Golly, do not put things in other peoples' fields -- not even on the very edges past where the crops are planted -- without ASKING first! They will probably not let you, either. Horse manure is usually pretty weedy (especially if you feed any form of grain), and farmers do NOT want extra weed seeds put in their fields
Compost it in one big pile (as cubical as possible, ie. tall and compact not flat and spread everywhere) over the winter, then either leave it there to compost further before adding it to garden beds in fall (which is the best idea), or spread it THINLY on needy areas of pasture in mid-spring or early summer.
If you want to use it for garden beds but are worried about the weed issue, you can use it as a DEEP DOWN amendment if you are double-digging a bed. Remove the top shovel-depth of soil, remove one shovel-depth below that too, replace the lower layer with composted manure (mixed with the subsoil or not, as you prefer), then put the top layer of soil back on. As long as you never dig or till down into the manure-carrying depth of soil, the weed seeds won't get light and will not sprout and cause problems. You want to do this in the fall, not in the spring, as some plants would not enjoy being planted into it when relatively mroe fresh.
Me, I let the winter's shed cleanings sit in the otherwise-disused old tractor shed til late summer or fall, then I wheelbarrow and hand-spread them on the pastures. Mind that spreading your manure on pastures will increase your horses' exposure to parasites and needs to be taken into account in your management scheme.
The big advantage of composting the stuff down for as long as possible is that it reduces into a smaller volume and weight for you to have to move
Pat
