Digestion — A Piece of the Deer Nutrition Puzzle

Analysis of the Ruminant Digestive System – Part 1

By WHITETAIL INSTITUTE STAFF

It is often said that the way to a man’s heart is through his stomach. Since I fall under that particular gender category, I can say with some expertise that this is a fairly accurate statement. After all, the male variety of Homo sapiens is a pretty simple beast. Supply a few basic needs and we are content.

 Whitetail deer are not that much different. If you are offended by my comparison, I apologize but you have to admit it’s a very close analogy. Give them food, cover and water and they will more than likely hang out close to where these basic needs are met. To narrow it down, let’s take water and cover out of the equation and just focus on food. Unquestionably, if you supply large amounts of highly attractive, highly nutritious food sources, deer will stay close to these food sources. I think it is fairly well accepted that planting food plots is an extremely effective way to supply these food sources. Much has been written about various aspects of food plots with articles ranging from how to plant to what to plant. This is all good information but little has been said about the subject of the system that utilizes these food sources; the deer’s digestive system. 

I am a firm believer in the value of seeing the entire picture of any endeavor I am involved with. Knowing how a deer’s digestive system works and how it can influence a whole host of things from food plot preference to deer activity can assist you in finishing the deer nutrition puzzle. So, in this article, we are going to begin a three-part series on the deer’s digestive system. In this issue, we will examine each part of the digestive system and how it works. 

Part 1 – The Parts and Pieces

Creation is an amazing thing. If you study the intricate detail in which living creatures are designed to have hundreds of parts working together in one complete synchronized mechanism, you will truly be astounded. The digestive system of a deer is one such example of a complex system of many parts working together to convert food into usable nutrients. 

It begins at the mouth. Deer are concentrate selectors or browsers, which means they pick and choose specific types of plants or even specific parts of a plant. The shape of a deer’s mouth coincides well with this particular feeding behavior. Deer have a long pointed muzzle with a fairly long tongue. This shape enables the deer to more easily pick and choose food stuffs. Compare this to the muzzle of a non-selective grazer such as a cow which has a wide muzzle and acts more like a living lawn mower. Inside the mouth you will find incisors located only on the bottom. On the top is a hard pallet which is used by the deer to nip off plants and plant parts as the bottom incisors lock the food stuff between them and the hard upper pallet. In the back are molars where the food is masticated by a figure-eight grinding and chewing motion, a similar chewing pattern used by most herbivores. This mastication of food stuffs is the very first step in the digestion process. Mastication is needed for efficient digestion to occur as the food stuffs consumed must be broken down into smaller particle sizes, thus increasing the total surface area of the food stuff. More surface area results in better enzyme activity and microorganism digestion as the food stuffs move further down the digestive system. All of this process is aided by saliva which has several functions. First, saliva is needed to aid in mastication. Also, saliva contains some enzymes such as lipase and pregastric esterase which is involved in the hydrolyses of short chain fatty acids. Yet another function of saliva is to act as a buffering agent. Finally, saliva contains mucin, urea, P, Mg and Cl all of which are needed by rumen microbes.

After the food stuffs are chewed and swallowed, they move down the esophagus. The esophagus is the portion of the anatomy that is between the pharynx and the rumen-reticulum. The main function of the esophagus is food transport. However, this does not only mean food transports down the digestion system but also back up as food boluses are regurgitated from the rumen-reticulum and brought back to the mouth for further mastication. This process is called rumination. The esophagus has the ability to expand and contract to move the food bolus either up or down during the rumination process.

From here we move to the powerhouse section of the digestion system; the four-chambered stomach of the deer. Many people think that a deer has four stomachs. A more accurate description is that deer have one stomach with four chambers. The first of these chambers is a called the reticulum. The reticulum is often called the “honeycomb” due to its many irregular mucosal layers found there. These layers function as a filtering agent, trapping larger particles so that they can be further broken down before passing on thru the digestive system. Other than filtration, the major function of the reticulum is contraction for moving food stuffs back up the esophagus or to the rumen. 

Next in the digestive system is a structure that is the largest part of the four- chambered stomach called the rumen. Any one who has ever field dressed a deer knows exactly what the rumen is even if you may not have known what it was called. The rumen is the large paunch, sack or bag (whichever terminology you prefer) that you avoid cutting into at all cost. For if you cut into this sack, a foul smelling, normally green substance will erupt from it, making the rest of the field-dressing job substantially more unpleasant. The smell comes from gases produced by fermentation of food stuffs. The fermentation is facilitated by billions of microorganisms that inhabit the rumen in a synergistic relationship with the deer. These microbial colonies consist of several different types of organisms such as bacteria and protozoa and a vast amount of the other microbes. Each microbial type has the ability to digest specific types of compounds. For example, some are cellulolytic microbes while are others are amylolytic and yet others are lipid-utilizing species. Each has specific functions and requires certain rumen environmental conditions in order to maintain healthy colonies. Therefore it is important to maintain a rumen environmental balance that is conducive to efficient rumen function and corresponding digestion. We will discuss rumen health in greater detail in part two of this series.

As food particles enter the rumen these microbial populations begin to digest and break down the particles. Through this fermentation process, the food stuffs are broken down into nutrients that can be digested by the deer. It is this microbial action in the rumen that allows deer and other ruminants to be able to digest fibrous materials. Not all food stuffs are immediately digested but are regurgitated in the form of a bolus and pushed back up the esophagus by the reticulum. There the bolus is once again chewed and swallowed making its way back down to the rumen. This rumination process gives the deer the ability to get as much nutrition as it possibly can out of the food it consumes.

After food stuffs leave the rumen, they enter the omasum, the third part of the ruminant stomach. The omasum is probably the least understood of all the ruminant system. However, we do know that the omasum acts as a filter and controls the flow of digesta from the rumen to the abomasum. It is also speculated that some nutrient absorption occurs in the omasum. 

From the omasum, digesta next enters the fourth and final section of the ruminant stomach called the abomasum. The abomasum functions very much like a mono-gastric stomach such as what you or I have. The abomasum has secretory tissue that excretes HCL and other gastric juices that finalize the breaking down of food stuffs. Because of the high acid level, the abomasum is highly acid as opposed to the rumen which is fairly neutral in pH. 

The next stop in the deer’s digestive system is the small intestine. The small intestine is comprised of three specific areas called the duodenum, jejunum, and ileum. The duodenum receives excretions from the pancreas, liver and intestinal wall. These secretions further assist in digestion. The other two areas, the jejunum and ileum are major nutrient absorption sites. Some of the nutrients absorbed include protein, minerals and vitamins. This is not the only absorption site but one of the main ones. 

From the small intestine, digesta moves to the lower gut or the large intestine. The large intestine is comprised of the cecum, colon and rectum. The large intestine has several functions including water absorption and filtration of indigestible material followed by excretion.

So there you have it—from when the food is first consumed to when the waste is excreted out. As you can see, the ruminant digestive system is a very complex but highly efficient system. It is perfectly adapted to the efficient digestion of plant material. This knowledge should help you to develop a foundation in which to plan your nutritional management program. Join us in part two of this series as we take what we have learned and apply it to how we develop and design our management program.