In the last issue we began a discussion of what makes a cow work – specifically her digestive system. A bovine's uniqueness lies largely in the fact that she possesses a four-compartment stomach system identified largely by the presences of the first and largest compartment known as the rumen. In this issue we are going to continue this discussion to help you gain a better understanding of this system and how it works.
Development of the Rumen and the Rumen Lining
Proper function of the rumen is dependent upon proper development. As we discussed last time, when a calf is born it basically has a non-functioning rumen and can almost be considered a monogastrics (having only one stomach). All the compartments are there; they are just small and undeveloped. This facilitates the calf being able to maximize digestion of milk where it gets the bulk of its nutrients from in the early stages of life. The initiation of rumen development is dependent on the animal's exposure to a number of key elements as it grows.
Bacteria. It's interesting that we normally spend much of our time trying to avoid or kill bacteria. We buy bactericidal hand soap, wash our clothes with bactericidal detergent and scrub our dishes in scalding hot water with compounds that kill everything in sight. When the calf is first born, the rumen is sterile -- there are no bacteria present. However, by one day of age, a large concentration of bacteria can be found -- mostly aerobic (or oxygen-using) bacteria. These are introduced by exposure to the environments and by the cow licking the calf, especially around its muzzle (nose and mouth) and by the calf's nibbling on grass or other materials. Thereafter, the numbers and types of bacteria change as dry forage and feed intake occurs and the substrate available for fermentation changes. The numbers of total bacteria (per milliliter of rumen fluid) do not change dramatically, but the types of bacteria change as the calf begins to consume different types of pasture, hay and feed and depends less on milk in its diet. This results in a dramatic loss of aerobes and predominance of anaerobes (bacteria that live without oxygen) with increasing dry feed intake. When we examine the contents of the rumen we find billions of bacteria as well as other organisms such as protozoa, fungi, molds, etc. These groups are then made up by hundreds if not thousands of other sub-groups, many of which perform a specific function. Some are known as cellulolytic (cellulose degrading) bacteria. These are largely responsible for breaking down the fiber in the animal's diet. Some are proteolytic (protein degrading) and breakdown protein. Others break down starch, produce methane, and so on. The presence of the different bacteria and their function is related largely to the diet the animal is on. We'll get to that more in a minute. All in all, the number of "typical" rumen bacteria -- those found in adults -- become established by about two weeks after dry forage and feed intake commences.
Liquid in the Rumen. To ferment substrate (grain and hay material the has been taken into the rumen), rumen bacteria must live in a water environment. Without sufficient water, bacteria cannot grow, and ruminal development is slowed. Most of the water that enters the rumen comes from free water intake. Milk or milk replacer does not constitute "free water." When milk or milk replacer is fed to calves, as we mentioned last issue, it bypasses the rumen and reticulum by the action of the esophageal groove. The esophageal (or reticular) groove is active in the calf until about 12 weeks of age. The groove closes in response to nervous stimulation, shunting milk past the reticulo-rumen and reticulum (2nd compartment) and into the abomasum (true stomach). Closure of the groove occurs whether calves are suckling the cow or are fed from buckets or bottles. Therefore, the feeding of milk replacer should not be construed as providing "enough water." Feeding water can increase body weight gain, starter intake, and reduce scouring. This tends to be a greater issue in dairy operations where calves are grown in confined areas for the first several weeks of their life
Outflow of Material from the Rumen. Proper ruminal development requires that material entering the rumen must be able to leave it. Measures of ruminal activity include rumen contractions, rumen pressure, and regurgitation (cud chewing). At birth, the rumen has little muscular activity, and few rumen contractions can be measured. Similarly, no regurgitation occurs in the first week or so of life. With increasing intake of dry feed, rumen contractions begin. When calves are fed milk, hay, and grain or other dry bulky materials from shortly after birth, normal rumen contractions can be measured as early as three weeks of age. However, when calves consume only milk, normal rumen contractions may not be measurable for extended periods. Cud chewing is a normal and required part of ruminant digestive function. When a cow consumes hay or grass, often she does not “chew her food” the way her mama told her to. Often forages enter the rumen in large particles. In order for these particles to be broken down more extensively the cow will regurgitate or “cough-up” a portion of this material and will chew it more when in a resting situation. This is also one way to visually determine if a cow is feeling good or not. If you watch her while resting and she does not “hock up a lugie” after a while and begin chewing, there may be something amiss. Another important characteristic to understand is that cud-chewing is much more predominant in pasture or hay feeding situations where the larger particle sizes require additional mechanical processing (chewing). Cattle on feedlot-type rations will not engage in much cud chewing simply because the particle sizes of the feed material aren't normally that large.
Absorptive Ability of the Rumen Tissue. The process of the bacterial breakdown of the forage and feed materials is referred to as fermentation. The absorption of end products of fermentation is an important criterion of ruminal development. The end products of fermentation take many forms and compose many of the nutritional building blocks for the digestive process. One of the most important components of the fermentation process are known as Volatile Fatty Acids or VFAs. VFAs (acetate, propionate, and butyrate) are the basic components of energy metabolism in a cow. In other words they are where the energy generation process in the cow start. The VFAs are absorbed into the rumen epithelium (rumen wall), where propionate and butyrate are metabolized in mature animals. These compounds are then transported to the liver and onto the tissues as an energy source.
