## 4 Discussion  
When starting this study, the UE common catalogue of agricultural plant cultivars (CCA) included 300 soybean varieties, while in Poland, there were 14 cultivars in the national variety register [37; 38]. Currently, the number of cultivars has increased three-fold and reached 40 in the Polish catalogue. Thus, the hypothesis was put forward that progress in the selection of cultivars will also have practical significance for Central Poland. The demand for feed protein is covered only in 20% of domestic production. To reduce the import of soybean meal from GMOs, a national program was implemented for 2016-2020 in terms of increasing the use of domestic fodder protein for high production quality of animal products in conditions of sustainable development. It is a common idea of many European countries [39], with the main purpose of creating opportunities to increase the protein security of the country for feed and food purposes. The original data of a three-year study in six locations in Central Poland allowed for the drawing of several important conclusions regarding the reaction of soybean yield to cultivar selection. Our study confirmed previous world reports [40; 41] that the soybean response was primarily dependent on genetically fixed maturation, which we defined here as the vegetation period days (VPD). In the Central Poland region, the highest-yielding cultivars belong to group 3 rd, where VPD was defined here between 130-142 day. This is an update until the recent opinion that in Poland, between 52 nd and 53 rd latitudes, in which the research was conducted, the soybean cultivars must be the early and medium-early group [42]. Moreover, a significant relationship was found on 20 soybean cultivars between VPD and yield, which indicated that each day of vegetation period extension (in the range 100-150 days) increased the yield by 20 kg per ha. Yield improvement of 26 soybean cultivars released over the past 50 years in southern Brazil was described by [PERSON] et al. [41]. The average rate of yield gain was 45.9 kg per ha per year (relative growth 2.1%). The authors see this progress in breeding through attributes such as seed number per area and harvest index, less susceptibility to lodging, and reduced plant mortality. On the other hand, an American study confirmed that the main abiotic factors influencing soybean yield are solar radiation and the availability of rainfall [43]. According to American research, delaying soybean sowing causes a yield loss of 0.09 to 1.7% per day. It is known that the cultivars grown in America (Brazil, US) belong to the GM group, and the seed traits cannot be compared to the nGMO genotypes, although certain issues such as sowing delay or tendency to lodging may be discussed here, as a Polish study [44] also confirmed that delayed sowing leads to a significant decrease in yield. Our research was guided by the principle of adapting the soybean sowing date to the phenological phase of cherry bloom, which is convergent with the soil temperature needed for soybean germination in the range of 8 to 10 \({}^{\circ}\)C [17]. According to this rule, all plantations were sown within 10 d between 30 April and 9 May, which ensured the proper development of seedlings and further ontogenesis. A study carried out in this region in 2000-2015 led to inconclusive results regarding thermal conditions for soybean [45]. First reports concluded that temporal variability concerning moderate and strong frosts in late spring increases the risk of soybean plantation disruption. We state that adapting soybean cultivars to the thermal conditions in Central Poland does not seem to be a problem; however, vegetal seasons with insufficient rainfall become problematic. Water conditions for soybean cultivation in Central Poland in this study and the variability of the hydrothermal coefficient confirmed the results of an intensified occurrence of drought [7]. The evidence of nine periods without precipitation or with low precipitation ranging from April to September 132-207 mm indicate drought syndrome.