A cow eating grasses may not be so interesting. It may be just an ordinary activity of a creature trying to continue its life. But, have you ever wondered and compared to yourself, why and how can that happen? How and why you can not eat even a tiny bit of grasses as sources of nutrients?
Well, the answer lies in the digestion system which is different between a cow and us. A cow and other similar animals (buffalo, camel, and elephant) can eat grasses and other tough feeds because she gets help from microbes living inside her stomach. There millions of microbes living in the stomach of a cow. It is this microbes that produce enzymes that are able to breakdown the polymer components of grasses, especially structural carbohydrates. Such substances are tough and not easy to digest. We can not eat grasses because we do not have microbes inside our stomach that produce such enzymes which are capable of breaking down tough substances. In the cow, the microbial digestion is the first phase of digestion process which converts polymers into simpler substances. The second phase is the further digestion of by the cow's own enzymes of the simpler substances. Results of the second phase are simpler compounds that are ready for absorption and then used for tissue metabolism in the animal's body. The first phase occurs mainly in fore stomach of the cow called the rumen while the second phase takes place primarily in the small intestine. The cow and the microbes actually develop a kind of mutual living collaboration that benefits both creatures. For the microbes, the cow's rumen is a convenient place for them to live and grow. The environments such temperature and osmotic pressure are just right as required by the microbes. Regularly, the microbes receive substrates in the form of feed eaten by the cow, which they digest to obtain their own nutrient requirements. For the cow, as mentioned above, she obtains the microbes' assistance in digestion process. The microbial cells, like feeds, are also nutrient sources for the microbes because the cells flushed out regularly from the fore stomach and reach the small intestine of the cow for where it is digested and absorbed.
Interestingly, the microbes are absent from the stomach when a calf is just born. That is way a calf is fully dependent on the cow's milk for nutrient at birth because there is no way for the calf to eat grasses. Only after the calf turn into some weeks of age, the microbes start to establish in the stomach. The microbes enter the calf's stomach through contaminated feed the calf starts to eat and from the cow through saliva contact. The microbe population may be not be so large at start, but they will rapidly grow and reach millions of cells as the calf grows and become accustomed to grass feed.
It is a blessing that a cow eat grass, we eat bread or rice. What if a cow has to be fed with bread too? Life would be so competitive!