2005 Annual Report 4d.Progress report. This report serves to document research conducted under a Specific Cooperative agreement between ARS and University of Idaho, Hagerman, ID. Additional details of research can be found in the report for the parent CRIS 5366-21310-003-00D Integration of Nutritional, Genetic, and Physiological Approaches to Improve Production Efficiency of Rainbow Trout and progress over the last year. Collaborative studies were conducted to determine digestibility and nutrient availability for a number of feed ingredients from grains, including barley, oats and several blends of fermented plant products. Each ingredient was added to a semi-purified control diet containing an inert, marker compound (yttrium oxide) and fed to post-juvenile rainbow trout. Feces were collected and analyzed, along with each ingredient, test diets containing each ingredient, and the control diet, and apparent digestibility coefficients (ADC) for dry matter, organic matter, and protein were calculated. Results indicated high ADC values for all test ingredients. The ADC values will be used to design feeding trials with rainbow trout to evaluate upper limits for each ingredient in practical feed formulations; these trials will be conducted when experimental rearing tanks become available at the University of Idaho's Hagerman Fish Culture Experiment Station. A feeding trial with rainbow trout was conducted to compare growth performance and feed utilization of feeds containing blends of plant ingredients and supplemental amino acids to replace all fish meal in the formulation, e.g., a fish-meal free trout diet. Ten diets were fed to triplicate lots of 30 fish for 12 weeks. Weight gain of fish fed the best blends of plant proteins was not significantly different from that of the control diet, a conventional fish meal-based formulation. However, weight gain was approximately 8% lower and feed conversion ratio about 10% higher than values from fish fed the fish meal control feed. At present, the cost of ingredients in the plant protein feed is significantly higher than that of the fish meal-based feed, but this study demonstrates that it is feasible to grow rainbow trout on an all-plant protein feed. The most promising all-plant feed will be used as a control feed in future feeding trials, providing a benchmark for comparative purposes.