Contribution of digestible energy from carbohydrates and estimation of protein/energy requirements for growth of rainbow trout ( Oncorhynchus mykiss)

Abstract

Four diets having different digestible protein (DP) to digestible energy (DE) ratios were formulated to study the protein-sparing effect of DE from carbohydrates and to estimate the DE requirements for growth of rainbow trout. A growth trial was conducted for 8 weeks. Digestibility measurements were made over 2 weeks. Trout (initial weights 37 and 100 g for growth and digestibility trials, respectively) were fed to satiation twice a day at a constant water temperature of 17.5 °C. Apparent digestibility coefficients (ADC) for dry matter, starch and energy decreased with an increase in dietary raw starch level. The metabolic fecal nitrogen loss was estimted to be 150 mg N/ 100 g of dry diet. Fish fed a high protein diet with 48% DP, 9% digestible fat (DL) and 20% digestible carbohydrates (DC) showed the best performance for weight gain, feed conversion ratio and daily growth index. However, the highest protein efficiency ratio (2.3) and protein retention efficiency (41.4%) were observed in fish fed a diet containing 38, 9 and 30% of DP, DL and DC, respectively. The whole body composition of the fish was not affected by dietary treatments. Liver glycogen and hepatosomatic index were positively related to dietary digestible carbohydrate levels. The digestible protein requirement per unit weight gain (g DP required per kg production) varied depending upon the DP/DE ratios. On the other hand, DE requirement per unit weight increment was relatively constant (about 17.5 MJ for the production of 1 kg of rainbow trout), irrespective of the dietary treatment.