Increased problems with grassweed herbicide resistance and a lack of new active ingredients mean growers may need to reconsider crop rotations, according to speakers at a recent HGCA workshop near Bishop's Stortford, Hertfordshire.

ADAS's James Clarke said growers would struggle to reduce resistance risk through changing herbicide chemistry alone, and so must look to cultural factors such as rotation.

"We are seeing increasing cases of target-site resistance to Atlantis (iodosulfuron + mesosulfuron) where weeds are already resistant to fops and dims. With no new actives, more questions are being asked of existing products."

A typical rotation involving cereals into oilseed rape left a very short window for weed control and meant almost all control options were within the crop, putting pressure on sprays, he explained. But drilling different varieties later in the autumn or into the spring, for example, provided an excellent opportunity for weed control, either through chemical (eg, glyphosate) or mechanical means, he said.

Challenge

"There isn't a right or wrong answer, but you need to challenge some of the principles you're working to and ask, can I do better?"

Break crops such as spring barley, potatoes and sugar beet provided the best opportunities to reduce blackgrass control costs in following wheat compared with wheat after wheat - possibly by as much as 30-40%, he noted. By contrast, oilseed rape was often regarded as a "cleaning crop", but trials had found that subsequent blackgrass control could cost over 10% more than wheat after wheat.

Potential

Changes to crop choice, labour and machinery also offered much potential for improving farm profit, added Sebastian Graff-Baker, from Andersons. "Over half the farms in the UK are generating less than £100 of output per £100 of input, so I'm confident there's huge scope to improve profitability. This will be largely through choice of cropping and the labour and equipment you commit to to grow those crops."

Rotation and variety choice could also mitigate the effects of diseases such as take-all, noted ADAS head of agronomy, John Spink. "No varieties are resistant to take-all, but they do differ in their tolerance to the disease." Einstein, Gladiator and Ambrosia were among the more tolerant as a second wheat, while Robigus, Mascot and Claire were less so, he said.

Related Links:

• Cooler conditions ideal for osr grassweed control
• Label extension for grassweed herbicide