The role of feed additives in ruminant nutrition

The rumen is the first stomach of cattle, sheep and goats. All digestion in this organ is carried out by a microbial population which consists of ciliate protozoa, anaerobic bacteria and anaerobic fungi. The bacteria are genetically very diverse, deriving from many different origins, whereas the protozoa are monophyletic, thought to have evolved from a symbiotic protozoan which established in the rumen 30-40 million years ago. The fungi have several morphotypes, but these are very closely related to each other. Fungi are associated with fibrous materials in the rumen and are the- refore difficult to see in the mixed community. This mixture of organisms combines to digest the food, and it is the products of microbial digestion which form the nu- trients that become available to the ruminant animal itself.

What would we like to change about ruminal fermentation? Fibre is digested incom- pletely: increasing its digestion would improve productivity. Proteolysis destroys high quality protein in the feed and should be slowed. Protozoal activity is detrimental to the efficiency of microbial protein synthesis: this activity should be suppressed. Bloat and acidosis are distressing disorders which result from malfunction of microbial di- gestion in the rumen. Methane, a potent greenhouse gas, and ammonia, which forms urinary urea, arise from normal rumen fermentation. They cause atmospheric and groundwater pollution respectively.

Feed additives for manipulating ruminal fermentation

A number of strategies have been used to enhance ruminal fermentation. Antibiotics and ionophores have been effective but were banned in the EC at the beginning of 2006 for safety reasons.Biological additives have included microorganisms, enzymes and plant products. None of these manipulating additives has been introduced in a rational way until now, in the sense that the target function was not identified before they were introduced. Their effects were discovered only after overall benefit had been observed. Their effects will be described. Natural plant compounds offer a wide range of opportunities to develop such a rational, target- based feed additive.

'Rumen-up': The potential of a wide range of plant materials in modifying rumen fer- mentation was investigated in an EC-sponsored shared-cost action under Frame- work Programme 5, ‘Rumen-up’ [Contract number: QLK5-CT-2001-00992]. Partners in the project were the Rowett Research Institute, Aberdeen (coordinator); University of Reading; University of Hohenheim, Stuttgart, Germany; University of León, Spain; Alltech Inc., Ireland; Crina S.C., Switzerland. The aim of the overall project was to develop new plants or plant extracts as dietary supplements for ruminants to replace chemical additives and growth-promoting antibiotics. The plant materials were collected from botanical and industrial collections, and evaluated for their ability to manipulate ruminal fermentation. The project was intended to deliver plant-based, sustainable solutions to production problems, thereby benefitting European biotechnological and agricultural industries, and the new plants increasing the diversity of crops used in agriculture. Twenty-two positive hits were obtained, and the potential of these materials is being explored further, in particular in relation to the precise chemical component which is responsible for the observed effects.

'REPLACE': Following the success of Rumen-up, REPLACE was a Framework 6 project which diversified the applications and extended the study to pigs, aquaculture and poultry. From 2004-8, the samples collected for RUMEN-UP were tested for properties not screened in the original project. Their possible impact on human and animal health (E. coli, parasites), food quality (saturated fatty acids versus conjuga- ted linoleic acid) and efficient use of natural resources (increased forage use by ruminants) were investigated.

The work on ruminants focussed on improving the content of conjugated linoleic acid (CLA) in beef, lamb and dairy products. New plants such as Chrysanthemum coronarium were found to modify ruminal biohydrogenation of fatty acids, suggesting their use possibly used as a feed additive, or more simply interspersed with pasture, to improve the fatty acid composition of meat and milk.

Essential oils are steam-volatile or organic-solvent extracts of plants, used traditio- nally by man for many centuries for the pleasant odour of the essence, or its flavour, or for its antiseptic and/or preservative properties. Their use has been investigated recently as a feed additive for ruminants. Dietary inclusion of a commercial blend of essential oils caused significantly decreased NH3 production from amino acids in ruminal fluid taken from sheep and cattle. This effect was mediated partly by effects on hyper-ammonia-producting (HAP) bacteria and on the protein and starch fermenting ruminal bacterium, Ruminobacter amylophilus. Thus, essential oils are an example of natural plant secondary compounds which can be used to manipulate ruminal fermentation in a specific manner which may benefit animal production.

Direct-fed microbials offer arguably the greatest potential for manipulation of ruminal fermentation. They offer a huge spectrum of metabolic activities and enzymes as well as metabolites. They also enable selection of strains or mutants best suited for particular applications. The yeast, Saccharomyces cerevisiae, is an especially attractive organism. It has widespread use in other industries, such as baking and brewing, so its properties and genetics are very well understood. It has GRAS (generally regarded as safe) status. Its cell envelope is quite different to that of ruminal microorganisms, which may be important in the delivery of nutrients or the absorption of toxic or undesirable compounds. Lastly, it is metabolically active in the rumen but does not grow, which means that its concentration and activity can be readily controlled by its dietary inclusion level, ensuring maximum efficacy.