Hello Steve
I find that weed seeds in manure are hard to ‘cook’ in a heap as it rarely gets hot enough. I also find that seeds growing out of weedy manure on the surface are reasonably easy to pull or knock around with a rake or hoe as seedlings, when small. I would spread your manure as soon as it is reasonably rotted – dark brown, straw dark brown but probably still identifiable – and deal with weeds as you see them. Sometimes weed growth looks more daunting than it is…