How do cows rely on plants?

Answer 1

They keep the grass down without the farmer having to turn on a tractor and hook up a mower to mow the grass him or herself. While he or she's got cows eating the grass, the farmer can be free to do other things that need to be done. Plus the grass benefits from the grazing cows, enabling the grass to grow more healthier when it is being grazed; the feces and urine from the cows also aid in grass growth. The deposition of manure leaves the farmer one less job to do as well; he/she doesn't need to spread manure that would've otherwise accumulated in the drylot if he/she had to feed the cows in a drylot instead of having them get their on food out on the pasture.

Answer 2

The cow's feces and urine are a source of nutrient for grasses to grow in. Also, the hoof-action and grazing stimulates grass to grow more vigorously than it would if not grazed regularly. By regularly, I mean being grazed once a month (every 30 + days).

Answer 3

Cows are herbivores, they are anatomically and physiologically made to rely on plants like grass to sustain themselves. Their rumen is a large fermentation vat and a host of millions of protozoa and microflora that help break down the plants cows eat and thus make energy, protein and other nutrients available for cows to live and function from.