I don't know what horses in the US are feed, but we always recon that Horse
manure is cleaner than most manures as Horses tend not to eat seed heads,
and the hay fed to them is of better quality than that feed to Cattle so has
little or no weed seed in it. Also the straw used is also almost weed free,
and if they are bedded on shavings or saw dust then that is weed free.
Mushrooms are chiefly grown on Stable (horse) manure which is very rich in
straw, and the resulting compost is just about 100% weed free.
Could the weeds be because the soil fertility is improved from the manuring?

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk