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Reducing potential negative impacts of plant growth regulator use in wheat and barley

By Sheri Strydhorst, M.Sc., Ph.D., P.Ag., Agronomy Research Specialist | Alberta Wheat and Barley Commissions

Plant growth regulators (PGRs) are synthetic compounds that can alter plant growth and development. PGRs change plant hormone activity to produce shorter stems and reduce lodging. PGRs should only be used in environments with abundant moisture and high levels of fertility. Moddus, Manipulator and Ethrel are the three PGRs registered for use to manage lodging in western Canadian cereal crops.  

Manipulator 620 is “for treatment of spring, durum and winter wheat, spring and winter barley, and spring and winter oats to manipulate apical dominance and producer shorter, thicker, stronger stems for improved lodging resistance.” Manipulator contains the active ingredient chlormequat chloride. The Manipulator label states “DO NOT apply Manipulator 620 to crops under stress from waterlogging, drought or nutrient deficiency”.

Moddus will aid in the growth and lodging management of wheat, barley, and oats. Moddus contains the active ingredient trinexapac-ethyl. The Moddus label states “Due to risk of injury to the crop, avoid overlapping and do not apply to wheat, barley, oats or perennial ryegrass that are stressed, such as by low fertility, high temperatures, drought, frost or diseased or insect-damaged”.

Ethrel is registered for use on wheat and cannot be used on barley.

PGR use in stressful growing conditions

PGRs help manage lodging risk in highly productive environments where agronomic inputs target high yields, or where genetic lodging resistance is inadequate. However, this does not mean PGRs are beneficial every year. If environmental conditions are not favourable for lodging (i.e. drought), PGRs should not be used. In these situations, no PGR application may be the best approach.

The type of stresses referenced on labels include low fertility, high temperatures, drought, frost, disease and insect damage. For example, PGRs should not be applied if the crop has experienced cold stress (<0oC) five days before the PGR application. Additionally, PGRs can take 14 to 28 days to fully breakdown within the plant. While it is very difficult to forecast heat stress one month out, weather forecasts should be checked and PGRs should be avoided if heat stress is expected 14 to 28 days after PGR application.

Selecting the right PGR for your crop type

Cereal crops and varieties differ in their sensitivity to PGRs. Wheat tends to show larger height reductions, and barley tends to show smaller height reductions in response to PGR applications. Responses differ with different products. Wheat tends to show larger height reductions with Manipulator compared with Moddus. However, both tools can effectively manage lodging.

Research from western Canadian and international studies found that Moddus is more effective on barley (Figure 1). An international review paper found that barley is less responsive to the active ingredient in Manipulator than wheat. Therefore, products containing the active ingredient in Moddus should be used to reduce the risk of barley lodging (Rademacher, 2015).

Similarly, not all varieties have the same response to PGRs. There is not a full dataset to show which varieties are most responsive to PGRs. Unfortunately, a variety’s height or lodging rating is not a reliable indicator of its response to a PGR. Just because a variety is rated ‘Good’ or ‘Fair’ (as indicated in the provincial seed guide), does not mean it will, or won’t, respond to a PGR.

Positive, negative and unexpected PGR effects on plant growth

PGRs are applied to the leaves and reduce cell elongation, reduce stem length, shorten the uppermost internodes and peduncle and they may alter stem diameter. The levels of one plant hormone affects other hormones, so PGRs may have secondary effects. These may include delayed maturity or possibly increased root growth. PGRs can positively or negatively affect yield, but results are inconsistent depending on crop lodging, environmental conditions, crop type and cultivar.

PGR applications often cause delayed maturity (up to three days) and decreased thousand kernel weight. Growers must weigh the benefits of PGRs against the chance of extended days to maturity, reduced test weight and possibility of reduced protein content when deciding to use a PGR.

Plant growth regulators are inconsistent. Responses tend to differ between years, locations, cultivars and species. Alberta research suggests that 93% of the time, PGRs will cause significant wheat height reductions (between 3 and 13 cm) compared to no PGR application. However, there are occasional instances when some cultivars show no height reduction in response to PGR application.

Grain marketing considerations

Growers should be aware that not all grain markets will accept grain which received a PGR application. Growers should check the Keep it Clean website for product advisories. There are potential market risks associated with certain PGRs on certain crop types. It is important for growers to check with their grain buyer to confirm contract obligations and acceptance before using PGRs.

Summary

To ensure the most value from your PGR application, consider both environmental conditions and select the best product for the intended crop. In the case of barley, Moddus has better efficacy in reducing height and improving standability. When growing seasons start off dry, growers are cautioned to limit PGR applications to fields where there is significant lodging potential.

Please follow all label directions for the application of plant growth regulators. To assist with staging your crop for the correct timing of PGR application, please check out this Growing Smarter video and review Figure 2.

Figure 1. Gadsby barley grown at Barrhead, Alberta on August 8, 2016. Left: Manipulator. Right: Moddus. PGR treatments were applied at GS 31-32. Please note that this article and these images do not reflect the performance of Manipulator in wheat. Manipulator has shown excellent performance in wheat, and it is still important to remember that PGRs should not be used when the crop is under stress.

Figure 2. Growth stage 31-32, ideal application timing for Manipulator and Moddus PGRs. Check out this video on PGR staging for wheat and barley

References

Bayer Crop Science Inc. 2021. Ethrel Liquid Plant Growth Regulator. Accessed 3 May 2021. Available at: https://www.cropscience.bayer.ca/-/media/Bayer-CropScience/Country-Canada-Internet/Products/Ethrel/Ethrel-Label---English-02_04_21_ac.ashx?la=en&hash=AEA622831836B3FFAED1E9F342D859D295067681

Belchim Crop Protection Canada. 2020. ManipulatorTM 620. Accessed 3 May 2021. Available at: https://www.belchimcanada.com/Label/59/E/1.pdf

Rademacher, W. 2015. Plant Growth Regulators: Backgrounds and Uses in Plant Production. J. Plant Growth Regul. 34:845-872.

Syngenta. 2020. Moddus®. Accessed 3 May 2021. Available at: https://assets.syngenta.ca/pdf/ca/labels/Moddus_33930_en_pamphlet.pdf