Resistance management: rediscovering phenoxies

Growers aware of growing resistance among a range of broad leaved weeds to ALS herbicides should consider phenoxies this season.

Independent consultant, Keith Norman

Growers aware of growing resistance among a range of broad leaved weeds to ALS herbicides should consider phenoxies this season, according to independent consultant, Keith Norman.

“With resistance to sulphonylureas appearing in weeds such as poppies and chickweed, it is important not to let this situation drift,” he says. “It’s time we looked at supporting chemistry to SUs. We need phenoxies, they are both good value and effective; they are flexible in terms of timing and they have been tried and tested for years.”

Nufarm’s Dan MacDonald continues: “Phenoxy chemistry is nothing new, in fact it has been commercially available for over 70 years and remains amongst the world’s most widely used herbicides. Phenoxies is a general term to describe a group of herbicides including MCPA, 2-4 D, CMPP-P, 2,4-DB, MCPB, 2-4 Dichlorprop-P, which are manufactured at Nufarm’s site near Bradford and other sites across Europe.

Nufarm’s Dan MacDonald

“Formulated from acids, normally as salts and sometimes as esters, phenoxy herbicides have the lowest risk of fostering resistance development due to their multiple sites of action. They mimic plant auxins – the plant growth regulator indol-3-acetic acid (IAA), which controls cell enlargement, division and development throughout the plant’s life cycle; it over stimulates plant cells causing abnormal plant growth and subsequent death.

“Following application, leaf absorption occurs within four to six hours and concentrates within the plant’s actively growing regions – the meristematic tissue within the stem and roots; the phenoxy interferes with the plant’s metabolism, protein synthesis, cell division and nutrient transportation within the plant which has no mechanism to control the concentrations of these ‘imposters’.”

He says phenoxies are well tolerated by cereal crops within a range of crop growth stages for winter and spring cereals, it’s possible to apply MCPA up to GS39 in wheat; they have the potential to kill almost all plants – simple perennial and creeping perennial weeds with just one or two foliar applications.

Mr Norman adds: “Since phenoxy herbicides are quite specific in the spectrum of weeds they control, it’s important for growers to double check what is covered by each of the options. Furthermore, since Clearfield oilseed rape varieties are securing a larger part of the market there is also a need for alternatives to SU to keep volunteer weed resistance in check,” he says. “We need non-SU options and phenoxies might fit this job.”

Best use

  • Weed must be growing. Avoid cold frosty conditions.
  • Phenoxies take about four to six hours rain free to get into the plant.
  • Check the species is actually susceptible.
  • Get the timing right – ideally young plants, but not very small to avoid droplet bounce
  • Make sure the crop is not under stress due to poor nutrition, drought or disease otherwise weed control will be reduced and there is the possibility of damage to the crop.

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