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S.D. Nitrogen Fertilizer Study Summary
Saturday, March 1, 2025
filed under: Fertility
A three-year sunflower fertility study conducted by researchers at South Dakota State University provides more affirmation of the role of nitrogen fertilizer inputs and placement in the production of optimal sunflower crops. The study, led by SDSU cropping systems agronomist Thandiwe Nleya, was conducted in 2022 through 2024, with results evaluated from five site-years. The South Dakota Oilseed Council provided financial support for this study.
Combined across the five-site years, the analyzed data showed sunflower seed yield peaking at about 75 lbs applied N per acre, with no significant increase occurring at higher rates. “However, location and year significantly impacted seed yield,” Nleya reports, “with the highest yields obtained at Brookings in 2024 (3,048 lbs/ac) and the lowest at Highmore (near Pierre) in 2023 (1,812 lbs/ac).” Seed oil content decreased as the N fertilizer rate increased, with the highest N rate of 125 lbs/ac having the lowest oil content (41.7%). Protein concentration increased as the N fertilizer concentration increased.
“Decreasing seed oil content due to increase in N fertilizer rates was compensated for by the seed yield increase up to 75 lbs/ac,” Nleya says. “For example, across five sites, an application of 75 lbs/ac N— plus inclusion of the carryover N —resulted in approximately 13% to 18% more seed yield and 6% to 16% more oil yield in comparison to the control (0 lbs N/ac).”
The SDSU results suggest that the placement methods (broadcast vs. band) evaluated in this study did not make a significant difference in terms of sunflower plant growth and yield, according to Nleya. “The only difference we observed was slightly larger head diameter (by 0.2 cm) for broadcast placement compared to band placement,” she observes. She simultaneously emphasizes that the N fertilizers used in the study were treated with urease inhibitor, “and this might have prevented N losses due to volatilization — which would have affected the broadcast treatment more.”
The SDSU test sites also showed that N fertilizer rates higher than 75 lbs/ac increased chlorophyll content in leaves; the higher rates likewise increased plant stem diameter and sunflower head diameter — but at the expense of seed yield and oil content. Not surprisingly, “excessive addition of N causes an imbalance between the vegetative and reproductive phases of the crop,” Nleya points out, “stimulating uncontrolled vegetative growth and delaying plant maturation.” The overall combination of these factors increases the likelihood of lodging, disease and pest attack while reducing sunflower yield, she concludes.
Tom Young, longtime sunflower grower and current executive director of the South Dakota Oilseed Council (SDOC), says that while producing no big surprises, the SDSU nitrogen fertility study affirms a couple things: First, sunflower growers can increase their yields — and profitability — by adding a basic amount of fertilizer. And second, the variation in crop response to added N in this study is reflective of what often happens in growers’ fields.
“Prior to SDOC funding this study, the National Sunflower Association had funded a fertilizer study in which the results showed no need or no response to applied fertilizer,” Young adds. “However, that study did not test the amount of nitrogen located in the soil before the tests. That was a head scratcher. Soil tests are a must in most operations and will help growers determine if it would be helpful to add nitrogen for increased yield.
“Manage sunflower like the big crop it is, and it will provide a big return,” Young concludes.