The objective of this thesis was to determine the effect of yeast supplements on the performance and health of beef cattle during the receiving and finishing period. Furthermore, another aim of this study was to evaluate the applicability of rumenocentesis on beef cattle and the viability of transabdominal ultrasonography of the rumen mucosa as a suitable, non-invasive diagnostic tool to identify beef cattle affected by SARA. Three trails were conducted in order to do achieve that. Trail I and Trail II were conducted simultaneously using the same animals, while the third trail used a larger sample but always in the same herd. Trail I and II assessed the effect of dietary supplementation of yeast cell wall (YCW) and live cell yeast (LY) at different dosages on rumen’s metabolites. Sixty Charolaise steers were divided into two groups on the basis of their feeding phase: growing and finishing. Growing and finishing groups were each randomly divided into equal three subgroups (n = 10): no supplement (growing control), supplemented with YCW, and supplemented with LY + YCW, no supplement (finishing control), supplemented with LY and supplemented with LY + YCW. Ruminal fluid has been collected before, after 21 and 42 days of experimental period in order to evaluate the volatile fatty acids concentrations and pH values. Faeces samples were collected before (T0), after 21 (T1) and 42 (T2) days of the start of the study, which took place simultaneously and analysed for dry matter (DM), Ash, crude protein (CP), ethereal extract (EE), neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL) and Starch detection. Dietary supplementation of LY and YCW increased (P