FRANKFORT, Ky. – Kentucky State University (KSU) researchers may have found a possible solution to the decline in seed protein concentration, which affects the quality of soybean meal used in livestock feed: late-season nitrogen applications.

Led by Deepak Khatri, KSU Agronomy Lab research associate, under the mentorship of Anuj Chiluwal, KSU Agronomy Lab principal investigator and assistant professor of agronomy, the two-year study was conducted at KSU’s Harold R. Benson Research and Demonstration Farm.

The study used two soybean cultivars (specific breeds or varieties) from different maturity groups, and examined how biochar and late-season nitrogen applications affect soybean yield and seed quality. Biochar refers to a carbon-rich, charcoal-like substance produced by heating agricultural and forestry waste in a low- or zero-oxygen environment, according to KSU.

The research evaluated the effects of biochar and late-season nitrogen fertilization on soybean yield and seed composition across two maturity group soybean cultivars. The study found late-season nitrogen fertilization increased seed protein concentration by 1.2 to 2.8 percent across both growing seasons.

Researchers said oil concentration was not affected, indicating the strategy improved protein without creating a trade-off in oil quality: “Declining soybean protein concentration is a major concern because it directly affects the quality of soybean meal used in animal feed,” Khatri said. “Our research provides evidence that lower nitrogen applications during the seed-filling period can help reverse this trend.”

Chiluwal told Farm World soybean yield in the U.S. has increased gradually over the past few decades; however, there is also a constant decline in seed protein concentration during the period, which has become one of the concerns for the U.S. soybean industry.

“Recent studies have suggested nitrogen limitations during seed filling period in modern high-yielding soybean cultivars are the main reason for the decline,” he said. “Therefore, we conducted this study where we tested different doses of nitrogen fertilizations during its seed filling stage to see if it can improve seed protein concentration.”

He said the findings provide a potential solution for soybean farmers and the industry to address the declining trend in seed protein concentration: “However, it is important to note that soybean obtains the majority of its nitrogen through biological nitrogen fixation, which is the most efficient source of nitrogen for soybean. Therefore, any additional nitrogen application should not suppress this process.”

According to Iowa State University, biological nitrogen fixation is the process by which specialized, free-living microorganisms convert inert atmospheric nitrogen into ammonia, which makes nitrogen accessible to plants for critical growth functions.

“To address this concern, we are currently conducting further studies to evaluate the effects of nitrogen fertilization on soybean biological nitrogen fixation, and determine whether this approach can sustainably improve seed protein concentration before recommending this practice to farmers,” he added.

Researchers said the study’s research takes a different approach from previous studies by applying varying doses of nitrogen fertilizer during the seed-filling period, when the crop’s nitrogen demand peaks and biological nitrogen fixation and soil nitrogen supply may no longer be sufficient under high-yielding conditions.

Chiluwal said the findings are significant because earlier efforts to improve soybean protein through late-season nitrogen applications often relied on fertilizer rates too high to be practical for producers.

“Previous studies aimed at increasing soybean protein through late-season nitrogen applications relied on extremely high rates – ranging from more than 200 to 870 kilograms per hectare – making them neither economically viable nor environmentally sustainable,” he said. A hectare is a metric unit of land area equal to exactly 10,000 square meters.

In contrast, he said the study tested lower nitrogen rates of 40 to 120 kilograms per hectare, applied in three split applications during the seed-filling period: “The findings demonstrate that even modest nitrogen applications can improve seed protein concentration, offering a more practical and scalable solution to a significant industry challenge.”