I think what farmers do not like is called - Field Bind Weed. Looks 100% like Morning Glory( is a member of the MG family) except to those who know their plants. I was admiring some beautiful Morning Glories on my driveway when my neighbor came over and advised that they were Bind Weeds. He advised that it would be best that I start an aggressive program to eliminate them. That was about ten years ago - they have not spread any since then - I've just left them alone.
I think the problem with Bind Weed & probably Morning Glory, as well, they are tough to eradicate and can cause real difficulties when trying to harvest a crop - such as hay. They can also bring diseases to potatoes, cereal grain crops, beans.............