Reduction of phosphorus (P) levels in aqua feed without impacting growth, feed efficiency and health is key to the development of sustainable aquaculture. This is currently being achieved by the use of low-ash fishmeal and highly available P supplementation. However, phytase enzymes can efficiently release P which is locked up as phytate and is found in high vegetable inclusion salmon feeds and can significantly help reduce eutrophication.
The objective of the present study was to measure the efficiency of a 6-phytase (OptiPhos®) using three different concentrations: 250, 500 and 750 FTU/kg in feed for a growth performance trial utilizing rainbow trout (Oncorhynchus mykiss). Feed was formulated with 48.4% crude protein, 22.3% crude fat and 22.3 MJ/kg gross energy. The enzymes were applied post pelleting using a vacuum coater.
The fish were separated into five treatments:
- Positive control (PC) with 1.14% P (1.7% mono calcium phosphate (MCP) in the feed formulation) and 0.48% phytate-P.
- Negative control (NC) with 0.75% P, without any MCP addition
- NC + 250 FTU/kg
- NC + 500 FTU/kg
- NC + 750 FTU/kg
The growth performance indicators, body weight (BW), specific growth rate (SGR), feed conversion ratio (FCR) and feed intake (FI) were recorded during the trial. Body weight varied between 58.0 and 69.2 g, and the SGR varied between 1.78 and 1.98%/d. Fish fed with MCP, 500 FTU/kg and 750 FTU/kg in the formulation showed a significantly higher BW and SGR compared with the NC and the 250 FTU/kg groups (P<0.05). Feed intake varied between 1.42 and 1.47 (% BW/d) and FCR varied between 0.93 and 0.96. Any dietary treatments had no significant effect on FI or FCR (P>0.05).
Exogenous enzyme (OptiPhos®) added at 500 FTU/kg feed and 750 FTU/kg feed can significantly reduce the usage of inorganic P in trout feeds and be used as a tool to minimize the excess P discharged into the environment, or allow higher biomass in compliance with the current local P discharge regulations.
A lower use of finite marine-harvested resources is a sustainability challenge facing the future growth of the aquaculture industry. Plant-based ingredients any by-products are promising sources of protein and energy for aquaculture feeds. However, high dietary inclusion levels of plant proteins generally depress growth and feed efficiency. The poor growth performance commonly found in fish fed plant protein rich diets is generally related to the lower biological value (essential amino acid imbalance, impaired P availability, presence of anti-nutritional factors, higher carbohydrate fraction) of the plant-protein sources.
The use of feed enzymes has shown the opportunity to enhance production economics resulting from the upgrade of the nutritional value of vegetable ingredients, a greater flexibility of ingredient use by the feed formulators and more important, a reduced environmental impact minimizing the excess of P discharged to the environment, allowing higher biomass to be farmed.
The present trial assessed the efficacy of graded supplementation doses (0, 250, 500 and 750 FTU/kg feed) of the phytase OptiPhos® 8000 L on the growth performance of rainbow trout.
Materials and methods
The product under testing was a phytase enzyme (OptiPhos® 8000 L) in a liquid form produced by Huvepharma.
The trial comprised five experimental diets (Tables 1 and 2).
* Yttrium oxide was only incorporated in a fraction of feeds used for digestibility measurements
Diets were manufactured by extrusion (pellet size 3.0 mm) by a pilot-scale twin-screw extruder (CLEXTRAL BC45, France) with a screw diameter of 55.5 mm and a temperature range of 111 - 114 oC. All batches of extruded feeds were dried in a vibrating fluid bed dryer (model DR100, TGC Extrusion, France). After, pellets were allowed to cool at room temperature, and subsequently the test enzyme, at the various doses, and oil were applied by coating under vacuum in a DINNISEN Pegasus vacuum mixer (PG-10VCLAB).
Regarding the post-extrusion coating procedure of enzyme and oil, the target amount of test enzyme OptiPhos® 8000 L was diluted in 2.5% demineralized water, emulsified with the oil on a high-shear mixer (Silverson L5T, United Kingdom) and sprayed onto the pellets under vacuum (760 mbar) for 3 minutes. The PC and NC control diets without enzyme supplementation were also coated using the same procedure. 1.7% MCP was added to the PC diet.
Quadruplicate groups of 35 rainbow trout with a mean initial body weight (IBW) of 11.7 ± 0.5 g were fed one of the five experimental diets for 90 days. Fish were grown in fiberglass circular tanks (volume: 300 L) supplied with flow-through freshwater, with a temperature range of 13.9 ± 0.2 oC and dissolved oxygen levels kept above 7.6 mg/L. Fish were hand fed three times per day at 09:00, 14:00 and 17:00 during weekdays and twice a day during weekends (10:00 and 17:00) to visual satiety. The utmost care was taken to avoid feed wastage and allow quantification of feed intake. Anesthetized fish (20 μl/L of AQUI-S™, New Zealand) were individually weighed at the start of the trial and group weighed on days 30, 61 and 90.
At the end of the trial (90 days of feeding), fish showed a 6-fold increase in their initial body weight (Figure 1). Specific growth rate ranged from 1.78 to 1.98%/day. In comparison to the NC treatment, both the PC and all phytase supplemented diets led to a significant increase in final body weight (FBW) and SGR. Moreover, fish fed the PC diet and those supplemented with phytase at 500 and 750 FTU/kg showed a significantly higher FBW and SGR than those fed the diet with phytase at 250 FTU/kg. FCR values were low (0.92 - 0.96) suggesting good feeding practices. Feed conversion rate, FI and protein efficiency ratio (PER) values were not affected by dietary treatments (P>0.05).
The overall growth performance can be considered as satisfactory for rainbow trout of this size. Fish fed diets supplemented with MCP and OptiPhos® led to a significant increase in final body weight and SGR compared with the NC. Fish fed diets supplemented with OptiPhos® at 500 and 750 FTU/kg achieved a performance, in terms of FBW and SGR, the same as observed in fish fed the PC diet. Feed conversion rate, FI and PER values were not affected by dietary treatments.