Considerations for variable maturity conditions pre-harvest
Heat waves and drought stress have once again occurred in Alberta this growing season. Some areas received very little rainfall until June or July. This has brought on a flush of regrowth in the form of new tillers. These new tillers are making heads just a few inches from the soil surface (at the moment) and growing up toward the riper tillers. If the crop is less than a month away from harvest, perhaps keeping the combine header high will just pass over these much shorter under-ripe heads, but some fields that received the rainfall earlier have these immature heads up above the “riper” original heads. This scenario will leave these tillers at least two weeks behind in maturity from the original set.
More times than not, the “green” colour can become “locked in” if frost or some other form of artificial ripening is attempted. As a result, the whole field may be rejected as a CWRS (bread wheat) or CWAD (durum), at the elevators. “Green seed” comes in many forms, from “grass green” that is docked separately, to “immature seed” that looks ripe for all intents and purposes. Note that there are very low tolerances to “grass-green” seed in say No. 1 CWRS where it must be 0.75% or less, or no more than 2% for No. 2 CWRS. This is the same for all other spring wheat classes of wheat except CWES, CPS-R and CPS-W where tolerances are higher at 2% and 10% for No. 1 and No. 2 respectively.
Swathing such a field early enough not to damage the ripe earlier maturing seed (shattering) and yet early enough to try and bring down the under-ripe second flush is going to be difficult. Leaving the field stand, given enough time before frost, is the only way to resolve such a difficult situation. The trick is not having the riper heads start to shatter out or have the whole field start to show signs of germination, such as for durum wheat, whilst waiting for the second flush to ripen. When immature seed is involved, a decision has to be made as to whether there is time to wait for such green heads to mature naturally before swathing and or direct combining, or if they are too late, then to graze, bale or silage the crop now for animal feed. If the new growth is just too dominant and or too late to make mature seed prior to a killing frost then these latter options would make the most sense.
When inspecting a field for ripeness, remember under drought stress conditions the colour change in plant tissues does not truly represent the actual seed maturity. One has to check beyond the “visual appearance” of the wheat field and check the actual “seed maturity”, especially under these circumstances. When one shells out some of the wheat seed in such affected fields, often a significant portion of the seed shelled is only at the soft dough stage still (higher than 30% moisture content). If we wish to prevent unintended shrinkage of any immature seed, this stage is too early to swath or to spray a pre-harvest glyphosate application for weed control. To prevent issues on these fields, one should manage the crop based on the latest maturing areas (via a look at the seed), and not based on the earliest maturing areas even if such areas are the majority.
Wheat seed is physiologically mature when the seed is about 30% moisture content, a stage when input into the seed by the plant (grain filling) has halted and moisture starts to be removed from the seed (dry down or desiccation phase). The “fingernail dent test” works in determining this 30% moisture point. Press your fingernail into the seed across its width, and you should leave a dent mark behind, but it should not break the surface of the seed. It is at this 30% stage or later when swathing should occur, or when glyphosate products for use for a pre-harvest weed control method can be applied. Point being in a year like this with many fields experiencing mixed maturities, one needs to know how a field is progressing based on the “actual” maturity of the seed and not just on appearance of the crop.
It is crucial that farmers using glyphosate for pre-harvest weed control spray only when the seed moisture content has reached 30% or lower in the least mature parts of the field as this will ensure our grain shipments adhere to all international regulations for residue limits and maintain continued access to markets.