Comparing N-FACT to the Yield Goal Method: A Look at Nitrogen Recommendations

A few years ago, I took a deep dive into two common approaches for determining nitrogen application rates: the Yield Goal method and the Maximum Return to Nitrogen (MRTN) approach, illustrating which was higher in each county in Iowa and how similar or different they were. That comparison highlighted the strengths and limitations of each method, offering insights into how they influence manure management planning and nitrogen use efficiency. With Iowa State University’s recent release of the Nitrogen Fertilization Application Consultation Tool (N-FACT), it seems like the perfect time to revisit this discussion, comparing N-FACT to the Yield Goal method.

 

N-FACT represents a step forward in nitrogen management, leveraging updated on-farm data and modeling techniques to refine nitrogen recommendations. But how does it compare to the long-standing Yield Goal approach in the Iowa Manure Management plan form? My goal here isn’t to summarize either method but to compare what the two methods suggest. In so doing, it gives some insight into who may be selecting each technique and why that may be.

In this post, I’ll walk through how N-FACT differs from the Yield Goal approach, present some figures to illustrate their recommendations across different scenarios and share key takeaways on their practical applications.

 

Manure planners are probably extremely familiar with the Yield Goal method. Still, the county average corn and soybean yield were briefly estimated using the last five years of county yield data (2019-2023). The amount of continuous corn and corn following soybean in each county was calculated from the area of corn planted and the area of soybean grown in each county. Corn following soybeans are set equal to the amount of soybeans in each county (but not allowed to exceed the acreage of corn for that county); continuous corn acres were calculated from the excess corn acres not yet accounted for in each county. Corn yields in continuous corn and corn following soybean were estimated by assuming a 10% yield drag in continuous corn as compared to corn following soybean and requiring the county-weighted average corn yield to equal the reported county average corn yield from the survey of agriculture. Yield goal methodology was then applied, setting the yield goal for corn equal to the estimated yield for that rotation plus ten percent, multiplying by the area-weighted nitrogen use factor for corn (typically 1.2 lb. N/bu for most Iowa counties, but adjusting as necessary for areas that are 1.1 lb. N/bu and 0.9 lb. N/bu, see figure 1). For corn-soybean rotations, a “rotation” credit of 1 lb N/acre per bushel of soybean yields up to a maximum of 50 lb. N/acre was used.

Figure 1. Nitrogen use zones for corn in Iowa for use in the Yield Goal method in Iowa Manure management Planning. Zone 1 has a nitrogen use factor of 0.9 lb. N/bu, Zone 2 has a nitrogen use factor of 1.1 lb. N/bu, and Zone 3 has a nitrogen use factor of 1.2 lb. N/bu.

 

N-FACT works differently, following an approach similar to Maximum Return to Nitrogen (MRTN) but based on modeling efforts that simulate the yield response of corn to nitrogen in each county under various weather conditions, planting options, and management scenarios. For this tool, you select a county, the rotation, weather conditions for spring and summer, the amount of residual nitrate in the soil, a planting date, and a nitrogen and corn price. Inputs used in these simulations were done in all counties for corn following corn and corn following soybean, but then consistent weather patterns (average spring moisture, average summer moisture, a residual soil nitrate of less than 20 lb. N/acre, a planting date before April 30^th, a nitrogen price of $0.45 per pound N, and a corn price of $4.64 per bushel. N-FACT provides a range of outputs from the 25^th percentile economic optimum nitrogen rate, the 75^th percentile economic nitrogen application rate, and the average economic optimum nitrogen application rate. A modeled yield is provided for the 25^th and 75^th percentiles and the average economic optimum nitrogen application rates. The N-FACT tool uses a concept from MRTN: the response from applied N determines the economic optimal rate, not the yield itself. As such, even if actual yields differ from those suggested by N-FACT and the underlying modeling behind it, the response curve generated is still the best estimate of the N rate required. All that to say, when the N-FACT reports the 25^th percentile economic optimum N-rate, it suggests that 25% of the modeled site-weather years in the county would require a lower nitrogen rate to be economically optimal. Similarly, when it reports the 75^th percentile N rate, it suggests that 75% of site-weather years within the modeled scenario would have a lower economically optimum N rate.

