*Simple Summary*

*In a recent Ontario dataset of 15 N response curves, reducing N rates 20, 40 and 60 lb-N/ac from traditional optimum N rates (10-year average costs/prices) resulted in:**average yield losses of 4, 9 and 16 bu/ac, respectively*

*average net losses of $0, $11 and $37/ac, respectively, using what might represent 2022 costs/prices ($1.20/lb-N, $6.75/bu corn) at the time of writing*

*Assuming N responses are relatively normal, N prices increasing more than corn prices would justify some reduction in N rates (~10-15 lb-N/ac for this dataset using above costs/prices)**Yield and net loss reductions for reducing N rates can be relatively flat to start, but eventually increase exponentially**Building a dataset of N responses on your own farm can build confidence for adjusting N rates*

With supply chain disruptions and global events over the past year, one question floating around winter 2022 has been “if we aren’t 100% certain today that we can source all the nitrogen (N) fertilizer we normally would, what are the risks if we don’t apply our normal N program in corn?”.

If you are in the uncertainty camp, perhaps you are weighing whether:

- corn acres stay at original intentions, but under the risk of uncertain final N supply and rates
- corn acres are reduced as a hedge to ensure that acres planted will have relatively normal N supply and rates.

**Yield Risks for Underapplying N**

What are the risks for underapplying N? To provide some insight, I will use a 2011-2014 N response dataset from 15 trials conducted on-farm across parts of Southern Ontario by former OMAFRA corn specialist Greg Stewart.

**Figure 1. **15 corn N response curves with MERN for 10-year average and 2022 price scenarios from Southern Ontario, 2011-2014.

These N response curves (Fig. 1) include the Most Economic Rate of Nitrogen (MERN) for what might represent a:

- 10-year average cost/price scenario (green dot, $5.00/bu corn and $0.60/lb-N fertilizer)
- 2022 cost/price scenario (red circle, $6.75/bu corn and $1.20/lb-N fertilizer)

For more background on MERN, see the article “Understanding MERN (Most Economic Rate of Nitrogen) for Corn” (Rosser, 2022b).

Yield risks for underapplying N in this dataset can be made by comparing yield loss for these curves as N is shorted below MERN. Since most on-farm N rates are likely selected to not limit yields in most years, I will compare yield declines as N rates are shorted relative to the longer term 10-year average MERN (green dots).

**Single Response Example**

As an example of where we are going, Fig. 2 shows one N response curve from the above dataset. It is segmented into 20 lb-N/ac reductions below 10-year average MERN (green dot), with the corresponding amount of yield loss by each point. Reflecting the quadratic nature of yield response, we can see that yields decrease faster as N rates are reduced below MERN.

**Figure 2.** Quadratic-plateau yield response with 10-year average and 2022 cost/price scenario MERNs, and yield losses for reducing N rates relative to MERN for a corn nitrogen trial near Ilderton in 2012.

**Full Dataset Analysis**

Figure 3 graphs what yield losses look like as N rates are shorted below 10-year MERN for all 15 curves from above.

**Figure 3.** Corn yield loss for N rates less than MERN (10-year average cost/price scenario) for 15 Ontario N response curves.

This can help demonstrate what yield risks for underapplying N might look like, at least for this dataset. For example, as N rates are shorted from 10-year MERN by:

- 20 lb-N/ac, average yield loss is 4 bu/ac, with 13 of 15 curves losing less, and the greatest losing 6 bu/ac
- 40 lb-N/ac, average yield loss is 9 bu/ac, with 9 of 14 curves losing less, and the greatest losing 13 bu/ac
- 60 lb-N/ac, average yield loss is 16 bu/ac, with 8 of 14 curves losing less, and the greatest losing 25 bu/ac
- With the quadratic nature of yield response to N (Fig. 1), yield losses for the first increments of N below MERN can be relatively small but increase exponentially the more N is reduced
- Yield losses across the 15 curves are similar to begin with, but vary more as N is reduced, reflecting differences in shape of response curves

