Like aminopyralids?
https://www.rhs.org.uk/soil-composts-mulches/weedkiller-in-manure
It might only be an issue if you are using hay or cow manure. Most people seem to use sugarcane.
I spoke to a friend that worked as a plumber at an abattoir and next door was a Searles. They used thebgrass from the cows stomach in some of their products. I had no idea potting mix wasn't vegan. There is a potential contamination there.
Farmers and Grazon; name a more iconic duo.