Plant growth regulators trials

By Damian Jones, ICC

The ICC has been trialling the use of Plant Growth Regulator (PGR) for many years. Results have been mixed, but their use on barley appears to be promising with yield increases in 3 out of 4 seasons, using the PGR trinexapac-ethyl (Moddus Evo or Marvel) despite little effect on crop height or lodging. Other crops have had mixed results – wheat has seen variable response to crop height control and little yield response, minimal height response in canola with no change in yield and no response at all in fabas.

PGR Overview Plant Growth Regulator is a term that describes many agricultural and horticultural chemicals that influence plant growth and development. This influence can be positive, eg larger fruit or more pasture growth, and negative eg shorter stems or smaller plant canopies. Most of the broadacre use of PGRs is to have a negative influence on plant growth, ie they are applied with the intention of producing a smaller plant that is resistant to lodging or reduce excessive growth in the crop. There are 4 broad groups of PGRs in use in Australian crops. NOTE: Not all products are registered for use on all crop types, and some products are registered for use but not as PGRs which may have different rates and timings from that on the label.

i. Ethephon eg Ethrel®

ii. Onium types eg Cycocel®, Chlormequat®, Pix®

iii. Triazoles eg propiconazole, tebuconazole, paclobutrazol

iv. Trinexapac-ethyl eg Moddus Evo®, Marvel®

These PGRs act by reducing plant cell expansion, resulting in, among other things, shorter and possibly thicker stems. If the stems are stronger and shorter, then the crop is less likely to lodge.

The majority of the PGRs (groups ii to iv) reduce crop height by reducing the effect of the plant hormone gibberellin. These are applied at early stem elongation (Z30-32). Ethephon is applied from flag leaf emerging (Z37) to booting (Z45) and reduces stem elongation through the increase in concentration of ethylene gas in the expanding cells.

Other benefits claimed by the producers of various products include;

  1. better root development that allows for increased
    root anchorage
  2. better root development providing greater opportunity for water and nutrient scavenging
  3. may offer improved grain quality
  4. reduction in shedding in barley
  5. increased Harvest Index (the ratio between grain and total dry matter)
  6. faster harvest speeds and reduced stress at harvest.

An alternative to the chemical PGRs is grazing. Demonstrated in the Grain and Graze project on a number of sites was the effect grazing had on the crops where the grazed treatments/crops were shorter than the nongrazed and were less prone to lodging.

Barley Results

The barley trial examined the use of trinexapac-ethyl (Moddus Evo or Marvel), plus high rates of triazole fungicides. To date, our PGR work has focussed on Commander, but in 2016, Commander and LaTrobe were sown.

The PGR treatments were:

  1. Moddus Evo at 400 ml/ha at Z32 (second node)
  2. Moddus Evo at 400 ml/ha at Z32 and 200 ml/ha at Z39 (full flag emergence)
  3. Prosaro at a high rate at Z32
  4. Tebuconazole at a high rate at Z32

Plots treated with Moddus Evo became evident with reduced plant height as stem elongation progressed. No height effect was noted with the triazole fungicides. At harvest, most plots were lodged to varying degrees. No treatment stood out as improving lodging resistance as the rain event in mid-September seemed to overcome any benefit of a shorter plant produced by the Moddus Evo (20 cm in Commander and 10 cm in LaTrobe).

The trial was harvested on November 28th.

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What does it mean?

Commander again responded with increased yield to the use of Moddus Evo, as well as the high rates of triazole fungicides, although this yield response is more likely to be as a result of the fungicide action that any growth regulator effect (see barley fungicide trial report). Commander also saw an increase in screenings and reduction in retention, although it would have had no impact on delivery standards. However LaTrobe yield and grain quality did not respond to the PGRs in any form apart from slightly improved retention which would not have impacted on delivery grade.

Canola Results

There are no PGRs registered for use on canola, although there are products used overseas that may be registered for use in Australia in the future. PGRs are used in Europe to reduce the depth of the canopy so light can reach the lower pods and aid grain fill. Compared to European conditions, Australian crops ripen under much higher sunlight levels that can penetrate through the canopy.

With little result in the last two seasons, the trial was repeated again looking at alternative timings for the application of the PGRs and a new triazole fungicide paclobutrazol not tested last season.

