"I had a friend tell me that horses can survive by just eating grass. Thought that didnt sound quite right. Dont horses need grain and hay too for survival? Isnt grass more of a complimentary food compared to hay and grain?"
The answer to this question is somewhat complex. A horse can survive on natural vegetation if there is enough variety. Just like people, trying to live on one source of food alone will not provide the variety of nutrients required for good health. That is why horses pastured in paddocks of lush grass of a single variety have problems. A variety of grasses and leaves of moderate nutritional value being grazed on throughout the day and night is the natural diet of a horse. Of course, even in the wild, vegetation can vary, and some horses do better than others.
Also, realize that most hay is just dried grass. I say most, because there is also alfalfa hay. Alfalfa is a legume rather than a grass. Too much rich food like alfalfa and grains can actually be more detrimental to horses than a variety of natural vegetation of less nutritional value. Too much rich food can cause ulcers among other problems. The idea of feeding less of this rich food at greater intervals does not solve the problem. A horse's digestive system is designed for more continuous intake of foods of lower nutritional value along with plenty of fiber.