Legislators in Europe have moved to prohibit the use of growth-promoting antibiotics in animal feeds from the end of 2005. This decision was based on public and political concerns that the heavy use of antibiotics in general can give rise to transmissible resistance factors which can compromise the potency of therapeutic antibiotics in man. Growth promotion was a clearly avoidable use. Thus, alternative means are being sought to modify ruminal fermentation to benefit production efficiency and product quality. Natural plant products offer a wide range of potential as manipulating agents.