Bovine biology series

Part - 8 Omasum and abomasum

The omasum and abomasum

The omasum is the third stomach, and it is the final non-secretory stomach; the first two are the rumen and reticulum. The omasum is a rather spherical stomach, like a ball, that expands and contracts depending upon the rate of fill. The omasum serves as a muscular stomach, in that it assists the movement of digesta into the final stomach, the abomasum.

In the omasum volatile fatty acids and water are absorbed into the bloodstream. This stomach provides no secretory function, and thus is similar in function to the first two stomachs.

In contrast to the reticular-rumen, the digesta, once reaching the omasum consists of small material, usually less than one-half inch in length. For it is the responsibility of mastication and cud chewing to physically reduce the particle size of the ration. As digesta is moved into the omasum, the material resembles a slurry, a mixture of solid and liquid based upon the mixing function of ruminations, or motility of the reticular-rumen.

Fermentation continues in the omasum, that is, the conversion of sugars into volatile fatty acids. This occurs also in an environment without oxygen (anaerobic). There is a pH change in the omasum. The contents become more acidic here because the rate of fermentation is rapid. Again, the length of digesta is smaller, and thus the surface space is increased. Also, the digesta has undergone much mixing already in the first two stomachs, and now is fully inoculated with rumen microbes.

The motility or movement action of the omasum is important. The digesta in this stomach undergoes some squeezing action as water is removed and absorbed into the bloodstream. The main function of the omasum is the absorption of volatile fatty acids into the bloodstream. The omasum is lined with many, many layers of muscle tissue that contain the type of epithelial cells capable of absorbing fermentation products, the volatile fatty acids. Another function of the omasum is controlling the rate of digesta movement into the abomasum. This is accomplished largely through involuntary nervous control, and that is directed by the liquid content of the digesta. Generally, the more liquid diet flows faster through the omasum. A ration made up of primarily hay and dry grains will require more ruminating, increasing the time spent in the reticular-rumen, and the omasum as well. Rate of passage, again, becomes extremely important in optimizing nutrient absorption. This fact points to another point as well.....the importance of drinking water as the most important nutrient in the ration. The relatively dry ration must have water consumed so the proper moisture content can be made in the reticular-rumen. So when the omasum finally receives this slurry, fermentation continues, yielding more digestible nutrients in the form of volatile fatty acids.

The abomasum, the fourth and final stomach, is in the ruminant similar to monogastrics, such as the pig or a human. In this stomach two secretions enter the mix of digesta. The first is pepsin, an enzyme that helps metabolize proteins into smaller units called peptides. The other is hydrochloric acid, which reduces the pH of the digesta.

The reduction of pH to a level of about 3 kills of the remaining bacteria of the first three stomachs. They simply cannot survive such an acidic environment. It is here, then, that the ruminant begins digestion of feed in a different manner. The secretions listed above begin this process. For now digestion is more chemical, brought about by changes in pH and enzymes that chemically break apart the bonds that hold together very large molecules. Yes, there is some volatile fatty acid absorption here, and also some ammonia is absorbed into the bloodstream, but the primary function of this fourth and final stomach is in preparing the digesta for absorption in the small intestine.

The abomasum digesta is emptied into the small intestine, the duodenum specifically, by muscular contractions called peristalsis. These contractions are controlled by several means, including the presence of food in the three other stomachs that must be emptied into the abomasum, the maintenance of proper digesta pH, otherwise too high pH will reduce digestibility and too low pH will cause an upset stomach, and finally the type of ration. A ration that is primarily silage based, where the pH is already lower than a hay ration, will move through the abomasum quicker. This is obvious in that the material entering this stomach is already low in pH and thus does not need as much hydrochloric acid added. Conversely, a hay ration may spend more time not only in the first three stomachs, but this fourth one as well, because the pH of the digesta is reduced to 3 or so in order for optimal nutrient uptake in the small intestine.

So we have once again arrived at the very significant point of ration making: the balancing of rate of passage and optimal nutrient uptake. And this is truly a balance. If passage is too rapid, the result of high concentrate feeding along with a lack of forage length, then optimal nutrient absorption is reduced because in the first three stomachs the symbiotic relationship of the cow and microbes is not given enough time. And in the abomasum, the digesta may not be adequately mixed with the proper gastric enzymes and acids for optimal small intestine absorption of nutrients. If the rate of passage is too slow, perhaps brought about by a ration that is too dry, too full of poor quality hay, or too much forage length, then even though water may be consumed and is not limiting, there is simply not enough feed volume going into the cow to satisfy not only maintenance but that of growth or weight gain, milk production and pregnancy. It is this balance that is the single most important factor in determining the proper ration for any cow, regardless of lactation or gestation stage.

Next month we examine the small intestine: duodenum, jejunum and ileum.