I thought they just baked the manure to kill off the seeds. Does that add too much to the cost?
Composting manure can kill some seeds, but baking won't work...if it gets too hot, it burns, and, in fact, if piled too deeply it will heat up on its own and spontaneously combust. Feedlots have to keep spreading it to keep this from happening...so, even after composting-in-place or mixing with other organics, weed seeds survive. The bigger knock against it is that chemical fertilizers are so much easier to apply, give more uniform distribution and usually require no tilling in, which saves on fuel/labor. Also, unlike manure, they don't stink for days or draw flies.