The Incredibly Complicated Cow

Jim Morrill grew up on a dairy farm and went to Murray State University (KY) for B.S., University of Kentucky for M.S., and Iowa State University for his Ph.D. He taught and did research (with emphasis on nutrition and management of dairy calves) at Kansas State University for 33 years, retiring in 1995.

And God said, "Let the land produce living creatures according to their kinds: livestock, creatures that move along the ground, and wild animals, each according to its kind." And it was so. God made the wild animals according to their kinds, the livestock according to their kinds, and all the creatures that move along the ground according to their kinds. And God saw that it was good (Genesis 1:24-25, NIV).

The animals that God created are remarkable--almost beyond belief. After much study, we have come to appreciate just how complex the various systems in animals are, and yet there is still very much that we do not know. This article provides a simplified description of the digestive system of ruminants to show something about God's marvelous creations.

Many animals can be classified as either simple-stomached or complex-stomached. An example of a simple-stomached animal is a pig, and a cow is an example of an animal with a complex stomach. (Other animals have digestive systems that differ from either of these in various ways.) The cow has a stomach with four compartments: the rumen, the reticulum, the omasum, and the abomasum. Animals having a rumen are called ruminants, and those that do not are sometimes called non-ruminants. Because it looks like and functions somewhat like the stomach of a non-ruminant, the abomasum is sometimes called the true stomach.

The complex stomach allows the cow to use feedstuffs such as hay, grass, etc., that contains cellulose, a component very resistant to digestion. In fact, the cow does not secrete the enzymes necessary for digestion of cellulose. Rather, the rumen and reticulum provide space and necessary conditions for bacteria and protozoa to live and multiply. These microorganisms break down the cellulose to products that the cow can use for a large part of the energy she needs. Contrasted to this, a large part of the energy consumed by the pig is in the form of starch, which is more easily digested and can be digested by enzymes secreted by the pig itself. This is a brief oversimplification; for example, bacteria are important in the digestive tract of the pig and the cow herself can digest some starch.

The primary purpose of this article is to show the remarkable design of the ruminant digestive system. This system allows a newborn calf to efficiently use an easily digested food (milk), a growing calf to digest milk efficiently and digest dry feed which contains increasing amounts of complex carbohydrates, and the adult cow to have the fully developed ruminant digestive system.

The very young calf is growing rapidly and needs lots of energy. However, milk is not efficiently converted by bacteria to products that the calf can use. Therefore, it is not desirable for milk to go into the rumen. At birth, there is a structure (called an esophageal groove) in the reticulum of the calf which conveys milk directly to the omasum and abomasum, bypassing the rumen and reticulum. The esophageal groove is actually a pair of parallel folds of tissue that, with the right stimulus, come together to form a tube that extends from the end of the esophagus to the opening into the omasum. The closure of the groove is achieved when the calf swallows certain liquids. In this way, the calf can swallow milk that goes to the abomasum where it is digested by enzymes secreted by the calf, then a moment later swallow a bite of hay that goes to the rumen.

When the calf is very young, it does not have a good appetite for dry feed and the rumen is not needed. At birth, the rumen is very small and the papillae, which enables absorption of end products of bacterial digestion are not developed. The rumen size increases and the papillae develop only when consumption of dry feed increases. The rumen of a 6-month-old calf will still be undeveloped if it has not been fed dry feed, whereas there is considerable development by 6 weeks if dry feed consumption is encouraged. There are other ways by which the calf is provided only what it needs to digest the feeds it is likely to consume. For example, a young calf produces ample amounts of enzymes to break down protein and fat, but has very limited ability to digest carbohydrates. It cannot digest much starch or any sucrose (table sugar), which is a simple carbohydrate. It can easily digest lactose, though, which is the only carbohydrate in milk, which makes up most of the diet of the very young calf. In nature, when the calf would normally start consuming starch or sucrose, it would be part of the dry feed and would go into the rumen and be digested there. In other ways, the design of the bovine digestive tract is amazing. For example, antibodies (proteins which are in colostrum, the first mammary secretion produced by the cow after the calf is born) can pass through the abomasum intact and be absorbed in the small intestine to provide immunity to the newborn calf. Beyond about 24 hours after birth, proteins consumed by the calf are broken down in the abomasum and intestines; and if protein somehow escaped breakdown, it would not be absorbed in the intestine. This is an incredible design feature, since the calf is born without immunity to disease.

Another example concerns what can happen if simple carbohydrates other than lactose go into the abomasum. Glucose is a very simple sugar and is called blood sugar because it is the sugar that circulates in the blood. Normally, there would not be much glucose in the diet of the calf or mature cow. If glucose is introduced into the abomasum (for example, during research), it is fermented to ethyl alcohol by yeast. This alcohol is absorbed and causes intoxication, which is undesirable. Lactose, however, is not fermented by yeast, and any other carbohydrate that the calf or older cow is likely to consume will usually be in dry feed that will go into the rumen and be broken down there, thus avoiding any problem.

Yet another example is regurgitation, which is the ability of the cow to move a small portion of feed from the rumen back to the mouth where it can be chewed again and more saliva added. This is advantageous because the cow typically consumes very fibrous feeds. Still another example is the ability of the rumen and reticulum to undergo strong contractions according to certain patterns. These contractions provide mixing of contents to aid fermentation, provide a way to eliminate gases that are produced, and facilitate regurgitation. Other examples about the rumen and reticulum, and interesting things about the omasum, could be given. However, it should be apparent from the examples already given that the design of the digestive tract enables the cow to function efficiently at different ages when the requirements of the animal (and the feeds consumed by the animal) are vastly different. These design features allow the cow to use feedstuffs humans cannot use and to provide high quality products that humans can use. Where there is a design, there must be a Designer.

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