I am kind of wondering about mint poisoning now. It was mentioned in another thread. We always have quite a bit growing in areas that flood in winter, but it seems like the cows don't eat it. It is flowering right now and smells very strong, so I would not think they would consume it as long as there still is grass. I have never seen a cow eat it and they leave the grass where Mint is intermixed, but calves tend to be willing to try more things. Mine will chew on every twine string they find tied to a gate and they are always mouthing things to see if they are edible. There is a winter pond right next to where the calf was laying in the shade. It is dry now and the entire bottom is covered in a beautiful purple carpet of mint. Apparently pneumonia is a symptom of Mint poisoning. I've never had an issue with Mint before, but I have never had an issue with pneumonia either.
Based on your location, your mint is likely Pennyroyal and consequently not the culprit in your calf's situation. Highly unpalatable as you know and from everything I've read, non toxic/poisonous. Overview: https://wric.ucdavis.edu/information/natural areas/wr_M/Mentha.pdf
As an aside, hay twine is a magnet for those little suckers (pun intended) even when its snugged up tight and the ends trimmed short. :roll: