In terms of improving animal productivity and profitability, the use of monensin and related ionophoric antibiotic growth promoters has unarguably been one of the major successes of rumen microbiology in contributing to ruminant nutrition (Russell & Strobel, Reference Russell and Strobel1989). Addition of ionophoric antibiotics to ruminant diets has been associated with beneficial changes in the ratio of volatile fatty acid produced in the rumen, reduced susceptibility to digestive upsets, a decrease in the loss of energy from the animal as methane and a reduction in the ruminal degradation of dietary protein, amongst other effects (Russell & Strobel, Reference Russell and Strobel1989). However, whilst the production benefits associated with these changes have created a considerable market for the sale of monensin and other ionophoric antibiotics for use in ruminant diets, and the study of these ionophores has greatly enhanced our understanding of rumen microbiology, it is perhaps salutary to remember that monensin was originally identified and developed as a coccidiostat for chickens and only later used in ruminants (Richardson et al. Reference Richardson, Raun, Potter, Cooley and Rathmacher1976).

Legislation has recently been introduced within the European Union to prohibit the use of growth-promoting antibiotics, including monensin and related compounds, in animal feeds (EC, 2003). The scientific basis for these restrictions, based around concerns that the use of antibiotics in animal agriculture can give rise to transmissible resistance factors that may compromise the therapeutic use of antibiotics in humans, may be questionable (Callaway et al. Reference Callaway, Edrington, Rychlik, Genovese, Poole, Jung, Bischoff, Anderson and Nisbet2003; Russell & Houlihan, Reference Russell and Houlihan2003). Nevertheless, the removal of antibiotic growth promoters in Europe has led to a perceived ‘gap’ in the animal feeds market and an increased interest in alternative means of manipulating rumen fermentation.

A paper by Selje et al. (Reference Selje, Hoffmann, Muetzel, Ningrat, Wallace and Becker2007) in this issue reports results from a substantive EU-funded project (‘rumen-up’; http://www.rowet.ac.uk/rumen_up) which has systematically screened a large collection of plants and plant extracts to identify potential new rumen-manipulating agents. The use of extracts and oils from plants to control microbial populations both in the gut and in feeds is rooted in human history (Erdogru, Reference Erdogrul2002). Kamel (Reference Kamel2001) suggested that use of plants and plant extracts to manipulate animal production dates back to the ancient Egyptians, Chinese, Indians and Greeks, whilst Leeny (Reference Leeney1921) described the use of ‘Youatt's Cordial’ containing ginger, peppermint and opium to treat digestive upsets in cattle. There is a small, but fascinating, literature from the 1960s investigating the potential of plant essential oils as rumen manipulators (Nagy et al. Reference Nagy, Steinhoff and Ward1964; Nagy & Tengerdy, Reference Nagy and Tengerdy1968; Oh et al. Reference Oh, Sakai, Jones and Longhurst1967^, Reference Oh, Jones and Longhurst1968) although interest seems to have faded thereafter possibly reflecting the introduction of ionophoric antibiotics on to the market in the 1970s.

What is however new in the study of Selje et al. (Reference Selje, Hoffmann, Muetzel, Ningrat, Wallace and Becker2007) is the use of a highly targeted reductionist screening programme to investigate the activity of a wide range of materials in order to identify plants or plant extracts that inhibit ruminal protein degradation. Such an approach has much to recommend it; experiments can be precise and well controlled, and the resultant data are clear and unambiguous. However, as the authors acknowledge, plants identified as being able to control rumen proteolysis may also have wider effects on ruminal fermentation and indeed animal performance, and thus subsequent to identifying suitable candidate plants and extracts, additional experiments both in vitro and in animal models will be required. There is also the wider question of gaining regulatory approval. European regulations require new additives to be extensively assessed for efficacy, toxicology and environmental safety (EC, 2003), and as such bring considerable financial barriers to new additives entering the market. Given that the market for such additives may well be geographically limited (North America and much of the rest of world still allow the use of monensin and its competing ionophores) and that the target itself defines a maximum price point in terms of the value of the protein supplement fed to the animal that is saved from ruminal degradation and is thus available for intestinal absorption, there are obviously very real commercial as well as scientific challenges to be met in bringing a new additive to market.

In conclusion, recent changes in European regulations have created opportunities to develop new products as rumen manipulators to be used in ruminant diets. However, in addressing this opportunity, ruminant nutritionists will need to overcome regulatory and commercial challenges in addition to addressing the obvious scientific and technical issues.