This is the first time I've heard it, and it's rather strange. We are all used to feeding livestock with grass and grain. What is the use of mushrooms - I don't even know. But I will not argue that they are harmful, since I have not studied this issue in detail. So I tried to look for information on feeding cows with mushrooms and found nothing in essence. There is no information anywhere that it is harmful (well, if an animal eats a poisonous mushroom, the result is obvious and you don't need to write about it). But at the same time, no one writes that it is useful. Personally, I would not experiment, because a lot of effort and money has been invested in animals. I am a supporter of traditional feeding.