Protein-rich soybean meal is a premier feed source for poultry and livestock around the world. However, the concentration of protein in soybean seed has declined significantly over the past few decades, and US soybean has fallen below South American soybean in seed protein concentration. For US soybean to maintain its value in the global marketplace, this trend of declining seed protein must be reversed. Seed protein concentration depends on both genetics and environment. Soybean breeders are working to genetically improve seed protein concentration, but breeding is a long-term process, and a more immediate response would be valuable. This project was funded for $142,240 in 2019, to investigate whether or not growers can improve seed protein concentration through environmental manipulations—specifically, by optimizing their farm management practices.

The objectives were to determine how farm management decisions, such as fertilizer application and tillage, impact soybean seed protein concentration in the final crop. Seed protein concentration is closely linked to nitrogen metabolism, so in addition the impact of these management practices on nitrogen metabolism in diverse soybean genotypes was investigated. Developing a better understanding of these factors, and the relationship between them, will not only give growers better information on how they can optimize seed composition on their own farms, but will help producers develop new soybean varieties with higher seed protein.

The project began with a meta-analysis of previously published, peer-reviewed scientific data. Information was aggregated from over 70 studies that examined the impact of various farm decisions on soybean seed protein content. The benefit of this meta-analysis strategy was that results based on data from multiple studies, conducted under a wide variety of conditions, are more likely to be useful for growers, who farm in many locations and with many soybean varieties, than are results from a single study that likely used just one or two sites and soybean genotypes.

In addition to the literature analysis, field studies were conducted to more closely examine these relationships between environment and seed protein concentration. In 2018, testing began on the impact of nitrogen application, sulfur application, tillage, and irrigation on seed protein concentration in soybean. These tests were conducted at multiple locations in both North Carolina and South Carolina. Physiological and agronomic data, such as pod number, yield and quality (protein and oil), were collected in six soybean genotypes (Benning, Benning Hi-Pro, Nitrasoy, AG59X, NLM09-77, N7003CN) at four sites in the two states. In 2019, a plant density test was added to the project, and again data was collected for six genotypes at six sites representing diverse soil types throughout the Carolinas. Samples from the 2019 soybean harvest are currently being processed. Data from both years across multiple sites will be analyzed to provide new insights into seed protein concentration.

This project will have both short-term and long-term impacts for growers. Upon completion, the data from this project will identify management factors that influence soybean protein concentration, which growers can immediately use to make decisions on their own farms. In the longer term, information that this project uncovers about nitrogen metabolism and differences is physiological and agronomic responses among soybean genotypes will improve efforts to increase soybean seed composition through conventional breeding and through biotechnology-based improvements.