In the USA, are pesticides regulated differently for animal vs human consumption?
I'm not familiar with pesticide legislation in the United States, but I do know that all oats are grown with human consumption in mind. Only the lower grades are used for animal feed. In the rare event that high grade oats become contaminated with insect pests, they would also be consigned to animal feed. Animals are less fastidious than humans, and don't mind a few beetles in their oats. It is highly improbable that your animal feed would be contaminated with pesticides, except to the extent that all non-organic farm produce has minute traces of it.

Most mushroom growers in UK use horse manure or stable muck as a substrate for growing their crop. The manure has to be suitably matured and not fresh. I would imagine it is cheaper than animal grade oats.