Advice on using PGRs to manage roots and tillers

A difficult autumn means that plant growth regulators (PGRs) are going to be a vital tool for many growers this spring, but using them to best effect requires tactical application...

More commonly used to reduce the risk of lodging in lush crops, plant growth regulators are also a vital tool to aid root growth and manage tillering in cereal crops.

This spring, where many crops are struggling after a wet winter, is a good example of when growers will benefit from the correct and tactical use of these products.


Dick Neale, technical manager at Hutchinsons highlights that wheat crops are all over the place this year. “Any crops drilled through September and early October can probably be treated as normal in terms of their PGR programme, with a focus on lodging reduction.” However, later-drilled crops will benefit from a different approach.

“A lot of people talk about using PGRs to create more tillers, but you won’t create more. Tillers are linked with leaf production and this is linked with thermal time,” he explains. “So if crops are not drilled until November, effectively emerging in December, they have less thermal time to produce leaves and tillers.”

So, what can growers do with PGRS? Though no amount of growth regulator will increase the number of tillers on a plant, they can be used in conjunction with early nitrogen fertiliser applications as a way of maintaining more tillers though to harvest.

“By suppressing apical dominance, PGRs can balance tiller growth,” says Mr Neale. Also, if plants have tiller buds that are ready to burst, PGRs can be used to encourage their growth. “But only if the tiller bud is actually there,” he adds.

The most effective way to do this is to balance the tillers by suppressing apical dominance and creating more root growth, which PGRs can be used to do when applied early (a bit before growth stage 30). However, many PGRs can’t be used before growth stage 30, he advises. “So check approvals on the label.”


Early PGR for tillering

He suggests first using prohexadione straight or formulated with trinexapac-ethyl. Straight trinexapac-ethyl can also be used, but not before growth stage 30. “These actives both come from the same chemical group and work in a similar way.

Straight trinexapac-ethyl should be applied at no more than 100ml/ha to achieve really good manipulation of the tiller population, but this won’t regulate the plant’s stem extension. At the same time, the plants need a stiff dose of nitrogen to make the tillers grow, push on and balance out.”

Mr Neale suggests that he personally would not use chlormequat for the first PGR tiller manipulation application. “I find it too slow in action and the product degrades too quickly.”


Second-stage PGR

Moving on to second-stage application of PGRs, growers should be looking more at growth regulation of stem growth.
“Growers will need to be careful this year, as when late-drilled wheat wakes up, it’s going to go for it,” warns Mr Neale. “It’s highly likely that leaf three could arrive at growth stage 31 and not 32, so growers will need to carefully identify the leaf emerging at growth stage 31.

The timing between the manipulative PGRs could be just 14 days this year,” he says.

“Using a mixture at growth stage 31 will make sure the plants have good stem strength without over shortening them. I would use prohexadione, trinexapac-ethyl, or the mixture with up to one litre/ha of chlormequat,” he explains. Rates need to match individual crops, but a gentle approach is best.

Mr Neale expects that this year, this will be all the PGR that growers need. “Using these applications will mean growers haven’t over-done it and the PGRs will regulate the plant as intended rather than shorten it.”


Need for follow-up PGR?

However, he advises keeping a 2-chloroethyl phosphonic acid-based product in the back pocket. “We can’t be sure what spring growth will do next.”

If a dose of late-season PGR is needed, the rates will need to fit the season. “If we still have moisture in the soil and the weather is nice and warm, with long growing days, late crops could romp away.”

However, whatever the spring weather throws up, late-drilled crops are going to have a smaller root plate, says Mr Neale.

“The biggest risk this year will be root lodging and not stem lodging, as soils are already in poor structural condition and could just give way around the supporting roots.” He warns late plant growth in wet soil could create enough leverage to drive root lodging.

This is where providing the stem with strength will be essential, which is why just a gentle application of PGRs is all that Mr Neale advises this season.

“Don’t wait and see and then be heavy-handed,” he warns. “Plant growth regulators are exactly that-straw shortening is not the primary objective.”

He also advises looking at bio-stimulants to stimulate the survival of tillers and roots. “Mix the initial PGR with a phosphite product.

Take care not to shock the plants this year,” he says. “Think about having enough nutrition under the plant to be able to maintain and manage them at the same time.”


A guide to the different PGR products

Plant growth regulators (PGRs) target a plant’s hormonal system and can be used to regulate the development of the plant.
There are a number of different chemical groups that affect plants in different ways and growers always need to check the label before using each product.

Chlormequat is effective from about 8C in the crop, says Dick Neale. “Growers do need to check the label as there have been significant changes and some can’t be used until growth stage 31 and some others not beyond 31, while others might not cut off until growth stage 39.

“Chlormequat is slow to react in the plant, essentially putting the brakes on slowly, but once the brakes come off, they completely fail, leading to bounce back,” he explains.

Prohexadione can come straight or as a mixture. “These can function in colder conditions than chlormequat and are fast-acting, but much slower to degrade, leading to less bounce back.”

Trinexapac-ethyl and prohexadione help create a thick cell wall and so plants get a denser, thicker stem. These are also effective down to 5-6C in the crop.

Chloroethylphosphonic acid is the active of both Terpal and Cerone, but Terpal is also mixed with mepiquat chloride, meaning growers need to be careful with rates.

“I would not suggest using more than O.4 litre/ha of Cerone, which is equivalent to 1 litre/ha of Terpal.

“It impacts the upper stem growth and the timing window is narrow – between growth stages 39 and 45. So particularly in winter barley, growers need to be careful not to wait too long and miss the latest growth stage.”

Dick Neale's approach in later-drilled wheat crops

Purpose Products
For early season tiller management Mix of straight prohexadione or formulated with trinexapac-ethyl
For stem strength Prohexadione, trinexapac-ethyl, or a mixture with up to one litre/ha of chlormequat
Optional late season application to tackle an increased risk of root lodging if there is rapid late plant growth in wet soil 2-chloroethylphosphonic acid

What about barley?

For barley, Dick Neale suggests doing mostly the same as with wheat, but watch out for growth bounce from some products.

“At growth stage 30, do the same, but then at 31, higher doses of prohexadione or trinexapac-ethyl, but no 3C or Cycocel.

“The reason being that barley always bounces back from Cycocel and it could induce more lodging using chlormequat.”

Mr Neale would then always finish winter barley at growth stage 39 with a 2-chloroethylphosphonic acid-based product.

“At this stage, barley is only at 50% of its final height, so if there is a lot of late-season growth, you could get caught out.”


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