Can't help you with a supplier but here is the sorry tale. The contamination is from a hormone weedkiller, aminopyralid, that was introduced 2 years ago. It is sprayed to control broad-leaved weeds on pasture that is then eaten by horses and cows or on hay crops that are harvested and sold as feed. The weedkiller survives being eaten and excreted and if applied as a soil improver, kills or damages 'sensitive' plants - potatoes, tomatoes, peas and beans, aubergines, lettuce, raspberries and certain ornamentals. If contaminated manure has been dug in, it is likely to affect crops for more than one growing season. The weedkiller binds very strongly to the lignin in the sprayed plant material. It is only when the hay or grass has completely rotted away which can take many months that the weedkiller is released into the soil where it is broken down into harmless compounds by soil organisms in around 3 weeks.
If you really have to use manure, ask the supplier if they can absolutely guarantee that nothing the animals have eaten has been sprayed. They may say they don't spray which is fine but if they buy in feed, they're unlikely to know if it has been sprayed. If they can't or won't guarantee it's safe, don't use it is my advice. There is no way to tell in advance if it is contaminated and there is no way to make it safe if it is.
Do a search on the forum for 'aminopyralid' and read about all the people who have lost their crops this year.
And Google it to get an idea of the size of the problem.
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