Delivering best practice for successful weed control with propyzamide while preventing it from entering water courses and reservoirs remains a difficult balance.

What’s good for weed control frequently increases the risk of water contamination.

It’s a challenging situation with the active ingredient in the oilseed rape and bean herbicide easily moving into water after rainfall, says Kim Hemmings, a catchment adviser for Anglian Water.

See also: Five steps to protect water from the herbicide bentazone

“It has a long half-life, making it persistent in water and requiring biological processes to break it down.”

A particular hotspot is in Northamptonshire where higher than usual levels of propyzamide have been detected in three reservoirs, Pitsford, Hollowell and Ravensthorpe, which are fed naturally with water from the surrounding land.

Detections were high in March 2024 following high rainfall over the previous winter contributing to soil movement off fields, carrying the herbicide into the reservoirs, Kim says.

“That means complicated treatment processes are required to remove the herbicide, which affects water quality and potentially puts the supply of water at risk,” she says.

In response to this year’s problems, Anglian Water has worked with around 20 local growers, plus agronomists in Northamptonshire and propyzamide manufacturer Corteva on strategies to reduce risks to water.

Rotation changes

Initial engagement led to farmers reviewing crop rotations to reduce use of propyzamide in the current season, with cropping changed to winter or spring oats, winter wheat or legume fallow under the Sustainable Farming Incentive (SFI).

That’s worked to some extent, Kim says, with a reduction in detection levels, but not quite as effectively as hoped.

“We anticipated by removing high risk crops from high-risk land for a year we would see the reservoirs get back to normal levels, but we haven’t seen that.”

In the long-term, rotational changes are challenging, with profitable alternative break crops in short supply. It also can lead to the unintended consequence of detections of other herbicides that need to be treated.

“We know farmers are in a difficult situation because weather is increasingly affecting operations and rotations,” Kim says.

SFI uncertainty

The uncertainly over the future of SFI options is also a problem.

As well as alternative whole field options, SFI could help with the use of larger buffer zones, which research at the Allerton Project by the Game and Wildlife Conservation Trust has shown to be effective at reducing soil erosion.

But taking 20m of land out of arable production has implications for farm businesses, especially without some form of compensation, Kim recognises.

While minimising risk by growing alternative crops is one way of tackling the issue, it’s not likely to be a long-term solution, particularly given potential costs.

Instead, ideally solutions can be found that enable farming and clean water objectives to align.

Such potential solutions are also hindered by the loss of SFI with another mitigation water companies and propyzamide manufacturer Corteva have seen some success with, in trials, companion crops (see “Companion crop trial reduced run-off risk”), also somewhat popularised by funding through a SFI option.

“We know that companion cropping is good for integrated pest management, but it also helps slow the movement of water or protect it from soil movement.”

He says most farmers have soil health as a focus, and they’re open to direct drilling and minimising soil movement, but inclement weather is challenging whatever they try to do.

“So I think keeping soil covered is going to be a key strategy.”

In oilseed rape that’s easier to achieve where propyzamide is sprayed post-emergence, giving the companions as well as the oilseed rape time to establish.

In winter beans where the product is used pre-emergence, it would probably mean establishing a catch crop of something like oats before drilling the beans into the oats.

Northants grower

It’s something Ian Matts, managing director of Brixworth Farming, has tested with some success in a dry year.

“If you have a catch crop with some roots down, it should hold onto soil particles a bit better giving more stability, but we haven’t tested enough to know how successful that would be in a more challenging year.”

There are no easy answers as Ian is only too aware of.

With only limited oilseed rape grown at Brixworth Farming because of the challenges to establish and grow the crop profitably, winter beans has become a key crop in the battle against long-term, high-pressure blackgrass.

“We’ve managed to improve blackgrass control through employing various methods, including the use of spring cropping.

“But winter cropping is still favourable on our heavy clay soils, which can be challenging to establish spring crops in.”

That leaves winter beans a key rotational option, with propyzamide a strong active with a different mode of action to use to control blackgrass.

