Assuming that you're not trying to be humorous, grass is their natural 
food, but cows on feedlots are fed mainly grain for the fat and marbling 
that it produces.  Only a minority are fed solely on grass throughout 
their lives.

On 8/7/11 4:44 PM, william wrote:
> On 08/07/2011 06:20 PM, theta wrote:
> check with your local area farmers to see who grass-feeds their
>> cows.
>
> No need. All cows eat grass.
>
> William
>