Farmers are at risk of losing the post-emergence herbicide bentazone – which is frequently used in pulses – because too much of the chemical is being detected in ground- and surface water.

After four decades of use, bentazone is now the herbicide that most commonly exceeds legal limits in groundwater, with contaminated surface water levels also on the rise, according to an official UK survey.

With the chemical up for reapproval in 2025, there is a serious risk that its use may be restricted or lost altogether, unless extreme care is taken to protect water supplies.

See also: Guide to growing a successful spring bean break crop

About bentazone

Commonly used on beans, peas, linseed and potatoes, bentazone is a highly soluble compound in water and is very mobile in soil.

The chemistry is absorbed through the leaves of target plants, disrupting photosynthesis, and causes a reduction in carbohydrate reserves.

As such, bentazone has been detected in both ground- and surface water for many years and, despite the agricultural industry implementing a stewardship programme in 2014 to mitigate against this, records show no serious decline in detected levels.

Bentazone is one of the few post-emergence herbicides for beans and the only post-emergence available for some challenging weeds in pea crops, such as black nightshade.

With growers reintroducing pulses into their rotation, some after a long break, and others growing it for the very first time, the loss of the herbicide could seriously affect the future of bean and pea production.

With this is mind, Paul Goddard, stewardship and value chain manager at BASF, offers his five top tips on how best to reduce the risk of bentazone contaminating ground- and surface water. 

1. Ensure the field is suitable

Ensure the field is suitable for application of bentazone before planting crops, as the herbicide should not be used in high-risk areas with shallow, light soil types.

Do not use bentazone on:

• Soils on chalk/limestone that are shallow (less than 30-35cm) and stony (more than 10% of surface area)
• Shallow (under 30-35cm) soils on sandstone
• Soils with shallow groundwater (less than 1m below surface)
• Soils with low organic carbon (less than 1%).

“Shallow, stony soils on chalk, limestone, or sandstone allow water to move more readily, taking the chemistry with it. Likewise, soil with low levels of organic matter is a challenge, as there is less to bind the soil and chemistry together,” Mr Goddard explains.

2. Avoid high-risk areas

Avoiding high-risk areas is another important consideration.

The maximum recommended dose for bentazone is 1,000g/ha. Applications should only take place from April onwards and not be made in the autumn/winter.

Mr Goddard says farmers can download Magic Maps from Natural England or Check Zones from the Gov.uk website, which provide geographic information about the natural environment, including watercourses, high-risk areas, safeguard zones and source protection zones for bentazone.

“As bentazone can enter watercourses through both ground- and surface water, there are multiple chances for leaching, so we must do everything we can culturally and chemically to reduce the levels,” he says.

3. Consider the weather

Farming faces many pressures, with the weather a major factor. It is important to avoid spraying crops on wet days or when there is significant water sat in the field.

Do not apply bentazone if heavy rainfall is likely within 48 hours. Avoid application when drains are flowing or likely to flow within seven days.

“Fields with a shallow water table are also to be avoided. The chemistry has high solubility and mobility, so it will move into the watercourse or groundwater,” Mr Goddard adds.

4. No-spray zones

Creating a 6m buffer zone or a 5m no-spray zone around field edges can help in high-risk areas or those adjacent to watercourses.

Growers can also follow basic water protection advice by avoiding point pollution sources such as farmyard runoff.

Particular care should be taken when filling and cleaning the sprayer.

“Most farms now have a dedicated area of concrete that can easily be cleaned when using the granular formulation,” says Mr Goddard.

“For those using liquid, we have been working with industry partners to develop an EasyConnect closed-transfer system that will soon be available,” he adds.

5. Keep chemistry on the surface

Mr Goddard concludes that where there is good supply of light and oxygen, bentazone breaks down relatively quickly, so keeping it on the surface for as long as possible limits the chance of transfer into water systems, while also increasing efficacy of weed control

If soils are dry, cracked or saturated, the herbicide should not be applied.