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Garden Organic information sheet:

Herbicide Contamination of Manure and Straw

July 2008

Garden Organic has received an unprecedented number of calls from members and concerned public regarding widespread damage to home-grown crops.

Large manure heap
Large manure heap

Herbicide damage is a very common problem this year and the root of the problem seems to be herbicide residues in grass or silage fed to livestock which subsequently contaminates the manure and bedding straw which is then supplied to gardeners and allotment holders. The herbicides responsible are mainly sold under the trade name Forefront and Pharaoh containing aminopyralid as an active ingredient although other herbicides containing a similar chemical, clopyralid (e.g. Vitax Lawnclear 2, Verdone Extra, Charter) can have similar effects. The herbicide manufacturers of Forefront and Pharaoh (Dow Agrosciences) recommend that manure and slurry from treated land should not be sold for garden use but this seems to have been overlooked in the supply chain. The Pesticide Safety Directorate say that the risk from eating any crops resulting from these plants is minimal.

It is still unclear why the problem has been much more noticeable this year although aminopyralid has only been widely used in the UK from 2006 when the first product became available. Some of the references supplied below indicate that it has been known about for some time in the US, where damage due to clopyralid has been widely reported since about 2000. Although there are a number of routes by which contamination may occur the herbicide is absorbed into plants and can bind to lignin and other plant cellular structures, only being released when they breakdown and so can be a contaminant of animal feed. As it is water soluble it is rapidly excreted intact in urine by farm animals (cattle and horses) when they are fed on contaminated material and also causes contamination of manure and bedding straw.

The herbicide is released once the manure or bedding material breaks down and can exert its herbicidal action at this point. The manufacturers advise that it is safe to plant sensitive crops when the manure is no longer visible in the soil. The actual length of time for manure to breakdown depends on soil conditions. By this measure stacking manure or bedding is unlikely to completely destroy the herbicide. However, proper aerobic composting (at 60°C) is more likely to degrade the herbicide in manure and straw and it should not be such a problem in thoroughly composted manure. If sensitive plants are present when the organic material breaks down they will be damaged but otherwise soil microorganisms rapidly break down the aminopyralid so crops may grow if the contamination is not too serious.

Clorpyralids (e.g. Vitax Lawnclear 2, Verdone Extra) are also licensed for amenity and amateur use in the UK and there is a possibility that they could end up either in the green waste stream or even home composting heaps. However the manufacturers warn that all grass clippings from the first mowing must not be used as a mulch, either fresh or after composting, since it may damage desired plants. They advise that it should be disposed as household waste and not via council composting schemes. Subsequent mowings can be used as mulch only after composting well for at least nine months. This should not be an issue for organic gardeners who should not be using lawn herbicides but may be an issue if you get green waste compost.

Symptoms of contamination include:

Distorted growth, cupped leaves or fern-like growth. Growth is often stunted.

Sensitive crops include:

Peas, beans and other legumes, sugar beet and fodder beet (i.e. beets), carrots and other umbellifers, potatoes and tomatoes, lettuce and other compositae.

Insensitive crops include

Wheat (if sown one month after application) and maize, grasses and brassicas (if sown with a four month delay).

Herbicides responsible are:

Mainly sold under the trade name Forefront and Pharaoh containing aminopyralid as an active ingredient. Manufacturers, Dow Agrosciences, recommend that manure and slurry from treated land should not be sold for garden use but this is being overlooked in the supply chain.

As this is not a new chemical it is still unclear why the problem has been much more noticeable this year although some of the references supplied below indicate that it has been known about for some time in the US, especially in active ingredients from the same family of chemicals.

Garden Organic advises:

  1. If you suspect herbicide damage due to manure do not use any more of this manure.
  2. Make your manure supplier aware of the problem so that it can be addressed in the supply chain in future years.
  3. Do not use the land treated with this manure until Spring 2009. You can plant a green manure in this period (e.g. rye grass immediately or grass-clover after some delay)
  4. Before sowing expensive seeds or planting transplants in the spring, test the soil by trying to geminate seeds of a sensitive (but cheap!) crop like lettuce – if the germination is weak or non-existent, leave the soil in a green manure for another 12 months.
  5. The manure should be returned to the vendor for disposal or spreading on cereal stubble. Alternatively it could be composted and spread on suitable land but this would not be advisable at home where the heap could not be regularly turned.
  6. Take steps to reduce your reliance on external composts or manure by making your own at home or by planting more green manures in your rotation.

Obviously steps should be taken by vendors to prevent contaminated straw and bedding entering the garden supply chain in future seasons. Alternatively farmers should be encouraged to use alternative methods of weed management that does not involve these chemicals.

Useful links:

Knowledge Development Team
15 July 2008

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