Assessing genetics, biophysical, and management factors related to soybean seed protein variation in Brazil.

Abstract: The demand for high-quality soybeans is increasing. The composition of soybean grain can vary with genetics, biophysical, and management factors. In particular, studies on protein concentration are increasing worldwide. The objectives in this study were: (i) to quantify the genetic effects on seed protein concentration and (ii) to identify the biophysical and management factors affecting seed protein concentration in soybean production systems in Brazil. We collected soybean samples and crop management data through surveys in 194 soybean farms in two growing seasons (2018/2019; 2022/2023) across eleven states in Brazil. Seed protein was determined by the Kjeldahl method. Random forest regressions and comparisons between high and low protein fields to identify the main causes of variation in soybean protein concentration were used. Fields with highest protein concentration were observed in older cultivars released in (2011), at lower yields (3082 kg ha−1), late sowing (DOY 313), higher temperatures (25.6 °C−1) and a lower photothermal coefficient (0.79 MJ m−2 d−1 °C−1). Conversely, low protein concentration was observed in fields with higher yields (4220 kg ha−1), early sowing (DOY 313), lower temperatures (24.8°C−1) and a higher photothermal coefficient (0.84 MJ m−2 d−1 °C−1) and newer cultivars released in (2016). The regression tree and random forest explained 58 % of the protein variability, including cultivar (39 %), latitude (12 %) and sowing date (7 %). Cultivar was the most important factor affecting soybean protein concentration, followed by sowing date. The year of cultivar release, breeding company, latitude, temperature, photothermal coefficient and water supply also affected the final concentration of soybean seed protein. The results emphasize the need for breeding programs to evaluate protein concentration in new soybean varieties. Additionally, we now have clear biophysical and management indicators to help achieve higher protein concentrations in soybean crops.