Impact Assessment of Main Stem Elongation and Wind Speed on Lodging of Soybean Cultivar 'Miyagishirome'

Abstract Problem: Soybean lodging (plants that fall over) results in a 9–34% yield loss. In Japan, there is high demand for indigenous cultivars, and it is often difficult to switch to lodging-resistant cultivars. As a countermeasure against lodging, "pinching", which is pruning the upper part of the trunk when overgrowth is expected, is attracting attention. However, pinching reduces the yield when the risk of lodging is low. Therefore, it is important to determine the risk of lodging before pinching is implemented. Although previous studies have shown that lodging is caused by the effects of main stem length and wind speed, there are still some questions that require clarification, e.g., the growth stages that have a high influence on lodging. Objective: The objective of this study was to analyze the effects of main stem elongation and wind speed on lodging for the establishment of a future method to predict lodging. Methods: We used data obtained from experimental plots with different cultivation management in the years 2018, 2020, and 2021 (n = 32). The cultivar tested was “Miyagishirome”, which is a major cultivar in Miyagi Prefecture and has characteristics that make it easy for lodging. The lodging angles were studied at R3 and R8, and classified the R3 lodging as the “Early lodging” and the R8 lodging as the “Late lodging”. Results: In the multiple regression analysis of “Early lodging”, the main stem length was not significant, only wind speed was significant at the 0.1% level. In the “Late lodging”, the effect of main stem elongation from V6 to R1 was high, and the result of a single regression analysis was R2 = 0.70 (p <0.001). Multiple regression analysis showed that the R2 (R2 = 0.81) was highest in the model with R6 main stem length added as an explanatory variable, and wind speed was not significant. Conclusion: The results indicated that “Early lodging” was strongly affected by wind speed, and “Late lodging” was strongly affected by main stem elongation (especially the main stem elongation from V6 to R1), resulting in more severe lodging. Implication: Prediction of main stem elongation after V6 should be focused for judging the application of pinching as a countermeasure of late lodging.