Effect of dietary bacterial protein or L-tryptophan supplementation on welfare and growth performance in silverfox

The suitability of bacterial protein meal as a feed ingredient in silver fox diets was examined in an experiment comprising 72 juvenile silver foxes. Bacterial protein meal has a high content of tryptophan, which is the precursor for the neurotransmitter serotonin. The biological hypothesis on which this study was premised was that increased brain serotonin production reduces the fear response, which may lead to better welfare and performance through lower energy expenditure related to fear-induced defensive responses. The effect of substituting 15% fish meal with bacterial protein meal was measured by two behavioural tests, growth performance and fur quality, by comparison with a control diet and a diet supplemented with a high level of synthetic tryptophan. The welfare of the foxes fed the diet supplemented with synthetic tryptophan was considered to be improved, as they used shorter time to approach feed in the presence of a person; thus displayed less fear, than the other two groups after treatment. Weight gain of the foxes during 55 d did not differ among diets, and feed consumption was similar. Live grading of the foxes showed that the dietary treatments did not affect fur quality (P > 0.05). It is concluded that 15% bacterial protein meal can replace fish meal in dry silver fox diets and that a large supplement of tryptophan reduces fear of silver foxes kept in cages.