Effect of phosphorus intake on faecal and non-faecal phosphorus excretion in rainbow trout ( Oncorhynchus mykiss) and the consequences for comparative phosphorus availability studies

Abstract

The effect of dietary phosphorus (P) concentration on faecal and non-faecal P excretion of rainbow trout was studied with P concentrations ranging from 1.0 to 6.2 g/kg dry matter, the upper level calculated to be slightly above the P requirement. Eight semi-purified diets were prepared with varying proportions of dibasic sodium phosphate and an inorganic binder to achieve intended P concentrations. Diets were fed at a restricted rate to triplicate groups of trout initially weighing 51 g/fish for 43 days. P retention was determined by comparative whole body analysis. Faecal material was collected from a sedimentation unit attached to each tank and digestibility was determined using acid-insoluble ash as a marker. Non-faecal P excretion was calculated as the difference between intake of apparently absorbed P and retained P. Blood samples were taken at the end of the experiment. Responses of trout to increasing P intake were described by non-linear functions. Digestibility of lipid and carbohydrates and, consequently, energy were lower in fish fed the basal P level than in fish fed any supplementary level of P. The partial efficiency of digestible energy for retention, however, was not affected by the dietary P level. With increasing P intake, faecal P excretion increased non-linearly. Basal non-faecal P excretion was estimated at 3.7 mg/kg BW per day and was unaffected by P intake until the estimated P requirement was met. Above a P concentration of about 5 g/kg DM, non-faecal P excretion increased. Serum P i concentration remained unchanged once this dietary P level was achieved. It is concluded that trout adapt P absorption rate to increasing P intake already before the P requirement is met, whereas non-faecal P excretion happens when the supply of available P is sufficient for P retention. Marginal P absorption, determined as ΔP apparently absorbed/ΔP intake, showed a maximum of 0.73 at an intake level, which was approximately 33% of the estimated P requirement and dropped towards 0.40. Similarly but more pronounced, marginal efficiency of utilisation, determined as ΔP retained/ΔP intake, showed a maximum of 0.88 and tended towards 0.10 at high intake. Apparent absorption of P proved a suitable measure for comparative evaluation of P availability. However, for testing P availability, the concentration of available P in the diet should not exceed 2.5 g/kg DM. As a consequence, the difference method is necessary for routine studies on P availability in trout. This method implies that individual P sources are mixed with a low-P basal diet and that the availability is determined for both the basal diet and the mix, allowing for the calculation of availability for the test source as the difference.