Effect of defatted and dephytinized soy proteins fortified with phytase on the growth, nutrient digestibility and phosphorus load of Rainbow trout.

Lawrence Nwanna



Aqua-feeds with minimal phosphorus (P) and nitrogen (N) stressors on the environment are paramount for eco-friendly aquaculture, as P and N are the major sources of aquatic pollution. This study investigated the effects of defatted soybean meal (DSM) plus phytase and dephytinized soy protein (DHP) plus phytase on the growth, N and P retention, N and P excretion and total P loadings by rainbow trout, Oncorhynchus mykiss. Seven isoproteic diets were formulated to contain, DSM (diet 1), DSM plus 5 g P kg -1 diet (diet 2), DSM plus 1500 U/kg phytase (diet 3), DSM plus 3000 U/kg phytase (diet 4), DHP (diet 5), DHP with 1500 U/kg phytase (diet 6) and DHP plus 3000 U/kg phytase (diet 7), making up treatments (T1-T7). Chromic oxide (1%) was added to the diets as inert marker for digestibility studies. The diets were tested on rainbow trout (25.6 ± 0.24 g) for 12 weeks. Results indicated no significant differences in the mean weight gain (MWG), specific growth rate (SGR) and food conversion ratio (FCR) of the fish subjected to the different dietary treatments. However, the MWG, SGR and FCR of the fish treated on diets of DSM were marginally higher than those maintained on DHP. Similarly, the fish treated on DSM retained higher N which resulted in lower excretion values of N. Fish in T 7 retained most (P<0.05) P and generally fish treated on DHP retained slightly higher P than those treated on DSM diets. P excretion was closely related in treatments with DSM (3.64-5.41kg/Ton) and that with DHP (3.65-4.85kg/Ton), with T(s) 3 (3.64) and 7 (3.56) having the least (P<0.05) values, while T2 with P supplement produced the highest (P<0.05) P excretion of 5.41kg/Ton, least P and Zn retention and N excretion values. Effluent P discharge was least (P<0.05) in T 7 (5.33mg/kg). This study revealed that addition of DSM in the diets would promote better fish growth, while addition of DHP would promote better environmentally friendly aquaculture.

Keywords: Eco-friendly diets, defatted, dephytinized soybean meal, phytase

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