Public concern over use of antibiotics in livestock production has increased in recent years because of their possible contribution to emergence of antibiotic resistant bacteria, and their transmission from livestock to humans. Accordingly, ruminant microbiologists and nutritionists have been exploring alternative methods of favorably altering ruminal metabolism to improve feed efficiency and animal productivity. Plant extracts contain secondary metabolites, such as essential oils (EO), that have antimicrobial properties that make them potential alternatives to antibiotics to manipulate microbial activity in the rumen. Essential oils are naturally occurring volatile components responsible for giving plants and spices their characteristic essence and color. Over the last few years, a number of studies have examined effects of EO, and their active components, on rumen microbial fermentation. However, many of these studies are laboratory based (i.e., in vitro) and of a short-term nature. Nevertheless, results from in vitro batch culture studies provide evidence that EO and their components have the potential to improve N and/or energy utilization in ruminants. Effects of EO on ruminal N metabolism is more likely mediated by their impact on hyper-ammonia producing (HAP) bacteria resulting in reduced deamination of amino acids (AA) and production of ammonia N. However, these responses are only observed with high doses of EO, which also can inhibit the process of ruminal fermentation as reflected by a decline in total volatile fatty acid production. Effects on methane production are inconsistent, but evidence to date indicates that there is potential to select EO, or active components, that selectively inhibit ruminal methanogenesis. Results from in vitro continuous culture studies suggest that rumen microbial populations may adapt to EO, which may explain the lack of an effect of EO on ruminal metabolism and animal performance in long-term in vivo studies. Several studies have examined the activity of a number of EO against a wide variety of food-borne pathogens. Data available show a strong bactericidal activity against pathogenic bacteria such as Escherichia coli O157:H7 and Salmonella spp. Essential oils hold promise as feed additives in ruminant nutrition to improve feed efficiency and control the spread of pathogens in livestock. However identification of EO, or their active components, that favorably alter fermentation without resulting in broad overall inhibition of rumen fermentation, continues to be a major challenge for researchers.
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