Plant Growth Regulators – The Why, Where and When

English Version revised April 15, 2021.

With new plant growth regulators (PGRs) entering the Ontario market, many are asking, when and where should a PGR be used? First, it is important to know what the purpose of a PGRs is. PGRs can be applied to winter wheat to reduce plant height and increase stem thickness. This reduces the risk of lodging, making managing and harvesting a tall winter wheat crop easier. Additionally, it can also help with reducing harvest losses that come from lodging. Although PGRs are a helpful tool, they are not necessarily needed every year or in every field.

The use of a PGR will bring the most benefit to a winter wheat crop when:
• You have an early planted winter wheat crop with lots of growth
• You are growing a variety that is prone to lodging
• You can determine the lodging potential of your variety by visiting
• Your winter wheat field has a history of manure applications and is highly fertile
• You are implementing an intensive wheat management program (i.e. early planted, high seeding rates, aggressive nitrogen rates) and have a high yield potential

The use of a PGR has less value when:

• Your crop was planted late and too much growth is not a concern
• You are growing a variety that is not prone to lodging
• You are not implementing an intensive wheat management program (i.e. aggressive nitrogen rates) and you have a low yield potential

Generally speaking, PGRs are not a tool for improving your plant population and creating more straw. On a year such as 2019 when the crop was planted late, the spring was cool and wet and nitrogen applications were delayed, lodging was generally not a concern and therefore, a PGR was not necessary or in many cases, was not economical. However, if the winter wheat crop had been planted early, had many tillers going into winter, and the spring was warm and promoted lots of growth, a PGR may have been a tool to consider if trying to manage lodging.

Currently there are a number of PGRs available in Ontario. Selecting a PGR for your field is often based on whether you are able to get the product on according to its labeled application window and whether you want the PGR to reduce lodging by reducing plant height or by promoting fall growth which results in a more robust plant.

Ethephon (Ethrel)
Ethephon, commonly known as Ethrel, is a PGR that when applied to wheat releases ethylene into cell tissue. Ethylene then causes a reduction in cell elongation and crop height which is effective in reducing lodging.

The application window for Ethephon can be quite tight, with the ideal timing being between GS37 to 45 (flag leaf just visible up to boot just swollen). If you miss the application window and more than 10% of the awns have emerged Ethephon cannot be applied. Applying Ethephon outside of the ideal application window can cause crop damage and ultimately reduce yield. That is why it is important to scout your fields regularly as the optimal timing approaches to ensure the product is applied at the appropriate time.

Chlormequat chloride (Manipulator)
Gibberellins are plant hormones that regulate various developmental processes, including the stimulation of cell elongation and cell division which gives plants their height. Chlormequat chloride, commonly known as Manipulator, is a plant growth regulator that inhibits the early stages of gibberellin production in winter wheat. What this means is that the application of chlormequat chloride to your wheat crop results in a reduction in plant height while the stems are thickened. This in turn results in a reduction in lodging.

The full application window for chlormequat is GS 12-39. However, the ideal timing is GS 30-32 (stem elongation – 1st to 2nd node). Additionally, chlormequat can be applied when temperatures reach as low as 1°C making it ideal for when spring temperatures dip down at night. The application window for chlormequat chloride is earlier and longer than ethephon and may be better suited for your operation if a tight application timing is a challenge for you.

Trinexapac-ethyl (Moddus)

Trinexapac-ethyl is a newly registered PGR that also inhibits the production of gibberellic acid.  This causes the internodes to shorten and stems to thicken resulting in a reduction in lodging. 

The full application window for trinexapac-ethyl is GS 30-39. However, the ideal application window is GS 30-32 (stem elongation – 1st to 2nd node). This staging is important for nitrogen, herbicide and fungicides applications as well so it is important to stay on top of staging. Tracking Growing Degree Days (GDDs) can be helpful in determining how quickly the crop is advancing. A helpful resource using Environment and Climate Change Canada weather stations can be found here:

Gibberellic acid, GA3 (Proliant)
Gibberellic acid (Proliant) is slightly different from some of the other types of PGRs available. Unlike other PGRs, gibberellic acid is used to enhance early growth in winter wheat rather than hinder it. Gibberellic acid elongates plant cells and encourages cell division. The result is more robust plants that can handle stress (i.e. cold temperatures and drought).

Gibberellic acid can be applied up to GS30 (stem elongation). However, the biggest benefit has come from when it is applied in the fall because gibberellic acid is generally used to promote early growth of the crop. While the early growth of the plants is enhanced, lodging is not a concern as long as it is used at the correct time.

If choosing to use a plant growth regulator, always follow the label for application recommendations.


Product Application Window Ideal Application Stage Function
Ethephon GS 37 – 45 GS 37 (flag leaf) Reduces cell elongation and plant height
Chlormequat chloride GS 12 – 39 GS 30 – 32 Reduces cell elongation and plant height
Trinexapac-ethyl GS 30 -39 GS 30 – 32 Reduces cell elongation and plant height
Gibberellic acid Up to GS 30 Fall application Encourages cell division, resulting in a more robust plant

In 2021, a research project being led by Dr. Dave Hooker will continue with the following objectives:

  1. To determine the effect of the newest plant growth regulators (PGRs) on winter wheat performance and profitability, including lodging potential, grain yield and quality.
  2. To determine the impact of PGRs within an integrated approach to increase profitability, reduce lodging potential, increase grain yields and to maintain or increase grain quality.

The project will involve both small plot and on-farm trials.  Anyone who is interested in participating in the on-farm portion of the project please refer to the following protocol: PGR Grower Protocol 2021.


Ethrel Label
Manipulator Label
Proliant Label

Moddus Label