Glyphosate ban would lead to difficult trade-offs

NEW modelling shows that some crops will have reduced yields if widely used herbicides are withdrawn, and alternative approaches to weed control offer mixed outcomes.

A new modelling study suggests that a ban on glyphosate could increase weed abundance and decrease crop yields. Although the environmental risks associated with this herbicide would be eased, alternative approaches to weed control have mixed outcomes for the environment, food production and profitability, although some show potential benefits.

Weed communities with evolved resistance to non-glyphosate herbicides were not projected to be disproportionately affected by removing glyphosate despite the lack of alternative herbicidal control options.

“Our findings emphasize the need for careful consideration of trade-offs if a ban were to be enacted,” said Rothamsted’s Dr Helen Metcalfe, who led the study.

“Glyphosate, the most widely used herbicide, is linked with environmental harm and possibly human health issues, but its 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.”

Glyphosate is widely used in arable farming. However, in regenerative systems focused on improving soil health, it 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 benefits of moving to more sustainable systems that reduce tillage and integrate cover crops.

The study team modelled the impacts of discontinuing glyphosate use and replacing it with alternative weed control methods based on winter wheat arable systems typical in northwest Europe.

Crop rotations with more spring cereals or grass leys for weed control increased arable plant diversity. However, they produced less food compared to simpler rotations using glyphosate.

An increase in spring cropping also increases environmental risks associated with herbicides due to the high toxicity and bioavailability of chemicals typically applied to these crops.

Stale seedbed techniques, for example, 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 longterm 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 Dr Metcalfe.

“The uncertainty associated with the nonchemical 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 crop rotation inevitably led to decreased food production due to replacing highyielding crops with less productive alternatives. While the additional benefits of these diversified systems may outweigh the loss of food production in some cases, there may also be additional avenues to increase productivity.

For example, adding grazing animals to grass leys will provide additional revenue sources and improve soil structure and nutrient cycling.

The study team expected to observe the benefits of adjusting crop rotation for weed control. However, these did not consistently materialize in the simulations.

It was anticipated that introducing crops with very different management techniques from 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 from that 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 Dr Metcalfe.

“For instance, overwinter stubbles from spring cropping provide conditions for germinating crucial winter food sources for seed-eating birds. In addition, spring crops 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.”

The study team hopes the modelling exercise will encourage more farmers to experiment with alternative weed control strategies.

“Many farmers 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,” said Dr Metcalfe.