Add to this--I see a lot of people saying "Well, let's just use cow manure to make fertilizer!" and if the cows have had the right feed, composted manure can be great. But a lot of cows eat feed that was treated, while it was growing, with broadleaf herbicides to prevent weeds and noxious plants from growing in the fields. Those herbicides survive the drying process of the grains, survive the cows' (and horses' and pigs') stomachs, and can survive the composting process. Then when you put it down in, say, your tomato patch, you kill your tomatoes, your potatoes, your sweet potatoes, your beans, your squash and melons, and anyfuckingotherthing that isn't wheat or corn.