This research consists of three experiments whose objective was the evaluation of different substrates and yeast strains obtained from the fermentation of apple by-products. The first work consisted of the development of a yeast inoculum, using aerators to evaluate eight treatments, using different levels of ground apple, cane molasses and two yeast strains, Saccharomyces cerevisiae and Kluyveromyces lactis. The second work consisted of evaluating the in vitro fermentation behavior of eight yeast strains isolated from apple by-products, based on the results of the first study. The third work consisted of determining the optimum level of a yeast-based additive added to diets for high-producing cows by means of the in vitro gas production technique, using four treatments.