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Sixty-Bushel Soybeans: Exploring Yield Potential

Kristen P. MacMillan, Agronomist-in-Residence, University of Manitoba/MPSG

THE ADAGE “RAIN MAKES GRAIN” certainly held true for soybeans in 2022. Following a historic drought across the prairies in 2021, soybean yield potential was realized across many regions of Manitoba in this past growing season with a provincial yield record of 45 (bu/ ac). The previous record was 42 bu/ac in 2016. Soybean is a high water use crop and it’s yield performance greatly reflects moisture availability. With renewed enthusiasm, it’s time to explore yield potentials and the opportunity for 60-bushel soybeans.

Yield potential non-limited is the yield of an adapted crop in a local environment where water and nutrients are non-limiting; diseases, insects, and weeds are effectively managed; and overall best agronomic practices are used to maximize yield. Yield is only limited by genetics and atmospheric conditions (i.e., temperature, CO2, solar radiation). However, here on the prairies where we are dependant on rainfall for water availability, yield potential water- limited (Yp) is a more useful benchmark. As we explore what Yp looks like for soybeans in Manitoba, our starting point is actual farm yield (Yf). At the farm and field level, multiple limiting factors (in addition to moisture) determine how we manage the crop. Pests, nutrition, and agronomic practices prevent us from reaching yield potential (Yp).

Let’s begin a crude analysis of those numbers for Manitoba. A look at historical on-farm soybean yield takes us back to 1998. Over the past 25 years, soybean yield has increased by 0.45 bu/ ac/year and the 10-year average soybean yield is 36 bu/ac (Yf). How does that compare to our yield potential? There are several methods used to quantify yield potential, but the most accessible data is that from variety trials and research plots. In small-plot research, we choose uniform, highly productive areas of the field; manage pest pressure; and use best management practices. So, we can expect yields in research plots to surpass farm yields and provide a reasonable estimation of soybean yield potential (Yp) for Manitoba. The 10-year average soybean yield among four small- plot variety trial locations is 49 bu/ac.

Historic on-farm soybean yield (1998–2022, MASC) and small-plot variety trial yield (2013–2022) in Manitoba

The difference between on-farm yield (Yf = 36 bu/ac) and variety trial yield (Yp = 49 bu/ac) can be referred to as the yield gap.

Based on this crude analysis, the yield gap for Manitoba soybeans is around 13 bu/ac or 36%. Yield gaps reported for major crops around the world range from 20–40%. Crop yields achieved at the farm level will never close the yield gap entirely, but there is the exploitable yield gap, which is defined as the attainable yield increase through refinement of management practices. The exploitable yield gap is likely between 10% and 20%. This means we should be able to reach a long-term provincial soybean yield of 39–43 bu/ac.

In Manitoba, soybean moisture deficit is the primary limiting factor for both Yp and Yf. Soybean yield has been strongly correlated to growing season precipitation, both in small-plot and on-farm. Generally, an inch of rain produces about five bushels in small plot/ultra high yielding environments and about three bushels at the farm level. Precipitation is an easily available parameter, but soil moisture availability is likely more accurate since it accounts for variation in soil characteristics that impact how well the crop uses precipitation (e.g., infiltration and water holding capacity). Thus, soybean yield (and crop yield in general) within a region and field is highly variable depending on soil.

Simplified concept of yield potential (Yp), actual farm yield (Yf), and limiting factors.

To overcome the exploitable yield gap, the primary yield-limiting factors related to agronomy need to be identified and understood. Through a comprehensive farm survey, we could compare management practices among high and low yielding farms (this may be a future research project). Farmers and agronomists could also do a self- assessment of their adoption of best management practices. Precluding adoption is the continuous assessment of the availability (is relevant research available?) and communication (is knowledge effectively transferred?) of best management practices. Variety selection, weed control, row spacing, and seed depth are easily adjusted management practices to increase soybean yield. Other yield-limiting factors that require broad integrated efforts are herbicide resistance, soil and water management, and disease management – specifically Phytophthora root rot and soybean cyst nematode.

In 2022, the moisture deficit in most regions of Manitoba was reduced and despite a very late start, soybean growers were pleased to see record yield potential. With a farm average soybean yield of 45 bu/ac (Yf) in 2022, does that mean we reached yield potential? Not quite – yield potential is a moving target because it’s driven by an environmental factor. In other words, our Yp benchmark (variety trials and small-plot research) also received greater precipitation, thus driving Yp to a record level as well. At 63 bu/ac it’s still 40% greater than the farm average.

10-year average soybean yield potential (Yp), on-farm yield (Yf), and exploitable yield compared to
compared to the 2022 growing season averages and top percentiles.
the 2022 growing season averages and top percentiles.

Further analysis of the 2022 yield data shows that the top 15% of soybean fields in Manitoba averaged an incredible 61 bu/ac. Indeed, sixty-bushel soybean is a realistic goal for some farms in Manitoba. Equally impressive is the top percentile of variety trial yields, which reached 80 bu/ac. So, there is certainly a range of crop yield performance that can be used to identify yield-limiting factors. Circling back to precipitation and soil moisture availability, the highest yielding soybean fields in 2022 were in the Red River Valley and east Central Manitoba. Risk Area 5 produced 49 bu/ ac and the highest yielding municipality was Morris with 55 bu/ac. This generally coincides with regions that received the greatest precipitation and heat accumulation and inherently have highly productive soils.

What makes soybean unique? In three separate studies of agronomy practices, environment accounted for the most variation in soybean yield. This means that the effect of environment on yield resulted in a greater range of soybean yield than agronomic practices such as seeding date and depth. But doesn’t environment account for most yield variation in all crops? Other major crops, like wheat and canola, use nitrogen, a very influential agronomic practice that accounts for much variation in yield. Wheat and canola also have an overall lower crop water demand. Thus, the effect of environment, specifically soil moisture availability, is particularly important in making soybean yield.

A review of crop yield potential and contributing factors is useful for individual farms to benchmark their production practices and yield goals. It is also important for industry, government, and academia to identify knowledge and communication gaps, and develop solutions to increase production efficiency. Overcoming the exploitable yield gap to increase yields by 10–20% through refinement of management practices is a reasonable goal.

Watch for Kristen’s next annual report booklet of results – available at manitobapulse.ca and Kristen’s U of M home page – and follow @kpmacmillanUM on Twitter to stay in touch with this research.