Apparent amino acid digestibility of feed ingredients for juvenile shrimp (Litopenaeus vannamei): a new method of determination using soybean meal as an example

Abstract

Literature information on apparent digestibility coe cients (ADC) of dietary nutrients in feedstuff for shrimp species dates to 1980´s though the nutritional consistency of ADC values determined in individual feed ingredients continues under scrutiny. This may be attributed to: a. the large variety of ingredients tested under a single standard methodology (i.e. the partial replacement of a reference diet mix by a xed proportion of the target ingredient), b. the complex effects of this dietary strategy upon palatability, digestion/digestibility, and consequently feeding rates, and c. nutrient leaching of diets during experiments. While the biological phenomenon behind ADC, the so-called "nutrient disappearance" between diet and feces through animal´s digestive system, is measured by the difference of nutrient and inert marker content in diet and feces, ADC of a feed ingredients has been mathematically calculated considering the difference in ADC from test (e.g. 30% replacement of a reference diet mash by the target ingredient) versus reference diet ADC and their nutrient content. The present study proposes the determination of individual amino acid ADC of soybean meal (SBM) based on the effect of inclusion of SBM in increasing dietary levels in four practical test diets (5, 10, 15 and 20% SBM, named diets T4, T5, T6 and T7, respectively) upon ADC of test diets. For comparison, the study also included three diets formulated under the standard replacement method (i.e. 10, 20 and 30% replacement of the reference diet mash by SBM, named diets T1, T2 and T3). The feeding trial was carried out under high shrimp performance with automated feeding (20-22h pellet delivery/day, minimized leaching), and daily feces collection over the trial period (55 days). The estimation of ADC of amino acids in SBM was based on ADC determined in test diets: ADC AA SBM = Mean [(ADC AA diet (T4, T5, T6 or T7) / ADC protein diet (T4, T5, T6 or T7) ) x ADC protein SBM ], diet ADC experimentally determined and an assigned ADC value of SBM true protein (ADC protein SBM ) estimated by three criteria: effect of increasing inclusion of SBM upon true protein ADC of test diet; additivity of digestible protein supplied by proteinaceous ingredients composing test diets T4 to T7, literature values and author's experience. Results by the new proposed method showed SBM ADC values between 76 and 88% for indispensable amino acids and contrasted with values obtained by applying the conventional replacement method at 30% inclusion of SBM into a reference diet (ADC AA : 87-96%). It was also checked either the improvement or reduction of certain individual amino acid ADC with increasing SBM inclusion level in diets. In conclusion, the new proposed methodology produced reduced and more realistic ADC values of amino acids compared the conventional method of ingredient replacement into a reference diet at one xed level. This new methodology for ADC determination is not intended to become an unquestionable reference but rather to offer an alternative view for more...