Method

• Two nitrogen (N) studies were established to assess the effect of N rates on corn yields under dryland and irrigated conditions at Oakes in 2019.
• N Treatments were: 0, 40, 80, 120, 160, 200 lbs applied as urea, on the soil surface, before strip-tiling corn.
• Previous crop was soybeans. Variety was: REA 4B953-RIB.
• Soil N and SOM before fertilizer application were 24 lbs N and 4.1%, respectively at the dryland site.
• The soil N and SOM content at the irrigated site were 38 lbs and 2.5%, respectively.
• Dryland corn was seeded on May 16^th and harvested on December 2^nd.
• Irrigated corn was seeded on May 17^th and harvested on November 26^th.

Results

• Yields were greater at the irrigated site than at the dryland site (Figure 1).

• At the dryland site, yields ranged from 126 bushels without N applied to 195 bushels at 120 or 160 lbs N, for an average yield of 172 bu/ac.
• Under irrigation, yields ranged from 131 bushels, without N applied, to 271 bushels at 200 lbs N, for an average yield of 238 bu/ac.
• Under dryland conditions, a comparison between average yields (Table 1) showed that the yields produced from 80 lbs N were not different from the yields produced at higher N rates, but was significantly greater than yields at 40 lbs N or without applied N.

• Under irrigation, the average yield at 40 lbs N was not different from yields at higher N rates, but was not also different from the 0 lbs N.
• Based on the graphs, the 142 lbs N was estimated to produce maximize yield of 179 bushels under dryland. Under irrigation, estimated maximum yield was 256 bushels, if 165 lbs of N was applied.
• However, these estimated N fertilizer rates were not economic N rates to maximize yields. Nitrogen contribution from soil and N credit from soybeans likely contributed the high yields at lower N rates.

Conclusions

• Even though N fertilization improved yields significantly, N in soil before fertilization (38 lbs under irrigation, and 24 lbs under dryland) and N from soybean credit (about 40 lbs N), no more than 80 lbs N was necessary because of N contribution from soil and soybean credit.
• Like for any fertilizer input, improved yields do not always translate to economic gains.
• Therefore, farmers are always encouraged to make their fertilizer decisions not based solely on yield increases from fertilizer application, but very importantly, should consider the net returns considering cost of additional input beyond recommended rates.  

Acknowledgement: Thank you to the North Dakota Corn Council for funding this research.

Jasper M Teboh, Ph. D.
Jasper.Teboh@ndsu.edu
Research Soil Scientist