N
Nondual
Only thing I can say is the REv told me not to use horse manure. That stuck with me. A local friend lets someone dump their manure on his property so I watched. Sure enough within a year a type of weed showed up that pretty much took over. Not the time of year I can get a pic of what it is. That's just my experience. Not saying it happens all the time but can.I have been around horses most of my life and it is very rare that a horse will eat weeds, in fact they are very fussy eaters and that is why it is good to run cows after horses so all things get eaten.
My belief is that where ever he got the horse manure from had penty of weed seed flying around and was tranfered that way.
Very possible the weeds came from seeds blowing around onto the manure. I can't say just know they showed up along with the manure.