Nutraceutical potential of essential oils in dairy animal diets: challenges and opportunities

Antibiotics at sub-therapeutic levels are often used as feed additives to increase the efficiency of feed conversion into milk and meat and/or to prevent disease and metabolic problems. Under realistic feeding conditions, ionophoric antibiotics such as monensin have proven to be the most efficient manipulators of rumen microbiota and fermentation. Despite these desirable effects on animal performance, the use of antibiotics in animal diets has become a public health concern because of their potential role in the emergence of antibiotic-resistant bacteria. In recent years, there has been public concern about restricting or limiting the use of these antibiotics in animal feed and replacing them with natural sources such as essential oils (EO). Essential oils produced by many aromatic plants have antimicrobial effects on a range of microorganisms. Considerable research has been devoted to utilizing the antimicrobial properties of EO to manipulate rumen microbial fermentation to improve feed utilization and reduce the greenhouse gas (GHG) emissions to decrease adverse environmental impacts of the livestock production. Despite in vitro studies have demonstrated antibacterial activity of essential oils, exhibiting their ability to induce changes in rumen fermentation and improve feed utilization, most in vivo studies have failed to show significant effects on nutrient utilization and animal performance owing to the delayed adaptation within the rumen microbial communities. Current review underscores the opportunities and challenges associated with the use of EO as a potential feed additive for ruminant animals to enhance their health and productive performance. Additionally, this review also highlights the challenges related to the practical application functional efficacy of EO.