Serious Eats: Talk
Horse manure can lead to herbicides in your compost
May affect you in other states:
This year Vermont has been dealing with a problem with persistent herbicides in compost, which cause a tight curling of tomato leaves, but also poor germination, stunting, and low productivity in some other vegetables.
There have actually been 3 herbicides found, including Clopyralid and Picloram. Nobody had registered with the Ag Dept that they were using these herbicides. Horse farms were one of the sources, and it turned out that some Purina feed included the Clopyralid.
The herbicide is not harmful to humans (in theory, and according to the VT Dept of Health). It passes right through the body, which is the problem for compost. The herbicide in the feed ended up in the manure, and survived the composting process.
The herbicides have a half life of 1-2 years. It affects plants in very, very small amounts, much less than the amount allowed in human foods. The use of this compost is not going to affect organic certification, but it sure messes up the garden.
I got some bad compost last year, from a different company than produced most of this year's problems. (The UVM and state plant pathologists thought my tomato problem was herbicides, but it wasn't confirmed, and because no one had registered its use it was a mystery.)
THE POINT OF ALL THIS IS, if you're using horse manure in your compost, or buying compost, it's really, really, important to pursue this, and particularly to find out if the horse feed being used is Purina.
Some federal agency or other is pursuing this nationally, because it may prove to be a national problem.
The problem here was actually identified by the principal seller of compost, the Chittenden Solid Waste District, which collects a wide variety of organic wastes from the Burlington area. Their web site has a lot of information:
And a link to an article on the local WCAX news: