Hi, I was just hoping I could help by clarifying some things on your questions about Pasture Pro. I happened to stumble across your post and I work for PBI/Gordon. There are no grazing restrictions for horses or cattle, but use your own best judgment when it comes to your animals.
Pasture Pro will work best if you hit the thistles in the bud stage: a calm, 65-80 degree day works best. You can spray in the spring and again in the fall if you still have a problem. For pre-emergent control you will want a product specifically for that purpose: we don't currently make a pre-emergent for pastures but your local extension office can recommend one. The pre-emergent will prevent the dropped cocklebur seeds from sprouting. If you use those two in conjunction plus good mowing and management practices you should be able to get your pasture back to the way it should be.
And of course - read and follow the label directions 100%. Don't think more will be better - our label recommendations are written for best results and sometimes adding more chemical has the opposite effect you want! And if you want to speak to someone in person, call our 1-800-236-5415 number and tell them you have a question about Pasture Pro. We have two gentlemen with years and years of experience that answer tech questions during regular business hours (8-4:30 CST M-F). They will be happy to help.
Hope you don't mind my dropping in - I just want to assure you that I only meant to help answer your questions, not to sell anything!