Fermentation-Based Preservation of Okara and In Vitro Evaluation of Its Application in Dairy Cattle Diets

Okara, a protein-rich byproduct of soymilk production, is highly perishable because of its high moisture content. This study evaluated the preservation and nutritional value of okara fermented by lactic acid bacteria for use in dairy cattle diets. Fermentation effectively reduced pH within 2 weeks and maintained quality for up to 6 weeks. However, aerobic exposure increased the concentration of ammonia, indicating a decline in stability. In vitro assessments revealed no significant differences in in vitro true dry matter digestibility, in vitro neutral detergent fiber digestibility, or gas production between fermented and fresh okara, although fermented okara had a higher concentration of ammonia nitrogen. In situ analysis revealed slightly lower dry matter effective degradability (ED) in fermented okara, but similar rumen-degradable and undegradable protein fractions. When fermented okara was used to replace soybean meal in total mixed rations, 25–50% inclusion-maintained digestibility and fermentation characteristics, with 25% replacement yielding the highest ED at a low ruminal passage rate (0.02 h−1). Taken together, these results suggest that fermented okara can be strategically incorporated into dairy rations as a sustainable protein alternative, supporting both rumen function and bypass protein supply.