The challenge of keeping pesticides, especially autumn- applied weedkillers, out of surface and groundwaters is vital for farmers and water companies alike.

The threat of reduced availability, or even withdrawal from sale, of essential herbicides for oilseed rape could threaten the viability of the crop. While for water companies, there is the need to comply with the European drinking water standard which requires less than 0.1 parts/bn of any pesticide at the tap.

Late autumn is a critical time for tackling weeds in winter oilseed rape and winter beans. In many parts of the UK, carbetamide and propyzamide are used to address the menace of grass weeds, especially blackgrass, across the rotation. This applies particularly to areas where resistance to cereal herbicides is increasing. Thus further restrictions on the use of these products could threaten the viability of oilseed rape as a critical break crop in cereal-based rotations.

For all autumn-applied herbicides the challenge is similar, to guard against loss of product to water either through run-off from the soil surface or by filtration through the soil, such as drainage. The challenge is made harder by the increased frequency and intensity of winter rainfall along with periods of frozen ground and snow cover - both key factors that can lead to loss of herbicide from the field.

Heavy rain can bring soils to field capacity or if the ground is frozen lead to run-off. Rapid thaws of snow are also likely to lead to run-off.

Research

Recent winter trials funded by Dow AgroSciences showed 1.5-2.5% of the herbicide applied was lost from the field. The trials at Cockle Park, Northumberland, and Loddington, Leicestershire, both reflected that the size and intensity of rainfall had the most influence on losses. However, buffer strips, cultivation and field slope also affect how much herbicide is likely to be lost, especially due to run-off.

Buffer strips

Buffer strips between crop and watercourse, ideally established by grass seeding or natural regeneration, during the year before the oilseed rape or winter bean crop should be at least 6m wide. And up to 20m wide where the field has a slope above 12°. In the trials, well-established buffers reduced run-off significantly - both in terms of pesticide concentration and total volume lost in run-off.

Cultivations

In terms of cultivations, at Cockle Park min-till reduced both maximum concentration and total amount of herbicide lost when compared with total inversion ploughing.

Careful use of buffers and cultivation choice can help reduce the risk of herbicide loss. Other factors can affect losses such as soil type and whether tramlines follow slopes or contours. But, the key factor remains the weather. Well-established buffer strips can do much to reduce loss to watercourses. However, very heavy rain can overwhelm such strips if applications are made on waterlogged or frozen soils.

The basic advice, developed by The Voluntary Initiative (VI) some years ago, remains sound.

Do not treat when:

• Heavy rain is expected within 48 hours
• Drains are running
• Ground is waterlogged, frozen or snow-covered
• Ground is dry and cracked

In addition, attention to detail in the field can also help minimise the risk of losses. This includes care with fieldwork and layout to minimise overlaps and short working. With the increasing uptake of precision technology and the ability to shut off specific nozzles in short workings, this becomes increasingly achievable. Similarly, GPS mapping can ensure sprayer booms are kept well away from watercourses.

Water Protection Advice Sheets - WPAS

Best practice on water protection is being developed constantly. For the key pesticides that are detected with some frequency in raw water, pesticide manufacturers and the VI have compiled Water Protection Advice Sheets (WPAS). These give guidance on how pesticides can reach water both from filling areas and field application. They also provide, in an easy to understand format, ways to assess the likely risk to water sources of planned treatments.

The latest WPASs deal with the spring applied herbicide bentazone and the slug pellet active, metaldehyde. These, along with other updated WPASs, are available from the VI website - http://www.voluntaryinitiative.org.uk/