That's nonsense. Herbivores will not "eat any food thats more calorically..." they eat what they have specialised to eat. Some even just eat one species of plant. Where do you find cows that hunt chickens and mice or chickens and mice that willingly sacrifice themselves to cows?
And this is all a fools errand away from the idea of cannibalistic cows!
I get they can - on rare occasion. I'm indignant about the stretch of language, presenting "can" and "sometimes" as "will" and "do", and the way this was tacked onto placenta eating to help say they "already eat parts of cows". To me this is all plain argument through exaggeration.
Herbivores don't need to eat meats and do so rarely, it has hardly been noticed. Sure, some slugs and insects may be an expected nutritional boost and help the digestive system cope with more, on rare occasion. But feeding cows cows does result in prion disease similar to other mammals, like humans suffer from the vanishingly rare practice.
And this is all a fools errand away from the idea of cannibalistic cows!