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.