Evaluation of methodology for determination of availability of amino acids and energy in feedstuffs for poultry

Abstract

In vitro and in vivo methods were evaluated for determining protein quality and availability of amino acids (AA) and energy in various feedstuffs or feed ingredients for poultry. In vitro assays included pepsin digestibility and lanthionine analysis. In vivo methods involved protein quality chick assay, AA bioavailability chick assay, and true AA digestibility and TMEn assay using conventional and cecectomized cockerels. Cecal retention of AA and effect of hindgut microbes on AA excretion were evaluated.

The chick growth assay is accepted as the standard for estimating AA bioavailability. This study illustrated the limitations of the growth assay when used for feedstuffs severely deficient in one or more of the important AA. Different calculation methods will often yield different bioavailability values.

AA digestibility and TMEn values for cecectomized cockerels were frequently lower than those for conventional birds, with the magnitude of difference varying among feedstuffs. This suggested that significant amounts of undigested dietary AA and other energy-containing compounds flowed into the ceca and were subsequently metabolized by microbes. The quantity of dietary AA present in the ceca at 48 hour postfeeding was insignificant. These results suggested that cecectomized birds might be a better model than conventional birds for AA digestibility, whereas cecectomized birds may underestimate TMEn values. AA digestibility and TMEn values between bird types were highly correlated.

AA digestibility values generally agreed well with the bioavailability values. However, differences did exist in some cases, and the magnitude varied among feedstuffs and AA within feedstuff. The digestibility assay may overestimate bioavailability for some AA in some feedstuffs of poor quality.

The pepsin digestibility and lanthionine assay yielded values highly correlated with most in vivo assay for feather meals. The reduced pepsin concentration (.002% vs.2%) was more sensitive for detecting differences in protein quality.

Intestinal absorption of DL-methionine and DL-methionine hydroxy analogue was essentially 100%.