Geoff I estimate 95% plus of weeds seeds in manure are from the feeds eaten, in this case seeds in horse hay. Very few arrive after, relatively.
Unless the farmer/horse owner left a manure heap long enough (a year or more) for weeds to grow on it and seed.
And old manure heaps have not necessarily got hot enough to kill weed seeds. One does not know. So ‘well rotted muck’ may be full of weed seeds.
