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Aug 26

Can the peduncle determine cereal pre-harvest glyphosate timing?

Posted on Aug 26 By: Jeremy Boychyn | MSc P.Ag | Agronomy Research Extension Specialist
Photo by Kara Oosterhuis 

The time for pre-harvest glyphosate on wheat and barley crops is right around the corner. Every year, discussion seems to be the same with the common question of ‘can the peduncle provide indication of appropriate pre-harvest glyphosate timing?’

Let’s break this question down.

The label recommendation for pre-harvest glyphosate on wheat and feed barley is less than 30% moisture. At 30% moisture, the kernels are 30% water, by weight. If you weighed that kernel and then put it in an oven at 100oC until it stopped losing weight, it will retain 70% of its original mass due to a loss of moisture within the kernel.

The peduncle is the area of the stem directly below the wheat or barley head, through which energy and water is shuttled to the head. The peduncles loss of green colour in the latter part of the season indicates movement closer to physiological maturity (see Staging for Pre-Harvest Glyphosate Application here). However, the rate at which colour change of the peduncle occurs can vary depending on a number of factors such as disease and environmental conditions. Therefore, its colour may not always directly correlate to physiological maturity as the colour change could have been accelerated by external factors.

Sourced from Minnesota State University Extension, Figure 1 demonstrates the typical medium maturity spring wheat variety kernel maturity process. You will notice that as the wheat reaches physiological maturity, the point at which dry weight increase slows and stops, the kernel moisture is still at or above 40%.


Figure 1: Kernel growth for a typical medium maturity spring wheat variety in Minnesota. Sourced from University of Minnesota Extension.

Even if no external factors affected the peduncle colour change, timing of pre-harvest glyphosate based on peduncle colour change would align application with physiological maturity (40% or greater kernel moisture) rather than the appropriate staging of less than 30% kernel moisture.

Peduncle colour change can provide a great indicator of progress toward maturity, however, it should not be used for timing a pre-harvest glyphosate application. Doing so can lead to potential maximum residue limit (MRL) concerns. One of the best ways of determining whether or not a crop is staged for pre-harvest glyphosate, is through the fingernail test. Watch Keep it Clean’s video on cereal pre-harvest glyphosate timing here. Enlisting this method, as well as ensuring your entire field is properly timed, will mitigate MRL concerns.

Be sure to check out for additional information on pre-harvest glyphosate management.

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