I am a former Yellowstone County extension agent who worked with John Ranney in the ‘80s. My work portfolio included working with weed control and making presentations to the farmers and ranchers in Yellowstone County on subject matter that they requested. I recall being asked about Roundup herbicide along with other products.
There were questions about use, storage, application rates, etc. Of particular concern was the question of toxicity and whether or not products were carcinogenic.
As I recall, there seemed to be unusual clusters of cancer occurring in the valley — whether or not this was strictly anecdotal information I can't say. What I can say is that there was definitely a concern among those folks.
To try and answer these type of questions county agents referred to a manual we called "The Blue Book" which listed all current materials used in ag. The manual gives descriptions of the chemical, including molecular structure, LD 50 rates, etc. I do recall coming back and telling my audience that, according to the manual, Roundup was a form of salt and was neutralized by contact with soil. I don't recall anything being said about it being carcinogenic at that time.
Recent events seem to indicate that information was either unknown or omitted. I'll leave it to others to research whether Monsanto did carcinogenic animal testing on Roundup before releasing it in the market.