Revisiting the Yield Goal Method for Nitrogen Management in Corn Production

What Is the Yield Goal Method?

The yield goal method calculates nitrogen application rates based on an anticipated crop yield. The simplicity of using a historical average made it accessible and appealing for widespread adoption. From 1970 to 2005, university extension personnel almost exclusively used “yield-based” algorithms, often based on the work of Standford (1966, 1973). Essentially, this method estimates a field’s yield and then multiply by a Nitrogen need factor, subtracting off any “nitrogen credits” the farming system would provide (such as legume credits).

In the mid-2000s, Iowa State Extension switched its nitrogen rate methodology to the Maximum Return to Nitrogen (MRTN). Iowa State University is currently performing the Iowa Nitrogen Initiative to envision future nitrogen recommendation rates and tools further. However, most Iowa manure plans are still filled out based on the yield goal method. As such, it is important to revisit some of these factors and how they may influence what is allowed in manure plans compared to our best-recommended nitrogen practices. Within this article we are revisiting the yield goal method to help empower farmers and technical service providers to make informed decisions supporting productivity and environmental health. As yields continue to advance, so must the methods we use to estimate and meet their potential.

Implementing the Yield Goal Method

Nf = n * YG – Ncredits

where Nf is the per-acre N application rate in lb and n = 0.9, 1.1, or 1.2 (based on the Iowa DNR map in Appendix A), YG is the yield goal. Ncredits are the adjustments made to the N requirement based on N “credits” left behind by previous leguminous crops, such as soybeans or alfalfa.

Within manure plans, the yield goal is often derived from historical yield data and adjusted to account for potential improvements. In Iowa, the yield goal is typically set as the average yield from the last five years, plus an additional 10% to account for advances in crop genetics, management practices, and technology. Farmers could account for incremental yield improvements by applying a fixed adjustment without overly complicating the process. However, as yields continue to increase, this 10% adjustment factor for attainable yields does as well, effectively allowing more nitrogen application rate as a function of time beyond what increasing yields alone would otherwise allow.

How well does this estimate do at predicting corn yield?

To estimate this, I’m using Iowa Agricultural Survey data on corn yields for the state and then later at the county level. At the state level, we can compare actual yield data in any given year to the estimated “yield goal” for that year. If we use the current method, the average yield over the last five years plus ten percent, on average, the state-level deviation is 12.2 bu/acre. However, the best yield estimate is to add only 4% instead of 10%; in so doing, the average yield deviation is 11.3 bu/acre. Another approach is to use trendline yield to estimate the yield in the next year; while this method is slightly more accurate, the average deviation is still 9.5 bu/acre.

This same approach can be done on a county-by-county basis for all Iowa counties, but the findings are similar, the best yield fit is 4% with a 0.4% standard deviation among counties. However, even at the county level, county trend line yield was always a better indicator of the average county yield and didn’t require calibration for use as the estimated yield.

Implications

So why am I doing this or talking about it? Within the yield goal, we’ve used 10% to determine the achievable yield, but yields continue to increase, and as a result, this 10% factor is getting more significant as a function of time. However, there is no evidence that deviation from trend line yields is increasing, but instead has been constant with time (figure 1).

Figure 1. Corn yield deviation from trendline yield as a function of time for Iowa corn yields. Deviation from the trendline has been nearly constant.

Moreover, if we are trying to fertilize the crop we expect to harvest, adding this 10% doesn’t give us the best crop estimate; instead, a 4% increase would be better. Stronger still would be estimated based on trendline yield. If we use the yield goal method as a regulatory tool, this 10% increase makes sense to allow flexibility for individual fields. However, if you are using that as the basis to understand what yield you expect to achieve next year and how much fertilizer you need, there are better estimates. 

There is a lot of uncertainty in nitrogen fertilizer recommendations. I’m excited to see where the Iowa Nitrogen Initiative takes us. Still, in the meantime, if you are filling out Manure Management Plans using yield goals and this to estimate your manure application rate for next year, it might be time to rethink the yield number and if that is the best use of the manure resources on your farm, and hopefully the Iowa Nitrogen Initiative will help direct us towards more effective nitrogen application rate methodologies.