What do Iowa Nitrogen Innovative on-farm trials say about optimal N rate and N per bushel?

 Every year, we ask the same question: “How much nitrogen do I really need to grow this corn crop?” And despite improving technology, we still often give the same, honest answer: “It depends.”

That’s why the Iowa Nitrogen Initiative (INI) is running hundreds of on-farm trials. By testing in real fields under real conditions, we can get closer to answering the “how much N” question in a way that fits Iowa corn growers. I dug into nearly 500 of those trials from 2023 and 2024 to see what they tell us about yields at the economic optimum N rate (EONR), how much N it takes to get there, and how efficient that N is when measured on a per-bushel basis.

I’m going to make this article a little different, I’m going to highlight what I found first, and if you want to see the statistics behind it and the data processing, I did to get there read to the end.

Why Does This Matter for Iowa Farmers?

·         Rotation helps because it boosts yield, and higher yields mean better N efficiency. On average, when corn followed soybeans, yields were about 19 bu/ac higher (239 vs. 220 bu/ac) than when corn followed corn.

·         Higher yield potential means more total N, but less N per bushel. When I looked at the economic optimum N rate across trials, a pattern popped out: for every additional bushel of yield, it took about 0.45 lb more N per acre.

·         Regional and year effects are real. Your neighbor’s optimum N rate might not match yours, because soils and weather shape the response.

This is why INI trials matter: they help us put real numbers to what we’ve all seen in the field. It’s not a perfect crystal ball (the models only explained about 25% of the variation), but it’s a step closer to giving farmers confidence in their N plans, and showing that chasing higher yield potential, when realistic, can also improve nitrogen efficiency, and more importantly, that there is work to do to understand what is driving the other 75% of the variation in need.

Alright, that’s the 30,000-foot view. But if you’re like me, you want to peek under the hood and see how we got those numbers. Here’s how the data was pulled together and what the statistics say.

Data & Scope

I analyzed 493 on-farm nitrogen (N) response trials from the Iowa Nitrogen Initiative (downloaded from N-FACT). Trials with alfalfa as the previous crop (n=1) was changed to an unknown crop, to alleviate the non-replication struggle I was having, and one 2024 Western Region outlier (99 bu/ac yield at 226 lb N/ac; 2.8 lb N/bu) was excluded. Four factors were available to explain variation: region, year, previous crop, and yield at the economic optimum N rate (EONR).

That leaves 499 trials left in the dataset that we can use to try to understand the yield, optimum N rate, and the N-use factor. In the currently existing dataset, there are four variables that we can use to understand these factors; they are MLR, year, the previous crop, and the yield of corn at optimum N rate.

Yield at Optimum N Rate:

We can write a statistical model of:

Using this statistical model, we can describe 22% of the variation in the corn yield at Optimum N. The Region, region x year interaction, and previous crop were all significant (p = 0.0001, 0.0005, & p < 0.0001) respectively.

The previous crop indicated that corn following soybean averaged 239 ± 4 bu/acre, while corn following corn averaged 220 ± 5 bu/acre. That 19 bu/ac bump isn’t just nice, it’s the main reason corn-after-soybean looks more efficient on a per-bushel basis. It’s yield driving efficiency.

The year x region interaction was driven by a large yield increase in 2024 relative to 2023 in the “Illinois and Iowa Deep Loess and Drift” (27 bu/acre) and “Central Iowa and Minnesota Till Prairies” (14 bu/ac) while other landform regions didn’t change significantly with time, though the “Northern Mississippi Valley Loess Hills” declined by 26 bu/acre in ’24 relative to ’23, the low sample size kept it from being statistically significant.

Figure 1. Average corn yield by region and year. 2023 yields not sharing the same lower-case letter are statistically different α = 0.05, 2024 yields not sharing the same upper-case letter are statistically different, and yields within the same landform region not sharing the same number are statistically different.

Optimum N rates:

This analysis describes 26% of the variation in optimum N rate. The previous crop term was not significant so it was removed from the analysis. The yield at economic optimum and the region x yea interaction were all significant or near significant (p < 0.0001, p = 0.396) respectively.

In this analysis, the economic N rate was a function of the expected yield at economic N. For every bushel increase in yield at optimum N, the estimated increase in the economic optimum N rate was 0.45 ± 0.06 lb N/acre. In this analysis there was no difference between the economic optimum N rate for corn following corn and corn following soybean, though trial did indicate that corn was planted approximately 12 days earlier on average following soybean than if it followed corn. That means if you’re pushing 250-bushel yields, don’t be surprised when the EONR is a good 20–25 lb higher than a 200-bushel field, yield does matter.

Again, their was significant interaction between year x region was primarily driven by the large increase in Economic Optimal N Rate in the “Eastern Iowa and Minnesota Till Prairies” (27 lb N/acre) and no other regions seeing a statistically significant difference, though the “Iowa and Minnesota Loess Hills, Iowa and Missouri Deep Loess Hills” and the “Iowa and Missouri Heavy Till Plan” both saw large optimum nitrogen need in ’24 relative to ’23 (16 & 32 lb N/acre respectively).

Figure 2. Average economic optimum N rate by region and year. 2023 yields not sharing the same lower-case letter are statistically different α = 0.05, 2024 yields not sharing the same upper-case letter are statistically different, and yields within the same landform region not sharing the same number are statistically different.

N Use Factor:

Using this statistical model, we can describe 24% of the variation in the Nitrogen Use Factor at Optimum N. In this model most terms were significant or near significant, including year (p = 0.1044), previous crop (p = 0.0599), year x region (p = 0.0394), and yield at economic optimum nitrogen rate (p < 0.0001).

One of the key findings of this analysis is that the N use factor decreases with increasing yield, by about 0.0021 ± 0.0002 lb N/bu. While this may not sound like much, it suggests a decrease of about 0.1 lb N/bu in moving from 175 bu/acre corn to 225 bu/acre corn. In other words, the higher your yield potential, the less N each bushel needs to get made. That’s the story behind the push for better genetics, drainage, and management; it’s not just more yield, it’s better efficiency.

We also saw there was a significant difference in lb N/bu in corn following corn (0.97±0.03 lb N/bu) as compared to corn following soybean (0.93 ±0.02 lb N/bu). Earlier we reported that there was a 19 bu/acre yield advantage that corn following soybean had relative to corn following corn, which corresponds to this difference in nitrogen use efficiency.

Figure 3. Average nitrogen per bushel by region and year. 2023 yields not sharing the same lower-case letter are statistically different α = 0.05, 2024 yields not sharing the same upper-case letter are statistically different, and yields within the same landform region not sharing the same number are statistically different.

Conclusions

The Iowa Nitrogen Initiative is helping us move beyond rules of thumb and into real-world, Iowa-based numbers. We’re learning that yield is the biggest driver; more yield means more N total, but less N per bushel. Rotation helps mainly by boosting yield. And soil and weather patterns mean your field’s story may not match your neighbor’s.

The other lesson? We can only explain about a quarter of the variation in optimum N. That means three-quarters is still a mystery; soil differences, hybrid genetics, weather patterns, timing, and maybe things we don’t even have on our radar yet. That’s why more trials, more farmers, and more data are the only way we’ll keep sharpening this picture.