An Overview of the Nitrogen Response of Corn and Wheat at the Carrington Research Extension Center from 2013 to 2024

The Soils Program at the Carrington Research Extension Center (CREC) has been heavily involved in fertility research since its inception. Almost every year since 2013, wheat or corn trials have included a nitrogen curve with plain urea as a comparison to test other treatments. Yield response to nitrogen varies widely year to year even on the same field, mostly due to climatic factors such as rainfall, temperature, and length of season. As the studies are conducted, we have the opportunity to address the objectives of each individual trial and to continue learning about the nitrogen response we can expect to see on fields. Even though the data presented only applies to the fields immediately surrounding the CREC, it provides a real-world example of the variability of nitrogen response that any farmer may experience.

The tables presented show the yield and nitrogen rate at which the highest yield was achieved, as well as the starting soil nitrogen levels. As a reference, the table also displays the nitrogen rate and corresponding yield, at which nitrogen fertilizer had the highest return on investment (ROI). The ROI was calculated using the recent local prices of $0.4 per lb of N ($368/ton of urea), $3.57 per bushel of corn and $5.92 per bushel of wheat. Each row represents an individual trial. In some years there were multiple nitrogen fertility trials. Each one has its own entry.
 

Dryland corn (Table 1).

Under dryland conditions, the average nitrogen rate producing the highest yield was 110 lbs. However, the individual values vary greatly between trials and years and range from 0 to 200 lbs of N. The average of the nitrogen rates for each trial at which return on N investment was the highest was only 57 lbs. The lowest value was 0. This was the case for 5 out of 12 trials in which the most profitable scenario would have been to forego nitrogen application altogether. The highest most profitable nitrogen rate was 200 lbs of N in a trial in 2020.

Table 1. Dryland corn nitrogen yield response from 2013 to 2024.

Irrigated corn (Table 2)
Under irrigation, the highest yield was achieved at 157 lbs N on average. The average nitrogen rate for highest return on nitrogen investment was at 107 lbs N. Unlike on dryland, there were no trial sites or years in which the highest yield was achieved with no nitrogen, neither was foregoing nitrogen the most profitable option. Both the high yields and the nitrogen rates to achieve them tended to be higher for irrigated corn than dryland corn. Notably, there was less variability in optimum nitrogen rates.

Table 2. Irrigated corn nitrogen yield response from 2013 to 2024.

Dryland wheat (Table 3)
Dryland wheat nitrogen response was more consistent than the response seen in dryland corn. Out of 15 trials only one had its highest yield associated with no nitrogen application. Furthermore, there were only three trials in which no nitrogen application would have been the most economic option. The average nitrogen rate at which yield was maximized was 91 lbs. The range for this value was 0 to 200 lbs of N. The average nitrogen rate at which profit was maximized was 47 lbs with a range of 0 to 105 lbs of N.

Unfertilized passes of wheat next to fertilized passes.

Table 3. Dryland wheat nitrogen yield response from 2013 to 2024.

Irrigated wheat (Table 4)
Under irrigation there was no trial in which yield was maximized without nitrogen application and there was only one trial in which foregoing nitrogen would have been the most economic option. The average nitrogen rate at which yield was maximized was 119 lbs with a range of 70 to 200 lbs of N. The average of the most economic nitrogen rate was 75 lbs with a range of 0 to 120 lbs. The differences between dryland and irrigated wheat nitrogen response were less noticeable than in a similar comparison in corn. Maximum yield was 71 bushels in both dryland and irrigated wheat. However, irrigation produced good yields more consistently.

Table 4. Irrigated wheat nitrogen yield response from 2013 to 2024.

Discussion
Spring residual nitrate levels in the top two feet of soil for all the trials ranged from 14 to 84 pounds (Tables 1 through 4). Residual nitrogen seems to have had no predictive value on nitrogen response in any of the scenarios. At our site, which has a loam soil and an average annual precipitation of 18.79 inches, the availability of moisture likely affected the profitability of nitrogen application as evidenced by the fact that under irrigation some amount of nitrogen application was necessary to maximize profit in almost every case, but the same was not true for dryland. This could be due to the fact that under dryland, yield potential was limited by the availability of moisture. Rainfall amounts had some correlation with highest yields in dryland corn (Figure 1) and with economic N rates in dryland wheat (Figure 2). Still, the fact that these correlations are not stronger and more consistent, suggest that the timeliness of the rainfall was a major factor determining yield and nitrogen response. Irrigation likely had a stronger impact because of the consistency with which water was supplied throughout the season.