Karpes,
In your first post you said the horses were fed mainly oats-- if they are kept in stables and fed grain or processed feed, you are not likely to have problems with weeds. Weed seeds would come from pasture and hay depending on the quality and types of hay, so you may just need to compost the manure to prevent burning.

JoeReal-- thanks for the input on fighting phytphora-- since cold is a problem here as well I will do like you and add some in late winter.

Ned-- just so you know -- most farmers are not that selective about manure. In general they are just putting it back where it came from (back on the pasture), which is a convenient place to get rid of the mountains that accumulate--(I used to work on a dairy farm).

-- Skeet