Adaptation to soybean meal in diets for rainbow trout, Oncorhynchus mykiss

Abstract

Three experiments were conducted with rainbow trout maintained in fresh water. In Experiment 1, growth was measured in groups of 34-g trout fed either a fish meal control diet (Diet 1) or a diet containing 60% soybean meal (Diet 2) for two periods of 28 days each. In Experiment 2, Diets 1 and 2 were fed to groups of 100-g trout for 10 days prior to stripping of faeces and digestibility determination. In Experiment 3, groups of 200-g trout were fed either a fish meal control diet containing 100 mg Y 2O 3 kg −1 diet as an inert marker (Diet 3), or a diet containing 40% soybean meal and 100 mg Yb 2O 3 kg −1 diet as an inert marker (Diet 4) for an adaptation period of 7 days. Thereafter, both diets were offered in excess in a 1:1 mixture for 2 days. Qualitative preference for the two diets was estimated as the proportions of Y 2O 3 and Yb 2O 3 in faeces. Subsequently, digestibility was determined for all diets. In Experiment 1, growth was 1.4 times faster for the trout fed Diet 1 than for those fed Diet 2 during the first 28 days. Adaptation occurred, and the difference in growth was not significant during the last 28 days. In Experiment 3, the trout which were adapted to Diet 3 preferred this diet, consuming only a small portion of the soybean diet. The fish adapted to Diet 4 consumed 59% of the fish meal diet and 41% of the soybean diet, indicating a rapid adaptation in feed intake to the soybean diet. Only minor differences in digestibility were found between the fish meal control diets and the diets containing soybean meal in Experiments 2 and 3. These results indicate that given an adaptation period, trout previously adapted to fish meal diets consume and grow well on diets containing high levels of soybean meal. Temporary lower growth after offering diets with soybean meal than when continuously feeding fish meal diets is rationalized by adaptation in qualitative preference to the soybean diets.