Q: Is it safe to use hay, as mulch and in a compost pile, that has been treated using 2-4-D?

A: I don’t think I can say that it’s “safe” in all situations. The weed killer 2,4-D has a half-life of seven to sixty days, depending on whether it’s applied to water or soil. Soil microbes do much of the work to break down the chemical. However, several plants, tomatoes in particular, are extremely sensitive to very small amounts of this herbicide in mulch. I think it’s too risky to use herbicide-treated hay in a vegetable garden or in flower beds. There might be places in a landscape where the hay could be used but I think you’d be better off not to use it at all.