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Ontario Grain Farmer Magazine

100-Bushel Soybean: What’s really holding us back?

Principal Investigator: Hugh Earl

Research Institution: University of Guelph

Timeline: April 2021 – March 2024  

Objectives:

  • Under conditions otherwise representing known best management practices for maximizing yield (choosing sites with adequate residual P and K fertility; selection of full-season varieties; use of seed treatments (fungicide, insecticide and rhizobial inoculant); seeding rates of 175k / acre in narrow (15”) rows; adequate weed and insect control) test the effects of variety selection, comprehensive fertilizer supplementation, foliar fungicides, and high rates of in-season fertilizer N additions on soybean yield, at three locations in each of three years.
  • In each of two years, compare the effects of standard vs high-input management packages, under both irrigated and rainfed conditions, to quantify the yield limitation from soil water availability and determine how this varies with input level.  Additionally, compare the irrigated, high-input system with an outdoor soilless culture system designed to, in so far as possible, eliminate all below-ground constraints to yield.
  • For all of the above experiments, and considering differences both within locations (treatment effects) and between locations (location effects), characterize the physiological basis of yield differences when they occur, such as: increased light interception, increased radiation use efficiency, delayed senescence leading to extended biomass accumulation, increased numbers of pod-bearing nodes, increased pods per node, increased seeds per pod, and increased seed size.

Impacts:

  • Quantifying yield limitations that can reasonably be attributed to management practices, as opposed to those determined by location effects that can not be overcome with management, at any cost.  This type of result is useful for combatting “silver bullet” thinking (“If I could just identify the one management factor that is holding me back, my yields would explode…”)
  • Identifying upper limits to the benefits of certain specific management practices (foliar fungicides, comprehensive fertilizer packages, greatly improved in-season N availability), and how those vary within the total management context (i.e., is the benefit of that practice much greater when adopted in a system that is otherwise high-input?)
  • Providing a better understanding of the physiology of very high soybean yields, when they do occur in Ontario.  What does an ultra-high-yielding crop look like, at different points during the season? This type of understanding can support management decisions, and is often non-intuitive.   For example, we may find that dramatically improved early vegetative growth, while visually impressive, may be inconsequential to yield determination, while two crops that look identical at mid season (R3 to R4) actually have much different crop growth rates and pod number establishment at that time, for reasons that are not visually apparent.  Detailed physiological measurements of this type have not been made for intensively-managed soybean in Ontario.
  • Clarifying whether or not rainfall during early seed-fill (August) is a primary determinant of yield potential.  Soil water availability during seed-fill is often cited as a major yield-determining factor for soybean in Ontario, but there is little experimental evidence to support this contention.  Rainfall may be a less important determinant of our success than conventional wisdom dictates.  Clarifying this could lead us to look for other potential limiting factors that are more easily addressed in a rainfed production system.
  • Demonstrating the potential yield of soybean in Ontario, when limited only by plant genetics and the aerial environment (solar radiation, temperature, humidity).  What fraction of the “yield gap” is attributable to all below-ground factors (water availability and crop nutrient acquisition)?  How far are actual yields from this theoretical attainable yield, when soil limitations are eliminated?

Scientific Summary:

In theory, soybean yields in Ontario can exceed 100 bu / acre, but this is rarely if ever achieved.  Much research effort has been directed towards evaluating practical (economical) management practices to close the yield gap, but we lack an understanding of the typical magnitude of specific factors limiting soybean yield in our production environment.  It is often assumed that soil water availability is the primary limitation.  Five years of trials at the Elora research station between 2009 and 2013 showed that irrigation usually increased soybean yield only by about 10%, and even in a very dry year (2012) the yield increase was only 16%.  This result points to the possibility that there are other, poorly-understood constraints to soybean yield.

We propose a series of field experiments to explore the upper limits of soybean yield, without the constraint of economic viability.  The logic behind this approach is to uncover the maximum possible benefit from various agronomic interventions, and in the process reveal the true environmental and physiological constraints to yield.  At three locations in each of three years, we will test the response of soybean yield to combinations of aggressive agronomic interventions, including comprehensive fertilizer additions, high rates of supplemental N, and foliar protection, using three different commercial varieties.  Additionally, at one location in each of two years we will compare standard and high-input packages under irrigated and rainfed conditions, and also a soilless culture system included in an attempt to quantify all soil-based restrictions.  A key feature of this work is a suite of season-long measurements to follow crop development, light interception, biomass accumulation, canopy senescence and yield components.  These will permit us to determine the physiological basis of yield increases when they occur, and identify the major yield constraints most worthy of further attention. 

External Funding Partners:

The project was funded in part by the Ontario Agri-Food Innovation Alliance, a collaboration between the government of Ontario and the University of Guelph.