Why can cows live on grass but humans can't?

Answer 1

Humans cannot obtain energy by eating grass because the constituents that make up the plant material make it much more difficult for a human to obtain any nutritional value from. Grass is a plant, and it is often a coarse plant that has a rigid structure that contains cellulose, hemi-cellulose, lignin and fibre, all of which impede a monogastric (simple-stomached animal) like the human to properly break down and utilize the energy and nutrients from such a plant. Humans do not have a large cecum like horses, zebras and rabbits do, nor do they have a three or four-chambered stomach like a llama, a camel, a cow or a sheep does. Without these critical organs, humans cannot obtain energy from grass and thus be able to live off of grass.

To shorten this statement up, the simple answer to this question is this: Humans are not adapted to spend a lifetime eating grass. Humans are hunters and gathers by nature, not grazing animals like bison or antelope are.

See the related question for more on the ruminant physiology side of eating and digesting grass.

Answer 2

This is an important question. Many animals can digest foods that we people can't use at all. If we ate the grass on our lawn, it wouldn't do us much good - we would slowly starve if that were all we had to eat. Grass and other plants have plenty of the chemicals that we need to live and grow, but our digestive tracts can't digest them and so we can't use those nutrients. But a cow will eat grass or hay and little else and do just fine. What's the difference between us and the cow? The answer is in those extra "stomachs" and what lives there.

The biggest part of the cow's stomach is called the "rumen" and it contains billions and billions of friendly germs. These germs are microorganisms that we can see only with a microscope. These germs - there are many different types in the rumen - are not harmful, in fact, they are badly needed by the cow so that it can use the nutrients in grass and other plants. The nutrients in grass are in the form of complicated big molecules that must be broken down into smaller pieces that can then be used by the cow's digestive tract and so used by the cow for growth of the body and to maintain life. The germs can do that job of breaking down those plant molecules, and that is why they are so important. So cows can live on grass and we can't - and it's all because of those extra "stomachs" and their friendly germs!

Answer 3

Cows have a more complex stomach than us humans do which allows for forage, or in this case, grass, to be digested more efficiently in several different steps. Cows are ruminants, which means that they have a rumen capable of housing millions of microbes and bacteria which help in the fermentation and nutrient extraction process of the partially eaten and re-chewed digesta. Fermentation is a evolutionary process of all herbivores that enables the break-down of tough plant fibres including cellulose, hemi-cellulose, and fibre. Cows typically eat grass at first by simply swallowing it whole: later they lay down or stand around regurgitating the partly chewed/partly digested forage and rechewing it again as cud. The rechewed bolus goes back into the rumen to go through further fibre and cellulose degradation and digestion. Once the digesta in the rumen is digested as much as possible, it is moved into the omasum, which acts as a filter that absorbs all the "water" or liquid out of the digesta. That dried up, sponged up digesta moves into the abomasum, the true stomach, to undergo further digestion in terms of digesting proteins and amino acids. The one stomach I left out is the reticulum, as it only acts as a stomach where foreign materials are stored and broken down by the acid in the stomach of the cow.

Humans, on the other hand, are monogastrics. That means we only have a simple stomach with primarily one efficient purpose: to digest proteins and carbohydrates. Now, the reason we humans cannot digest grass nor other plant matter efficiently is because our stomach is not designed to break down the tough outer plant material that I mentioned above. Very little is gleaned from tough plant material like lettuce or even grass. We may have stomach acid, but the enzymes of peptidases and starch enzymes are simply not meant to break down plant matter. So it simply passes through. Even our small intestines cannot absorb much nutrient from the plants.

Answer 4

Im pretty sure your appendix helps you eat grass and dirt and rocks and stuff.

Answer 5

Well humans don't eat grass because it just isn't on the food pyramid or anything plus cows and horses eat grass. Plus grass is in the ground and the ground is not clean.