Have you ever used aged manure from a horse farm and had strange things happen to your vegetables? I have.I had one year where some my potatoes and some tomato beds developed thickened curled leaves and contorted stems after top dressing. I read elsewhere that certain chemicals can pass right through a horses digestive system causing their manure to contain a percentage of things like meds and even herbicides.
What follows is an article from Mother Earth News from 2013 and it’s worth a read. The gist of it says that composts and mulch may well be contaminated with persistent herbicides. Livestock grazing on treated fields are affected and their manure, even when composted, still contains herbicides, some worse than others. Companies that make bagged compost or soil amendments by gathering manure from multiple sources were getting complaints from gardeners about plant damage.
Another source point was from commercial feeds–the pellets were likewise found to contain certain herbicides which again came through the manure. Feed samples were found to contain certain of the worst offending chemicals from 142 to 465 parts per billion when all it takes to harm your veggie garden begins around 30 ppb!
Read more here: Killer Compost Update