We’re small acreage- not many optionsPersonally, as a horse owner, I wouldn't use it on pastures, as 2,4D bioaccumulates. Without knowing what it is that you are trying to suppress it is hard to advise.
If you are having issues with some weeds, I would think about either changing the grazing patterns with electric fence subdivisions in the pastures, or think of having something like a few goats or cows to graze what the horses don't/won't graze. We use cows, and we have a little mustard and the occasional thistle, but nothing else.
I have friends who spread the manure every fall, till it in, and reseed for their horse pastures. Their pastures look awesome, but it is a lot of work and it doesn't work for sloped pastures.
All the best,