Interesting citation. The toxicity report in Canada was from 1943. The incident seems to have been associated with the consumption of large quantities of mature mustard seed pods within the hay. (Imagine eating an entire jar of mustard at one meal!)https://fr.wikipedia.org/wiki/Moutarde_des_champs
The mustard of fields was indicated as responsible for poisonings of the cattle in Canada, due to the consumption of hay by containing a big quantity.
The manner in which the farmer mows the pasture beside my house, and his method of bailing the hay should result in less than about 10% of several different bales being wild mustard. It will likely be mowed before the seed pods have fully formed. His cattle should tolerated this easily.
All brassica vegetables contain varying quantities of thiol-based toxins, like isothiocyanate. An exclusive diet of brassicas (in people as well as livestock) can result in hemolytic anemia and in thyroid imbalance (goiter). That's not to mention gastrointestinal upset and possible nephritis. The prepared mustard that many of us use contains those same toxins. That's what gives mustard its unique, pungent flavor.
Everything in moderation.
Bob