Aminopyralid, which is found in several Dow products, the most popular being Forefront, a herbicide, is not licensed to be used on food crops and carries a label warning farmers using it not to sell manure that might contain residue to gardeners. The Pesticides Safety Directorate, which has issued a regulatory update on the weedkiller, is taking samples from affected plants for testing.

Problems with the herbicide emerged late last year, when some commercial potato growers reported damaged crops. In response, Dow launched a campaign within the agriculture industry to ensure that farmers were aware of how the products should be used. Nevertheless, the herbicide has now entered the food chain. Those affected are demanding an investigation and a ban on the product. They say they have been given no definitive answer as to whether other produce on their gardens and allotments is safe to eat.

It appears that the contamination came from grass treated 12 months ago. Experts say the grass was probably made into silage, then fed to cattle during the winter months. The herbicide remained present in the silage, passed through the animal and into manure that was later sold. Horses fed on hay that had been treated could also be a channel.