There are many things I don't know much about, and agriculture is right up there near the top of the list. But as a guess, I'd say that grain and molasses are the most cost-effective weight-adding foods they can find that the cows would eat. Being herbivores, cows aren't going to eat meat. And fettucini Alfredo and chocolate eclairs are too expensive. Compared to their standard diet of grass and hay, a grain and molasses diet is calorie rich. Anyway, I'm not so sure interspecies comparisons are valid. For one thing, we only have one stomach.