A UK ban on glyphosate, a herbicide used by many potato growers, could lead to an increase in weed abundance and a decrease in crop, according to a new modelling study.
Rothamsted’s Dr Helen Metcalfe led the study. She said: “Glyphosate, the most widely used herbicide, is linked with environmental harm and possible human health issues, but it’s use is central to no-till farming approaches. Public pressure is now building for it to be replaced in agricultural systems. We wanted to find out what the implications of a ban might be.”
Approval for glyphosate use in the EU expired in December 2023, following an extension to the renewal assessment process in 2022. In the UK, the current expiry date is set to be December 2025, following a three-year extension as the UK’s post-Brexit pesticides regulatory regime was developed.
The treatment is widely used in potato farming, but in regenerative systems focused on improving soil health in particular, glyphosate is important for weed control in no-till stubbles and the management of cover crops and leys. The environmental and health issues associated with glyphosate may trade-off against some of the benefits of moving to more sustainable systems that reduce tillage and integrate cover crops, the study found.
The study team modelled the impacts of discontinuing glyphosate use and replacing it with alternative control methods.
Crop rotations with more spring cereals or grass leys for weed control increased arable plant diversity compared to simpler rotations using glyphosate but produced less food. An increase in spring cropping also increased environmental risks associated with herbicides owing to the high toxicity and bio-availability of chemicals typically applied in these crops. Stale seedbed techniques such as delayed drilling and opting for ploughing instead of minimum tillage had varying effects on weed abundance, food production and profitability. Ploughing was the most effective alternative to glyphosate for long-term weed control while maintaining production and profit.
“Integrated Weed Management with more use of cultural control methods offers the potential to reduce chemical use but is sensitive to seasonal variability and can also have some negative environmental and economic impacts,” said Helen. “The uncertainty associated with the non-chemical approaches we tested supports the view that adoption of IWM requires multiple options adapted to the local environment. This will however require careful consideration and a strong founding in the principles of weed ecology and biology.”
In the study, introducing more grass leys or spring cereals into the crop rotation inevitably led to a decrease in food production owing to the replacement of high-yielding crops with less productive alternatives. Whilst in some cases the additional benefits of these diversified systems may outweigh the loss of food production, there may also be additional avenues to increase productivity in these systems. For example, the addition of grazing animals to grass leys will not only provide additional revenue sources but may also improve soil structure and nutrient cycling.
The study team expected to observe weed control benefits of adjusting the crop rotation, yet these did not consistently materialise in the simulations. It was anticipated that the introduction of crops with very different management techniques to typical winter cereals would have allowed weed abundance to fall. This is because the management operations required for these alternative crops will select a different weed community to that which is adapted to winter cereals.
“Although we saw increased environmental risks related to herbicides used in spring crops, the benefits of spring crops in improving biodiversity in general are clear,” said Helen. “For instance, overwinter stubbles from spring cropping provide conditions for the germination of crucial winter food sources for seed-eating birds. In addition, spring crops themselves can provide breeding habitats and food sources for farmland birds. It is therefore important to consider these trade-offs when implementing such a strategy on farm.”
Overall, the study team hope that the modelling exercise will encourage more growers to experiment with alternative weed control strategies. Helen said many are beginning to investigate how they can best control weeds with fewer herbicides.
“Simulation studies like this one can help to carefully assess any management changes, as it is not always possible to predict outcomes when so many variables – including the weather – are playing a key role,” she said.
