I no longer use horse manure or any other manure from any animal other than my own
chickens - because I don't use chemical de-wormers on them.
I visited a
local farm where on one side of the road there are
cattle being dewormed with chemical dewormers and eating
hay sprayed with herbicides.
On the other side of the road are pigs that are not eating food that has treated with herbicides and and are not receiving chemical dewormer.
I asked the farmer "What is growing in the pile of cow poop?" "Nothing" he said. Actually there was some grass growing in it but nothing else.
Then I asked "What is growing in the pigs poop?" "Lots of stuff"..........
All we have to do is observe.
Over 30 years ago piles of poop would be full of an amazing variety of plants from seeds that had not been digested and a whole host of earth worms and insects. Nowdays that same pile of poo at the farm just sits there with maybe a few grasses - but no broadleaf weeds growing out of it.