Dietary formic acid enhances apparent digestibility of minerals in rainbow trout, Oncorhynchus mykiss (Walbaum)

The effect of dietary formic acid on the availability of phosphorus (P) from a fishmeal-based diet by rainbow trout, Oncorhynchus mykiss (Walbaum), reared in brackish water (5-6 g L-1) was determined. Diets containing a low amount of P (6.0 mg P g(-1) dry matter) were acidified with 0, 4 and 10 mL kg(-1) formic acid and fed to trout (average weight, 520 g) for 4 weeks. The measured pH of the diets were 6.3, 5.8 and 5.3, respectively. The intestinal solubility of P and the digestibility of P were measured by stripping the faecal matter from the fish. The apparent digestibility coefficient (ADC) of P significantly (P < 0.05) increased from 69.5% to 75.0% of the basal diet in fish fed diets containing 10 mL kg(-1) formic acid. The solubility of P in the intestine was highly variable within each treatment, and the differences were not significant. The pH of intestinal content increased with the increase in dietary formic acid concentration. The ADC of magnesium and calcium also showed a significant (P < 0.05) increase with the acidification of diet by formic acid.