All livestock, cattle included, evolved on a diet of green leafy plants (mostly grass). That is God's way. That's because the green plant came before animals. The green plant was the first sustainable life form. Only with it could an animal kingdom exist. Consequently, for all of time animals ate virtually no grain. (Grain is the seed of a land-based annual grass plant.) They either ate green plants and/or they ate other animals that ate green plants and/or other animals. Consequently, the green plant has always been at the bottom of the food chain.