EFFECTS OF DIETARY PHYTASE AND CHOLECALCIFEROL ON PHOSPHORUS BIOAVAILABILITY IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

A study was conducted to examine the influence of dietary inclusion of phytase and high levels of cholecalciferol on phytate phosphorus (P) utilization of rainbow trout. Triplicate groups of rainbow trout, init ial weight 51.6 g, were fed in excess diets containing 0 or 1500 units phytase kg(-1) and 2500, 250,000 or 2,500,000 IU cholecalciferol kg(- 1) for 12 weeks. The basal diet provided 5.8 and 3.2 g total and phyta te P kg(-1) dry matter, of which most was derived from soy protein con centrate. Weight gain of fish was increased by phytase supplementation but decreased by higher cholecalciferol concentrations. Inclusion of phytase improved P availability as indicated by significantly higher a pparent availability of P and bone ash and plasma and body P concentra tions. Dietary cholecalciferol content did not influence P utilization . An increase in dietary cholecalciferol concentration caused higher d eposition of calcium, magnesium and zinc in the kidney. Both cholecalc iferol and phytase supplementation significantly (P < 0.05) increased the hepatic cholecalciferol concentration. In summary, dietary phytase supplementation was effective in reducing P load of rainbow trout fed soybean protein concentrate-based diet. High levels of cholecalcifero l (250,000 and 2,500,000) had no beneficial effect on P utilization. ( C) 1998 Elsevier Science B.V. All rights reserved.