And surely in the cattle (feeding on the pastures of the revived earth) there is a lesson for you: We give you from that which is within their bodies, (marvelously distinguished from) between the waste and blood, milk that is pure and palatable to those who drink. (Nahl 16:66)

Ruminants, probably the most abundant of the herbivores such as cattle, sheep and goats, are foregut fermenters with a four-chambered stomach (rumen, reticulum, omasum and abomasum) and are an essential component of utilizing marginal land in the world in a sustainable way.

In the verse above from the Qur’an, the Creator and the Sustainer of the universe draws our attention to many of the benefits we get from domestic animals. While the main message of the verse is easily understandable to the general reader, it also contains some concise hints, even descriptions about the physiological details of milk production in ruminants that would be fully understood and explained by science only centuries after the Qur’an was revealed. The purpose of this article is to explain this process in a general sense and milk production in some detail.

Mammals are generally categorized according to the dietary habits into three classes-flesh-eating (carnivore), plant-eating (herbivore) and both flesh and plant-eating (omnivore). In a sense, humans (omnivores) and carnivores depend on herbivores for their nutrition. Generally speaking, all humans and animals in the world have directly or indirectly benefited from the plants. The ability of herbivores to utilize plants as their main energy source is dependent on symbiotic microorganisms which live at various sites within their gastrointestinal tract. The animal provides the microorganisms with food and habitat for growth and the microorganisms provide the animal with fermentation acids and microbial protein.

Herbivores are divided into two types, those with post-gastric (hindgut) fermentation and those with pre-gastric (foregut) fermentation. Fermentation is a chemical process during which microorganisms obtain energy from organic products. Ruminants, probably the most abundant of the herbivores such as cattle, sheep and goats, are foregut fermenters with a four-chambered stomach (rumen, reticulum, omasum and abomasum) and are an essential component of utilizing marginal land in the world in a sustainable way. Rumen and reticulum contain millions of microorganisms, which form about 3 to 10 percent of rumen fluid.

A major reason why human beings keep ruminants is their ability to convert food which humans find inedible-or at least unpalatable-to food (meat, milk) which humans can eat. They play an important role in the livelihood of farmers throughout the world, providing sustenance such as milk and meat, manure for crop production, cash income from sales of their products and a safety net of capital assets to face risks and misfortune in harsh environments. Currently, humans obtain about fifty percent of the meat and most of the milk they consume from ruminants. Scientists who have conducted studied on ruminants have developed cow breeds, which have higher milk and meat production than traditional cow breeds, and thus supplied an important development to meet the nutritional requirements of humans.

Pre-gastric fermentation provides three important nutritional advantages to the host animal.

First, cellulose and other plant polysaccharides are brought into solution and become available as energy sources. Cellulose is the most abundant natural carbohydrate polymer in nature, but mammals do not produce enzymes that can degrade it. Ruminant animals utilize cellulose via a symbiotic relationship with ruminal cellulolytic microorganisms. During ruminal fermentation, microorganisms ferment the carbohydrates to produce energy, gases (methane and carbon dioxide), heat, and volatile fatty acids (VFA) in the rumen. Effective digestion of plants requires a means of dealing with cellulose, the most important structural material of plants, which is extremely insoluble and remarkably resistant to a chemical attack. Cellulose digesting enzymes that are called cellulases and produced by microorganisms are also present in the intestinal tract of several invertebrates that feed on wood and similar plant products. Rumen harbors the different functional groups of the microbial population, which is responsible for about seventy percent of total digestion in ruminants, and the ability to digest cellulose has been ascribed to a large number of bacterial, fungal and protozoal species isolated from the rumen.

The energy content of plants is low, and the herbivore must consume a large quantity in order to satisfy its energy requirements. Therefore, herbivores spend a lot of time eating; eight or more hours per day may be spent eating.

Secondly, the rumen microorganisms can utilize non-protein nitrogen for growth, converting it into microbial protein which becomes available to the host. Proteins provide the amino acids needed for maintenance of vital functions, reproduction, growth and lactation. Non-ruminant animals need pre-formed amino acids in their diets, but ruminants can utilize many other nitrogen sources because of their rare ability to synthesize amino acids and protein from non-protein nitrogen sources via a symbiotic relationship with ruminal microflora.

Ruminants possess a rumeno-hepatic nitrogen circulation mechanism, which does not exist in non-ruminant animals, in order to save nitrogen. By this mechanism, ruminants can be fed non-protein nitrogen sources such as urea and nitrate when nutrients are in short supply to obtain high quality milk protein. Feed proteins are degraded by microorganisms in the rumen via amino acids into ammonia. Ammonia is used by bacteria to build their proteins and any excess of it is absorbed through the rumen wall into the blood and then converted to urea in the liver. When a diet is low in nitrogen, large amounts of urea (which is normally excreted in the urine) return to the rumen where it can be used by the microbes. In non-ruminants, urea is always entirely lost in the urine. If ammonia levels in the rumen are too low there will be a nitrogen shortage for bacteria and feed digestibility will be reduced. Too much ammonia in the rumen leads to wastage, ammonia toxicity, and in extreme cases, death of the animal.

Thirdly, vitamin synthesis by the microbial population makes the ruminant animal virtually independent of dietary sources of all vitamins, except for vitamins A and D.

However, rumen fermentation also brings some disadvantages. First of all, rumen metabolism causes environmental pollution. Methane is produced as a natural consequence of the anaerobic fermentation; it is a potent greenhouse gas. Dairy farming is the largest agricultural source of methane, one of the greenhouse gases. Furthermore, the major environmental concern associated with the animal industry is ammonia volatilization, which increases atmospheric acid deposition because of the impact of nitrogen-rich excreta on the environment. Therefore, worldwide, scientific research projects have been carried out to find sustainable strategies for reducing emissions of the greenhouse gas methane and ammonia volatilization from domestic ruminants to the environment.

In conclusion, the symbiotic relationship between ruminants and ruminal microorganisms plays an important role in the recycling of nutrients between humans and plants. This relationship also contributes to human life by converting low quality nutrients (grass and hay) to high quality food (meat and milk). Humans will benefit more from ruminants as scientific knowledge about relationships between ruminants and rumen microorganisms advances.

Zubeyir Altuntas has a PhD in Molecular Medicine. He is a research associate in Immunology Department of The Cleveland Clinic.