Effects of dietary organic chromium on rainbow trout (Oncorhynchus mykiss) performances and body composition

INTRODUCTION: Over the last decades, chromium has been recognized as a new and important micronutrient, essential in human and animal nutrition , influencing many aspects of metabolism. In particular, it has been shown to have a positive influence on the growth, reproductive efficiency and carcass composition of pigs and cattle. Studies in swine and poultry suggest that inorganic forms of chromium are poorly absorbed from gastrointestinal tract whilst the organic forms are more effective. As very few data are currently available in the literature about the influence of the organic forms on the metabolism of fish, the aim of the present study was to assess whether dietary supplementation with chromium picolinate could modify performances, carcass and fillet composition of rainbow trout (Oncorhynchus mykiss). MATERIAL AND METHODS: Four semipurified diets (Protein: 46 %, Lipid: 21 %) were formulated and produced using a pelletizer machine at the Department of Veterinary Morphophysiology and Animal Production (DIMORFIPA, University of Bologna). The diets were designated as either (L) containing picolinate chromium at 500 ppb; (I) containing picolinate chromium at 1000 ppb; (H) containing picolinate chromium at 2000 ppb or (C) the control diet containing no added chromium. Experimental feeds were administered for 100 days to juvenile trouts (Oncorhynchus mykiss) with an approximate initial average weight of around 25 g. Animals were hand-fed twice a day (at 9.00 a.m. and 5.00 p.m.) at a daily ratio of 1.8. The facilities consisted in twelve 300 litres steel tanks supplied with a continuous flow of well water (5 litres min-1) at 13  1 °C of temperature. Photoperiod have been held constant at a 16-h day length. Each diet was tested in triplicate. At the end of the trial, zootechnical and nutritional indices, carcass and fillet proximate composition and haematocryt were measured. RESULTS AND DISCUSSION: At the end of the trial, animals fed diet L and diet I showed statistically higher final weights (130 g – diet L; 127 g - diet I;) in comparison with the other groups (117 g – diet H and diet C). Nutritional indices were also different between treatments. In fact, PER and PPV were higher in fish fed diet L (2.28; 58.7) compared fish fed diet I (2.27; 57.4), diet H (2.16; 53.8), and the control diet C (2.12; 51.3). Again, in terms of carcass composition, fish fed diet L showed the highest protein percentage (52.8%) in comparison with the other groups. Slaughter indices, fillet proximate composition and haematocryt were similar among groups. The results of the study show that chromium picolinate is able to positively influence the utilization of protein in trout and this effect appears to be dose dependent. The low and intermediate levels seem to exert a better effect in comparison with the highest level where no differences were noticed in comparison with the control.