You are right to wonder. I thought the same because any farmer who uses it has to sign an agreement to keep the hay on their holding, as it is acknowledged to be so poisonous to other plants than grass, cereals and brassicas.
However I have found some in a compost heap where I added some horse manure and inbitial enquiries suggest it is more widespread than I had realised.
However you can do a simple test, with half- or well-rotted manure: plant a potato in it, or sow peas, runner beans, which are fast-growing but susceptible. If early leaves show curling and the main growing point is deformed or curling, that is a.p.
It is almost certainly only in horse manure as hay-farmers have been persuaded to clean their hay of thistles, dacks and nettles. Whereas cow farmers feed silage which is cut younger, before those plants get going, and few of them use herbicide on grass (so far!).