What’s your strategy for stubble management?

Effective stubble management is an essential part of controlling difficult grass-weeds. Cultivation and Roundup (glyphosate) are the two best tools at farmers’ disposal to keep control of weeds.

But no-till farming, product stewardship and a better under-standing of the weed seedbank all influence stubble management decisions.

Cultivation strategy

Establishment method has the biggest impact on how farmers manage stubbles. In conventional plough or min-till systems, there are three opportunities to control weeds, Bayer’s Roger Bradbury explains.

“The basic strategy is to spray off weeds and volunteers in the first flush of growth after harvest and then cultivate, followed by a second Roundup application before drilling. This approach fits with WRAG resistance management guidelines (see box), but many farmers are moving away from deeper cultivations because it mixes weed seed within the soil profile.”

Mr Bradbury thinks farmers adopting no-till or only doing very shallow cultivation need to be mindful of the dangers of glyphosate resistance, a view shared by Agrovista’s Chris Martin.

“Regen agriculture is a positive development, but in the short-term it might put more pressure on glyphosate. Many farmers are cultivating less, but cultivation is a good alternative to chemical weed control. Likewise, cover crops generally need glyphosate for destruction. Diversity in cropping and establishment dates will help farmers get the benefits of regen-ag without over-reliance on chemistry.”

Resistance management guidelines

The Weed Resistance Action Group (WRAG) guidelines, recommend a maximum of two glyphosate applications after harvest before drilling the next crop. Between applications, use cultivation to eliminate survivors. When there is no cultivation, for example no-till systems, only one application is recommended.

https://ahdb.org.uk/knowledge-library/the-weed-resistance-action-group-wrag

However, Mr Martin says the danger is wider than regen-ag. “Using multiple stale seedbeds with fairly low rates is a problem. We need to take a step back and develop a system less dependent on glyphosate. We’ve seen with other actives that there is a tendency to overuse the chemistry that works until there are problems.”

Valuing control

Achieving good control and preserving long-term efficacy are equally important when using glyphosate in stubbles. Agronomist Patrick Stephenson acknowledges glyphosate stewardship is not farmers’ highest priority, but general weed control strategies are managing the danger.

“Any time you use a product there is risk, but I don’t think it is a significant problem currently.”

In many countries, resistance was a result of GM crops which encouraged as many as five applications a season without any other form of control, he notes. In the UK, cultural controls and other actives are widely used in weed control programmes which helps mitigate the risks.

Attitudes towards glyphosate are starting to change though, because of recent supply issues. “Farmers are asking where they can get the most benefit and paying more attention to targeting rates to the problem.”

But this cuts both ways, Mr Martin says. “Current price pressures in farming are a worry; there is a temptation to cut rates or use cheaper products. But since the withdrawal of tallow-amine, cheap formulations are not working as well.”

Maintaining appropriate rates is the single most important practical step for good performance.

“Get the right dose for the weed and growth stage,” says Mr Bradbury. “Annual grasses typically require a minimum of 540 g a.i./ha for seedlings up to 2-3 leaves, to a minimum of 720 g a.i./ha when tillering.”

A second resistance management consideration is to focus glyphosate use at the most valuable timings across the cropping cycle. “The most important timings are pre-drilling and oilseed rape harvest management,” says Mr Stephenson.

“Rates have to be high to ensure proper desiccation in rape; pre-drilling, there is some flexibility depending on the weed spectrum and cultivation strategy.”

He says that unlike other insecticide and fungicide resistance, farmers have a big influence on the problems on their farm. Machine hygiene is important, but aside from that, any resistance farmers see, is a result of their management. With this in mind, proactive resistance management can have long-term benefits.

Key messages

• Avoid over-reliance on chemistry

• Support with cultivations, diverse cropping and varied establishment dates

• Use well-timed, quality glyphosate products, at an effective rate for the specific task