I had a problem last spring with what I presume was a persistent herbicide in the hay fed to my wife's horse. I could not get any legume to sprout well in the SFG bed that contained compost made that spring from the horse's manure. I believe that clopyralid was the problem substance, and apparently legumes are the most sensitive of our garden vegetables to this substance. The good news is that the contamination in the compost did not persist through a very mild Florida winter, and I have had no problems with this spring's crops grown in that very same bed.