Use of nonlinear regression to evaluate the effects of phytase enzyme treatment of plant protein diets for rainbow trout (Oncorhynchus mykiss)

The effects of dietary phytase enzyme treatment and increasing ration on growth, body composition, nutrient digestibility and retention and phosphorus release in effluent water were studied in rainbow trout (Oncorhynchus mykiss). Rainbow trout (initial weight: 115.3 +/- 0.49, mean +/- S.D.) were randomly distributed among 24 tanks and fed two experimental diets (A and B) each at six feeding levels (0.5, 1.0, 1.5, 2.0, 2.5 b.w. and satiation for 49 days). The two diets contained 33% soybean meal and differed only by the addition of 1000 U phytase/kg to diet B. The apparent digestibility coefficients for dry matter, crude protein, lipid, N-free extract, gross energy and ash of the two diets did not differ statistically. The phytase in diet B increased apparent digestibility of phosphorus (P) from 58.6 to 68.1% (P<0.01). Daily gain and FCR improved in fish fed diet B with feeding levels greater than 1.5 and the best fit of the data was by using logistic and quadratic regression, respectively. Body protein, ash and P content decreased in fish fed diet A and with increasing ration up to 2.0% feeding rate and then reached a plateau. In contrast, body energy concentrations increased up to the same feed ration level and than levelled off. A significant difference (P < 0.05) between diets was only observed for body ash content. Body protein, ash and P retention decreased while energy retention increased in fish fed diet A and with increasing feeding levels. Best fit was obtained by using an exponential model for body protein (P < 0.05) and a quadratic model for the other body composition variables. Phosphorus released into the environment was higher for fish fed diet A (P<0.05) and increased with increasing feeding level. A semilogarithmic model showed the best fit of the experimental data for P release.