Addition of yttrium oxide as digestibility marker by vacuum coating on finished pellets — A method for assessing digestibility in commercial salmon feeds?

Abstract

Abstract Two experiments were carried out with Atlantic salmon in sea cages to study the possibility of assessing digestibility of commercial feeds by means of an inert marker added by top coating in a vacuum coater. A complete salmon feed (39% protein, 34% lipids) containing ytterbium oxide (Yb 2 O 3 ) added pre-extrusion was top-coated with yttrium oxide (Y 2 O 3 ) dispersed in fish oil. The feed was fed to salmon (2–3 kg) in two experiments. Digestibility estimates using the two methods of marker addition were compared in stripped faeces in Experiment 1, and in Experiment 2 temporal variations in marker-to-marker and nutrient-to-marker ratios in the content of different gastrointestinal segments after a single meal were studied. Digestibility measured using the two markers showed consistent results from stripped fish. However, data from Experiment 2 showed that yttrium (Y) dispersed in oil and added by top-coating was evacuated from the stomach together with lipids a short time after ingestion, while evacuation of protein and ytterbium (Yb) added to the meal mix pre-extrusion followed later. The Y:Yb ratio in the mid-intestine remained higher than in the feed until 17 h after feeding and until 24 h in the distal intestine. In conclusion, yttrium oxide added by top-coating can be used as a marker for digestibility assessment in commercial salmon feeds. Measurements may possibly be biased, however, by the differences in gastrointestinal evacuation rates amongst nutrients and markers, so frequent feeding and/or careful timing of the faeces collection may be required for a reliable result.