The rumen wall consists of two layers -- the epithelial and muscular layers. Each layer has its own function and develops as a result of different stimuli. The muscle layer lies on the exterior of the rumen and provides support for the interior (epithelial layer). Its primary role is to contract to move the ruminal contents around in the rumen and move digesta out of the rumen, into the reticulem and on into the omasum (3rd compartment). The epithelial layer is the absorptive layer of tissue that is inside the rumen and is in contact with the rumen contents. It is composed of a very thin film of tissue holding many small finger-like projections called papillae. To get a better feeling of what the inner rumen wall looks like, examined under a microscope it looks a lot like a carpet. These papillae provide the absorptive surface for the rumen and greatly increase the surface area. At birth, the papillae are small and non-functional. They absorb very little and do not metabolize significant VFA.
Many researchers have evaluated the effect of various compounds on the development of the epithelial tissue - in relation to size and number of papillae and their ability to absorb and metabolize VFA. The results indicate that the primary stimulus to development of the epithelium are the VFAs - particularly propionate and butyrate. Milk, hay, and grain added to the rumen are all fermented by the resident bacteria; therefore, they contribute VFA for epithelial development. Plastic sponges and inert particles -- both added to the rumen to provide "scratch" -- did not promote development of the epithelium. These objects could not be fermented to VFA, and thus did not contribute any VFA to the rumen environment. Therefore, rumen development (defined as the development of the epithelium) is primarily controlled by chemical, not physical means. This is further support for the hypothesis that ruminal development is primarily driven by the availability of dry feed, but particularly starter, in the rumen.
Factors Affecting Rumen Function
The function of the rumen can be affected by a whole host of things. Health, forage quality, stress levels, feeding and supplementation program, contaminants, etc. can all have positive or negative effects on how well the rumen works to serve the digestive and overall nutritional needs of the cow. Remember the microbial population in the rumen and the animal itself have a symbiotic relationship both need to survive. In order for us to effectively manage the nutritional needs of the cow we have to manage the bacteria. In all actuality, we probably do not “manage” the rumen microbes; we simply have to understand how they function and how they respond to various stimuli. Let's discuss that for a minute.
The microbes in the rumen are countless in number one of the main reasons for this is that they are constantly reproducing and dying off as well as being passed on down the digestive tract as material is forced from the rumen. The bacteria go along for the ride. The population profile, in other words, which bacteria are present at any given time depends largely on the type of diet the animal is on. In a pasture situation, the microbes present will be largely the types that digest cellulose and other fiber components since this is what makes up the lions share of what is entering the rumen. In these situations there is a lot of rumen movement, a lot of cud-chewing, a lot of saliva production. There tends to be a higher percentage of acetic acid produced compared to propionic acid. Overall, not as much acid in general is produced and the rumen pH is higher – 6.0 to 6.5 or so. Flow of digestive material through the rumen is relatively slow. This is not always the case, however. When cattle are on lush winter or spring pastures, when forages are characterized by high moisture content, movement through the rumen and on down the digestive tract can be pretty rapid (never stand close behind a cow on wheat or ryegrass pasture).
By contrast, cattle in the feedyard consume a diet which has little roughage and is characterized by the intake of grains such as corn, milo, wheat, etc. which are high in starch. The bacteria which become predominant at this time are the starch digesters. They produce a greater proportion of propionic acid as compared to the acetic acid level. They also produce a lot of other organic acids (i.e. lactic acid) that drop rumen pH. Normal pH ranges in the rumens of feedlot cattle can be 5.0 or less. This drop in pH as cattle are shifted onto high grain rations reduce the number of fiber digesting microbes which exist at the higher pH ranges. We'll discuss this in a minute. With high grain rations we see less rumen movement and faster movement of digesta through the digestive tract. Also, since most of the particle sizes are smaller we see less cud-chewing. All of these things are inter-linked. Cud-chewing increases saliva output in the animal. Saliva acts as a natural buffer which helps pH from getting too low. With cattle in the feedlot, as we mentioned, cud-chewing is reduced which reduces saliva production. This on to the fact that we have lower pH levels due to the high starch diet further serves to depress pH. This is one reason that we see a greater incidence of conditions like acidosis, bloat and founder in confined cattle feeding situations.
These circumstances also create some problems in cattle on pasture that we are supplementing. It's fairly common to see producers go out and supplement with range cubes or other high grain supplements once or twice per week as time allows. In many cases they will put out enough for three or four days. Cows being cows, they may very well eat the entire amount in one day. This can create a whole new set of problems and defeat much of the purpose.
In the next issue we'll discuss this situation and how to overcome these problems. We'll evaluate some of the tools and management techniques that are available to us to maximize rumen performance and to give your cattle every benefit.
Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, Texas. He can be reached at P. O. Box 653 Sulphur Springs, TX 75482, by phone at (903) 885-7992 or by e-mail at firstname.lastname@example.org.