Abstract: The invention relates to a composition for improving the efficiency of ruminant feed utilization, which comprises as active ingredient one or more microbial cultures, capable of adjusting the weight ratio of acetic acid to propionic acid to an optimum value, preferably to 1.5-4.0:1, and of growing in the rumen and persisting there at least for 60 days, optionally in admixture with carriers, diluent, preserving agents conventionally used in animal husbandry and nutritive and/or other substances conventionally administered to ruminants.According to another aspect of the invention there is provided a process for the preparation of microbial cultures used as active ingredient in the above composition.The invention further relates to a process for the preparation and use of said compositions.