Aminopyralid is a weedkiller mostly used on grass and hay fields. The herbicide binds strongly to plant material. The poison can enter horse manure if the horses eat hay that has been sprayed with the weedkiller. Manure from animals fed on treated hay contains chemical residues sufficient to damage certain crops, especially broad beans, beans, potatoes and tomatoes. With the phasing out of peat, some commercial composts use manure which may have been infected with this weedkiller.
It seems to be a special problem in horse manure. Anyone buying fresh horse manure need to know whether the fields where horses grazed had been sprayed with this weedkiller.
The main symptom is new leaf growth that curls inward, yellowing leaves and stunted growth.
According to Charles Dowding, this is a growing problem:
“I am so upset to hear of increasing problems from this. It’s getting even more serious. The main identification is a deformation of plants’ growing tips. And crinkling/yellowing/distortion of older leaves. Labs often cannot measure the tiny amounts of this lethal poison, which are enough to damage our vegetable and fruit plants. Roses and apple trees suffer too.”
He claims to have seen weedkiller damage on plants grown in many prominent brands of compost. There’s also a video on the subject here.
Dowding lists a number of weedkillers where this aminopyralid is present: Banish, Forefront, Grazon, Halcyon, Pharaoh, Pro-Banish, Runway.