The “rumen”, comprising the reticulum, the dorsal and ventral sacs of the rumen, the omasum and the abomasum, together with the combined contents of digester, accounts for as much as 100-120kg of the total live weight of dairy cows, perhaps 80kg of a finishing beef steer and even 15kg of an adult sheep.

After ingestion (feed which has been mixed to varying degrees with saliva during prehension), enters the rumen where the processes of microbial digestion commence. It joins previously ingested feed which is at different stages of digestion, together with ingested water and saliva. Rumen digester has an average dry matter (DM) content of between 10 and 12% and whilst relatively fluid, some stratification is generally evident, with ventral sac contents usually having a higher DM content. At the interface of the dorsal/ventral sac, the rumen mat normally develops and is important in the initiation of good bouts of rumination. Digestion occurs as a consequence of microbial fermentation, supported by rumination, where feed bols are regurgitated for further physical dissimilation by chewing.

As previously stated, the rumen is highly anaerobic, with low levels of oxygen resulting in a highly reducing environment. The main route of entry for oxygen is with ingested feed but in a healthy rumen this will not have any marked effect on the reducing properties of the rumen. The rumen is dominated by large populations of anaerobic micro-organisms which exist in a symbiotic relationship with the host animal, the nature of the rumen microbial population being significantly influenced by the type of ration being fed. On high fibre rations, fibrolytic bacteria will dominate, and together with limited colonisation of dietary fibre by anaerobic fungi, are responsible for most of the digestion that occurs in the rumen of cattle and sheep fed high forage rations. In contrast, on starch-rich rations, the bacterial population will be dominated by amylolytic species which together with protozoa effect most of the starch digestion which occurs.

Carbohydrate digestion

The principal role of the microbes is to degrade dietary carbohydrates and more specifically the dietary fibre which cannot be digested in the small intestines and to only a limited extent in the hindgut. This does not preclude the digestion of starch and sugars in the rumen which is generally quite extensive. The ruminal dissimilation of dietary carbohydrates comprises initially of the degradation of polysaccharides to simple monomers (hexoses and pentose’s; example below) followed by extensive fermentation of these to yield energy (Adenosine triphosphate; ATP) which the micro-organisms utilise for the purposes of maintenance and growth (example 2-6). As the rumen environment is anaerobic, the yield of ATP per mole of degraded carbohydrate is much lower than that which would be derived from the aerobic oxidation of carbohydrate, and depending upon the type of fermentation is usually between 4 and 5 moles ATP/mole carbohydrate, compared with 38 moles/mole for complete oxidation. Clearly the production of volatile fatty acids (principally acetate, propionate and butyrate) accounts for the major part of the energy contained in ruminal digested carbohydrate, generally of the order of 80-88% on a stoichiometric basis. Not all carbohydrate digested in the rumen however is fermented and a significant but variable proportion of the  degraded  carbohydrate is used to support microbial biomass synthesis, principally microbial polysaccharide, protein, nucleic acids and some lipid.

The principal pathways of carbohydrate dissimilation are provided below;

Polysaccharides = Monosaccharides (Hexose {C6} and Pentose’s {C5})

Hexose = 2 Pyruvate.

Pyruvate = Acetate (CH3COOH) + CO2 +H2 + ATP    Pyruvate + H = Propionate (C2H5COOH)

2 Pyruvate = Butyrate (C3H7COOH) +2CO2 +2H2+ATP

CO2 + 2H2 = CH4 +ATP

Examining these reactions in respect of carbon utilisation provides clear insight of the energetics of carbohydrate fermentation. Reaction 2 shows a carbon input/output ratio of 1.0, as each molecule of pyruvate contains 3 carbon atoms. The production of acetate (2 carbon atoms; reaction 3) however results in the net loss of 1 carbon atom with an associated production of hydrogen. Reaction 5, the production of butyrate has a similar loss of carbon for whilst it takes two molecules of pyruvate (6 carbon atoms), butyrate has four carbon atoms, the other two being converted into carbon dioxide. There is also an associated production of hydrogen.

In contrast, the production of propionate requires 1 molecule of pyruvate and with propionate being a three carbon molecule, there is no net loss of carbon. In addition, the conversion of pyruvate to propionate requires hydrogen which is provided from the rumen environment, and as such propionate production is considered to be a net utiliser of hydrogen, thus competing with other routes for the disposal of rumen hydrogen. From this it follows that propionate production is inherently more efficient than either acetate or butyrate production, especially as these reactions contribute both carbon dioxide and hydrogen unlike propionate production. Finally any carbon dioxide and hydrogen produced during the ruminal fermentation of feed (principally carbohydrates) is converted to methane by methanogenic bacteria, which in turn derive a small amount of ATP from this reaction.

On high forage rations, acetate will be the dominant VFA (>65% of all VFA), usually with modest levels of propionate (approx. 16%) and butyrate (10%), with higher carbon length VFAs as well as branched chain VFA providing the balance. Increasing levels of starch in the ration will increase propionate levels, generally at the expense of acetate but only rarely will propionate account for more than 25% of total VFA, whilst acetate will still be the major VFA (circa 55%). On high sugar rations, especially when significant amounts of molasses or fodder beet are being fed, butyrate levels may increase to approximately 15%, with most notable increases occurring immediately following feed ingestion. In those situations where carbohydrate degradation in the rumen is extensive both in terms of amount and rate of digestion, significant amounts of lactic acid may be produced from the conversion of pyruvate and as lactate is a stronger acid than any of the VFA this can have a more pronounced effect on rumen pH. Lactate is cleared from the rumen either through further metabolism by lactate-utilising bacteria or by absorption across the rumen wall and subsequent metabolism in the liver. In those situations where lactate production significantly exceeds lactate clearance by metabolism or absorption, rumen pH levels will fall quite dramatically, leading ultimately to subclinical rumen acidosis which can have serious adverse effects on both rumen and animal health.

Author: Denis Dreux