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Olive mill wastewater (OMW), a major by-product of the olive oil production industry, is rich in polyphenolic compounds that impart health benefits to several animals. Here, we evaluated the effects of OMW addition on the nutrients, quality, and bacterial composition of fermented total mixed ration (TMR). Different amounts of OMW (0%, 5%, 10%, and 20% in fresh matter) as a substitute for water were mixed with the formulated TMR and fermented for 4 weeks. At the initial stages of fermentation, OMW significantly altered the dry matter, pH, and lactic acid content; however, it had minor effects on most macronutrients, quality parameters, and bacterial flora in the final TMR product. OMW (10%) supplementation improved the polyphenolic compound content in the fermented TMR. However, OMW (10% and 20%) increased lipid levels beyond the recommended level of 3% in cow diets. OMW supplementation did not affect the microbial composition of TMR, except for an increased abundance of Weissella. These findings suggest that supplementation of 5–10% OMW improves the quality parameters of TMR without affecting the nutrient composition of fermented TMR. OMW can be used in cow feed; however, further studies involving feeding trials are needed to validate the effects of OMW on cows. [ABSTRACT FROM AUTHOR]