Here’s what really drives soybean yield variation

Even when environment and management both increase yield, they may do so for fundamentally different reasons.

Ontario soybean fields often differ sharply in yield from one year to the next, but the reasons driving those differences are rarely straightforward. New research shows that inputs aren’t the biggest driver of yield variation; rather, environment plays the lead role.

The study, conducted by University of Guelph and Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA) researchers, and partially funded by Grain Farmers of Ontario, also found that high-yielding fields share a consistent physiological marker: strong crop growth at the R5 stage.

The study set out to quantify how much of soybean yield variation can be attributed to environment and how much is determined by management practices. The project was led by University of Guelph researcher Dr. Hugh Earl and formed the M.Sc. thesis research of Matt Rundle. OMAFA soybean specialist Horst Bohner was involved in implementing the field trials.

The researchers wanted to test whether environmental yield variation and management-induced yield variation work through the same internal plant processes or through different ones. This required hundreds of biomass measurements across multiple locations and several growing seasons. The calculations were complicated but the aim was simple: to finally resolve why some soybean fields outperform others, even when growers follow recommended practices.

The study ran for 3 years and spanned 9 “location-years.” Each location-year is a unique combination of soil type, weather, temperature accumulation and seasonal conditions. Three soybean varieties, selected in consultation with growers, were included to ensure the results applied generally across typical varieties adapted to the region.

The project compared a standard, good-management approach with an intensive-management treatment that Bohner once called the “everything-but-the-kitchen-sink-method.” Earl said the aim was to push the crop’s biological limits, to determine yield possibilities. The intensive treatment included high starter fertilizer rates, multiple foliar fungicide applications, nitrogen top dress applications, seed treatments, and inoculants.

If the approach sounds impractical and financially untenable, that’s because it is. The goal was not to build an economically sound treatment protocol, but to create enough variation in order to understand why yield changes.

Throughout the growing season, researchers measured plant biomass accumulation to track crop growth rates at different developmental stages. Later, they linked those growth patterns to final yields and specific yield components, such as pod number, seeds per pod and seed size. Once they crunched the data, clear and somewhat surprising patterns emerged.

ENVIRONMENT CREATES LARGER YIELD DIFFERENCES THAN MANAGEMENT

Across all locations, environmental variation proved to be the most powerful driver of yield differences. High-yielding environments outperformed lower-yielding environments by 25 per cent, even

though the management approach was the same in both cases. According to Earl, these differences are tied to how much biomass the crop accumulates and, more specifically, to how many seeds the crop ultimately produced.

The most important finding from this part of the study is that seed number (the number of seeds per square metre of ground area) was the main component separating high-yielding environments from lower-yielding ones. Seed size also varied, but to a much smaller degree. In other words, when yields differ widely from one location to another, the reason is usually that one field sets more seeds than the other.

FOCUS ON R5

According to Earl’s findings, the environments that delivered the strongest yields were the ones in which the crop achieved rapid growth at R5. This is the stage that governs seed number, he says. When growth during this window is strong, the crop produces more pods, more seeds per pod, and ultimately more total seeds. When growth is slow, the crop produces fewer seeds and a lower final yield. “There’s no way to fix that,” he says. “If seed number is reduced, you can partially make up for it by producing larger seeds, but you’ll still have a lower yield than if you had the maximum seed number.”

These patterns were consistent across all environments. Whether the year was wet or dry, or soils were heavy or light, the strongest predictor of high yield was rapid crop growth at R5.

Management also affected yield, but at a smaller scale. Across all treatments, the intensive “everything” program increased yield by about 17 per cent compared to standard good practice.

However, the researchers’ analysis showed that nearly all of this increase came from the nitrogen component of the treatment. Neither the high-starter fertilizer rates nor the foliar fungicides produced consistent yield gains. Furthermore, the nitrogen rates needed to hit that 17 per cent yield bump were far above economically viable thresholds for soybean production in Ontario.

The results reflected what Earl regularly sees in the field: the belief that a single input or practice can unlock yield gains.

“Growers are sometimes looking for some sort of silver bullet,” he says. “Part of the value of a project like this is to combat that silver-bullet thinking.”

With that said, Earl noted that growers can realize yield gains by addressing nutrient deficiencies. “In the absence of such an identified problem, just throwing inputs at a soybean crop is not likely to produce impressive results,” he says.

A DIFFERENT PATHWAY

Earl also discovered that management-driven yield increases worked through a completely different pathway than those driven by environment. Environment mostly increased yield by boosting seed number; intensive nitrogen treatments advanced yield mainly by increasing seed size.

Earl believes those extra nitrogen applications kept the plants green longer at the end of the season. Under normal circumstances, soybeans draw nutrients from their leaves to fill seed, which shortens the seed-fill period, he says. With extra nitrogen available, that process was delayed, allowing the plants to stay active longer and produce larger seeds.

This distinction – environment influencing seed number, and added nitrogen influencing seed size – is one of the clearest findings in the study. It shows that even when both environment and management increase yield, they may do so for fundamentally different reasons.

Earl and his colleagues noticed a second pattern in the data that raised questions. Growth measurements taken earlier in the season suggested that the crop’s performance around R2.5 to R3 – before any seeds had formed – might be linked to final seed size. This is earlier than expected; seed size is usually thought to be determined much later. Earl stressed that this signal needs more investigation, but said it may point to an additional window of development that contributes to how soybeans build yield.

EXPLORING BIOLOGICAL CEILINGS

To explore how far soybean yields might go under ideal conditions, the team also grew soybeans in pots using a sand-peat mix and daily fertigation. These plants shared the same above-ground environment as field plots, but the environment below ground was completely optimized. The pot system produced very large plants and suggested potential yields exceeding 100 bushels per acre; however, disease issues prevented clear results.

Earl says that while the results were inconclusive, they are still informative. They suggest that agricultural soils may have fundamental physical limitations that cannot be overcome solely with added fertilizer or irrigation. Understanding those limitations may become an important research direction.

Taken together, the project’s results are not intended to guide best management practice recommendations for farmers, but they do offer value for soybean breeders. Understanding the stages when yield is formed, and the conditions that support it, could help breeders shape future genetics that perform more consistently across Ontario’s fields.

“Understanding the physiology of yield formation in our environment is key for developing regionally adaptive varieties,” Earl says. •