Buffer strips are a valuable way to keep pesticides out of water and avoid stringent restrictions. The Voluntary Initiative's Patrick Goldsworthy explains why this is especially important is oilseed rape.

Winter oilseed rape is the UK’s second largest arable crop, worth some £400m a year at farmgate prices. It is also a vital break crop in heavy land rotations, providing farmers with good returns as well as an opportunity to control resistant blackgrass.

However, weed control depends on four key chemicals (see table) and the crop usually requires treatment with slug pellets. All four chemicals and the slug pellet ingredient metaldehyde have been found in water at levels in excess of the one part per billion drinking water standard. That means water companies have to treat the water to ensure it meets this legal requirement.

Without concerted action to reduce the risk of pesticides entering watercourses from fields of oilseed rape, there could well be more stringent restrictions imposed on the products which are so vital to the future viability of the crop.

Rape – A Special Challenge

Pesticides attached to soil particles or in solution can either enter water through field drainage systems and/or percolating through the soil, or by erosion and run-off.

Each of the four key oilseed rape herbicides used in late autumn/winter can enter soil water and bind to soil particles.

These products are often applied between October and January when heavy rain can fall onto moist soils. As a result there is a high risk of herbicides entering drains and being washed down slopes. A survey in 2001 suggested that almost 90% of farms have at least one field beside a watercourse.

Winter bean crops also depend on these products. While the winter bean area is far smaller, the same general advice applies.

Grass buffers

The concept of buffer zones at the edge of crops to protect against spray drift and to protect the environment is well established. But new research points to grass buffers having an important role in preventing pesticides entering watercourses, due to soil erosion, surface run-off and poor application practice. Indications are that grass buffers may reduce herbicide losses by more than 50%.

The buffers should be a permanent feature of a field, rather than being seen as a no-spray zone. While no-spray zones are specified on some pesticide labels, they can contain cropped areas fertilised and treated with other pesticides.

Where is a grass buffer needed?

Grass buffer strips are needed wherever oilseed rape is grown next to a watercourse, irrespective of slope, soil type or field drains. A watercourse is any ditch or channel that carries water – the same approach as for a Local Environmental Risk Assessment for Pesticides (LERAP).

Grass buffer strips need to be at least 6m wide. Where fields have slopes that are steeper than 5%, buffer strips up to 20m wide may be required.

While a hedge beside a watercourse will help prevent pesticide loss, especially if there is a well established grassy hedge bottom, the watercourse will still need to be buffered. Stewardship or cross-compliance requirements may mean the grass buffer needs to be measured from the edge of the 2m cross-compliance strip.

Establishing grass buffer strips is a must to reduce peaks of rape herbicides or metaldehyde in water in the coming year.

Establishment

Ideally, grass buffers should be established the year before oilseed rape is sown. However, it is better to establish the buffer in the same year, than to do nothing.

Use standard stewardship grass mixtures, choose tussocky grass species as these can help trap any soil particles moving across the buffer.

Carbetamide and propyzamide offer effective grass control. However, it is unlikely that run-off will be at sufficient concentrations to seriously affect grass established in buffers.

It is important to plan. Establish buffer strips this autumn against watercourses in fields where rape will be sown in 2010 and 2011.

If in any doubt, seek advice from a BASIS-registered adviser.

Management

Grass buffers may be eligible to be part of an agri-environment scheme, for example ELS option EE3 6m buffer strips on cultivated land.

If they are part of a scheme then any requirements about mowing, traffic and other treatments need to be observed; different rules may apply to cross-compliance and stewardship strips.

Ensure the strip does not become compacted and that there are no breaks, gateways, tracks or "grips" (temporary channels) through the strip as these will channel water and undo the benefits of the strip.

However, grass buffers are only one part of the solution for protecting watercourses. Other advice on best treatment, for example timing and dose, soil structure and management, should also be followed.

For instance, on sloping fields it is best to establish tramlines round the contour, rather than up and down the slope.

Buffers make economic sense

According to an ADAS review, if the four herbicides and metaldehyde were banned, rape margins could be cut by up to £365/ha, essentially making the crop unviable in many situations.

In addition, the same study estimates that farms with resistant blackgrass face further loss to cereal gross margins of £430-500/ha.

So, although it can cost £200-250 to establish each hectare of grass buffer, plus an annual mowing cost of £12.50, introducing them next to water makes sense, especially as they are often eligible for support through agri-environment schemes.

Buffers are a must

Establishing grass buffers is a must for the whole farming industry. Failure to reduce the peaks of rape herbicides or metaldehyde in water in the coming year could lead to a regulatory review of these products.

Already, the industry has seen valuable products, such as isoproturon and trifluralin lost. If these oilseed rape herbicides were lost then many current winter arable rotations would become untenable.

Not only would farmers face less profitable break crops, the industry would lose a valuable home-produced source of vegetable oils and livestock feed.