Paraquat pot was what it was called...yeah they crop dusted the outdoor fields with stuff to make people sick. People would still smoke it even knowing it was paraquat but it wasn't worth much destroyed the value and made people sick.
Look it up :
http://en.wikipedia.org/wiki/Paraquat
Complete text on above link:
Herbicide use
Paraquat is used as a quaternary ammonium herbicide, one of the most widely used herbicides in the world. It is quick-acting, non-selective, and kills green plant tissue on contact. It is redistributed within the plant, but does not harm mature bark. Being a herbicide, paraquat protects crops by controlling a wide range of annual and certain perennial
weeds that reduce crop yield and quality by competing with the crop for water, nutrients, and light.
"Paraquat pot"
During the late 1970s, a controversial program sponsored by the US government sprayed paraquat on
marijuana fields in Mexico.
[8] Since much of this marijuana was subsequently smoked by Americans, the US government's "Paraquat Pot" program stirred much debate. Perhaps in an attempt to deter people from using marijuana, representatives of the program warned that spraying rendered the crop unsafe to smoke.
However, independent bodies have studied paraquat in this use. Jenny Pronczuk de Garbino,
[9] stated: "no lung or other injury in marijuana users has ever been attributed to paraquat contamination". Also a
United States Environmental Protection Agency manual states: "... toxic effects caused by this mechanism have been either very rare or nonexistent. Most paraquat that contaminates
marijuana is pyrolyzed during smoking to
dipyridyl, which is a product of combustion of the leaf material itself (including
marijuana) and presents little toxic hazard."
[10]
So it is really just a herbicide, no injury due to smoking.
But it was the BIG thing back then.