For posterity here's the opening paragraph of the Mother Earth News article that farmerlon linked:
The aminopyralid herbicide known as Milestone, plus other related herbicides collectively known as pyralids (sold under the brands Confront, Curtail, Forefront, Hornet, Lontrel, Millenium Ultra, Reclaim, Stinger and Transline), are still surfacing unexpectedly in gardens throughout the United States, with devastating results. The EPA allows Dow and others to sell these potent weed killers to farmers, who spray them on their pastures and hayfields. When animals graze on the treated pasture or hay, the chemicals pass through the animals and persist in the manure for several years Ã¢â‚¬â€ even if the manure is processed into compost! Gardeners then use the contaminated hay or compost on their crops, bringing a slow death to carrots, lettuces, potatoes, beets, spinach, tomatoes and legumes, including (but not limited to) beans and peas.