Multicoloured wheat? At this stage in late May, winter wheat should be dark green and uniform, going flat out as it comes into head.
However, many wheat fields across Ontario are showing every shade of green imaginable, and too much of it isn’t dark green and photosynthesizing at maximum speed. We put the nitrogen and sulphur on, so what gives?
First, a few observations. In most cases, dry urea/AMS applications are worse than UAN/ATS combinations. Any small difference in spread pattern shows. In some UAN/ATS fields, the “every other row” syndrome is still showing: normally, this would be long gone. Next, fields that were “shorted” a bit on nitrogen and sulphur are much worse. Poorly-drained fields are worse than well-drained fields (that’s a root growth factor, folks). Any field that skipped sulphur is really yellow and requires an immediate application. Finally, a few fields with manganese deficiency are so deficient that the wheat appears to be dying. All of these are clues.
After the week of incredible warmth from April 9 to 15, it turned cold and damp. There was enough moisture to get the nitrogen into the ground, and there it sat. It was so cold that the transformation from urea to ammonium and nitrate was slow (remember, urea is all urea while UAN is 50% urea, 25% ammonium and 25% nitrate). Then May came in dry. So dry, in fact, that when Syngenta agronomist Greg Stewart went to take nitrate samples in some wheat fields last week, it was all he could do to get the probe in the ground — the soil was bone dry to the full 30 cm depth of the probe.
From all these clues, here is the WheatPete hypothesis: there is a lot of nitrogen and sulphur that is “positionally unavailable” in dry soil. Urea is worse, not enough of the urea transformed to nitrate, the form that moves with water, before it got dry. There has so little rainfall that even UAN has not moved sideways in the soil, and if the application wasn’t perfect — for example, 3 or 6 stream nozzles at the wrong height — every other row syndrome is still painfully apparent.
There are other factors to consider: if there was any nitrogen loss in late April, it would be twice as bad with urea than UAN. If the urea was stabilized, it would have slowed the transformation to nitrate even more, which could potentially make the situation worse. In reality, I doubt we lost much nitrogen, or whether stabilizers play much, but those questions do get asked.
What do we do now, if streaky wheat still plagues us? Fortunately, that 10-12 mm rain Friday, May 19 has helped a lot of wheat snap out of it. Where streaky fields remain, it’s a great idea to tissue sample: at least 50 flag leaves from the good, and 50 flag leaves from the bad to compare. That will tell you if the problem is nitrogen or sulphur or both: apply additional fertilizer accordingly.
Yes, we need some rain to make that fertilizer work, but without sufficient nutrients, yield will suffer. If you are worried about lodging in the good areas versus helping out the pale green wheat, there is a good chance the yield increase from the poor wheat will trump lodging in the good wheat. Just don’t go crazy with the added nitrogen.
Peter Johnson is RealAgriculture’s resident agronomist.
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