Perennial sowthistle

This is my first experience trying to grow hay. I purchased 20 acres that was planted in wheat the previous year. In early May I disced in the wheat stubble and planted a 30% alfalfa 70% grass mix fesuse, bromegrass, timothy and orchard grass. As of now half the field looks great with minimal weeds. The other half is thick with perennial sowthistle. Anyone have any advice?

Perennial sowthistle