The culprit is the widely used herbicide aminopyralid, an active ingredient in Milestone and Forefront.

This herbicide, as well as some others that belong to the same class of chemicals (such as clopyralid) developed and sold by DowAgroscience, was widely used on pastures with horses and cattle to control perennial weeds.

Although it was claimed that the herbicide should degrade within days, it has been found to persist for years. It passes through animals feeding on treated grass or hay.

Previously treated straw, well rotten manure from animals feeding on sprayed grass, or even compost converted from contaminated source may carry enough plant killer to kill your sensitive plants and harm the others.

To keep your garden save, ask questions before buying manure or compost.

To learn more about the problem and see what the company DowAgroscience, the EPA and the government have or have not done to control the problem, read this and this.