Cattle are remarkable creatures. If we didn't think so we probably wouldn't be in this business. Have you ever stopped to consider what makes a cow work? Now granted, that's a pretty silly statement but think for a second – what makes a cow different from a chicken or a pig or other farm animals? Well, over the next couple of issues, we're going to discuss these differences and what it is that makes a cow a cow. Many producers have a basic understanding of rumen function and physiology. I want to spend some time with you discussing what a ruminant animal is, how it develops and how this unique digestive system works and finally how this allows cattle to play such a significant role in the production of food.
What in the World is a Ruminant Anyway??
When I was a kid growing up and we'd sit down to the supper table I'd always be extremely careful to insure that none of the food on my plate touched. In other words, the meat didn't touch the potatoes which was separated from the peas which didn't dare come near the okra. If someone made the mistake of fixing my plate and allowing one dish to drift near the other, there was all heck to pay. My dad, on the other hand would pretty much dole it out on his plate and then stir it all together. He'd ask me “what, are you a cow and each of those dishes has it's own compartment?” While I'm not a cow I have been accused of spreading the bull a fair bit. However, my dad made an interesting point long before he had any idea his son would become a ruminant nutritionist. What makes a ruminant animal unique is that it has four separate stomach compartments as compared to other farm animals such as pigs, chickens, dogs, cats, horses, etc. which only have one. These compartments are known as the rumen, reticulum, omasum and abomasum and are found in cattle and other animals such as buffalo, elephants, deer, camels and kangaroos. Some animals are slightly different and only have three compartments but that's another story. This four-stomach system is what allows animals such as those listed to subsist on high quantities of roughage – grass, hay, leaves, browse, etc. Let's take a second and discus the four components of this system.
The rumen in the newborn calf is very small and nonfunctional. It begins growing at two to three weeks of age, depending on when hay and grain are fed in addition to the milk it receives from the cow. The rumen grows and develops rapidly during the first five to six months. From this point on, it grows at a rate proportional to other parts of the animal. During the early period of growth, the specialized function of the rumen is established. The rumen is considered to be fully functional at six to nine months of age. The rumen serves as a fermentation vat where microorganisms (bacteria, protozoa, fungi, etc.) break down the feed. In a mature cow it is also very large, in many cases capable of holding 40 to 50 gallons. The feed (hay, grass, silage, grain mix, water and saliva) serve as food for these microbes. They break down the feed and produce certain by-products. The microorganisms convert carbohydrates (mainly starch, cellulose and hemicellulose) to volatile fatty acids (VFAs). Rumen VFAs are absorbed from the rumen and are the major source of energy for the cow. The bacteria break down protein to keto acids and ammonia. The microbes then use the ammonia and keto acids to rebuild protein for themselves as they grow. This microbial protein is digested by the animal.
Minerals, salts, buffers and fiber are important in that they help control rumen pH. This is important because the bacterial and protozoa responsible for rumen digestion are very sensitive to pH change and have very specific pH requirements. Those which digest cellulose and hemicellulose, such as that found in grass, hays and high roughage rations, require a pH in the 6.0 to 7.0 range. Bacteria that digest rations high in starch, such as high grain rations, prefer a pH in the range of 5.3 to 6.5. It is estimated that 60 to 90 percent of the total digestion occurs in the rumen. Therefore, it is extremely important that cattle be fed properly in order for the rumen to function at the optimum and maintain good rumen health.
The reticulum is basically an extension of the rumen. The reticulum, by means of regular contractions, aids in keeping the feed in the rumen mixed with the water and saliva until it has a consistency that it can pass into the lower tract digestive areas.
The omasum serves mainly as a dehydration area. As the feed passes through the omasum it is squeezed and compressed by the contractions of the omasum. This removes 60 to 70 percent of the water from the partially digested feed (ingesta).
The abomasums is often referred to as the “True Stomach” and is very similar to our stomach and that of other monogastrics (animals having only a single stomach system). As the feed passes into the abomasum, gastric juices secreted by the abomasum are mixed with the ingesta, producing a material about the same consistency of that in the rumen. The high acid content of the gastric juices lowers the pH rapidly and kills the protozoa and many of the bacteria. The ingesta passes rapidly through the abomasum into the lower tract, where digestion is completed in a similar manner to simple stomached animals.
