The effect of vitamin premix in extruded plant-based and fish meal based diets on growth efficiency and health of rainbow trout, Oncorhynchus mykiss

Trout diet formulations have changed considerably in the last ten years and modern diets reflect changing ingredient markets, feed processing technologies, and faster growing strains of trout. In contrast, most of the vitamin requirements for rainbow trout were determined more than 30 years ago and whether these requirements are appropriate for modern diet formulations is unclear. For these reasons, a study was conducted to determine the effect of protein source and vitamin levels in extruded feeds on growth performance and nutrient retention. A 2 × 4 factorial treatment design was used with two protein sources, fish meal and plant meals, and four vitamin premixes. All vitamin premixes contained the same vitamins but at different levels and included; 1) 100% of NRC [National Research Council (NRC), 1993. Nutrient Requirements of Fish, National Academy Press, Washington, DC. 114 pp.] recommended vitamin level (NRC), 2) NRC levels with each vitamin independently increased for potential extrusion losses (variable retention ,VR), 3) NRC levels with all vitamins increased to assume equal retention of 40% after extrusion (ER), and 4) negative control with no vitamin premix added (None). Each of the 8 diets was fed to groups of 35 rainbow trout (4.8 g) in 150-L fiberglass tanks (5 replicate tanks per diet, except for the negative control diets with 3 replicates) for 15 weeks. Each tank was supplied with 6 L/min of untreated, constant temperature (14.5 °C), spring water. Signs of a pantothenic acid deficiency were observed within 6 weeks for the fish fed the plant-based diet without vitamin premix and signs of a vitamin E deficiency were observed for the trout fed the fish meal-based diet without vitamin premix. These treatments were terminated after 9 weeks. Among the supplemented diets, vitamin premix did significantly affect survival, feed intake, protein retention efficiency (PRE), energy retention efficiency, hematocrit and HSI, but not weight gain or FCR. Significant interactive effects between vitamin premix and protein source were observed for survival, hematocrit, HSI, and PRE. Vitamin retentions after extrusion were similar to reported values with the exception of vitamin A, folic acid, and thiamin. Results indicate that the vitamin levels recommended by NRC do not appear to be adequate for young, fast growing trout fed extruded feeds. Adjustment of individual vitamins in the premix to account for vitamin destruction during the extrusion process will maintain levels in the final feed at target levels while reducing vitamin costs associated with over-supplementation.