I had some friends who used to go shrooming in cow pastures and were lamenting that they couldn't find them anymore. I told a farmer about that and he laughed and said the commercial cow feed has an anti-fungal in it now that makes the cow shit toxic to the mushrooms.
He could have been bullshitting (har har) to discourage dumbass kids from going in his pasture, but I've never done mushrooms so I have no idea.
