I hope the manure was old and well-rotted. Horses don't have multiple stomachs like cows and they don't chew their food thoroughly, so horse manure can have LOTS of weeds. Voice of Experience, here! After my father-in-law had his legs amputated due to side-effects of diabetes, he still wanted to garden, so he ordered seeds and fertilizer and I supplied all the labor. And what a LOT of labor that was! The guy he ordered manure from brought horse manure, not the promised cow manure, and I was fighting wild mustard, pig-weed, goose-foot, and dozens of other noxious weeds all summer.