**Economic Risks for Underapplying N**

Yield losses are obviously important, but what really matters at the end of the day is economics. Figure 4 shows net return losses for the same curves by including the value of corn yield lost while removing the cost of nitrogen saved relative to 10-year MERN. Two sets of curves are included:

- green (dashed) curves showing net losses for what might be a 10-year average cost/price scenario ($5.00/bu corn, $0.60/lb-N)
- red (solid) curves showing net losses for what might be a current 2022 cost/price scenario ($6.75/bu corn, $1.20/lb-N)

**Figure 4.** Net losses for N rates less than MERN relative to 10-Year Average Price Scenario MERN for 15 N response curves in Ontario at both 10-year and 2022 cost/price scenarios.

Looking at the red (solid) curves (2022 cost/price scenario), as N rates are shorted from 10-year MERN by:

- 20 lb-N/ac, average net loss is $0/ac, with 11 of 15 curves losing less, and the greatest losing $18/ac
- 40 lb-N/ac, average net loss is $11/ac, with 8 of 14 curves losing less, and the greatest losing $38/ac
- 60 lb-N/ac, average net loss is $37/ac, with 8 of 14 curves losing less, and the greatest losing $96/ac

Like yield losses, there is minimal change in net losses for the first increments of N below MERN, but they eventually increase exponentially the more N is reduced.

**Net Losses for 2022 vs 10-year Average Cost/Price Scenarios**

You may notice that in this dataset we can reduce N rates 20 lb-N/ac from 10-year MERN and suffer no economic loss. Many of the 2022 (red) curves actually dip below zero losses (become net gains) for the first 20-25 lb-N reduction as cost savings in N are greater than the value of yield lost.

Because N costs have increased more than corn prices in the 2022 cost/price scenario, optimum N rates will be less than what they were using 10-year average costs/prices (as shown in Fig. 1). This is explained further in the article “How Low Should You Go? Adjusting Corn Nitrogen Rates for High Fertilizer Prices” (Rosser, 2022a).

As N-rate reductions continue, 2022 net losses eventually become greater than 10-year given the higher value of corn.

**Parting Comments**

When estimating risks for applying less N than normal, the last consideration should be “how close is my normal N rate to what is actually needed most years?”. The above loss examples are for N rates below MERN, not necessarily the actual rate being applied by the grower in each field. Of course, MERN is a bit of a moving target – it varies year-to-year and we don’t know exactly where it is going to be when applying N.

- If you are applying rates that might be well above a typical MERN, or not including N credits (manure, red clover, alfalfa etc.) where some are warranted, you may have more room before yield or economic losses apply.
- For example, if the normal N program in Fig. 2 was 160 lb-N/ac, there would have been 35 lb-N/ac of buffer even before reaching MERN.
- If you feel you are typically very close to MERN, perhaps the above values can serve as a general guide for risk.

One last consideration is if N responses (and MERN) end up being significantly greater than normal (N-losses, increased yield potential etc.) yield and net return losses could be greater than expected. Nothing beats good on-farm data to guide decision making. If you are looking for N rate guidance, or how to verify how close your N rates are to optimum on your own farm, see the article “What rate of nitrogen should I be applying on my corn?” (Rosser, 2022c).

**Acknowledgements**

Thanks to Greg Stewart, Agronomy Lead at Maizex Seeds for his comments and input on this topic and article.

**References**

Rosser, B. 2022a. How Low Should You Go? Adjusting Corn Nitrogen Rates for High Fertilizer Prices. FieldCropNews.com. https://fieldcropnews.com/2022/03/how-low-should-you-go-adjusting-corn-nitrogen-rates-for-high-fertilizer-prices/.

Rosser, B. 2022b. Understanding MERN (Most Economic Rate of Nitrogen) for Corn. FieldCropNews.com. https://fieldcropnews.com/2022/02/understanding-mern-most-economic-rate-of-nitrogen-for-corn/.

Rosser, B. 2022c. What Rate of Nitrogen Should I Be Applying on My Corn? FieldCropNews.com. https://fieldcropnews.com/2022/02/what-rate-of-nitrogen-should-i-be-applying-on-my-corn/.