For reasons of aspired naturalness and cost reduction, vitamin additions should be kept low in organic livestock systems. To define safe lower threshold levels of riboflavin supplementation in organic poultry feeding, 135 hybrid layers were allocated to three dietary treatments supplemented with either 4.5 (R4.5), 3.0 (R3.0) or 1.5 mg (R1.5) GMO-free riboflavin added per kg feed. This resulted in total measured concentration averages of 5.0, 4.5, and 3.1 mg kg−1 feed for R4.5, R3.0 and R1.5, respectively. For 18 weeks, each treatment was replicated in three groups of 15 hens. Feed consumption, laying performance, egg quality, riboflavin concentration in the yolk, and clinical health scores were assessed. Two hens per group (six per treatment) were slaughtered in weeks 11 and 18, respectively, for measurement of pancreas, spleen, liver and heart weights, and determination of liver riboflavin. No treatment effects on feed consumption, body weight, laying performance, egg weights, eggshell strength and yolk colour or plumage and keel bone integrity were observed. No symptoms of lameness or footpad damage were found. The riboflavin concentration in egg yolks in R1.5 was lower than in other treatments. Liver riboflavin concentration increased between week 11 and 18 in all treatments, but it was also lower in R1.5. Based on these results, the addition of 3 mg riboflavin kg−1 feed appeared to be sufficient with respect to the health and performance of laying hens. Supplementation with only 1.5 mg kg−1 resulted in some metabolic signs of deficiency. These results apply only to organic winter diets.

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Florian Leiber, Mirjam Holinger, Zivile Amslera, Ariane Maeschlia, Veronika Maurer, Barbara Früha, Christian Lambertz and Hannah Ayrlea
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