Plant growth regulators

Author: | Date: 31 Jul 2014

Take home messages

  • Crop responses to the use of plant growth regulators (PGRs) can be inconsistent.
  • In general, yield responses, if any, are produced by the reduction in lodging rather than as a direct effect of the PGRs.
  • In ICC trials however, yield increases that may be attributed directly to the use of PGRs have been measured in barley but not wheat.
  • Plant growth regulators must be applied at the correct crop growth stage according to product directions, which can be well before any lodging issues are apparent.

(n.b. Not all products are registered for use on all crop types, and some products are registered for use, but not as PGRs.  If used as a PGR they may have different rates and timings from that on the label).


Attempting to grow high yielding irrigated crops requires high levels of inputs; including water and fertiliser, which can promote large vegetative crops that increase the risk of lodging. Lodging can result in reduced yields and difficult harvest.  Plant growth regulators have been around for many years but results can be variable, even having negative effects on yield. The Irrigated Cropping Council (ICC) conducted trials in 2003 and 2004 that saw some reduction in lodging but little yield response. At the same time, trials on nitrogen management in cereals demonstrated that to achieve high yields, crops do not necessarily need heavy sowing rates and large amounts of nitrogen at sowing, with corresponding lush crop in early season prone to lodging.  This has seen many growers adopt a topdressing strategy that supplies the crop with N when it needs it; i.e. from stem elongation onwards. Less vegetation at stem elongation promotes stronger stems, which can support a crop yielding eight t/ha.

A trial conducted at the ICC trial block in 2012 which aimed to grow 10 t/ha of wheat and barley was deliberately sown heavy and fertilised early and sprayed with the plant growth regulator, Moddus Evo as lodging was likely to occur. The effect of the PGR was mixed; barley yields increased but wheat yields did not, despite the crops not actually lodging. A repeat trial sown in 2013 saw some lodging control and once again, a yield increase in barley.

Plant Growth Regulators

Plant growth regulator (PGR) is a term that describes many agricultural and horticultural chemicals that influence plant growth and development. This influence can be positive; e.g. larger fruit or more pasture growth, or negative; e.g. shorter stems or smaller plant canopies. This paper/presentation will focus on the PGRs that have a negative influence on plant growth; i.e. they are applied with the intention of producing a smaller plant that is resistant to lodging or they are applied with the intention of reducing excessive growth in irrigated broadacre crops. Currently, there are four broad groups of PGRs in use in Australian crops. The four broad groups of PGRs are as follows:

  1. Ethephon; e.g. Ethrel®.

ii.                    Onium types; e.g. Cycocel®, Pix® (n.b. Pix® is registered only for cotton).

iii.                  Triazoles; e.g. propiconazole (n.b. propiconazole is registered as a fungicide not a PGR)

  1. Trinexapac-ethyl; e.g. Moddus®, Moddus Evo (n.b. Moddus® is registered for ryegrass seed crops, poppies and sugar cane.  Moddus Evo is an enhanced dispersion concentrate of Moddus, it is not currently registered but has been submitted to the APVMA for registration in Australian cereals)

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 the 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 were regularly shorter than the non-grazed treatments and were less prone to lodging.

The use of PGRs

A survey funded by the GRDC on the use of PGRs has recently been completed by Tina Acuna et al, Tasmanian Institute of Agriculture, University of Tasmania. They contacted 142 agronomists working for eight major rural supply companies across the Australian grain growing regions. While the majority do not use PGRs, once yields were four t/ha or greater, the use of PGRs increased. The main reason given for the use of PGRs was to reduce crop lodging through reduced crop height. Other reasons for use included yield, (my interpretation of the data is more to maintain yield by avoiding lodging) and harvestability. In essence, PGRs were applied to produce a compact crop that was easy to harvest.

The survey results also looked at information published on the use of PGRs. There is considerable evidence that PGRs reduce plant height when they are applied at early stem elongation in cereals or rosette formation in canola. Shorter plant height increases crop resistance to lodging and can improve harvestability and possibly grain quality if lodging is associated with increased sprouting. In some instances PGRs have been linked to an increase in stem strength and the number of roots.

However, a consistent yield improvement in response to the use of PGRs was not observed.  This suggests the crop’s response to the application of a PGR reflects the complex interaction between crop species and variety with the type, rate and timing of PGR application with respect to plant phenology (Acuna et al. 2014).

Trial Results

Irrigated Wheat and Barley Trial, Hillston

This trial was conducted by Barry Haskins, formerly a District Agronomist with NSW DPI, in 2007. The trial used two PGRs treatments plus a control – No PGR, Moddus® at Z31 and Moddus® @ Z31 plus Ethephon® @ Z49.

Plant maturity, height, yield and grain protein were measured.

Plant Maturity: application of a PGR delayed flowering by between five and 12 days, with barley the most affected.

Plant Height: Both PGR treatments reduced plant height post application but the Moddus® at Z31 treatment ‘bounced back’ and the plants were approximately 40mm shorter at flowering. Where the crops received the Ethephon® at Z49, the height reduction of approximately 160mm to 230mm was maintained. Barley had the strongest response to Ethephon®.

