Since 2009/10, there have been cases of contaminated farmyard manure causing damage to vegetable crops in gardens and allotments. This contamination is caused by application of weedkillers to farmland used to grow hay and other forage which are then eaten by stock. The weedkillers are Aminopyralid (AP) and its close ‘cousin’ Clopyralid (CP). It is found in hay, grass and silage cut from sprayed fields and the chemical passes through feeding animals into their manure.
Typical symptoms include cupped or distorted leaves on sensitive plants. The shoot tips become pale, narrow and distorted, with prominent veining on the foliage. Growth generally is stunted, leaving most crops unusable. This can affect many crops including beans, tomatoes and potatoes.
What to do if you have suspect you have infected manure:
The HSE advise that Dow AgroSciences has set up an advisory website for allotment holders and gardeners to provide advice on the use or disposal of existing manure that may contain aminopyralid and will provide a ‘Bioassay’ (testing kit) to allow growers to check their manure. If the manure is found to contain residues of aminopyralid Dow AgroSciences will arrange for the manure to be removed. See here for the full HSE advice
Residue levels in the soil peak at three weeks after digging before breaking down relatively rapidly. This means affected ground is usually safe to replant by the following spring.
Scrape off any loose manure used as mulch around flowers, shrubs or fruit bushes. Return it to the suppliers or spread it on grassland. Failing this, put it in council refuse.
Fruit trees and bushes damaged by contaminated manures are likely to survive and grow well next season. Feed the plants in the spring to encourage good cropping next year.
It is not advisable to compost ruined crops. If they cannot be incorporated into the soil, bag them up and put out with household refuse (NOT green waste collection). Seek advice from your council if they won’t accept green waste in domestic refuse.
When buying manure in the future:
Products containing aminopyralid were temporarily withdrawn from supply, sale and use. Although it is now legally back on sale, measures have been taken to ensure the risk of contaminated manure becoming available to gardeners have been reduced. However, gardeners should still be cautious and ask manure suppliers if they can give an assurance that the manure has not come from animals fed on grass or forage treated with persistent hormonal weedkillers, especially aminopyralid products such as Forefront.
For more information about this problem go to the RHS pages on Aminopyralid