It’s difficult to tell from the picture but it could be an early stage of Florida Wild Lettuce (Lactuca floridana). The stems have a milky sap when broken and the plant produces a blue or violet flower. The plant is a member of the sunflower family and has not been found to be toxic to livestock or humans.

Thank you, but that would not be it.  The flowers are white and look almost like a miniature dandelion when it is ready to seed.  It almost looks like baby breath.  Here is another photo.

From your description and pictures it would seem to be a member of the genus Conyza. Most likely it is Hairy Fleabane (Conyza bonariensis), although other Conyza species look similar. While Hairy Fleabane does not have any toxicity reported, other Conyza species have caused toxicity in cattle and goats, and at least have the potential to be harmful to horses.

Regardless of the toxicity of the plant itself, it is taking up room in your pasture that could be growing grass, so it needs to be controlled. I would recommend checking with your local extension service again to see what broadleaf herbicides are recommended in your area. Commonly used products such as 2,4-D and Dicamba should give you very good control.