Nitrogen Application Timing – Is “Cheap” and “Easy” Really that Cheap After All?

Soil temperatures are finally approaching the mid 50’s across the state of Iowa after an unusually warm Sept. and Oct. This suggests that many more growers across the state may soon start applying fall anhydrous ammonia for the 2017 crop.

After seeing the results of a 3-year study conducted by Iowa State University, I was again reminded that the economic penalty associated with fall anhydrous applications far outweigh the conveniences of this practice.

Downsides to Fall Anhydrous Applications

  • Fall anhydrous ammonia application timings required 54 more lbs/N per acre to maximize economic returns, and yielded 6% less when compared to spring or early side-dress N applications timings.
  • Lost yields and substantially higher N requirements for the fall anhydrous applications meant economic losses of $47.10/acre when compared to the spring or early side-dress N application timings.

Economic Optimum Nitrogen Rates and Application Timing

Last week I was in Des Moines, IA, at the North Central Extension-Industry Soil Fertility Conference. At this conference, Dr. John Sawyer (ISU’s Soil Fertility Specialist) brought some data to my attention that I wanted to share.

Dr. Sawyer presented some research results regarding the impacts of nitrogen timing effects on corn grain yields and economic optimum nitrogen rates (EONR). In this research, Dr. Sawyer applied N rates from 0-to-200 lbs/N per acre at 3 different timings: in the fall after soil temps fell below 50 degrees F, as a pre-plant application, and at an early side-dress timing (V4). Dr. Sawyer did this for 3 seasons in central IA, the previous crop was always soybean and the nitrogen source for all 3 N application timings was anhydrous ammonia.

Averaged over the 3 seasons of the study, Dr. Sawyer found that the EONR for the fall applied anhydrous ammonia was 200 lbs/N per acre, but for the spring pre-plant N application and the early side-dress N application timings the EONR was about 146 lbs/N per acre. What’s more, corn that received all fall applied N had yields that were 6% lower than the corn that was side-dressed or had all the N applied prior to planting in the spring.

I wish I could say I was surprised by Dr. Sawyer’s findings, but I was not. What Dr. Sawyer shows here is not new, other University studies over the years have reached similar conclusions, and when you use data sets like this to make some simple economic conclusions, it appears that what initially seemed “cheap” and “easy,” may not be so cheap after all.

Economics for Different N Application Timings

To elaborate on the economics of these nitrogen timing practices, Table 1 shows that applying N as either a pre-plant or early side-dress N application is $47.10/acre more profitable than fall anhydrous ammonia applications.

To reach this conclusion I used the EONR found in Dr. Sawyer’s latest research for the pre-plant/side-dress (EONR = 146 lbs/N ac) and fall (EONR = 200 lbs/N ac) applied N application timings. In addition, I assumed that the fall anhydrous ammonia application timing yielded 6% less (188 bu/ac) than the pre-plant or side-dress N application timings (200 bu/ac), and that the price received for a bushel of corn was $3.50. Lastly, I assumed the fall anhydrous cost $0.31/lb of N and spring or side-dress sources of N cost $0.39 per/lb of N (typical price spread for AA bought in the fall vs. UAN that would be applied pre-plant or in-season).


What Table 1 shows makes it very clear which N application timings are more profitable. So, the question really is, “At what economic (to the grower) and environmental price does the convenience of applying N in the fall no longer seem reasonable?”

While lower yields and higher EONR for fall applied N applications are not new, wide-spread availability to high clearance, self-propelled sprayers is new and gives growers yet another reasonable option for applying N in-season.

Figure 1.

Fall, spring pre-plant and side-dress anhydrous ammonia application effects on EONR and corn yields. Figure from Sawyer et al., 2016.

Table 1.

Corn yield and profitability from fall applied or pre-plant/side-dress N application timings.


Sawyer, J.E., D.W. Barker, and J.P. Lundvall. 2016. Impact of nitrogen application timing on corn production. North Central Extension-Industry Soil Fertility Conference proceedings. 39:56-60. (accessed 11 of Nov 2016)