The average protein concentration of soybean grain has steadily decreased over the past 20 years, threatening the value of the commodity. Soybean protein concentration is nominally reported as 40%, but average protein concentration today is 34% and is considerably lower than this in the northern US. Because protein production requires large amounts of nitrogen, management and genetic strategies to slow or reverse the decline in protein concentration require accurate methods to determine the relative contributions for the two sources of nitrogen nutrition - nitrogen fixation and soil mineral nitrogen. These measurements require the use of non-nodulating genotypes that are of similar maturity to nodulating cultivars being grown by farmers that also have similar yields when provided with adequate nitrogen fertilizer. Currently, there are no non-nodulating genotypes in any maturity group that are comparable to high-yielding cultivars grown today. This research project is creating a suite of nodulating and non-nodulating near isolines of soybean in each of the major maturity groups (MGs) grown in the US (MGs 0 through 8). These lines are a critical tool for developing management methods or cultivars with increased seed protein. We are finalizing release notes for MG 4 isolines that when fertilized heavily with nitrogen fertilizer have yield similar to commercial MG 4 cultivars. Isoline pairs for the other MGs are in advanced stages of development.

Outlined below are the steps needed to capture benefits from this project for all US soybean Farmers. University of Arkansas (Larry Purcell and Caio Vieira): The nine pairs of MG 5.4 isolines (BC3F2:4) harvested this fall will be increased in winter nursery for field evaluation in 2024. We plan to have experiments at two locations with these nine isolines plus the recurrent parent (R13-14635RR:0010) and Lee-nonnod. At both locations, 150 lbs N/acre will be applied at V6 and then again at R2. These experiments will guide the decision on which lines to release as germplasm late in 2024 or 2025. University of Minnesota (Aaron Lorenz): During the next year, we will focus on increasing seed quantities for field evaluation. Ideally, we could obtain some funding to genotype induvial plants with whole-genome markers to create pairs of NILs that are more isogenic to one another as determined by marker similarity. We also need to prepare a Journal of Plant Registration article to announce the availability of these NILs. University of Nebraska (George Graef): After our analysis of the 2023 initial yield results and other agronomic data, we will narrow down the selection of final isoline pairs that will be deposited in the USDA Soybean Germplasm Collection and available for distribution for further research. We will need another seed increase in 2024 to get sufficient amounts of seed for distribution to the soybean research community. We also plan a replicated, multi-location yield test of selected isoline pairs during 2024. Kansas State University (Bill Schapaugh): An initial set of MG4 non-nodulating releases should be completed in early 2024. These lines have been extensively phenotyped and genotyped. Seed increases in AR and KS should be adequate in quality and quantity to provide to the USDA Germplasm Collection. And, as already noted in previous reports, some of these initial non-nodulating lines and the recurrent parent have been distributed to several other researchers in other states. So, initial benefits from this work are being captured for US soybean farmers as this project continues. In KS, backcrossing has been completed using four additional recurrent parents, but phenotyping, some additional genotyping, and seed increases need to be completed on this material before additional genotypes are available for release. This work will require at least one more growing season and year, and perhaps two years to complete. University of Georgia (Zenglu Li): Soybean has the ability to obtain its own nitrogen through a symbiotic relationship with nitrogen-fixing bacteria, Bradyrhizobium japonicum. Nitrogen fixation in the plant eliminates the need for nitrogen fertilizers and is fundamental to seed protein composition development. The NILs, markers and mapping positions of Rj5 and Rj6 genes generated through this project will help soybean breeders/researchers understand the genetic controls of nitrogen fixation which could further help identify genotypes that increase yield. The genes and marker information will be useful tools for the soybean researchers to study the uptake and utilization of nitrogen in soybean plants and mechanism of nitrogen fixation.