Pesticide residues in manure may cause damage to plants
Regulatory Update: 18/2008
Issued: 11 July 2008
This is an update to Regulatory Update 15/2008 (issued 16 June 2008) concerning possible pesticide residues in manure causing damage to plants.
Information about aminopyralid
Aminopyralid has been granted provisional authorisation for use as a herbicide/weedkiller to control broad-leaved weeds on grassland, following an evaluation of extensive data by the Pesticides Safety Directorate (PSD). The substance can remain in grass from treated land and pass into the manure of grazing livestock, where it remains tightly bound to the plant material until it decomposes. Similarly the substance can remain in grass fed as hay or silage to housed cattle, again passing through the animal into the manure. If manure is applied to soil or crops before the plant material in the manure has fully decomposed, susceptible crops may be damaged. These include peas, beans and other legumes, carrots and parsnips, potatoes and tomatoes, and lettuce and similar crops.
As the plant material in the manure breaks down it releases the aminopyralid, which is likely to be at its highest level in the soil about 3 weeks after applying the manure. However, soil bacteria then break down the aminopyralid so that susceptible plants may start to recover and grow again.
Most of the aminopyralid residue in the soil should have been broken down after 6 months if the manure been has fully incorporated (rotavated/mixed) into the soil to aid decomposition. Therefore, it should be safe for growing plants next spring (2009).
Labels of products which contain aminopyralid therefore include warnings not to use manure from livestock, which have eaten grass from treated land, on susceptible crops, or on land intended for growing such crops, until all plant material has fully decomposed. As with all plant protection products, users should always read and follow the label instructions. This is a statutory requirement.
Safety information
PSD has now assessed additional information from the manufacturer that confirms that using manure, which may contain residues of aminopyralid, does not have implications for human health.
Based on reasonable worst case assumptions: that cattle are only fed grass, or silage made from grass, treated with aminopyralid; that vegetables are grown in soil mixed with manure produced from the animals; and that all the aminopyralid released from the manure is taken up into the plants; the highest residues would not be a concern for health, so vegetables should be safe to eat.
The manufacturer also provided some information on studies of residues in treated food crops which indicates that actual residues will be much lower than estimated, which gives additional reassurance.
The substance is an effective herbicide/weedkiller, but of low toxicity to mammals. The problem has arisen in part because the substance can persist in grass, hay or silage, and passes through the mammalian digestive system largely unaffected, rather than being broken down.
Action being taken
Although the presence of aminopyralid has not been confirmed in any of the cases reported to us, circumstantial evidence indicates that it is likely to be involved in at least some of them. It would appear that the label precautions concerning the use of manure may not always have been followed when manure has been supplied to allotment holders and gardeners. PSD is urgently investigating the problem. A key issue in the investigation will be whether the conditions of use regarding manure are sufficient, or sufficiently well known, to prevent a recurrence.
The manufacturer, Dow AgroSciences Ltd, has set up an advisory note for allotment holders and gardeners at http://www.dowagro.com/uk/grass_bites/faq/allotment.htm. They also have a contact email address at ukhotline@dow.com. As regards handling affected plots, anything that will speed up the decomposition of the grass in the manure will help alleviate the problem. For example rotavating or digging over affected plots a number of times and ensuring the manure is turned readily. It is advisable not to replant susceptible crops until next year, to ensure that all plant material has had sufficient time to decompose. As regards unused manure, the best solution is probably to ensure that it rots down fully and is fit for use in the next few years; or it can be disposed of as waste.
Contact details
If you have experienced problems with your plants being affected by manure, please contact Dow AgroSciences Ltd. This will help in collating and gathering information on this issue. ukhotline@dow.com.