Another lesson learned: just because a bag of cow manure says "composted" doesn't mean it really is - composted properly, that is. I knew fresh cow - and horse - manure had weed seeds in it, but when properly composted, the heat means most of the seeds won't be viable.
When I cleaned out my small flower bed a couple of months ago, I used some bags of soil I had stacked beneath the carport. One of them turned out to be the cow manure I had bought at least a year before. I had already dumped it in the bed before I realized it. What was a pristine bed now is filled with weeds that sprouted a week or so ago. I keep pulling them, but it's obviously going to be a long process. My advice - skip the manure and use true compost you know to be good.
The seed problem occurs with both horse and cow manure. What they eat is going to end up in their manure - and wherever that manure ends up.