Horse manure is high in seeds.. That's why they say compost it for a 6 month minimum a year is better.. It gets so hot it kills most seeds then turn and repeat the process.. I need it to amend my soil it was Rick solid clay before I added it now it's like crumbly black soil I've never tested it but we'll see if it's fertile if anything grows.. The thing with the leftover seeds in the manure I figured wouldn't be a problem since my plan all along was to cover with weedmat..