Milkweed Toxcicity for Equines
Milkweed is considered to be unpalatable to horses. Horses will typically leave it alone if there is other available forage present. However, Milkweed is considered to be toxic to horses if consumed in the fresh and/or dry form such as hay.
Control in pastures: spot spray with glyphosate.
Reference article: https://www.canr.msu.edu/news/milkweed_in_no-till_fields_and_pastures_a_persistent_problem
As always, when using any type of herbicide always read the label to be sure what and/or if any grazing restrictions apply to the product used.