Agronomy Centre

1. Check with a best-rate recommendation

It is always good to take a look at recommendations based on good data. The Ontario N Calculator can serve as a reality check when other forces may convince you that you can’t grow 200 bu/acre corn without at least 200 lbs of fertilizer N.

Here is a middle of the road example. Loam soil, winter wheat last year (straw removed) in a 3000 CHU area, field generally yields right around 200 bu/acre. Producer applies 60 lbs broadcast N/ac and bands another 30 lbs N/ac through the planter.

The Sidedress Recommendation is for 60 additional lbs/acre of N. Total N = 150 lbs. Of course, if we talk clay loam soils, corn after corn, and leave everything else the same, the recommendation goes to 30 + 60 + 90 = 180 lbs/acre.

Your fields represent many other possibilities that you can check out by running your scenario through the Maizex N Tracker.

2. Rate recommendations using a PSNT

Pre-Sidedress Nitrogen Tests (PSNTs) were generally calibrated for soils that have not received any broadcast, planting time N. So in the above example, if the grower had only applied 30 lbs of N in the starter band, the PSNT would indicate the total sidedress N requirement.

With so many fields getting at least some broadcast nitrogen up front, the PSNT has become more of a benchmark guide. For example, with 30 lbs through the planter and 60 lbs broadcast, a PSNT in that field that is 25 PPM is a reasonable indicator that your requirements are in line with the 60 lbs/ac sidedress recommendation. As PSNT values increase from 25 to 35 PPM, your confidence grows that you need less nitrogen. Rarely is there a field that has a 32–35 PPM (or above) PSNT value (wherever that N came from—soil, manure, red clover, previously broadcast N, etc.) that needs any more sidedress nitrogen.

PSNT values should be integrated with yield expectations, as well as corn and nitrogen prices, with N banded (not influencing the sample) and with N broadcast (influencing the sample). These are all factored into the Maizex N Tracker 4.8.

3. Losses from early applications of N

Weather can be extremely variable, and we often worry that planting time N is at risk to losses from heavy rainfall events, especially on sands or clays. This sort of excessive or prolonged rainfall has been mostly absent in 2020 during the April 15 to June 10 window. A PSNT might confirm that residual N is high, but if you generally add 10 lbs N/acre to your sidedress/top-dress applications to cover off losses from earlier applied N, this may be a year to skip that practice.

4. Rate reductions due to lower plant stands

In quite a few cases, there are corn stands in 2020 that are lower than the targeted 31–34,000 plants per acre. This has sparked some consideration as to whether these fields should receive less sidedress N. With all other field factors being equal, you would need a population of less than 23,000 to cut yields enough to warrant a 10 lb reduction in N rates. However, this does not account for yield reduction that may be more associated with non-uniform stands in addition to strictly lower populations. It is true that lower density / lower yielding corn needs less N, but cutting 10 lbs out of a sidedress N rate should not occur unless stands are below 25,000 plants per acre.

5. Rate reductions due to lower corn prices

N-rate prediction tools should factor in the ratio of corn prices to N costs. With corn prices lower this year, should you be cutting back on nitrogen based simply on expected corn prices? Based on the N Calculator math, if we assume nitrogen prices are constant, corn price needs to drop fairly significantly (i.e. $.80 to $1.40 per bushel, depending on the starting point) to trigger a 10 lb/ac reduction in N rates. Or, if we use numbers from last year with $5.40/bushel corn and $360/tonne UAN and compare it to this year at $4.40 corn and $340/tonne UAN, N rates recommendations year to year would drop by 6 lbs/ac. Not big numbers either way, but both point to slight reductions to maximize profitability.

6. Protected N in sidedress applications

Slowing the conversion of nitrogen to the nitrate form, thus protecting it from losses, is a reasonable tool for N applications that occur around planting time and perhaps for later applications if you farm sharp sands or heavy clays where leaching or denitrification risks are higher. For an average sidedress operation where the UAN is knifed into medium textured soils, I doubt you can justify the cost of slower release nitrogen additives.

7. Protected N in surface applications

If you are surface banding UAN or granular urea, you should consider an additive that acts to reduce surface volatilization (urease inhibitors). Worst-case scenarios for surface-applied N losses are when the nitrogen is applied to damp soils and then rainfall does not come for 7–10 days. Volatilization inhibitors in surface-applied situations are good options to reduce N losses.

8. Impact of rainfall post June 15

One of the nicest pieces of N research that has come from the University of Guelph (Niemeyer and Deen) recently was the idea that rainfall in the V7-to-tassel stage (VT) can have a direct correlation on how much nitrogen the crop requires. This is the period when rainfall had the greatest impact on N requirements. Consider a grower that applies 120 lbs of N upfront; this allows for a fair bit of confidence that he or she can delay the late nitrogen application well into July to assess the total rainfall and apply accordingly. When the total rainfall from June 15 (V7) to July 20 (VT) is 50 mm (2 inches), the recommendation for the late N application (needless to say, high-clearance equipment required) is 41 lbs N/acre. If on the other hand the rainfall total is 100 mm (4 inches) in that same period, the N recommendation moves to 82 lbs N/acre.

9. Risk of late applications

You can make the argument, as in the previous point, that the later you leave the sidedress application, the more information you have (soil nitrates, rainfall, yield potential, etc.) and the better rate decision you can make. This may be true, but it needs to be pointed out that there are some risks associated with very late applications. If the soil is very dry and you do not receive some timely rains following very late applications, the nitrogen may be stranded at the soil surface and unable to meet the crop requirements. Although the advancement of high-clearance application equipment has really widened the sidedress window, the evidence has been underwhelming that very late applications or multiple late applications (i.e. spoon-feeding nitrogen) under average weather conditions has any net economic benefits to the grower.