How the Rumen Works
1. Nutrients for ruminants
Nutrients are ingredients of feed, which supply animals with the requirements for life, growth, health and reproduction. These nutrients may be listed into four essential elements:
Measuring the nutrient content of feed allows comparison of different feeds. It also allows us to estimate requirements of different groups of animals and determine whether these requirements can be met by various feeds. If requirements for one or several nutrients are not met, it is then possible to suggest a suitable supplement to balance nutrient requirements. A shortage of one of these is not favorably offset by a surplus of another - it is the balance that is most important. A feed in which nutrients are balanced is more efficient and effective.
Ruminants are unique in the animal world. They are able to live on pastures that have dried out and provide only cellulose and hemi-cellulose as a source of energy. This source of energy cannot be used as effectively by other grass-eating mammals and allows ruminants to be kept on country that is not suitable for monogastric animals. Ruminants do not compete with humans for food. In the ruminant, cattle, sheep, deer, goats and to a degree the camilids, the anatomy of the digestive tract is the important determinant of how the animals derive their nutrients from the foods they eat. The rumen is an adaptation for the treatment of fibrous feeds before it reaches the true stomach. This converts the grass into a form that the true stomach can digest, and then assimilate the nutrients into the animal. The rumen is infront of the true stomach (abomasum) and it is in the rumen that the fermentation of the energy (carbohydrate) from the diet is converted to volatile fatty acids by microbial action. The volatile fatty acids (VFA) are absorbed from the rumen and are the main energy precursors for the animal. The microbial population is very high and is made up mainly of bacteria, fungi, protozoa and viruses. The major protein needs for the animal are provided by the bacteria when they are washed out of the rumen into the abomasum.
The rumen is the organ that gives a ruminant its ability to convert cellulose and hemi-cellulose (dry forages) into energy. The rumen occupies about 85% of the volume of the four compartments comprising the stomach. This is about 50% of the abdominal cavity. The contents of the rumen comprise 15% dry matter and 85% water. Particles within the fluid component are fermented by rumen microbes to produce new forms of energy.
4. Rumen Microbes
Bacteria produce enzymes that ferment feeds eaten. There are hundreds of different types of bacteria, some dependent on the others for survival.
Different feeds require different bacteria to complete the fermentation process and it may take many days (when the feed ration is changed from dry pasture to grain for example) for sufficient numbers to multiply to be effective. This is why a small change in feed may set a ruminant back for weeks.
The bacteria ferment carbohydrates and some soluble protein in the feeds into volatile fatty acids (VFA). It is these VFA’s, that when absorbed from the rumen become either the fat or glucose for the animal.
Bacteria also provide the ruminant with between 50-100% of all its protein requirements, when they die in the rumen they are digested in to the true stomach. The remainder of the protein available to the animals is from that which escapes from the rumen without being fermented by bacteria. This is known as By-Pass protein.
Fungi physically break down the structure of dry forage and allow bacteria access to nutrients.
Fungi can also be responsible for some of the fermentation process.
Protozoa are microbes that appear to reduce the efficiency of rumen fermentation.
6. Saliva: Saliva production by ruminants is copious. A sheep can produce up to 20 Liters per day of saliva (average 10 Liters) and cattle up to 180 Liters (average 100 Liters). Saliva is rich in sodium and phosphorus and recycles urea and sulphur. The main function of saliva is to provide a buffer for the rumen; it acts as an antacid. The rumen must not be allowed to become too acidic when it ceases to function (this may cause death). Dry roughage stimulates the production of copious amounts of saliva, but grains acts as a poor stimulant.
Nutrition of Microbes
Because the ruminant is dependent on microbes as the source of most of its energy and protein, it is imperative that rumen microbes are as numerous and active as possible. This means that they must be well nourished. They must be supplied with what they need to multiply, and produce enzymes to convert energy to VFA (Volatile Fatty Acids). Energy requirements are obtained from the feed source. In addition, a small amount of readily digested energy (e.g. molasses) can stimulate microbes when they are sluggish.
About three-quarters (75%) of bacteria require nitrogen, sulphur and phosphorus to synthesize amino acids and protein. Protein in feed has to be broken down to these elements before bacteria can use them. Or they can be supplied as urea, sulphur and phosphorus in a supplement. There is a rumen concentration at which absorption into the microbe is most efficient, so each element should be in balance.
The other quarter of bacteria require amino acids and peptides which are already formed, so can survive only when protein is available.
In addition, minerals are required for utilization in the microbe cell’s structure and production of enzymes and vitamins.
Green roughage provides sugars and starches that are fermented by bacteria to VFA (Volatile Fatty Acids). It is relatively soluble and digestible. Because of its high solubility a proportion of protein usually escapes fermentation.
Dry roughage contains cellulose and hemi-cellulose that is bonded by lignin. This makes it less soluble and less digestible. Thus, digestion is slower and not as many nutrients are available. Fungi and bacteria have to penetrate and break the plant structure to get access to the nutrients. Cellulose and hemi-cellulose are then fermented to VFA and protein is utilized for microbial cells.
The bacteria that ferment roughage survive in an environment where the pH is about 6.8 (range 6.2 - 7.2). The pH is dependent on the rate of production, the rate of absorption of VFA and the production of saliva.
The important principle to remember is that the food eaten by the animal first feed the microbes and the microbes then feed the animal. The supplement is designed to keep the rumen functioning at its optimum, and then the protein to energy ratio of the feed allows the animal to produce wool, meat and offspring at its optimum level.