Rumen function

(even though this article by Protexin Animal Health  is geared toward cows, it can also be applied to goats)

The rumen, the first stomach of the ruminant, is effectively a large fermentation organ where the majority of feed is broken down by microbial action and where much of the animal's source of energy is absorbed. Every ml of ruminal contents contains as many as 10 billion bacteria (1x10¹⁰), 1 million protozoa and variable numbers of yeasts and fungi. There is a diverse range of fermentative bacteria with digestive capabilities including cellulolytic (digests cellulose), hemicellulolytic (digests hemicellulose), amylolytic (digests starch), proteolytic (digests protein), sugar utilising (monosaccharides and disaccharides), acid utilising (lactic, succinic and malic acids), ammonia producing, vitamin synthesising and methane producing.

Starches, sugars and protein are broken down by the microbes and absorbed from rumen across the rumen papillae (villi - the absorption surface of the rumen). The microbes utilise dietary protein for their own metabolism and the host gets its protein from digestion of a proportion of the microorganisms that pass with the chyme into the lower gastrointestinal tract. Ammonia, from the microbial fermentation, is absorbed in the rumen and converted in the liver to urea that can be utilised by the animal. Complex carbohydrates (cellulose, hemicellulose, xylans etc) are broken down by fermentation into absorbable products. Starch carbohydrates from grain for example are converted into glucose then metabolised by rumen microorganisms into volatile fatty acids and lactic acid.

Very little of the original protein from the feed is digested by the cow's own enzymes, and none of the sugars. The cow reconstructs sugar in the liver from absorbed VFA by a process called gluconeogenesis, and digests microbial protein in the abomasum. This allows it to feed on highly fibrous material that cannot be digested by mammals: the fibrous, indigestible feed is converted into microbial protein and VFA in the rumen.

While the overall microbial population remains relatively constant, the proportions of microbial species within that population varies with diet and reflects substrate availability. During the dry period, the rumen flora are adapted to a high forage fibrous diet. There is a large population of cellulolytic bacteria and low population of lactic acid utilising bacteria. As the amount of non-fibre carbohydrate in the feed is increased during early lactation, this easily available energy source causes a burst of activity by many species, including the lactic acid producing bacterium Streptococcus bovis. Normally, the lactic acid produced by this species is controlled by lactic acid degraders such as Megasphaera elsdenii and Selenomonas ruminantium. However, the surge in activity by Strep. bovis results in a rate of lactic acid production far in excess of the capabilities of the lactic acid degraders, and these take longer (3-4 weeks) to adapt to the higher levels of lactic acid. If the rumen pH is driven down too far by excessive lactic acid production, the lactic acid degraders are inhibited and the animal experiences full-blown acidosis. Otherwise, a sub-acute acidosis may develop which can seriously impair the cow's productivity. For this reason the cow's diet and rumen must be managed during the transition period in order to ensure adaptation to the early lactation high-concentrate ration. A high production dairy cow (say 40 litres) that is well managed during this period will achieve a ration intake of 22 kg dry matter per day, more than half of which might comes from concentrate, without any problems but the freshly calved cow will not be able to tolerate this diet without proper management.

Volatile Fatty Acids (VFAs), short chain fatty acids, such as acetic, propionic and butyric acids as well as small quantities of acetoacetic and lactic acid, are produced in large amounts through ruminal fermentation and provide more than 70% of the ruminant's energy supply. The ratio of production of VFAs in the rumen is diet dependent and exerts considerable influence on milk quality. A high fibre ration produces acetic, propionic and butyric acid in 70:20:10 proportions whilst a high grain/concentrate ration shifts the ratio towards higher levels of propionic and butyric acid. Lactic acid production may also become relatively high. A high fibre diet will result in a lower milk yield but a higher butterfat level, acetate being the main VFA for the production of butterfat. The high concentrate ration will provide more energy, a higher total protein yield and will lead to higher milk yield and typically less butterfat percentage, propionate and butyrate being the main VFAs for the production of milk protein.

VFAs are absorbed across the rumen epithelium and are then carried to the liver. About half of VFAs produced in the rumen are absorbed with the other half being removed by neutralisation with salivary or other buffers or by passage from the rumen. Continuous removal of VFAs from the rumen is important for distribution of this energy source and also to prevent excessive decreases in pH of the rumen fluid. The surface area of the healthy rumen papillae is proportionate with the concentrations of VFA's in the rumen. In the dry period, the surface area of the papillae is reduced by half due to low dietary non-fibre carbohydrate. Therefore, during the transition period, it is important to manage feeding in order to produce a correctly balanced population of rumen microorganisms and allow the development of the surface area of rumen papillae.

Ruminal acidosis occurs when the production of fermentation acids exceeds the ability of the animal to remove or neutralise the acids produced.

At calving, the amount of non-fibre carbohydrate increases, producing high levels of fermentable carbohydrate and the amount of VFAs produced exceeds the capability for the rumen papillae to absorb or for the rumen fluid to neutralise. This leads to elevated concentrations of VFA in the rumen resulting in a decrease in the pH that can induce subacute rumen acidosis (SARA). These acidic conditions will further reduce cellulolytic bacteria in the rumen and contributes to reduced feed digestion and thus a reduction in dry matter intake. If the papillae are not given time to adapt to the change in ration, they will not be sufficiently developed and thus the surface area will not be increased. The result is that the absorption of VFA from the rumen will be low, leading to a reduced pH post-feeding.

High levels of acid further reduces rumen motility and efficacy of mixing of contents which acts to reduce the levels of VFA near the rumen wall, resulting in a further decline in VFA absorption. As motility and mixing decline, so does rumination, which decreases the level of saliva flow to the rumen, reducing its buffering capacity. Clinical acidosis is measured as a pH of less than 5.5 for a prolonged period of time. SARA is typically diagnosed when the average herd pH is less than 5.5-5.6 during the daily fluctuation cycle.

At these more acidic pH levels, the papillae will be sloughed / damaged from the rumen surface further reducing the absorption rate. This damage allows bacteria to enter the blood stream, triggering histamine production in response to bacterial invasion. This causes blood vessels in peripheral tissues to double, increasing the risk of laminitis as well as liver and ruminal abscesses in the early post partum period. Following bouts of acidosis, repaired papillae thicken with a reduction in the surface area for absorption. This slows the rate of removal of acids from the rumen further, predisposing cows to acidosis.

The role that Probiotics will play in the calf and dairy cow are discussed in a detailed literature review by Dr Kevin Hillman. In summary, the benefits for the dairy cow can be outlined as follows:

Healthy Rumen Function = Optimum Productivity

• Assist in stabilising the rumen during ration changes (such as the transition period)
• Reduce weight loss at start of lactation
• Increase lactic acid metabolism
• Reduce risk of SARA and Clinical Acidosis and rumen dysfunction
• Increase levels of total VFA's
• Increase cellulytic bacteria to utilise high starch rations
• Improve FCR - increase utilisation of Dry Matter and fermentation of Organic Matter
• Important in aiding the establishment of the microflora in order to develop the rumen papillae during the transition period
• Maintain feed intake
• Increase milk yield (maintained butterfat and protein levels)

• Anti-pathogenic and anti-diarrhoea properties
• Immunomodulatory function
• Binding capability of mycotoxins within the gut