Effects of phytate, phytase and dietary phosphorus on feed intake, growth performance and phosphorus utilisation in Atlantic salmon (Salmo salar, L.)

One of the key research areas in aquaculture nutrition is fish meal replacement. Plant derived materials such as legume and oilseed meals and protein concentrates are being investigated and used in aquafeeds. However, the use of these ingredients is limited by antinutritional factors. Phytate is an important antinutritional factor that impacts on phosphorus utilisation and excretion in monogastric animals including fish. Phytate has negative effects on growth performance, protein and mineral, especially phosphorus, digestibility and utilisation. Both purified phytic acid and natural sources of phytate were used in the present study. Different criteria such as growth performance, chemical composition and nutrient apparent digestibility were measured to assess the effect of phytate and phytase on Atlantic salmon (Salmo salar, L.). Furthermore, due to the importance of phosphorus in environmental loading, the effect of phosphorus intake on phosphorus utilisation was also investigated. The first experiment established that phytate (0.8%) did not affect feed intake or growth performance, but significantly reduced protein digestibility without negative effect on trypsin activity in Atlantic salmon. Phytase per se enhanced the growth of fish and when phytase and phytate were added together feed efficiency ratio was higher. Phytase neutralized the effect of phytic acid on protein digestibility. Use of phytase in plant meal based diets prevented the negative effects of phytate. Phytase had positive effects on feed intake, growth, whole-body chemical composition and nutrient digestibility. When phytase was used in a diet with sufficient dietary phosphorus, the positive effects ofphytase were not observed. Efficacy of phytase strongly depends on endogenous and exogenous factors. Phytase inclusion in a fish meal based diet partially replaced with plant meal had no effect on fish performance, but when phytase was used in a soy protein concentrate (SPC) based diet, improved performance was observed. Probably, phosphorus apparent digestibility is the best criterion for determining phytase efficacy. Furthermore, phytase had a significant effect on reducing phosphorus waste from a plant protein based diet. Phytase supplementation of diet and pretreatment of ingredients were compared using a SPC based diet. Supplementation at levels greater than 1 OOO U phytase kg-1 diet was necessary to improve salmon performance and phosphorus utilisation, while pre-treatment of SPC with 250 U phytase kg-1 had similar significant effects. The effect of phosphorus intake on apparent digestibility of phosphorus was investigated in Atlantic salmon. Phosphorus apparent digestibility was negatively correlated with dietary phosphorus concentration. Digestible phosphorus was approximately the same for salmon in all treatments despite using different levels of dietary phosphorus that ranged from suboptimal to excess amount. The effect of feed intake on nutrient and mineral especially phosphorus digestibility was also investigated. Ration affected nutrient digestibility except energy digestibility in fish and there were significant correlations between dry matter, protein and phosphorus digestibilities. Experiments generally showed that phytase supplementation of diet or pretreatment of ingredients has the potential to improve growth performance and phosphorus utilisation of Atlantic salmon and reduce the phosphorus load from fish farm provided proper experimental design is used. Since dietary phosphorus concentration and phosphorus intake affect apparent phosphorus digestibility these factors must be taken into account when conducting phosphorus nutrition research.