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eturn to table of contents Table 1. The duration and GDU accumulation of vegetative and reproductive growth periods for soybean maturities Late II to Mid III from Parker et al. (2016). Planting Date Days to R1 GDUs to R1 Days R1 to R6 GDUs R1 to R6 4/15/15 50 768 52 1,198 5/4/15 41 805 45 1,032 5/9/15 40 821 38 859 5/11/15 41 861 37 836 6/11/15 34 851 28 836 7/7/15 28 759 26 609 Early planting and/or warm spring temperatures can induce earlier flowering and extend the flowering period. Soybeans initiate flowering based upon the interaction of daylength and heat unit accumulation (Lawn and James, 2011). Shorter daylengths hasten development. This can be observed with research from southern Pennsylvania in 2015 where shorter daylengths with both very early and very late planting reduced the growing degree units (GDUs) required to reach R1 (Table 1). Parker et al. (2016) showed that soybean planted on April 15th would begin flowering on June 4th, while May 11th planted soybean did not flower until June 30th after the summer solstice and when days begin to shorten (Figure 4). Longer daylengths extend reproductive development. This is evident by the longer R1 to R6 period with earlier planting (Table 1; Figure 4). Greater GDU and solar radiation accumulation from R1 to R6 have the potential to increase photosynthate production and, thus, seed number and yield. 15:50 15:21 Mid-April Planting Mid-May Planting R1 R1 R6 R6 After planting, growing conditions throughout reproductive development will have the greatest influence on final yield. Both too much and not enough water can have a large impact on photosynthesis and crop growth. Fertility and pH must also allow for optimal crop growth rates. This should be managed according to soil and plant analyses in conjunction with the yield goal and calculated crop demands. While weed control is important, herbicide applications during reproductive growth should be avoided due to the potential for reduced photosynthate production and seed number. This is especially true if the herbicide causes plant injury (Kyle, 2014). In-season management of insects and diseases are also crucial to limit their impact on crop growth and yield. All of the aforementioned practices and any others should focus on achieving optimal growing conditions from R1 through R7 to maximize photosynthate production during seed set and to lengthen the seed fill period in order to maximize yield. 14:52 Daylength (hr:min) 14:24 13:55 13:26 12:57 12:28 12:00 Mar 25 Apr 15 May 6 May 27 Jun 17 Jul 8 Jul 29 Aug 19 Figure 4. Daylengths during the spring and summer growing season for Pittsburg, PA, (40.4º N) with example planting, R1, and R6 dates from Parker et al. (2016). Full season varieties can have longer vegetative and reproductive periods and often have greater yield potential when planted early compared to short season varieties. In the Mid-South, short season varieties can be planted too early and have later optimum planting dates compared to fuller season varieties (Poston and Jeschke, 2015; Salmerόn et al., 2016). Further north, short season varieties can be planted later with less associated yield loss (Nafziger and Vossenkemper, 2015). In both scenarios, planting full season varieties first will maximize yield potential. CONCLUSIONS This physiological framework for grain yield determination in soybean provides a guide for understanding the effect of management practices and growing conditions on final yield. Stresses or gains in crop growth prior to R1 are not likely to have a large impact on final yield as long as full canopy closure occurs by flowering. Factors influencing photosynthate production during the period from R1 to R5 will have a significant impact on final seed number and yield. The following seed fill period from R6 to R7 will have a major impact on seed weight, which will also influence yield. This understanding of how yield is determined in soybean is the crucial first step in making management decisions for sustainable yield increases over time. 120

eturn to table of contents SOYBEAN MICRONUTRIENT UPTAKE, PARTITIONING, AND REMOVAL ZN, MN, CU, FE, B by Adam Gaspar, Ph.D., Field Agronomist, and Shawn Conley, Ph.D., Associate Professor, Department of Agronomy, University of Wisconsin-Madison RATIONALE AND OBJECTIVES • In recent decades, growers have become more specialized in specific facets of production. Application of manure has been eliminated on some agriculture land due to the geographical separation of crop production and livestock. Therefore, synthetic fertilizers are the main source of various micronutrients for crop production. • Furthermore, annual increases in soybean yield and marketing tactics of some agricultural retailers have growers concerned that soybean yield is limited by soil micronutrient supply. Consequently, applications of micronutrients have become more common in recent years, even though there is little research data to support these applications. • Knowledge of actual soybean micronutrient uptake requirements and utilization across a wide seed yield range coupled with soil and tissue tests would be beneficial for determining when applications of certain micronutrients can be profitable. STUDY DESCRIPTION • Test Environments: 2 years at 3 locations with nonlimiting fertility levels, resulting in 6 different testing environments • Soybean Varieties: 8 Pioneer ® brand soybean varieties (RM 1.0-2.5) • Planting Dates: Early and late May • Plant Sampling: Collected at the V4, R1, R4, R5.5, R6.5, and R8 growth stages and partitioned into the following parts: »» Stems » Petioles »» Leaves » Pods »» Seeds » Fallen Leaves/Petioles • Nutrients Quantified: Zinc, Manganese, Copper, Iron, and Boron • 6,672 tissue samples analyzed that span a yield range of 40 to >100 bu/acre Catch container used to collect all fallen leaves and petioles throughout the growing season from each plot. MICRONUTRIENT TOTAL UPTAKE Total Nutrient Uptake (lbs/acre) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 40 60 80 100 Seed Yield (bu/acre) Nutrient Equation R 2 S.E. Zinc (Zn) y = 0.003x + 0.009 0.33 0.05 Manganese (Mn) y = 0.004x + 0.136 0.15 0.11 Copper (Cu) y = 0.001x – 0.013 0.50 0.01 Iron (Fe) y = 0.006x + 0.132 0.13 0.18 Boron (B) y = 0.002x + 0.041 0.19 0.06 Figure 1. Total whole-plant micronutrient uptake at growth stage R8 (full maturity) across all environments and varieties. • Total uptake for a high yielding, 80 bu/acre soybean crop was less than 0.61 lbs/acre for all micronutrients. • Furthermore, total uptake for each micronutrient was heavily affected by the environment and/or variety, meaning that total uptake was variety by field by year specific. • These interactions resulted in large variability in estimating total uptake with yield across all varieties and environments (R 2 = 0.13 – 0.50) (Figure 1). However, this variability translates into small differences in actual lbs/ acre, and therefore, these equations are applicable for broad recommendations. • While actual (lbs/acre) differences are small, the variability may help explain the inconsistent yield responses to foliar micronutrient applications experienced by many growers and documented in multiple studies where different varieties and environments were employed. 121

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