Planting date is a commonly manipulated management practice in soybean [Glycine max (L.) Merr.] production; however, the impacts of past and ongoing agronomic improvements, such as earlier planting, on genetic yield improvement and associated changes in seed protein and oil have not been evaluated. The objectives of this study were to determine if a 30‐d difference in planting date affected measured rates of genetic improvement in (i) yield, (ii) seed mass, and (iii) seed protein and oil in the midwestern United States. Research was conducted at Arlington, WI, Urbana, IL, and Lafayette, IN, during 2010 and 2011, using 59 Maturity Group (MG) II cultivars (released 1928–2008) at Wisconsin, and 57 MG III cultivars (released 1923–2007) at Illinois and Indiana, with targeted planting dates of 1 May and 1 June. Earlier planting provided higher yields (+3.1 kg ha−1 yr−1) than late planting in MG III soybean. Seed protein concentration decreased linearly over cultivar year of release at a rate of 0.191 (± 0.069) g kg−1 yr−1 for MG II, and 0.242 (± 0.063) g kg−1 yr−1 for MG III. Seed oil concentration increased over year of release at a rate of 0.142 (± 0.037) g kg−1 yr−1 for MG II, and 0.127 (± 0.039) g kg−1 yr−1 for MG III. The interaction between planting date and cultivar year of release for MG III yield suggested that the trend toward earlier planting is one agronomic improvement that, when coupled with genetic improvement, has provided a synergistic increase in on‐farm soybean yields in the midwestern United States.

Soybean [Glycine max (L.) Merr.] yield has increased during the past century; however, little is understood about the morphological parameters that have contributed most to yield gain. We conducted eld studies to determine relationships between genetic gain of soybean yield and seeding rate. e hypothesis was newer cultivars would express higher yield than older cultivars when grown in higher plant populations. A total of 116 soybean cultivars equally representing Maturity Groups (MGs) II and III released over the last 80 yr were evaluated at high and low seeding rates in Wisconsin, Minnesota, Illinois, and Indiana. Seeding rates were 445,000 and 148,000 seeds ha -1 resulting in 311,000 and 94,000 plants ha -1 (high and low, respectively). Seed yield was greater for the high seeding rate vs. low seeding rate throughout all cultivars and years of release, but the di erence was larger in newer cultivars.e di erences observed primarily came from an increased number of pods and seeds plant -1 . However, newer cultivars grown in low seeding rates increased per plant yield linearly by 0.118 (± 0.02)x-208.0 g plant -1 , where x = year-of-release, which was three times greater than at the high seeding rate. e greater yield trend came from seeds produced on plant branches. erefore, newer cultivars produce more compensatory yield on plant branches under lower plant populations than older cultivars, so over the last 80 yr there has been a diminishing response to the expected yield penalty from reduced plant density.

Soybean [Glycine max (L.) Merr.] grain yield has annually increased nearly 23 kg ha−1, but the interaction of genetic advancement and improved agronomic practices has not been well quantified, including N utilization and fertilization. A field study with soybean cultivars released from 1923 to 2008 in maturity group (MG) II and MG III was conducted in multiple environments with a nonlimiting supply of fertilizer N to examine the main effects and interactions of N supply and release year on grain yield and seed quality. We hypothesized that grain yield and seed quality would be improved with the nonlimiting supply of N, especially for the modern cultivars. Supplemental N totaled 560 kg N ha−1 with 40% applied at planting and 60% applied at V5. Grain yield increased with release year in MG II (17.2 kg ha−1 yr−1). Application of N to MG II cultivars increased seed protein by 10 to 19.5 g kg−1 across all release years, but grain yield and seed oil was not affected. Grain yield gains of MG III cultivars fertilized with N was 27.4 kg ha−1 yr−1, which was 20% better than unfertilized (22.8 kg ha−1 yr−1). Application of N to MG III cultivars increased seed mass (11%) across release years with no changes in seed protein and oil. The nonlimiting supply of N increased seed protein across all release years in MG II cultivars, and the N supply from the soil and biological N fixation was insufficient to maximize grain yield in modern, MG III cultivars in the tested environments.

The trend toward earlier soybean [Glycine max (L.) Merr.] planting in the midwestern United States has interacted synergistically with genetic yield gain to provide improvement in on‐farm yields. However, the impacts of earlier planting dates and their interaction with genetic gain in physiological and phenological traits remain unclear. The objectives of this study were to determine if a 30‐d difference in planting date affected measured rates of genetic improvement in (i) total dry matter (TDM) production, (ii) harvest index (HI), and (iii) growth‐stage duration in the north‐central United States. Research was conducted at Arlington, WI, Urbana, IL, and Lafayette, IN during 2010 and 2011 using 59 Maturity Group (MG) II cultivars (released 1928–2008) at Wisconsin, and 57 MG III cultivars (released 1923–2007) at Illinois and Indiana, with targeted planting dates of 1 May and 1 June. A mixed‐effect regression analysis was used to model genetic change in TDM, HI, and growth stage duration as impacted by planting date. Breeding efforts have increased TDM(R7), HI, seed‐fill duration (SFD), and reproductive growth duration over time, as vegetative growth duration has been reduced. Early planting provided increased TDM(R7) and longer reproductive growth duration, but had no effect on HI or SFD. A synergistic planting date × year of release interaction existed for TDM(R7) in both maturity groups, but not for HI or SFD, suggesting that the higher yields in newer, early‐planted cultivars resulted from greater TDM production, not improved HI or SFD.

On-farm U.S. soybean [Glycine max (L.) Merr.] yields have increased at an annual rate of 23.3 kg ha -1 yr -1 since the 1920s. These gains have come from a variety of sources including genetic, agronomic, and environmental changes. Genetic gains arising from breeding efforts have likely contributed the most to the U.S. soybean yield increase; however, the relative contribution of each source of gain is difficult to estimate. The objectives of this study were to compare yield of soybean varieties with different year of release, understand the effects of fungicide applications on soybean seed yield, and evaluate the composition of soybean cultivars chosen to represent historically significant releases in maturity groups (MGs) II and III released during the last 85 yr. A set of 116 cultivars in these two MGs, released from 1923 to 2008, received a fungicide seed treatment followed by foliar applications at R1, R3, and R5 and were compared to non-treated controls. Seed composition changed over time with protein concentration decreasing 2.1 g kg -1 for every g kg -1 increase in oil concentration. The significant interaction between fungicide treatment and MG III cultivar release year for plant stand revealed that such treatments were more beneficial with respect to obsolete cultivars of MG III, though this plant stand interaction did not translate into a significant yield interaction. The rate of genetic yield improvement made by breeders was not influenced by fungicide management and matched the observed rate of on-farm yield improvement that occurred during the same period.