The Digestion of  Food in a Ruminant Stomach

Ruminants (including cattle, deer, and their relatives) are unable to digest plant material directly because they lack enzymes to break down cellulose in the cell walls. Digestion in ruminants occurs sequentially in a four-chambered stomach. Plant material is initially taken into the Rumen, where it is processed mechanically and exposed to bacteria that can break down cellulose (foregut fermentation). The Reticulum allows the animal to regurgitate and reprocess particulate matter ("chew its cud"). More finely-divided food is then passed to the Omasum, for further mechanical processing. The mass is finally passed to the true stomach, the Abomassum, where the digestive enzyme lysozyme breaks down the bacteria so as to release nutrients. The use of plant material is thus indirect, with primary processing by the bacterial flora maintained in the stomach.

The Digestion Of Milk In The Newborn Ruminant

Rennin

The enzyme rennin is found in the stomach juices of calves and possibly also in other young ruminants and causes milk to curdle. It is also activated by hydrochloric acid. In the newborn calf or lamb, digestion in the stomach is similar to that of the monogastric animal. By virtue of the oesophageal groove, milk consumed by-passes the rumen and reticulum and arrives directly in the abomasum. The milk is then coagulated by rennin and digested as in animals with a simple stomach. At the age of two weeks, calves and lambs will start to nibble and chew pieces of roughage. This intake of roughage is necessary for the development and functioning of the fore-stomachs i.e., rumen, reticulum and omasum.

Digestion of Cellulose

After thorough mixing of the food in the rumen, the cellulose is broken down by ruminal organisms into fatty acids. These organisms have an optimal activity and multiply only at a pH of 5, 0 to 7, and 0. The fatty acids that are produced, of which acetic acid and butyric acid are the most important, are inclined to increase the acidity (lower pH) of the rumen, which would have an adverse effect on the micro-organisms. The sodium bicarbonate secreted by the salivary glands acts as a protective agent by neutralizing the acids and keeping the pH constant. The fatty acids thus formed are then:

• Directly absorbed through the ruminal wall into the blood, or
• Moved down to the abomasum and small intestine to be absorbed there, or
• Taken up by the ruminal organisms themselves.

Small quantities of sugar (2 percent) stimulate more effective digestion of cellulose. When too much sugar is present this process is inhibited because the organisms would rather attack sugar than cellulose. If enough proteins are present, however, up to 6 percent of sugar can be fed with good results (Usually in the form of molasses or bagasse – derivatives of sugar cane). Carbohydrates can be absorbed directly by the organisms to produce glycogen or can undergo fermentation by yeast cells, which are one of the types of ruminal micro-organisms. Gasses, especially carbon dioxide (CO2), are formed by this process and they collect on top of the fluid layer in the rumen. The yeast cells synthesize vitamins such as vitamin B1 (thiamine) and vitamin B12so that these vitamins need not be included in ruminant rations.

Digestion of Proteins

The micro-organisms in the rumen of the ruminant can make use of two sources of nitrogen to build up their body protein. They either use free nitrogen in the form of non-protein nitrogen or protein-nitrogen from their rations. Some micro-organisms can synthesise protein out of free nitrogen and build up their own systems with these proteins. The organisms are themselves later digested by the ruminant in the abomasum. The ruminant then utilizes the synthesized microbial protein to its own benefit. The ruminant cannot use nitrogen unless enough easily digestible sugar is available to convert it to protein. Therefore urea, which is a good source of non-protein nitrogen, can be used successfully under the right conditions. In the absence of enough sugar, however, urea is converted to ammonia, which is very poisonous. Sugar as such need not be present in the ration. The ruminant can produce its own sugars if enough carbohydrate is present. The appetite of the ruminant is directly related to the activity of the ruminal microbes. Conditions, which might detrimentally affect these microbes, are:

• Sudden changes in the ration
• Lack of green feed
• Lack of water
• Changes in the pH of the rumen
• Poor quality feed e.g., during the winter
• Phosphate deficiency
• Dosing with antimicrobial drugs.

Luminal movements and the activity of ruminal organisms are also interdependent. If there are not enough active organisms in the rumen, the ruminal movements will cease (rumen stasis). This condition is commonly known as “dry gall sickness”. Treatment for these conditions is:

• Sufficient water.
• Acetic acid (vinegar) – to restore the pH of the rumen.
• Sugar or glycerol – an energy source for organisms.
• Brewer’s yeast – replacement of yeast cells in the rumen.
• Fresh ruminal fluid can also be dosed.

A simple preventative measure is to make any change in the diet gradually so that the micro-organisms can adapt to the changes.

Eructation

The oesophageal opening is not situated at the highest point in the rumen so that the surface of the fluid layer lies above this opening. During the eructation movement of the rumen, the reticulum relaxes to take a greater volume of fluid. The height of the fluid is thus lowered, and it allows gas to escape.

Click here to view a video that explains the ruminant digestive system.