Weeds in empty pasture
The weed pictured with the purplish flower is Spotted knapweed. The other weed pictured appears to be in the ragweed species group.
Tillage could be used as a control measure before planting and establishment of forage species.
For more information on horse pasture and hay field establishment, please refer to the following link for the Growing Hay and Pasture for Michigan Horses extension bulletin: https://www.canr.msu.edu/resources/hay_and_pasture_for_michigan_horses