Tallow in Atlantic salmon feed

• Replacing poultry by-product oil with tallow up to 50% had no impact on performance.
• Tallow inclusion reduced lipid and fatty acid apparent digestibility.
• Tallow inclusion resulted in decreased omega-6 fatty acid content in tissues.
• Tallow inclusion increased omega-3/omega-6 fatty acid ratio in fillet.
• Tallow inclusion increased in vivo bioconversion of EPA to DHA.

Amongst the various available alternative oils, tallow (TAL) has attracted limited research interest and is not commonly used in commercial aquafeeds. This is likely due to concerns of consumers' perception and its high saturated fatty acid (SFA) content, which raises concerns about its digestibility, particularly for species cultured in cold water conditions, such as the Atlantic salmon (Salmo salar). Conversely TAL is conveniently priced and has a very low n − 6 polyunsaturated fatty acid (PUFA) content, which may be beneficial to the final product quality. In many parts of the world, modern salmon aquafeed commonly contains poultry by-product oil (PbO) as the alternative lipid source to replace fish oil (FO). Accordingly, the control diet used for the present experiment contained 75% PbO and 25% FO, as the added dietary lipid sources. Five additional experimental diets were formulated to progressively increase the level of TAL inclusion substituting PbO in 10% increments (10–50%), with a constant amount of FO (25%). A feeding trial was conducted using triplicate groups of Atlantic salmon over a 14 week time period at 10 °C. No difference in growth performance was recorded between treatments, but TAL substitution impacted lipid and fatty acid digestibility. The apparent in vivo β-oxidation of SFA intensified with TAL inclusion, whilst n − 3 PUFA β-oxidation decreased. TAL inclusion also resulted in increased apparent in vivo bioconversion of 22:5n − 3 to 22:6n − 3. This was also reflected in fillet and whole body n − 3 long chain PUFA (LC-PUFA) composition. TAL inclusion impacted positively on the fillet n − 3/n − 6 PUFA ratio. This study suggests that TAL appears to be a viable alternative oil, with potential for inclusion in commercial aquafeeds.