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Yeast (different sources and levels) as protein source in diets of reared piglets: effects on protein digestibility and N-metabolism.

The aim of this study was to examine the feeding value of different yeasts as a substitute for soya bean meal, the main protein source in diets of weaned piglets. Tested two yeasts were already available on the market, Saccharomyces cerevisiae and Kluyveromyces lactis (beer and milk yeast), which replaced 40% of the soya bean meal in the diets. Furthermore, a yeast (Kluyveromyces fragilis) grown on whey, a side-product of cheese production, was used in increasing concentrations in the diets, so that increasing amounts of the soya bean meal (20%, 40% and 60%) could be replaced. As proved in these experiments, a replacement of 60% of the soya protein with whey yeast protein had positive effects on the performances (daily weight gain) and on the N-metabolism and did not have negative effects on the health or the faeces consistency. The whey yeast stands out because of its high protein quality (N-digestibility and N-retention). Furthermore, the replacement of soya bean meal with highly digestible yeasts is welcomed under the aspect of animal health, because of the reduction of anti-nutritive soya components (stachyose, glycinin) in diets of weaned piglets. The controlled production conditions of the yeasts result in a high feed safety; in addition, the yeast as an end-of-pipe-product is a resource conserving and valuable feed. A main stimulus for the use of yeasts, however, in a food production controlled by economic standpoints, is their price and the costs of other competing feeds.