</p><p><em>THE RUMINANT DIGESTIVE SYSTEM</em></p><p>&nbsp;</p><p>Cattle and sheep have a compound stomach divided into four compartments, which are closely linked together and function as a single unit. The entire stomach system in adult cattle has a capacity of 150 to 200 litres. The stomach system consists of the rumen, making up 80 % of the total volume, the reticulum (5 %), the omasum (6 to 8 %) and the abomasum (7 to 8 %). Cattle can be likened to a walking fermentation vat containing 20 to 40 billion bacteria and up to a million protozoa per millilitre of ruminal fluid.</p><p>&nbsp;</p><p>The bacteria in the rumen break down the organic matter in the feed, through fermentation, into ammonia or relatively simple compounds called volatile fatty acids, in this way supplying both their own nutrient requirements and those of the host. Some of these simple compounds are absorbed through the reticulo-rumen wall into the blood stream, while others are incorporated into the bodies of the micro-organisms. The micro-organisms are digested by the animal in the abomasum as they are swept along with the feed moving through the digestive tract. About 70 % of the digestible organic matter entering the rumen is broken down by the micro-organisms before the feed passes to the omasum.</p><p>&nbsp;</p><p>The volatile fatty acids (VFA) produced by the bacterial fermentation in the rumen are used by the cow to provide 50 to 70 % of her daily energy requirements and to form nearly 50 % of the butterfat produced by the mammary gland. Acetic, propionic and butyric acids constitute nearly 90 % of the ruminal VFAs and each is produced by a particular strain of bacterium that require a specific acidity (pH) range in the rumen for maximal growth.</p><p>&nbsp;</p><p>The feed must be broken down into small particles before it can progress from the reticulum into the omasum and abomasum where true digestion occurs. Any feed particles which are not small enough (less than 2 mm) to pass through the reticulum are regurgitated for extra chewing (chewing the cud). Chewing the cud (6 to 8 hours daily) not only helps reduce feed particle size, but also incorporates saliva into the feed. This saliva contains salivary salts which act as a natural buffer to maintain the rumen pH at normal levels (pH 6 to 7) and reduce acidosis. <strong>Cows on a typical dairy ration can produce 80 to 100 litres of saliva per day.</strong></p><p>&nbsp;</p><p>Cattle on low roughage/high concentrate rations, or finely milled rations, chew less and consequently produce less saliva. This reduced saliva production can result in excessive acidity (acidosis) in the rumen which reduces the rumen pH from a norm of between 6,0 and 7,0 to between 5,2 and 5,5. The reduced pH can:</p><p>cause reduced feed intake</p><p>reduce the population of bacteria producing acetic acid, which is used to produce milkfat by the cow, thus reducing the butterfat levels in the milk</p><p>damage to the rumen wall, a common condition with animals on feedlot rations. The damage to the rumen wall can lead to liver abscesses because bacteria normally limited to the rumen enter the bloodstream and eventually infect the liver.</p><p>The problems caused by the reduced buffering which is associated with feeding high levels of concentrate can be alleviated by supplemental buffers such as bicarbonate of soda or magnesium oxide. These commercial products simulate the buffering effect of the saliva. Splitting the daily concentrate portion of the diet into more, but smaller, feeds to reduce the pH drop in the rumen, has been shown to alleviate low butterfat (BF) problems, i.e. less than 3,3% BF for Holstein-Friesland cows. Research has also shown that splitting the concentrates into more than two feeds per day is usually beneficial only when the concentrate proportion of the diet exceeds 60% or an amount equivalent to approximately 2% of body weight.</p><p>&nbsp;</p><p>&nbsp;</p><p>The feed usually remains in the rumen-reticulum complex for up to 60 hours, in the omasum for 8 hours and in the abomasum, where true digestion occurs, for 3 hours. After leaving the abomasum, the feed enters the 40 metre long small intestine from which the nutrients are absorbed into the bloodstream. The last part of the alimentary tract is the large intestine, which is 11 metres long, where the water is removed from the feed immediately prior to excretion from the animal's body. In the case of grazing cattle, the faeces are still very fluid when they are voided, whereas with sheep, the faeces are normally dry due to the efficient manner in which water is removed by the colon.</p><p>&nbsp;</p><p>The reticulo-rumen acts as a fermentation vat which functions on a &quot;continuous production&quot; principle and not a &quot;batch&quot; system. It therefore requires a constant supply of feed material to keep it functioning. This is achieved in the natural state by the long periods spent grazing, both day and night. When cattle are placed on processed feeds, e.g. complete diet or total mixed rations, the time spent eating may be considerably reduced to about 3 to 4 hours, instead of the normal 8 to 9 hours spent grazing. It is considered preferable to split this shorter period of feeding into several feeding periods, so that the reticulo-rumen is frequently replenished with new feed material.</p><p>