Although we don't use herbicides any more, 2-4-D used to be our favorite.
The grazing withdrawal should be on the label; if it's not call the manufacturer to be absolutely sure. Keep in mind dogs will graze too, so they need to be kept out of it for the same amount of time as the horses.
2-4-D works well in the fall on perennial or first-year biennial broadleaf weeds, which are running sap down to the roots for winter. The chemical will carry down to the roots more effectively this time of year than in the spring. Annual weeds are going to be dying off anyway, so no sense spending the money and time spraying them. 2-4-D has no effect on seeds.
Follow the label directions for dilution. Fill your sprayer with water first, then measure the amount needed, add to the water and mix. I have a glass kitchen measuring cup just for 'outside' use. Mix up only what you'll use, it won't keep. If your weed problem isn't bad, it's cheaper and safer to spot spray individual weeds rather than just blanket spraying everything. When you are finished, triple rinse your sprayer to get all the chemical out.
Although 2-4-D is considered one of the 'safer' chemicals, please protect your animals and yourself from exposure. My vet mentioned recently that there have been recent links documented between 2-4-D and lymphosarcoma in dogs. One reason we quit using it was losing 2 dogs and a pony to lymphosarcoma in 18 months about 6 years ago.
We don't know if it was the 2-4-D or just bad luck, but thought better to play it safe.
I do miss being able to use it; it is an effective weed killer.
Suzanne