Why is the nutritive value of grass to humans compared to alternatives?

Cows thrive on eating large amounts of grass. That’s because they have specialized digestive processes, such as chewing a cud that maximizes the value of grass to her. Humans who don’t have these processes, don’t get as much a of a benefit.

Suppose a human being were "stuck" for food, and the most available food was grass. What percentage of the nutritive value of the grass would the human obtain, compared to what the cow would obtain? Also, how would the nutritional value of grass (for humans) compare to vegetables such as lettuce or spinach? And is the lack of nutritional value of grass one reason why humans prefer to eat beef rather than grass, even though 90% or so of energy is lost at every level of the food chain?