“The reality of high blackgrass pressure and the limited herbicide options in beans is that if you grow the crop, you’re committed to using the product. It’s important we maintain the ability to use in future,” he stresses.

“But as last year highlighted it is a big challenge.”

The field where he suspects torrential rain in November three weeks after propyzamide application may have washed soil treated with propyzamide into the bordering water body had been in a legume fallow stewardship option the previous five years.

Drainage

Beans were established following cultivations, with propyzamide applied in a relatively dry period in the autumn.

“We’re trying to improve drainage in fields, which helps the crop and soil health, but from a water perspective makes it more challenging with increased water flow.

“We have 6m grassy margins to stop or reduce surface water run off, but if we get extremes like last year, it’s hard to plan and creates a challenge in understanding how to use a product that is so useful but also so potentially harmful.”

Agronomic factors that help grow a successful crop of oilseed rape or beans, unfortunately are also exactly what will help water carrying pesticides to infiltrate to drains or be washed off.

That includes ensuring there are no restrictions to rooting.

David Felce, a Cambridgeshire farmer and spray application specialist says: “You need well-structured soil for a good crop of beans or oilseed rape, especially on heavier soils.

“But creating vertical fissuring to help rooting also provides a route for chemicals be washed through the soil profile.”

The initial 48 hours following application is when propyzamide is most at risk of being washed through the soil, hence the label warning about avoid spraying in that period if heavy rain is forecast.

“In that period propyzamide is only weakly adsorbed onto soil particles, so any heavy rain falling will likely wash it down the soil profile, possibly into soil drains,” David explains.

“If you can leave it longer so it becomes absorbed that doesn’t mean it won’t ever go anywhere, but it is more strongly bound to the soil matrix.”

Soil erosion risk

Any factors that increase the risk of soil erosion, such as slope of field and cultivations, also need to be carefully considered.

Drilling across the slope can help reduce soil erosion, while minimising cultivations when creating seed-beds are mitigations growers could employ to increase water protection.

However, it might not always be the most successful or practical way to establish a crop.

Where minimal cultivations are not ideal for the crop, it’s important to reconnect soil layers, David says.

“Consolidate so you’re not creating a sponge to allow water to move straight through is important, but avoid capping or compaction,” he stresses.

“Creating seed-beds with a good crumb structure – so a good mixture of pore sizes – also reduces speed of water movement.”

Cover or companion cropping can also help stabilise vulnerable soils, he adds.

Spray application

When spraying propyzamide, David recommends using a coarse spray to reduce drift, and higher water volumes to cover the soil surface.

“Also as shown by research by Agrii Polska, using the right adjuvant can be hugely beneficial in holding the active, not only to improve water quality but also to increase herbicide performance and crop safety.”

Oilseed rape trials carried out by Stephen Moss Consulting for Anglian Water looked at the use of clethodim.

The trial suggested it could reduce the rate of subsequent propyzamide applications to 500g/ha rather than the label rate of 850g/ha without any detriment to blackgrass control.

That sequence could potentially reduce risk to water on fields with a high risk of leaching or soil erosion, although acetyl-CoA carboxylase (ACCase) resistance in blackgrass is a potential risk on clethodim effectiveness.

Corteva only recommends the use of 500g/ha of Kerb Flo where blackgrass is not present, however.

As soon as you have blackgrass growers should apply at least 750g of active/ha, rising to 850g of active/ha in bad infestations or where it is difficult to control, the firm’s field technical manager for OSR herbicides Joe Martin says.

“The Anglian Water trials were on a site with low levels of blackgrass,” Joe claims. “Our data has shown fairly consistently that the 500g/ha rate doesn’t give good enough control.”

To help guide when to use the product, Corteva has developed an app, which currently uses soil temperature as the main decision-based factor.

Lower soil temperatures are important for the product to be effective, Joe explains.

“If it’s too warm the microbial activity will degrade propyzamide quickly potentially reducing efficacy.”

It’s another conundrum highlighting the difficulty in balancing product effectiveness with reducing risk – quicker degradation, in theory, reduces the risk of the product ending up in water.