The PGR treatments were:

  1. Chloremequat + Tebuconazole (pgr + triazole)
  2. Prosaro (triazole fungicide)
  3. Reward (mepiquat)
  4. Moddus (trinexapac-ethyl)
  5. Paclobutrazol (triazole fungicide)

GT50 canola was sown April 18th. The PGRs were applied at either green buds (G) visible on July 14th or the beginning of flowering (Y) on August 4th.

Timing of application of the treatments is not definitive. While searching papers for suitable treatments to include in this trial, successful results were achieved with the application of the PGRs at around buds visible through to the flower buds starting to yellow.

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The paclobutrazol applied at green bud did an extremely effective job in reducing the overall plant height, but did so by shortening the height to first pod. Teb + Chlormequat and Moddus both at green bud also reduced height to first pod. Any green bud applications did not affect the depth of podding and this was only influenced by paclobutrazol and Teb + Chlormequat at yellow bud.

Yield and oil content was reduced by the paclobutrazol applied at green bud. No other treatment had any effect on yield and oil. Lodging was reduced slightly in the Teb + Chlormequat Y, Paclo Y and Moddus G treatments, with the Paclo G treatment not lodging at all.

What does this mean?

Paclobutrazol is an unregistered product that can reduce plant height and lodging in canola, but at rates used, reduced yield and oil content. Further work needs to be done on rates to see if the PGR effect can be achieved without impacting on yield and oil.

Faba Results

Once again, there are no registered PGRs for faba beans. Previous work by the ICC on the use of PGRs on fabas showed no response to any of the PGRs used so far. Timing this season was brought forward well prior to flowering in an attempt to curtail growth with an “Early” application in June.

The timing of the treatments was:

  1. One application “Early” on June 29th.
  2. One application at “1st Flower” on August 11th.
  3. Two applications, one at “Early” and the other at “1st Flower”.

The PGR treatments were:

  1. Tebuconazole + Chloremequat (triazole + pgr)
  2. Paclobutrazol (triazole fungicide)
  3. Reward (mepiquat pgr)
  4. Moddus (trinexapac-ethyl pgr)

Post application, the PGRs appeared to be having an effect, but the rain event in mid-September saw all plots collapse to the ground. Subsequently the ends of the plants began to grow vertically again. Attempts were made to measure plant height prior to harvest, but due to the intertwined stems, accurate measurement was difficult without destroying the plants or losing pods. Of the measurements collected, plant height was variable between the treatment replicates, but the paclobutrazol appears to have shortened the fabas.

The trial was harvested on December 12th. No treatment was significantly better or worse than the control even though the paclobutrazol “1st flower” treatment was over a 1 t/ha better simply due to the variability of the data.

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What does it mean?

If using a PGR to prevent lodging was the aim, then all treatments failed. However the trial identified a potential PGR but further work needs to be done with rates and timing.

Wheat Results

The Wheat PGR trial was conducted in conjunction with the Wheat Fungicide Trial. One true PGR treatment was applied (Moddus Evo at 400 ml/ha) to Scout wheat at 1st node along with Prosaro fungicide at a higher than label rate applied at 2nd node. The trial was harvested on December 7th.

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No treatment yielded significantly different to the control. Similarly, no treatment affected height, although the Prosaro treatment is significantly higher than the Moddus treated plants. If the Prosaro treatment is included, lodging differences between the treatments was not significant.

If the Prosaro treatment is excluded (from the data, it did not act as a PGR), then the Moddus did reduce plant height (statistically speaking, not so much from a practical sense) and lodging.

What does it mean?

Like many of our wheat PGR trials, there did not appear to be any benefit from applying a PGR to Scout. Yield was not improved and plant height reduction was minimal. A reduction in lodging may have made harvesting easier. PGR work in wheat is also complicated by the various genes that control plant height and the effect the PGRs have on their expression. Our earlier work in wheat used H45, Chara and Derrimut, which while being a diverse group, all have the same genes controlling plant height, while the Durum wheats respond to PGR applications. We do not have the genetic data on the newer varieties and so results may vary depending on their genetics controlling plant height.

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The stubble project – maintaining profitable farming systems in Victoria and Tasmania with retained stubble (project number BWD00024) is funded by the GRDC.

A printer-friendly version of this research report is available here.

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About BCG

Birchip Cropping Group Inc. (BCG) is a not-for-profit agricultural research and extension organisation led by farmers in the Victorian Wimmera and Mallee.
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