Yield : Wheat yields were similar across the Moddus® only and control treatments as there was no lodging.  However treatment improved barley yield by 0.5 t/ha as the control crop lodged.  Ethephon® decreased yield in both crops. This may have been due to the Ethephon® being applied slightly late but it also caused leaf necrosis in the barley. Protein improved slightly with the use of PGRs, but the increase in the protein for the Moddus® plus Ethephon® treatment may have been due to the reduced yield.

High Rainfall Zone Canola Trials

These PGR trials were conducted on canola by Penny Rifkin of DEPI, in the western district of Victoria using an un-named PGR that reduced the effect of giberellins in the plant.

The trial conclusions were that the PGRs used did alter plant height, podding, seed numbers/pod and canopy structure but did not improve the Harvest Index or grain yields. The lack of yield response was attributed to no lodging in the trials. A comment was also made regarding the inconsistency of the responses, either from season to season or between sites.

ICC Irrigated Cereal Trials

In 2003, a wheat trial using H45 and Chara  was sown and Moddus®, Cycocel® and Ethrel® were applied. No significant height reduction occurred at harvest in Chara, and only Ethrel® had an effect on height in H45. Yield was similar in most treatments, with Ethrel® causing a slightly lower yield along with increased screenings in both varieties.

In 2004, a wheat trial comparing Ethrel®, Cycocel® and Moddus® saw crop height reductions in all but the Cycocel® treatment, with no effect on yield, lodging or grain protein.

In 2012, a trial looking to grow 10 t/ha of wheat (Derrimut) and barley (Commander) was sown at Kerang. A combination of sowing rates and nitrogen rates were used to ensure there were sufficient shoots (>1000/m2) at Z31 to produce the 10 t/ha target, as well as a 6 t/ha yield target treatment as a comparison. As this level of shoot density was highly likely to lodge, Moddus Evo was applied at Z31 at 400 ml/ha.

Barley Results

Moddus Evo reduced plant height (78.6 cm untreated vs 69.6 cm treated).

Moddus Evo reduced lodging slightly (7.7 rating untreated vs 9.0 treated) where a score of 9 = vertical, 0= flat on the ground, although the degree of lodging was not considered to be impacting yield.

Application of the PGR resulted in higher yields. Yields were improved by the application of Moddus Evo under the same N regimes, but the extra N from the “10 t/ha treatment” was only converted into yield where Moddus Evo was applied.

Table 1. 2012 barley PGR grain yields and protein.


No Moddus Evo

Moddus Evo @ 400ml/ha


Grain Yield


Grain Protein


Grain Yield


Grain Protein












n.b. Yield lsd = 0.38; Protein lsd = 0.44; Figures with different suffixes are significantly different from each other.

Wheat results

In comparison, treatment with Moddus Evo on the wheat had a small reduction in plant height (2.3 cm), but no other effects on lodging, yield or grain protein.

As a result the trial was repeated in 2013, but only looking at Commander barley.

2013 trial results

The Moddus Evo treatments were:

  1. 400ml/ha at Z31 only – the standard application
  2. 400ml/ha at Z31 plus 200 ml/ha at Z39. This second application is to reduce the “bounce back” response, where initial application resulted in initial reduced crop height but the plant compensated later in the season and the final height is only marginally below that of untreated plants.
  3. Control treatment with no growth regulator application.

The results in Table 2 illustrate a similar trend to 2012 with respect to the effect of the growth regulator on crop performance.

Table 2. 2013 barley PGR trial results.


Yield t/ha

Height cm

Harvest Score

Moddus Evo 400+200




Moddus Evo 400




No Moddus




p value




LSD 0.05








n.b. Figures with different suffixes are significantly different from each other.

Moddus Evo application increased yield in both treatments, with the later treatment resulting in further yield improvement.  Only the later application of Moddus Evo saw a reduction in crop height.  Moddus Evo also improved the harvestability of the plots. While the treated plots were an improvement over the untreated plots and were consistently given a score of 4, the difference between the Z31 application only and the dual application should be viewed with caution as the scores were quite variable. The harvest score reflected the lodging and degree of uniformity of the lodging. A score of 9 meant the crop was standing vertical and as the score decreased the lodging of the crop increased, as well as the variability in the direction of the lodging.

This trial data confirmed what was seen in 2012 that the yield increase was a result of applying the PGR. Lodging did not explain the increase, nor did the claimed benefit of Moddus Evo of reducing head loss, as any grain loss was not noticed prior to harvest and CommanderA is not particularly prone to head loss. Other possible explanations for the yield increase are the claimed positive effects on the roots. This may have some merit as prior to each spring irrigation, there was considerable cracking of the soil and many small roots broken off.


PGRs may have a place in the management of high yielding crops. Unfortunately their effects are not consistent and the decision on whether to apply the PGR has to be made at approximately three months before the lodging would be expected.

Alternative PGRs are available but are not yet registered for use on all crops or at rates and timings that would have a growth regulatory effect.

The yield improvements seen in barley in the ICC trials need further investigation as the reason behind the increase is not clear.


The effects of plant growth regulators on winter and spring canola types in the High Rainfall Zone of south-eastern Australia. Proceedings from the Australian Agronomy Conference 2012 -

Acuna et al. 2014: Plant growth regulators in broad acre crops from the Adelaide 2014 GRDC Update for Advisers -

Canopy management for durum wheat and barley -

Contact details

Damian Jones

Irrigated Cropping Council

PO Box 238 Kerang, 3579

0409 181 099