ISSUE FOCUS FEED & ADDITIVE MAGAZINE January 2023 27 decreasing. The decreased levels of selenium in the diet accompanied by the increased disease pressure highlights the importance of adding additional selenium to the feed. The addition of selenium in the organic form of L-selenomethionine is shown to be a good option, since selenium in this form can be stored in animal tissues. This allows for the best results, since it ensures a constant selenium supply by safe deposits of selenium that can thereafter be used by the animal during times of stress or when selenium uptake is limited. One example of this is a trial done in Thailand by Dr Wangkahart et al. (2022). During a feeding trial of 8 weeks, Nile tilapia were either fed a control diet, without added selenium, or a diet with 1, 3 or 5 mg/kg selenium from sodium selenite, or a diet with 1, 3 or 5 mg/kg selenium from L-selenomethionine (Excential Selenium 4000, Orffa Additives BV). After 8 weeks of feeding the seven different diets, it was observed that addition of selenium as L-selenomethionine significantly improved the growth performance and feed efficiency compared to the control treatment, with an optimum dosage at 1 mg/kg selenium addition. At the same time, selenium from sodium selenite did not improve growth performance and feed efficiency compared to the control treatment. After 8 weeks the same fish were challenged with Streptococcus agalactiae. For 21 days the cumulative survival was measured and compared among all treatments. What could be observed was that, regardless of the source, selenium allowed for an increase of the cumulative survival in Nile tilapia. However, when looking at figure 1, it can be observed that L-selenomethionine (SeMet) supplementation led to significantly increased survival compared to sodium selenite (SS). Figure 1. Cumulative survival of Nile tilapia fed selenium at 0, 1, 3 and 5 mg/kg from either L-selenomethionine (SetMet) or sodium selenite (SS) (P<0.05).
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