Porkpal, I know you must have checked this but just in case.......
I happened to be researching lambs quarters - Chenopodium and ran across this:
"Grazing animals will eat common lambsquarters when young, but as it matures it becomes woody and disagreeable. Under certain conditions, it can also accumulate nitrate, sulfate, and oxalates in its tissues and cause poisoning of livestock if eaten in large quantities."
From:
https://extension.usu.edu/weed...
I don't have any idea if any of that could cause liver problems in horses but wanted to mention it.
