The situation- Several calls have come in this week in North Dakota regarding mostly spring wheat that looks yellow that had N fall-applied.
The first lesson-The first lesson to learn is when to apply fall N in the future, because most growers and their suppliers jumped the starting gate last fall. There is no perfect time to apply N, but the following are guidelines that will lead to successful retention of N in all but the most unusual fall-spring time periods.
1. Do not apply ANY N until Oct. 1, even if the soil temp falls below 50 degrees for a time. (Weather history in the region shows us that cool periods are possible in September, but in many years, temperatures can rise later in the fall and can contribute to conditions that will result in transformation of ammonium/urea-N to nitrate-nitrification-that leads to possible N losses through the late winter and early spring).
2. After October 1, check soil temperatures between 8AM and 10AM. When soil temperature falls to 50 degrees, it is time to apply anhydrous ammonia. You might choose to add a nitrification inhibitor such as N-Serve™ to the ammonia if you fear it might warm up in the next few weeks, but in most years the ammonia is fairly stable without it, according to NDSU work conducted in the late ‘70’s. DO NOT START APPLYING BANDED OR BROADCAST UREA WHEN THE AMMONIA TANKS START ROLLING! Anhydrous ammonia has some initial nitrification inhibiting properties for a week or two after its application due to high ammonia content and high pH. These inhibiting properties first disappear at the edges of the ammonia band aura, and works its way into the center of the band later on.
3. A week AFTER the soil temperatures fall to 50 degrees after October 1, it is time to apply banded urea. This is a common practice especially in western ND and is becoming more common in some areas of eastern ND. There is a small nitrification inhibiting advantage to banding urea over broadcast, but not as much as with anhydrous, thus waiting a week after ammonia application begins is prudent.
4. Two weeks AFTER the soil temperatures fall to 50 degrees after October 1, it is time to apply broadcast urea. Broadcast urea has no nitrification inhibiting properties and as soon as the urease attacks and splits the urea into ammonium, the nitrification bacteria are free to transform it into nitrate.
Last fall (fall 2010), many growers looked at the calendar; worse, some just looked at their work load and started in September-bad year for that call!
If you followed the guidelines above, you would have begun anhydrous application in SE North Dakota about October 15. Banded urea should have started October 22, with broadcast urea delayed until October 29. Unfortunately, many N applications started well before these dates, and therefore contributed greatly to the yellow wheat seen in a number of fields.
The second lesson-What to do? Ideally, apply before the wheat or other small grain is less than the 5-leaf stage with stream-bar UAN. UAN tends to be cheaper compared to other liquid sources and it is half ammonium nitrate which does not volatilize. Even without a significant rain for a long time, there will still be about half the N around to at least contribute to greater protein if rain is received later. Also, applying urea-N in a stream reduces the rate of urease activity for reasons I do not understand, but studies show that this is true.
If the wheat is at the 5-leaf stage and yellow, this is a bad thing since heavy decisions regarding yield potential are being made inside the plant. If your supplier has access to a straight urea solution, broadcast urea-solution will contribute to immediate foliar feeding, and with the projected weather through the weekend forecast in the 60’s at most, leaf burning will be minimal. If leaf burn occurs, take the opportunity to make renewed acquaintance with neighbors and just accept it. If you stream-bar UAN at the 5-leaf stage, you need a rain very soon (1-2 days) for the wheat to recover enough to reach near-full yield potential. If it doesn’t rain very soon your yield potential is noticeably reduced.
At the earlier stages (2-3 leaf wheat) urea granules applied with a floater will also work if it rains and the wheat will be small enough that driving over it will not interfere appreciably with growth. Agrotain™ with the urea would be a good plan unless rain is forecast and received very soon after application. At later stages (4+ leaf wheat) I would not recommend floater application unless there is no other way to apply the fertilizer and you accept that wheat that is run over will be shorter and less productive than wheat that is not run over.