When does spring wheat reach physiological maturity and when is the proper timing for pre-harvest glyphosate application?
By Andrew Hector, Agronomy Extension Specialist – Cereal Crops, Manitoba Crop Alliance
Properly identifying spring wheat growth stages is important throughout the season, but growth stage identification becomes essential as we near plant maturity and approach the window for pre-harvest activities such as swathing or applying pre-harvest glyphosate.
What is physiological maturity (PM)?
At PM, wheat plants have accumulated the maximum amount of kernel dry weight. Therefore, at this point, final maximum yield has been determined, and no more dry matter is being accumulated in the kernels.
How to identify PM
When assessing growth stage, check representative areas of the field. Special attention should be given to the least mature part of the field, especially if you are applying a pre-harvest weed control product, to avoid maximum residue limit (MRL) issues.
The first indication of PM is the loss of green colour from the kernels,= and the appearance of dark pigment along the kernel crease (Figure 1). It’s important to note that kernels on the same head don’t mature simultaneously, with the middle kernels maturing first.
Figure 1. Wheat kernels prior to (top) and at (bottom) physiological maturity (note the darkened pigment strand).
The second indication of PM is when the peduncle (i.e., stem below the wheat head) changes from dark green to light green or yellow. Once the peduncle turns yellow, the plant is getting close to PM. However, change in peduncle colour is just an indicator (Figure 2). A fingernail test is also required to confirm growth stage.
Figure 2. Peduncle colour change associated with physiological maturity.
Pre-harvest glyphosate application for perennial weed control
Glyphosate is registered for pre-harvest weed control only, it is not a desiccant. Proper application timing can be challenging, especially if crop maturity is uneven across the field. Ensure the least mature part of the field is assessed for appropriate crop staging, to reduce MRL issues.
If applying, application should be conducted when grain moisture content (MC) is below 30 per cent, AKA the hard dough stage. Application prior to 30 per cent MC can reduce grain yield and quality. Furthermore, early application can lead to elevated residues in grain.
Conduct a thumbnail test:
- Shell out the wheat head. Press your thumbnail into the kernel. At < 30 per cent kernel MC, and under reasonable pressure, the thumbnail impression or dent will remain. This indicates that plant has reach hard dough (Figures 3 & 4).
- If the imprint of dent does not remain in the kernel, then the plant is not at the appropriate stage for a pre-harvest glyphosate application.
Always read and follow label directions. Check with your grain buyer before applying a pre-harvest product to ensure grain acceptance.
Figure 3. Thumbnail test assessing kernel moisture. Source: Keep it Clean.
Figure 4. Kernels at various times during grain filling: a. watery ripe, b. late milk, c. soft dough, d. hard dough (loss of green colour) and e. ripe for harvest. Source: Dave Hansen, Growth and Development Guide for Spring Wheat.