 

An essential consideration for manure planning is how different locations will be impacted if a switch from the yield goal method to the N-FACTs recommendation is evaluated. For both rotations, we grouped counties into three groups: counties where the yield goal nitrogen estimate was less than the 25^th percentile economic N rate from N-FACTS (N-FACTS suggests considering higher nitrogen rates). In these counties, the yield goal nitrogen estimate is between the 25^th and 75^th percentile economic optimum N (N-FACTS and the yield goal method are similar), and locations where the yield goal estimate exceeds the 75^th percentile economic optimum N rate. These counties were colored blue (Yield goal lower than the 25^th percentile N-rate from N-FACTS), white (Yield goal is between the 25^th and 75^th percentile N-rates from N-FACTS), and orange (Yield goal is higher than the 75the percentile N-rates from N-FACTS), respectively.

 

(a)	(b)
Figure 2. Comparison of the Yield Goal Method (based on county average yields) and N-FACTS nitrogen suggestions for (a) corn following soybean and (b) continuous corn. Counties colored blue have a yield goal N rate below the 25th percentile economic optimum N suggestion from N facts, counties left white have a yield goal N rate between the 25th and the 75th percentile economic optimum N rate suggestion from N facts, and counties in orange have a yield goal N rate above the 75th percentile economic optimum N rate suggestion from N facts.

 

The maps in Figure 2 suggest that counties on the Des Moines Lobe, the Iowa Erosion Surface, and much of the Northwest Iowa Plains farms should continue to fill out their manure management plans using the yield goal method, but at the time of manure application should be considering opportunities to lower their actual nitrogen application rates (through both manure and commercial manure fertilizer) to be more in line with the guidance suggestions coming from N-FACTS. In the remaining regions of Iowa, primarily the Alluvial Plains, the Loess Hills, the Southern Iowa Drift Plain, and the Paleozoic Plateau, we should carefully consider our nitrogen application rates and obtained corn yields to evaluate improvement opportunities. Specifically, N-FACT suggests that higher nitrogen application rates than those indicated by the Yield Goal method of the Iowa Manure Management Plan form may be required to reach optimum economic thresholds. However, the N-FACTS tool suggests that these locations can achieve higher yields. As such, choosing to apply higher nitrogen rates should be accompanied by evaluating corn yields and practices that may help increase corn yields.

While the monolithic evaluation provided in Figure 2 helps answer the question of writing a manure plan with a yield goal compared to writing a manure plan with N-FACT, it doesn’t address the magnitude of the change. Overall, the results are similar; the counties in red on the Des Moines Lobe and the Iowa Erosion Surface are locations where the yield goal is higher than the 75^th percentile N suggestion from the yield goal. In many of the remaining counties, the green color indicates that the yield goal would need to increase by this much to reach the 25^th percentile N suggestion. While the graphs are similar, these figures illustrate the magnitude of change that could cause.

(a)	(b)
Figure 3. Comparison of the Yield Goal Method (based on county average yields) and N-FACT nitrogen suggestions for (a) corn following soybean and (b) continuous corn. The scales demonstrate the magnitude of difference between the yield goal method and the N-FACT suggestion. Counties in red show N-FACT is lower than the yield goal suggestion, and counties in green show N-FACT above the yield goal suggestion.

 

One important point to clarify is writing your manure plan based on a method and selecting a nitrogen application rate. Writing a manure plan to allow flexibility in decision-making is often helpful. Choosing methods that would enable higher nitrogen application rates and include more acres than the minimum required to write an acceptable plan provides the needed flexibility. Such an approach puts us in a position to make the best agronomic decisions within a growing season to respond to unknowing weather patterns; this means not treating the manure plan rate as a maximum rate but choosing a target rate below this to make the best use of manure. It also plans to increase the number of acres our manure touches to maximize its value.

