The Choice of Soybean Cultivar Alters the Underyielding of Protein and Oil under Drought Conditions in Central Poland

The popularization of soybean cultivation in Central Poland is progressing due to the European Soy Declaration signed by 13 member states in Brussels on 17 July 2017. Hence, this research was initiated under the European Innovation Partnership on phenotyping soybean cultivars in two regions, i.e., Kuyavian-Pomeranian and Greater Poland for integrated systems. The aim of this study was to determine soybean potential in the agrotechnical and agroclimatic conditions, with the selection of the most suitable cultivars for tillage and no-tillage cultivation in the region of Central Poland. Strict field research was carried out in six locations from 2018–2020 on 20 cultivars selected in terms of their earliness of maturation to the climatic conditions. On the basis of meteorological data, it was found that half of the plantations suffered from drought stress, as evidenced by the hydrothermal coefficients (K) for the growing season of soybean K < 1.2. The significant multivariate relationship (R = 0.67; p < 0.001) between the hydrothermal coefficient K and the vegetation period days (VPD) as the predictors of the soybean yield was determined by the regression equation Y(yield) = 21.8 + 12.0 X(K) + 0.20 X(VPD). A significant multivariate relationship (R = 0.43; p < 0.01) was also found between the seed yield, hydrothermal coefficient K and the protein content, quantified with the regression equation Y(protein) = 32.6 + 0.25 X(Yield) + 0.28 X(K). In the no-tillage system of cultivation, the seed yield of soybean constantly increased with increasing K (+32.3% between dry/relatively dry and optimal periods, +22.4% between optimal and humid periods), while in the tillage system, the yield increased by 22.1% only when K rose from optimal to humid. In the seasons with optimum and humid conditions, no-tilled soybean produced more oil, in comparison to the season of dry conditions, and the opposite trend was found with the greater oil content in the tillage system. Meanwhile, a higher protein content was observed in the tillage system under humid conditions. All soybean cultivars were grouped according to the cluster analysis (k-means) with ANOVA in terms of vegetation period in days, seed yield, oil, and protein content for tillage and no-tillage cultivation. The study confirmed that soybean might be considered in the future as a profitable crop in Central Poland and create perspective for a low-input source of protein and oil.