Weed grows very fast. In a normal season one can grow it almost three times. Think about all the yield. Feeding the livestock would be no problem. Cow and chicken really love those leafs and seeds.
The seeds could be used to make oil, bread and even milk. You don't need to buy pesticides or any commercial seeds from Monsanto.

What made it shady back then? Effective mass media marketing/propaganda. Just like those ads about smoking some kinds of cigarettes is highly advised by doctors. With proper marketing one can rule the world.