Rumen acidosis is a common problem under feedlot conditions where cattle change from a roughage-based to a high concentrate-based diet. It is associated with an imbalance between lactic acid producing and lactic acid utilizing bacteria. Megasphaera eldenii is an important lactic acid utilizing bacteria which can now be produced for commercial use and supplemented for the prevention of lactic acid build up in the rumen and subsequent acidosis. The adaptation period is required in order for numbers of lactate utilizers such as M. elsdenii to increase to levels effective against subsequent build up of lactic acid. The purpose of this trial is to identify the effects of a strain, NCIMB 41125 (MeCH4), of Megasphaera elsdenii on the adaptation period of feedlot cattle on diets formulated for high starch levels. MeCH4 was isolated from the rumen of cattle adapted to high starch diets. By converting lactic acid in the rumen to volatile fatty acids (VFA’s) such as butyrate, M. elsdenii should be able to at least reduce the adaptation phase, if not totally eliminate the need for it without effects on intake patterns, feed conversion rates, health and carcass yield. . If results are positive, this strain of M. eldenii will have great economic implications to all feedlot farmers. There may be one major conclusion to be drawn from this trail and this was the fact that a decrease in adaptation days to reach the high concentrate grower diet had no statistically significant influence on key performance parameters relevant to the feedlot industry However, possibly the most attractive implication of these results is the lower need for roughage and therefore the implications tied in with the buying in and storage thereof. / Dissertation (MSc)--University of Pretoria, 2010. / Animal and Wildlife Sciences / unrestricted