Herbicide resistance in ryegrass is an increasing problem in arable rotations and growers can’t afford to overlook the threat this spring, experts advise.

Results from a new Rothamsted Research study [see panel] show that 70% of Italian ryegrass samples, “semi-randomly” collected from 50 farms, were resistant to diclofop (as in Tigress Ultra) and/or tralkoxydim, while 36% were resistant to fluazifop (as in Fusilade). Cycloxydim (eg, Laser) and pinoxaden (Axial) were significantly less affected, with about 20% of samples showing resistance to either active, says Rothamsted’s Stephen Moss.

While likely to overestimate actual field resistance levels, the findings are an early warning for growers to manage herbicide performance carefully. “In general, if you get resistance to diclofop, you virtually always get resistance to tralkoxydim, or vice versa. So, you’re highly unlikely to get around resistance by changing from one to the other,” Dr Moss says.

But there is a strong difference between fops, dims and dens, all of which have the same ACCase inhibitor mode of action. Fops and dims are affected by the most common enhanced metabolic resistance (EMR), and at present this has not been found to affect pinoxaden. “Pinoxaden used in cereals will control most populations that are resistant to fops and dims.”

But Dr Moss warns that the less common, but potentially more problematic, target site resistance has been found to affect all active ingredients, including pinoxaden and cycloxydim, so growers should not rely too heavily on those forms of chemistry alone.

Syngenta’s Jason Tatnell agrees. “With Axial we are seeing excellent control in the field in most cases, but you can’t ignore there is a selection pressure and resistance can develop if you don’t act appropriately.”

Spring advice

After a mild start to the year, prompting early weed and crop growth, Dr Moss advises growers to spray ryegrass as soon as conditions allow, particularly where enhanced metabolic resistance is present and fop or dim chemistry is planned. “The herbicide has a much better chance of working while the weed is small.”

He cautions against overreliance on any one form of chemistry, but suggests Axial is a good choice in cereals where EMR ryegrass is present, while in oilseed rape Laser or Aramo (tepraloxydim) are likely to be effective.

Many growers traditionally control ryegrass and oats together, so spraying early could mean you have to return later in the spring to take out late-emerging wild oats, Mr Tatnell adds.

Traditionally, ryegrass is a problem where grass was, or still is, in the rotation, but the Rothamsted study found this is not exclusively so. “We’re not quite sure where it is coming from,” Dr Moss says. “On some farms I think it has been at a low level for some time, but as resistance develops, you start to notice it more.

“So even if you don’t think you’ve got ryegrass, you’ve got to look out for it and make sure it is picked up early. It’s more competitive than blackgrass and populations can develop very fast. You don’t need many ryegrass plants to cause a problem about 5/sq m will give a 5% yield loss, compared with 12 blackgrass plants.”

Ryegrass resistance findings

• 55 samples collected from 50 farms across England during 2006-07
  
• 62% resistant to diclofop
  
• 60% resistant to tralkoxydim
  
• 70% resistant to diclofop and/or tralkoxydim
  
• 18% resistant to pinoxaden
  
• 36% resistant to fluazifop
  
• 20% resistant to cycloxydim
  
• Enhanced metabolic resistance affected fops and dims, but not den chemistry
  
• The Rothamsted study was funded by Syngenta and HGCA and conducted by Reading University PhD student Rocio Alarcon-Reverte.

Ryegrass control

• Control ryegrass as soon as possible
  
• Get a resistance test done
  
• Choose an appropriate herbicide
  
• Use the recommended rate
  
• Optimise application techniques