Rediscovering phenoxies – a solution to minimizing resistance
Whilst weed resistance to acetolactate synthase (ALS) chemistry continues to grow amongst broad-leaved weeds in particular Chickweed, Mayweed and Poppy, in varying degrees according to geographical location, growers have another weapon in the armory – phenoxies, says Nufarm’s Dan Macdonald.
“It’s important not to let this resistance situation drift. It’s time we looked at supporting chemistry to SUs, comments independent consultant, Keith Norman. “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.
“However, 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.”
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 that mimic the effect of natural plant hormones called auxins. They are formulated from acids, normally as salts and sometimes, as esters for cost effective broad spectrum weed control in cereals.
The phenoxy group includes 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.
Phenoxies have a significant role to play going forward in managing resistance because they have the lowest risk of fostering resistance development.
Introducing phenoxies to postpone resistance will lengthen the time before resistant prone herbicides become truly non-renewable.
Mode of action
Phenoxy herbicides have the lowest risk of fostering resistance development due to their mode 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. So far, the full mode of IAA has yet to be understood in detail, however researchers have confirmed that IAA binds to auxin binding proteins located in the cell membrane, endoplasmic reticulum, the cell nucleus and cytoplasm.
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 and cell division and nutrient transportation within the plant which has no mechanism to control the concentrations of these ‘imposters’.
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.
Nufarm’s Dan Macdonald
- Phenoxies are often my product of choice; they’re really effective on a range of weeds particularly some of those difficult to control with SUs including Speedwell, says independent agronomist, Ben Burrows, of Crop Management Partners.
- I’m really comfortable using phenoxies which have in some cases been forgotten in the UK, in Canada where I’ve worked as an agronomist, this older chemistry is in widespread use because it overcomes growers concerns regarding SU residue carry over.
- I recommend phenoxies, particularly in spring barley and also for Clearfield OSR volunteer control. Care should be taken, being mindful of latest growth stages for use together with the tank mix and the phenoxy’s compatibility.
- They also have an important role to play in an anti-resistance strategy. It’s vital you don’t wait until you witness resistance and carry on regardless; I believe it’s very important to rotate different modes of action – use products in sequence.
- 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
Controlling Clearfield volunteers
Phenoxy herbicides are providing a solution to the increasing issue of Clearfield volunteers.
Clearfield oilseed rape hybrids continues to expand; since 2014, the area has doubled each year to over 10% of the UK’s oilseed rape area amounting to an approximate 60,000 ha.
Whilst Clearfield hybrids offer various benefits including the ability to control broadleaf weeds without having to rely on pre-emergence herbicides, growers must contend with the challenge of controlling volunteers.
Traditionally, volunteer OSR is controlled using SU’s such as metsulfuron. Clearfield varieties have an ALS tolerance built in and therefore group B chemistry will have no effect on the volunteers. Nufarm has a range of non-group B chemistry, including phenoxies that will control Clearfield volunteers in a number of crops.
Safe application timing
Cereal crops can be sensitive to phenoxy application at several growth stages throughout the season. These sensitive timings usually coincide with periods of high growth or reproductive activity.
Six pointers to getting it right
- The critical period is the ‘double ridge’, when the shoot apex changes from leaf production to spikelet formation.
- Safety period occurs 1 leaf after the ‘double ridge’stage.
- Spring cereals: correct timing can be determined by leaf number.
- Winter varieties: spikelet initiation occurs later; the plant has more leaves by the time the crop has reached the correct time – leaf
sheath erect GS30. That stage can only be determined exactly by dissection.
- GS30 – the tip of the developing ear is 1 cm or more from the base of the stem where the lowest leaves attach to the shoot apex.
- GS31 – the first node can be seen 1 cm or more above the base of the shoot (with clear internode space below it) and the internode above it is less than 2 cm.
- Spikelet initiation is delayed in dry seasons.
- Ear distortion doesn’t always translate through to a yield reduction and it can sometimes be mistaken for frost damage.
Phenoxy cereal application timing