samples to determine if the original findings on N retention and correction were still appropriate. For all ingredients tested, a positive correlation (P < 0.0001) existed between N balance and the TME minus TMEn value. Unlike the original research, over 15% of the samples tested had a positive N balance. These samples were almost exclusively animal by-products (ABP) such as feather and meat and bone meals. About 45% of the tested ABP had a negative N balance. There was a positive (P < 0.0001) correlation between total digestible amino acid content and N balance in the ABP samples. For 68 of the ABP samples the TMEn value was decreased by 10% or greater from the TME value. However, unlike the original research, N correction increased the TMEn value relative to its TME value in 10% of the samples. Roosters fed these samples had a negative N balance that exceeded that of the unfed endogenous control roosters, and the samples such as soy and rice hulls, were low in protein and/or high in fiber. For some of these samples the N correction inflated the TMEn value by over 20%.

These results suggest that for the vast majority of ingredient samples, the N correction established by the original research is appropriate, but for samples with a very positive N balance or with a negative N balance that exceeds the endogenous controls, a change in the degree of the N correction may be necessary.