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Plant Growth Regulators

By Sara Tetland, MSc, PAg, Provincial Specialist, Cereal Crops, Regina

June 2022

Plant growth regulators (PGRs) are synthetic compounds that alter hormone production in plants, which can modify plant growth and development. They can improve crop standability and harvestability by shortening stems, which can reduce lodging.

Lodged Barley
Lodged barley field. Photo by Sherri Roberts.

Lodging may result in reduced quality, by lowering test weight and increasing sprouting. It can also cause yield loss due to reduced photosynthetic capacity of the plant and difficulty harvesting plants that are laying flatter in the field. Lodged plants are also typically more susceptible to diseases.

PGRs are typically used in environments with high moisture and high fertility where there may be an increased risk of lodging. This has not been typical in Saskatchewan for the last several years. With parts of the province receiving average, to well above average precipitation levels this spring, PGRs may be something worth considering. A PGR can be useful when growing high yielding varieties that are taller or don’t have sufficient straw strength.

While PGRs may be suited for fields with abundant soil moisture, they should not be applied to crops that are or may become stressed. This includes stress caused by excess moisture, drought or nutrient deficiency.

There are PGRs registered for use in spring wheat, durum, winter wheat, barley, oats and perennial ryegrass grown for seed. Different crop species and varieties can have varying responses to PGRs and the effects are not well known.

Manipulator 620 (Belchim Crop Protection Canada) and Moddus (Syngenta) are two PGRs registered for use in barley, oats, spring wheat, durum and winter wheat. While they have different active ingredients, both work by inhibiting gibberellin production, which is a plant hormone that promotes stem elongation, among other things. Cycocel Extra (BASF) is another PGR that has the same active ingredient as Manipulator 620 but is registered for use in winter wheat only.

Commercial, semi-dwarf wheat varieties have a reduction in height due to a change in plant gibberellin as well. These varieties contain different genes that either reduce production of gibberellic acid or make the plant less sensitive to it, resulting in a reduction in height.

Ethrel (Bayer CropScience) is another PGR registered for use in spring wheat but is more commonly used in horticultural crops. It reduces plant height by releasing another plant hormone, ethylene, into plant tissues which reduces cell elongation.

Timing of PGR varies slightly with the product and crop type, as well as if you are doing a single or a split application. Single applications are usually done sometime between the beginning of stem elongation and the second node stage, but exact timing for specific crops and products can be found in the Saskatchewan Guide to Crop Protection.

There are various reasons to avoid mixing PGRS and herbicides. The timing of the two applications is not compatible. The proper timing of most herbicide applications is too early for PGR application and the proper timing for PGRs too late for the herbicide to provide adequate yield benefits. PGRs also effect plant growth by manipulating hormones in the plant, and therefore could have unintentional impacts on how the herbicide performs on both the crop or on the efficacy of weed control.

Before using a PGR, contact your grain buyer to ensure they will accept grain from plants treated with the product. Although there are PGRs registered for use in Canada, not all markets accept grain treated with them.

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