The winter is not officially gone but temperatures over 60°F and cumulative rainfall approaching 2 inches in many MO counties indicate that the 2024 growing season may start earlier than previous years. The planting operation and crop stand establishment is the most critical event in every farmer's calendar. While late-season weather often influences final yields, the potential yield is primarily set during planting, considering the ideal combination of planting date, variety (and its maturity group), and plant population. Any stress the soybean crop encounters throughout the remaining growing season will either diminish or maintain the yield potential established during planting.
The University of Missouri Variety Testing Program (MUVT) and seed companies strive to offer support to growers in decision-making for variety selection by releasing field trial results of current relevant varieties. Typically, variety testing results are presented by ranking superior performances along with some management information for a given location-year trial. This approach provides important information; however, it is difficult to predict if the reported performance will repeat under grower conditions with different planting dates and weather. Nevertheless, when merged together, the collective information from these variety trials may reveal trends in the interaction of planting date and maturity group.
By focusing only on the top-yielding varieties in each MUVT and seed companies trials, there is a clear overall relationship between planting date and actual yield: the later the planting date, the lower the potential yield. The Southeast region depicts the strongest yield penalty, with a loss of 0.46 bu/acre/day from March to June, followed by the North and Central regions with penalties of 0.16 and 0.13 bu/acre/day, respectively (Figure 1). Interestingly, the Southwest region did not show the same trend, mainly due to not recording yields as high as the other regions before April. While the decrease in yield over time has been noted in previous studies, there is considerable variability in the data across different regions. This means that high yields can still be observed in April and early May across all regions, highlighting the significant influence of factors such as variety selection, management practices, and weather conditions on yield, in addition to planting date. Conversely, there are fewer instances of high yield records from mid-May to June, except in the southwest region.
Figure 1 Field trial sites (n=422) evaluating soybean variety performance in Missouri. The dataset includes data from the University of Missouri Variety Testing Program spanning 2010 to 2023 (n=206), along with publicly available data from Asgrow, Becks, Channel, FS, Golden Harvest, NK, Nutech, and Pioneer covering 2020 to 2023 (n=216). The yield of the best soybean variety in each Missouri region is displayed according to the planting date of the field trial. Colors indicate the maturity group of the best variety. The number at the top right is the negative trend in bu/ac/day.
How to adjust variety selection or at least MG as the planting season progresses? Unfortunately, establishing a clear relationship between the MG of top-yielding varieties and planting date is not always straightforward using a simple model. We can observe segmentation across locations (Figure 1), characterized by predominantly high yields with later maturity groups (MG) in the south and early MG in the Central and North. However, there is no strong trend indicating a particular MG associated with higher yield at the beginning or end of the planting season (Figure 1). This does not imply that such a relationship does not exist; rather, it suggests that capturing it is challenging. In response, the MU Soybean Farming System research group will initiate multi-environmental trials in 2024, specifically designed to optimize MG, planting date, and plant population across Missouri regions.
Figure 2 Simulation results for the recommended best maturity groups (early, intermediate, and late varieties) at the Missouri county level across six scenarios. These scenarios include three planting dates (April 10th, May 20th, and June 30th) under two weather conditions (wet and dry years). Early, intermediate, and late MG are relative for each state region.
In the meantime, let's attempt to temporarily establish basic trends by deploying a machine learning model on the dataset depicted in Figure 1, simulating different planting dates and weather conditions at the county level. In a wet year, early April plantings should predominantly use early maturity groups (MG) in most counties, except in the bootheel region where intermediate and later MGs are more likely to yield higher returns (Figure 2). As planting progresses into May and June, most of the state should transition to intermediate MGs. It's important to note that terms like 'early,' 'intermediate,' and 'late' are relative and depend on the region. A 4.0 MG may be considered intermediate in the north but early in the south.
In dry years, the prediction suggests a shift towards more intermediate MGs in the northern region and early MGs in the rest of the state for April plantings. This transitions to intermediate MGs in most of the state by May and late MGs by June plantings, particularly in the bootheel and certain areas in the northern region. The change in MG from wet to dry years likely reflects a strategy of drought avoidance by adjusting seed filling away from the hot and dry late July and August period.
In summary, the challenges in establishing a soybean crop with high yield potential are numerous. Opting for an early planting date, provided weather conditions permit adequate stand establishment, may be a proven strategy to secure high yield potential. The selection of the optimal maturity group according to the region is crucial. Moreover, the optimum MG may change with the planting date and later season weather conditions, adding complexity to variety selection and increasing uncertainty. Future research from the MU Soybean Farming System Program will address the interplay between planting date, MG, and plant population. More information about the UM Variety Testing Program: https://varietytesting.missouri.edu