Writing plans for higher nitrogen application allows flexibility without having to write amendments to the plan should weather conditions in any specific year put us in a situation where more nitrogen is required. An example of this might be if a wet spring occurs and additional nitrogen is needed to address this concern. One example of how to do this is that plans choosing to use N-FACT as their methodology could be written for average moisture conditions, but should wet spring conditions occur, the farm would be allowed to modify to wet weather conditions without filing an amendment.

One important question that remains is how a change from yield goal to N-FACT recommendations would impact overall nitrogen recommendations in the state. In the map, you can see areas where more nitrogen is required and areas where less nitrogen would be suggested, but it is hard to say what this means for state nitrogen use. Weighting the state by the number of corn acres in each county suggests that if all acres were applied based on a yield goal methodology and switched to the N-FACT suggestion, the average nitrogen application rate would decrease by about ½ lb. N/acre. As I follow up, it will be important to compare what this means for some of the manure-rich areas in the state and the implications for manure management planning.

Another aspect of N-FACT is the on-farm nitrogen rate trials. The on-farm trials provide data on the economic optimum nitrogen rate, yield at the economic optimum nitrogen rate, and the nitrogen use efficiency at the optimum nitrogen rate for each trial conducted. Thus far, data from two trial years, 2023 and 2024, have been available. The nitrogen use efficiency at the economic optimum nitrogen rate provides another point of comparison for the yield goal method. In 2024, the on-farm trials for corn following corn averaged an N use efficiency of 1.00 ± 0.19 lb. N/bu (ave.  ± s.d.). One interpretation of these results is that 84% of fields (assuming the fields evaluated represent Iowa fields) are below the 1.2 lb. N/bu.

As we look at the on-farm trials for corn following soybean, the obtained N use efficiency was 0.92 ± 0.19 lb. N/bu (ave.  ± s.d.). Again, this can be interpreted as 84% of fields (assuming the fields evaluated represent Iowa fields) are below 1.12 lb. N/bu, that on average corn following soybean crops would be allowed in Iowa Yield Goal based plans. This value is based on the 231 bu/acre corn yield (average of corn following soybean plots) and a 50 lb. N/acre rotation effect that allowable N rate is 1.10 N/bu.

Data is also available for 2023, but the previous crop isn’t listed. The measured N use per bushel of corn at optimum N rate was about 90% of the 2024 value, and the standard deviation was 0.14 lb. N/bu. As 2023 was drier in much of the state, these results are consistent with what we’d expect. Higher rainfall led to some nitrogen losses in 2024 and required higher nitrogen inputs per bushel. Moreover, the drier conditions led to more consistent results as the soil variation was slightly reduced. Assuming the 90% reduction is consistent for continuous corn and corn following soybean, then 0.92 ± 0.14 lb. N/bu was required. In continuous corn, 97.5% of all fields would be below the 1.2 lb. N/bu is used for much of the state for yield goals. For corn following soybean, the N needed was 0.85 ± 0.14 lb. N/bu. Given the yields in 2023, the YG method would have allowed 0.98 lb. N/bu. In 2023, 84% of fields in corn following soybean would be below the N level allowed by the yield goal method.

A few things come to mind as I look at the totality of the results. The yield goal method is essentially performing how it should be. It limits N application based on agronomically allowable amounts without constraining yield limits by limiting nitrogen application. I.e., those constraints were initially set to be a guide that didn’t limit N to the point yields would be unduly constrained, but it wasn’t meant to be a nitrogen recommendation system. It sets a maximum limit based on the best available science at the time, not an application rate suggestion. We should be thinking about facilitating better manure management and moving our rates to not the maximum allowable but more towards the best recommendation or best available guidance methodology. In some cases, this could mean increasing nitrogen application rates, but often, these must be accompanied by practices that are increasing corn yields.