Potential effects of supplemental microbial phytase on digestibility and utilization of phosphorus (P) originating from plant materials were studied in trout. Diets were mainly based on soybean products and contained P only from plant materials. Concentration of total P was suboptimal and about 60% of total P were bound as phytate P. Diets were prepared either without enzyme supplementation or after supplementation with an Aspergillus niger phytase (1000 units/kg diet). Apparent digestibility of P was determined at a water temperature of 15°C. Utilization of P was measured in two growth trials at water temperatures of either 15 or 10°C. P retention of trout was measured using the comparative body analysis. At 15°C, both digestibility and utilization of dietary P were considerably improved when microbial phytase was supplemented. P digestibility and P utilization increased from 25 to 57% and from 17 to 49%, respectively. At 10°C, P utilization increased from 6 to 25%, but intake of the diet as well as growth rate of trout were very low, irrespective of whether phytase was supplemented or not. Practical relevance of the findings seems to apply only to diets in which protein is supplied almost entirely by plant products.

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