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1st July 2010
Keeping metaldehyde out of surface water to prevent exceedances in drinking water supplies is something the whole farming industry has been working extremely hard to deliver. New research funded by the Metaldehyde Stewardship Group (MSG) has highlighted that diffuse field losses via field drainage and surface runoff are more significant than previously understood in terms of the possible routes by which metaldehyde is reaching raw water supplies. Dr Paul Fogg led the water catchment study set up to investigate the relative significance of the three major routes by which metaldehyde may reach surface waters. “Direct (e.g direct application into ditches), point source (e.g spillages when filling) and diffuse pathways (movement through soil) were all assessed and while factors such as direct and point source remain important, the study showed diffuse losses through soil drainage as being most significant. “We found that water moving down through the soil profile will take the metaldehyde from recently applied slug pellets into field drains and from there into streams, ditches and watercourses which may ultimately lead to a drinking water abstraction point,” explains Dr Fogg. In response to the new findings, the Metaldehyde Stewardship Group has announced significant amendments to the best practice guidelines already in place for the use of metaldehyde. The new advisory guidelines maintain the annual maximum dose rate for the calendar year at 700g of the active ingredient per hectare. However a new maximum rate for a single application of metaldehyde is advised at 210g metaldehyde per hectare, reduced from 250g. Most significantly the MSG are now advising that no more than 210g metaldehyde per hectare should be applied in total during the period August 1 to December 31 – a timeframe which encompasses the establishment phase of winter combinable crops, when plants are at most risk from slugs. NFU Plant Health Advisor, Paul Chambers warns that although the metaldehyde exceedances reported by water companies last season were significantly less than in the two preceding seasons, 2009 was a comparatively dry autumn with low slug pressure. “When the wet weather returns it may be an entirely different story and all growers must follow best practice and reduce the amount of metaldehyde being applied in the high risk autumn period.” So in practical terms what do the new guidelines actually mean to growers? East Anglian agronomist Colin Myram has the following advice. “There is still flexibility to use metaldehyde within the autumn window, depending on the products used. You need to know the percentage rate of metaldehyde contained in the product you are applying and set that against the kg/ha rate of slug pellets being applied. With these two figures you can then determine how many applications can be applied whilst keeping within the 210g maximum. “Unfortunately there is no ‘one size fits all’ solution because there are a range of different percentage metaldehyde based products available with a range of different application rates. But with recent improvements in formulation technology; there is a new generation of slug pellets that have been developed which deliver lower levels of active ingredient whilst maintaining efficacy.” The results of the water catchment study showed that most significant drainage losses of metaldehyde occurred up to a month after slug pellet application, reinforcing the existing guideline that slug pellets should not be applied when heavy rain is imminent. Keeping a close eye on the soil moisture status and weather forecast now need to become key determinants of the products for slug control, particularly on under-drained fields. “It’s vital that metaldehyde based slug pellets are not applied if drains are flowing or when soils are near field capacity and rain is forecast,” explains Mr Myram. “Growers should always aim to use the minimum amount of metaldehyde per hectare, up to a maximum of 210g/ha.” According to the MSG many agronomists and suppliers are likely to be advising application rates of 160g/ha metaldehyde ai or even lower this autumn. In spite of the emphasis now being on soil drainage as the main pathway for metaldehyde into water sources; the best practice guidelines for storage, handling and application of pellets and cleaning of machinery still stand. “The catchment study was conducted with best practice guidelines for handling, application and machinery cleaning in place. As a result the results for both direct and equipment contamination were very low. This shouldn’t be interpreted as these factors not being important, they are. Point-source contamination can and does cause very high levels of metaldehyde in water and everyone involved in slug pellet application must continue to be vigilant and take extreme care,” stresses Mr Myram. Combinable Crops board chairman Ian Backhouse urges all growers to follow the revised advice. “To retain profitable crop rotations metaldehyde is essential, therefore it is absolutely vital that growers follow the revised guidelines,” he comments.
Metaldehyde Stewardship Guidelines for 2010 *from any combination of products | ||||||||||||||
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