I've done a search for this subject on this forum but can't find it mentioned previously. I apologise if I have missed it and you are aware of this issue which I would like to bring to your attention.
The basics are that it has been found that 'manure' produced by cows grazing on grassland treated with a herbicide has been found to affect the growth of plants grown on land which has had the manure applied. This problem was first noticed among veg growers last year but is still being experienced. The particular chemical was found in a Dow herbicide and Dow did eventually withdraw the product after much alarm was raised, but they are seeking to have it relicenced.
Whilst not many herbs need manure, perhaps your fruit or veg, or rose shrubs for example could be affected. Also bear in mind, that manure is used by some compost producers - including organic - and some of them are implicated. I grow my own veg and whilst I don't use manure I have used bought in compost for raising and transplanting seedlings (including herbs) and have had great difficulty with a lot of non-germination and non-development of plants, which taking weather, mould, etc, etc into account, has been out of the ordinary.
There is also the issue of this chemical working its way intact through the cow and us humans drinking the milk or eating the meat! Or eating veg which has managed to grow on contaminated land - although the evidence is that not much actually grows. Dow says there is no problem or risk to us!
Regulation and the paramount safety of the public springs to mind!!
You can find more information on this subject at:
www.kitchengarden.co.uk/forum/index.php under the section "Weeds, Pests and Diseases" and then under the heading 'contaminated manure'.
There is more at: www.glallotments.btik.com/home.ikml along with a petition to prevent the product being relicenced.