The critical seed-filling period duration is dependent on soybean planting date and cultivar maturity selection: implications for crop management

Historically, planting soybean in early when compared to late May in the midwestern United States was not found to contribute to increasing soybean [Glycine max (L.) Merr.] yields. However, decades of genetic improvement concurrent with crop management research has resulted in a clear consensus among midwestern university researchers that soybean yield potential is increased when the crop is planted in late April and early May. These findings have encouraged earlier planting, but raise questions about how planting date might interact with other management factors. In 58 on-farm trials in Illinois and Indiana in 2011 and 2012, our objective was to examine if soybean seed treatments (fungicide + insecticide + rhizobia inoculant) and cultivar maturity selection interact with planting date. We found that using seed treatments increased yield, by an average of 80.7 kg ha-1, but did not interact with planting date. However, cultivar maturity selection did interact with planting date. Full-maturity cultivars yielded 182 kg ha-1 more when planted in late April vs. mid- to late May, but planting date had no effect on the yield of the short-maturity cultivars. Using small plot trials conducted at 12 site-years in Iowa, and in the northern half of Illinois in the 2012, 2013 and 2014 growing seasons, we examined the interaction between cultivar maturity selection and planting date more closely. Our objectives were to investigate how planting date and cultivar maturity selection influenced the critical seed-filling period duration, and to examine if planting in early May vs early June shifted the range in maturity group that produced close to maximum yields toward fuller-maturity cultivars. The seed-filling period duration was highly related to yield. Simple linear correlations coefficients ranged from r = 0.31 to r = 0.78 when the seed-filing period was expressed as accumulated incoming solar radiation and accumulated modified growing degree days. Planting date and cultivar maturity interacted to effect the duration of the seed-filling period. At the early May planting date, the seed-filling period for the full-maturity cultivars was 11.4 and 4.6 days longer than the short- and mid-maturity cultivars. For the early June planting date, however, cultivar maturity only had small effects on the seed-filling period duration. A slightly fuller range in maturity group was required for near maximum yields (within 67 kg ha-1 of maximum yield) at the early May vs. early June planting date, but even at the early planting date the fullest-maturity cultivars did not yield more than the mid-maturity cultivars, despite accumulating 32 MJ m–2 more incoming solar radiation over a 4.6 day longer seed-filling period. A lower seed-filling rate, perhaps related to lower late-season night temperatures for the full-maturity cultivars meant that the longer seed-filling period did not result in higher yields. To investigate more closely how late-season air temperatures may interact with cultivar maturity. We used 92 total site-years of variety trials in northern and central, IL conducted by the University of Illinois Variety Testing program to examine if site-years with warm air temperatures during the seed-filling period would result in full-maturity cultivars yielding more than mid-maturity cultivars. We also were interested to see if site-years with cool air temperatures during the seed-filling period would result in full-maturity cultivars yielding less than mid-maturity cultivars. In northern, IL, when mean daily air temperatures were warm during the seed-filling period, the full-maturity cultivar class (3.1 to 3.5) produced 232 and 167 kg ha-1 more grain than the 2.3 to 2.6 and 2.7 to 3.0 mid-maturity cultivar classes. In northern, IL, full- and mid-maturity cultivars generally had similar yields when air temperatures were cool during the seed-filling period. In central, IL, the 3.5 to 3.8 mid-maturity cultivar class produced 59 kg ha-1 more grain on average, than the 3.9 to 4.4 full-maturity cultivar class, and warm or cool air temperatures during the seed-filling period changed that very little. In northern, IL, full-maturity cultivars may have higher yield potentials than mid-maturity cultivars, but that yield potential may only materialize if air temperatures during the seed-filling period are warmer than normal