Soybean [Glycine max (L.) Merr.] growers are concerned that soybean yield is restricted by limitations on biological N2 fixation and soil nitrogen (N) mineralization. However, a comprehensive study characterizing actual soybean N requirements across wide‐ranging seed yield environments is nonexistent for modern soybean production systems. Using six site‐years and eight soybean varieties, plants were sampled at six growth stages and partitioned into their respective plant parts and analyzed. For each kilogram increase in yield, total dry matter accumulation, harvest index, and total N uptake increased by 1.45 kg, 0.0034%, and 0.054 kg, respectively, but all varied by environment at any specific yield level, whereas N removal did not (0.055 kg N kg−1 grain). Nitrogen harvest index (NHI) increased (0.0019–0.004% kg−1 grain) with yield but varied by environment and yield level, resulting in indices between 73 and 90%. Peak uptake rates for N were 3.6 to 4.3 kg ha−1 d−1 between R4 and R5, depending on the yield level. After R5.5, 66 to 69% of vegetative N was remobilized to the seed, which accounted for 50.4% of seed N at the low yield level (3608 kg ha−1), but only 38.9% at the high yield level (5483 kg ha−1). Moreover, higher yields attained a greater portion of their total N uptake after R5.5 (40.1%) compared with the low yield level (29.7%). These results highlight greater remobilization efficiencies and late‐season N uptake in conjunction with greater NHI to support higher yields per unit of N uptake in current production realities.

Core Ideas Planting date and maturity group decisions can greatly affect yield and composition. Temperature had a significant effect on seed yield and composition. Planting date × maturity group should be chosen based on the product's end use. Soybean [Glycine max (L.) Merr.] production has greatly increased in the upper U.S. Midwest over the last decade, but little information exists regarding the interactive effects of environment and spring management decisions on soybean seed yield and composition. Our objective was to assess the effect of four planting dates (PDs), four cultivar maturity groups (MGs), and ambient temperature between R5 to R8 (T5–T8) on soybean seed yield and composition. Field studies were established between 2014 and 2016 at three locations in Wisconsin and one in Minnesota. Across environmental conditions and management decisions, greater seed yield was positively correlated with protein and oil contents but negatively correlated with linoleic, linolenic, and sucrose contents. Multivariate data analysis showed positive synergies between early planting (late April–early May) and MG 2 for yield, oil, and oleic acid across the examined region. A MG 2 was the highest yielding and a ∼1200 kg ha−1 yield difference was observed between early and late PDs. These results underline the complexity of the soybean yield‐composition relationships. Additionally, the large variability in the responses of constituents to management decisions and temperature variations highlights the importance of a producer knowing the product's end use (e.g., high yield vs. high protein) and accordingly modifying the growing environment by selecting an appropriate PD and MG for the respective region. To provide more accurate recommendations to a broader range of producers, multi‐environment studies are imperative to capture large environmental variability in important soybean production areas across the United States.

Core Ideas Soybean seed yield response to plant density is dependent on yield environment.Low yield environments required higher plant densities than high yield environments.Plant density mainly affected per‐plant seed number.No differences in plant survival were observed among yield environments. Inconsistent soybean [Glycine max (L.) Merr.] seed yield response to plant density has been previously reported. Moreover, recent economic and productive circumstances have caused interest in within‐field variation of the agronomic optimal plant density (AOPD) for soybean. Thus, the objectives of this study were to: (i) determine the AOPD by yield environments (YE) and (ii) study variations in yield components (seed number and weight) related to the changes in seed yield response to plant density for soybean in North America. During 2013 and 2014, a total of 78 yield‐to‐plant density responses were evaluated in different regions of the United States and Canada. A soybean database evaluating multiple seeding rates ranging from 170,000 to 670,000 seeds ha−1 was collected, including final number of plants, seed yield, and its components (seed number and weight). The data was classified in YEs: low (LYE, <4 Mg ha−1), medium (MYE, 4–4.3 Mg ha−1), and high (HYE, >4.3 Mg ha−1). The main outcomes were: (i) AOPD increased by 24% from HYE to LYE, (ii) per‐plant yield increased due to a decrease in plant density: HYE > MYE > LYE, and (iii) per‐plant yield was mainly driven by seed number across plant densities within a YE, but both yield components influenced per‐plant yield across YEs. This study presents the first attempt to investigate the seed yield‐to‐plant density relationship via the understanding of plant establishment and yield components and by exploring the influence of weather variables defining soybean YEs.

Soybean [Glycine max (L.) Merr.] seed treatment adoption has increased dramatically over the past decade in addition to the number of pesticide components within commercially available seed treatments. The study objectives were to evaluate the effects of multiple seed treatments and their individual pesticide components (fungicide, insecticide, and/or nematicide) on soybean plant stand and seed yield across diverse environments. Trials were conducted at 10 Wisconsin locations during the 2011 to 2013 growing seasons. Soybean seed treatments containing fungicide + insecticide + nematicide increased plant stands over the untreated control (UTC), fungicide only, and fungicide + insecticide seed treatments by an average of 10, 9, and 5.5%, respectively. During 2013, yield was increased by the fungicide only seed treatment pyraclostrobin + metalaxyl + fluxapyroxad; however, across all environments, no consistent yield increase was shown for fungicide only seed treatments. Fungicide + insecticide seed treatments increased yield over fungicide only seed treatments by 55 and 76 kg ha -1 during 2011-2012 and 2013, respectively, and were similar to fungicide + insecticide + nematicide seed treatments. However, fungicide + insecticide and fungicide + insecticide + nematicide seed treatments only increased yield over the UTC in 2013. These results suggest that though fungicide + insecticide and fungicide + insecticide + nematicide seed treatments consistently increased plant stand, yield increases were variable and contingent on unpredictable factors. Therefore, producers will need to weigh potential yield gains with biological (resistance management) and economic (return on investment [ROI] and risk mitigation) concerns before implementing seed treatment practices at the whole farm level.

Soybean [Glycine max (L.) Merr.] planting date trends have steadily shifted earlier within the northern Corn Belt, while inclement weather, insect pressure, and disease pressure associated with spring planting can result in replanting some years. However, limited published literature exists about soybean replant thresholds for suboptimal plant stands and the effects of seed treatments on this decision. This study evaluated three planting dates, three seed treatments, and twelve seeding rate–replant combinations to determine replant thresholds in terms of maximizing seed yield, the effects of seed treatments on these thresholds, and the relationship of cumulative intercepted photosynthetically active radiation (CIPAR) and cumulative normalized difference vegetative index (CumNDVI) on seed yield. Trials were conducted during 2012 and 2013 in southern Wisconsin. Soybean planting in early May increased seed yield by 358 kg ha−1 compared with late May and by 805 kg ha−1 compared with mid‐June. Seed yield reductions were due to decreased seeds m−2 while CIPAR and CumNDVI measurements may partially explain this planting date effect. When initial plant stands were below threshold (<247,000 plants ha–1), filling in these stands with enough seed to bring the final plant stand above the threshold increased yield, whereas using tillage and replanting the entire stand only increased yield when initial plant stands were extremely low (<91,000 plants ha−1). Yield was related to both CIPAR (R2 = 0.54) and CumNDVI (R2 = 0.49) and the latter two variables were highly correlated (r = 0.89). We observed linear yield increases through 700 MJ m−2 of CIPAR; therefore, management practices to increase CIPAR should be used. This study showed that early May planting with a fungicide/insecticide seed treatment (CruiserMaxx) and generating adequate plant stands (>247,000 plants ha−1) maximized CIPAR.