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In previous examinations for both the required and toxicity levels of minor minerals studies concentrated on the zootechnical efficiency and the efficiency of the specific enzyme systems. The present examinations proved whether the selenium requirements for optimal efficiency of the animal result in positive effects on the ruminal fermentation. This study, therefore, examines the effects of different selenium levels (in form of sodium selenite) on gas building and fatty acid concentration under in-vitro-conditions.One rumen fistulated cow and three rumen fistulated sheep were used for the taking of rumen fluid and were marginally supplied with selenium. The examinations were realized using the Hohenheimer Futterwerttest (HFT) as a short-term incubation system and the Rumen Simulation Technique (RUSITEC) as a long-term incubation system. By feeding the donating animals a low-level of selenium, concentrations of 0.071 mg Se/kg DM in HFT and 0.068 mg Se/kg DM in RUSITEC could be achieved.In the HFT, positive effects on gas building were detected at every nutritive situation for the dosages of 0.2 and 0.4 mg Se/kg 13M. A significant (p<0.05) reduction in gas building occurred by selenium dosages of more than 12.8 mg Se/kg DM. The production of short chain fatty acids was optimal by dosages of 0.2 and 0.4 mg Se/kg DM but decreased significantly for concentrations of more than 12.8 mg Se/kg DM.In the long-term incubation (RUSITEC), an optimal production of fatty acids was also obtained by dosages of 0.2 and 0.8 mg Se/kg DM.Negative effects on the characteristic parameters for the rumen fermentation were obtained by dosages of 12.8 mg Se/kg DM and higher.Minimal improvements of the fatty acid production were obtained by dosages of up to 0,8 mg Se/kg DM, it can be concluded that mistakes in the dosages have o negative effects on the ruminal fermentation.