In association with Syngenta
There are now EU limits on the levels of mycotoxins allowed in cereals. Simon Edwards from Harper Adams University College advises on how they are transferred to stored grain and how to prevent them being a problem.
What are mycotoxins and why are they important?
Mycotoxins are toxic substances produced by a fungus which can reduce animal performance (eg reduced weight gain) and which can affect animal and human health.
There are EU legislation limits for cereals intended for food and EU guideline limits for cereals intended for feed.
Causative agent and types of mycotoxins
In wheat, fusarium mycotoxin contamination is produced in the field as a result of infection with fusarium head blight (FHB).
FHB can be caused by several fusarium species, most of which can produce at least one mycotoxin.
The precise species present fluctuate each season; however, the most important species worldwide (and it is thought in the UK) is Fusarium graminearum, which produces the mycotoxins deoxynivalenol (DON), nivalenol (NIV) and zearalenone (ZON), though NIV is only found at low levels in the UK. The distribution of fusarium mycotoxins also varies across regions, between seasons and between host crops. Recently high levels of HT2 and T2 have been detected in oats in Northern Europe.
Head blight can also be caused by the Microdochium nivale and Microdochium majus, but these do not cause mycotoxins.
Fusarium – infection cycle and ideal conditions
Infection starts when a warm, dry spring induces spore production on crop debris. Heavy rainfall in June then splashes spores on to newly-emerged ears, with infection of ears then occurring mainly at flowering under warm, humid conditions. High rainfall/humidity through summer allows infection to spread, particularly once the crop ripens.
Infection can also occur from Fusarium-infected seed, which causes damping off in young plants (seedling blight), followed by brown stem base lesions (foot rot) before spreading up on to ears.
Within Europe, the most conducive conditions for DON and ZON production in cereals are in central Europe, in particular France and Germany. Conditions are usually too dry in southern Europe and too cool in northern Europe to favour severe FHB infections.
In England, the main risk areas are routinely the south and east, though it does seem to be progressing steadily up the east coast. A few years ago the northern limit for ?F graminearum was Lincolnshire, but now it is North Yorkshire.
Year on year there is also fluctuation within those areas which are hotspots. Last year, Leicestershire, Northamptonshire and Nottinghamshire were particularly high risk because of the rainfall at flowering and the delayed, wet harvest.
Mycotoxin limits and sample rejections
Legislation for fusarium mycotoxins was introduced by the European Commission in 2006 for DON and ZON (see table for limits for wheat intended for human consumption). However, some processors use lower intake limits, and it is important to identify specific end-user requirements.
Limits for HT2 and T2 are under discussion and could be set by July 2009.
Legislation applies to unprocessed cereals from the first point of sale. There are lower limits for cereal products (eg, flour, finished products and infant food).
UK data show the number of wheat samples exceeding EU limits intended for human consumption varies each year. On average, about 5% exceed legal limits for DON and ZON. However, owing to the wet weather and delayed harvest, last year was the worst on record – with over 13% of samples above the legal limit for DON and over 28% above the legal limit for ZON.
On average, more milling wheat samples are rejected for traditional quality traits, such as specific weight, Hagberg or protein, than for mycotoxins. So it is important to be aware of mycotoxin issues, but without losing focus of the need to produce milling quality.
Risk factors and how to manage them
Risk assessment is now a requirement for wheat grain passports and mycotoxins must be managed by good agricultural practice (See HGCA Fusarium Guidelines).
It is important to note that risk can only be reduced, not eliminated. Fusarium head blight is a sporadic disease and difficult to control and the most appropriate method is to use an integrated approach of rotation and cultivation and a fungicide regime which targets fusarium inoculum throughout the season, from seed treatment to head spray.
Integrated control of FHB 1
1 Minimise sources of inoculum
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Use a non-host as previous crop for wheat in rotation – the more intensive the cereal rotation, the higher the levels, but maize as a previous crop is by far the highest risk factor. Practically every wheat sample following maize last year was above the legal limits.
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Remove crop debris – straw removal is beneficial.
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Bury crop debris – ploughing is beneficial. If using minimum tillage, the more chopping and burying of crop debris the better.
2 Target Fusarium within fungicide regime
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Foliar fungicides – principally T3. From field trials using a recommended triazole (eg tebuconazole, metconazole, prothioconazole, bromuconazole) the evidence is that control averages between 50-70% with a half to full rate.
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Timing is critical. Infection occurs throughout flowering, so application needs to be as close to the time of infection as possible, but before the rains.
Most products also work better as a protectant before infection rather than curatively. If adding a strobilurin at T3 for other yield or quality effects, it is important not to compromise the triazole rate.
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Seed treatments – as seed treatments reduce the amount of Fusarium present on the stem base of cereals during early growth stages, this would reduce the amount of inoculum present.
The benefit would depend on distribution of the inoculum. One observational study in Canada in 1984 showed a significant reduction in head blight incidence after seed treatment in fields of wheat following maize.
In the UK, the current mycotoxin survey is collecting data on seed treatment use, and trial work at Harper Adams University College is investigating whether a fusarium-active seed treatment can reduce head blight and mycotoxins as well as reducing early disease
3 Minimise conducive conditions
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