Apparent Digestibility of Gross Nutrients from Feedstuffs in Extruded Feeds for Rainbow Trout, Oncorhynchus mykiss

—The current experiment was performed to determine apparent protein and energy digestibility coefficients of feed ingredients by the fecal stripping technique using extruded diets. The ingredients tested included five fish meals, three terrestrial animal by-products, five plant protein concentrates, four plant meals, and seven low-protein plant ingredients. Protein digestibility differed among fish meals and ranged from 90% for fair and average quality menhaden meal to 97% for anchovy meal. Protein apparent digestibility coefficients (ADCs) in plant concentrates ranged from a low of 89% for rice protein concentrate to a high of 100% forwheat gluten meal. Apparent protein digestibility was lower in plant meals than fish meals with a low of 70% for flaxseed meal to a high of 89% for soybean meal. Low-protein plant meals had generally lowerproteinADCfrom64%for rice bran to85%for wholewheat.A similar pattern for energyADCswasobserved; ADCs ranged from 106% for anchovy meal to 32% for whole wheat. In the current trial, divergent protein and energy ADC values were obtained most notably in ingredients known to be high in fiber or have very high starch content. The comparability of ingredients/diets processed by cold pelleting or extrusion thus appears questionable at this juncture. Reducing the reliance of trout production on fish meals is an ongoing effort worldwide, and the utilization of sustainable plant-based ingredients has become a research focus. A crucial stimulant to these efforts has been the dramatic rise in fishmeal prices coupled with often limited availability (FAO Globefish 2007). Researchers around the world are making progress not only in defining the nutrients that are limiting to trout but also in identifying conditionally indispensable nutrients that are present in fish meals but absent from alternate protein sources (Gaylord et al. 2006). Glencross et al. (2007) recently reviewed strategies of evaluations for aquaculture feedstuffs and highlighted five key evaluation components: ingredient characterization, ingredient digestibility measurements, ingredient palatability, determination of nutrient utilization or interferences from an ingredient, and ingredient functionality. Chemical analyses coupled with ingredient digestibility measurements are often employed as a first step in evaluation of potential aquaculture feedstuffs. These values then serve as the basis for the necessity of further evaluation as described by Glencross et al. (2007). However, currently, most digestibility studies are conducted with cold-pelleted diets and thus may not give accurate estimates of the value of these ingredients in extruded diets. Therefore, the objective of this study was to simultaneously develop a data set of apparent digestibility coefficients (ADCs) in extruded diets for 24 ingredients that are commonly included in aquafeeds or have potential for future use. The ingredients included five fish meals, three animal by-products, five plant protein concentrates, four highprotein (.25% crude protein) plant meals, and seven low-protein (,25% crude protein) plant feedstuffs. Expansion of ingredient evaluation data sets is paramount for the continued sustainability and profitability of the rainbow trout industry and developing these data sets with diet processing techniques that mimic industry methodologies is vital. Materials and Methods Ingredients and Diets The 24 ingredients evaluated were grouped into five classes based on their source and utility 1 Corresponding author. JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 39, No. 6