I was intrigued by your question. The best answer I could find is on a Bible oriented web site.
No need to go there, because the summation of the cow's digestive system ended with the followingfrom the above reference)
The digestive system of the cow is designed to handle large amounts of forages. They do not do well on all-grain or high-fat diets. One article states, ‘cattle evolved digesting roughages that ferment slowly in the rumen’ and that ‘high grain diets … disrupt the normal microbial environment which precipitates acidosis.’
So I imagine that a rumen friendly diet is one of 'large amounts of forages'.