Rumen Development from Birth
As mentioned above, when a calf is born it is essentially a monogastric and does not have a functional rumen. The rumen is present but it is very small. While the calf is nursing the cow and consuming primarily cow's milk, the rumen does not develop very quickly. The milk, for the most part, is shuttled past the rumen and reticulum into the abomasum and on down the digestive system. This shuttling continues even as the calf grows and the rumen develops. Research has shown that as a calf suckles the rumen system creates what is referred to as an esophageal groove which acts like an extension of the esophagus, helping the milk by-pass the first compartments and the digestive activities which are found there. This helps the milk to be digested in a more complete form farther down the digestive tract.
As the calf gets a bit older and starts nibbling on grass and hay, the rumen grows and develops and take on a digestive role. The feeding of grass and hay actually helps stimulate the rumen's development. We'll discuss these more in a moment.
Many factors play a role in the ruminal system development. There are five "ingredients" that are required to cause ru-minal development. They are:
- Establishment of bacteria in the rumen
- Liquid in the rumen
- Outflow of material from the rumen (muscular action)
- Absorptive ability of the tissue
- Substrate available in the rumen
Let's discuss these and some others here.
Fresh, clean water is critical to rumen development. Without sufficient water, bacteria cannot grow, and ruminal development is slowed. Most water that enters the rumen comes from free water intake. In most cases water is available to calves at an early age and is not a problem. It becomes an issue in some dairy operations where calves are pulled off the cow at a very early age and are bottled and hand fed by the producer.
Milk does not constitute "free water." As discussed before, when calves consume milk it bypasses the rumen and reticulum by the action of the esophageal groove. The esophageal groove is active in the calf until about 12 weeks of age. The groove closes in response to nervous stimulation, shunting milk past the reticulum and rumen and into the abomasum. Closure of the groove occurs whether calves are fed from buckets or bottles. Therefore, the feeding of milk or milk replacer should not be construed as providing "enough water." Calves need fresh, clean water from as early as 3 days of age.
Rumen Bacteria in Calves
Rumen bacteria in the preruminant animal (before rumen development begins) are quite different from those in the mature ruminant. 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 bacteria appear to enter the rumen from swallowing items containing bacteria from the outside environment (bedding, saliva, etc). To a certain degree the calf is also “inoculated” by the cow as she licks it, especially around the muzzle area. Aerobic bacteria aren't the type of bacteria normally found in rumens of mature animals. You can think of these bacteria as temporary - they will be replaced by bacteria that can better compete in the rumen once the calf begins to eat dry hays and feed.
The numbers and types of bacteria change as hay and feed intake occurs and the substrate available for fermentation changes. There is a dramatic decrease in the number of aerobic bacteria that occurs shortly after calves begin to consume forage and/or grain. The change in bacterial numbers and types is almost always a function of intake of substrate. Prior to consumption of dry feeds, bacteria in the rumen exist by fermenting ingested hair, bedding, and from milk that flows from the abomasum into the rumen. The substrate ingested will also affect the types of ruminal bacteria that flourish in the young rumen. For example, calves fed mostly hay develop a different population of bacteria from those fed mostly grain, as is the case with young dairy calves.
The Role of Hay and Forage in Rumen Development
In a typical cow/calf program, calves have access to the same feeds and forages as the cow herd. One common reason hay and forage is useful in calves is that it provides a "scratch" needed to start the workings of the rumen. In fact, the development of rumen function is primarily chemical and is caused by VFA in the rumen. Providing forage has less of an effect on ruminal epithelial (inner lining of the rumen wall) development, thus on activity and function. The concept of "scratch" to develop the rumen is a myth. However, forage is important to promote the growth of the muscular layer of the rumen and to maintain the health of the epithelium. Rumen papillae (short extensions of the rumen wall which increase surface area) can grow too much in response to high levels of VFA - when this happens, they may clump together, actually reducing the surface area available for absorption. Also, some "scratch" is needed to keep the papillae from forming layers of keratin, which can also inhibit VFA absorption.
As we are seeing, many factors affect how the rumen in a cow develops. You can also get a grasp of how complex this system is. As we continue this series in the next issue we'll explore the ruminal system in even more depth to help you gain a better understanding of how a cow works.