I finally contacted the farm that had supplied me the horse manure, and described my experiences and test results.
They were shocked to hear it, although said they had never heard of Aminopyralid, said they’d never had that in 20 years etc. I asked if they bought in their hay or grew it themselves. They said they grew it all themselves but never used any chemicals at all, didn’t trust in them as they always feared these would affect the horses health. They said that the straw for bedding was bought in, suggested that maybe the contamination was there (nb they also use wood shavings). I said I thought that AP use was forbidden on straw or any cereal crops so it was unlikely to be that. Its possible that this information is incorrect, but I am inclined to believe them. I’m wondering whether I could try an additional bioassay ( I think I read this somewhere, maybe on this forum?) by taking some of the straw, and separately some of the hay, soaking it in water for a while and watering the beans
with the resultant solutions to see if the problem is there?
