Tighten the row and watch yield grow?
Megan Bourns, MSc, Agronomist – On-Farm Network, MPSG and Baljeet Singh, PhD, Assiniboine Community College – Spring 2021 Pulse Beat
From 7.5 to 30″, is there an optimal row spacing for soybeans? Does narrow spacing always lead to better yield? These are the types of questions the On-Farm Network (OFN) set out to answer, beginning in 2019. Over the last two seasons, the OFN hosted a total of 12 soybean row spacing trials comparing 7.5 vs. 15″, 10 vs. 20″ and 15 vs. 30″ rows.
Above the canopy – Row Spacing and Canopy Closure
The influence of row spacing on canopy architecture is often a driving factor in the decision to narrow soybean spacing. Generally, narrower spacing should increase the rate and extent of canopy closure. Why is this an important agronomic consideration? Canopy closure affects the efficiency of sunlight capture by the crop and can influence crop water availability through the effect of shading in the interrow space. Sunlight capture, which is critical for photosynthesis and water use efficiency, can then influence yield.
In 2019, canopy closure differences were observed in row spacing trials, and in 2020 the OFN set out to measure those differences. Closure measurements were captured at four of the five 2020 trials, measured in partnership with Assiniboine Community College. At three locations in each strip of each trial, canopy closure measurements were collected at the R1, R3 and R5 growth stages. These measurements were collected using an iPhone, with the Canopeo app, which measures the fractional green canopy cover from images captured one metre above the ground.
While this is a limited dataset, with just four sites in one growing season so far, the trends in canopy closure across spacings and crop stages behaved as expected. Generally, the trend is that canopy closure declined with wider row spacing (Figure 1). Additionally, the difference in closure between spacings tended to be smaller as the season progressed and the rows continued to close. In other words, the canopy closed more, and more quickly, in narrower spacing compared to wider spacing.
Below the Canopy – Row Spacing and Plant Architecture
In the OFN row spacing trials, seeding rates are the same throughout the trial. As a result, there are fewer seeds per row in narrow spacings, compared to more seeds per row but fewer rows overall in wider spacings. This difference in spatial arrangement of seeds leads to differences in individual plant architecture between narrow- and wide-row spacing.
Generally, on narrower spacing with fewer plants per row, individual plants have more space to develop thicker stems and more extensive branching than soybeans on wider spacing where more plants are within a row. The effect of row spacing on individual plant architecture was observed throughout the 2020 growing season.
Row Spacing and Yield
Out of the 12 total row spacing trials conducted through the OFN so far, narrower row spacing increased yield compared to wider spacing in five trials (Figure 2). There were significant yield differences in each of the three categories of row spacing comparisons — 7.5 vs. 15″, 10 vs. 20″ and 15 vs. 30″.
Across all trials, the frequency of yield increases from narrowing row spacing was 5/12 (42%). When we break this down to look at the frequency of response within each growing season, in 2019, the frequency of response was two out of seven trials (29%), but in 2020 the frequency of response was three out of five trials (60%). Why did the frequency of response double in 2020 compared to 2019?
The answer could lie in soil moisture and growing season precipitation differences between the two years. Generally, 2019 was fairly dry in most soybean growing areas of the province. On the other hand, in 2020, the moisture deficit was less severe. With more moisture, yield potential is increased and we may see the benefit of narrow-row spacing more frequently under less severe conditions. Narrower row spacing can help conserve moisture for the crop, and we may assume that the effect of row spacing on yield is greatest in dry years, but there still needs to be adequate moisture to support crop yield development. The effect of soil moisture and growing season precipitation on yield response in row spacing trials is something the OFN would like to monitor as the dataset expands with more trials in future seasons.
What is next for row spacing trials?
The OFN plans to continue with row spacing trials to build this dataset and examine the mechanism behind yield differences — is it mostly driven by canopy closure? Growing season conditions? How does plant architecture and physical yield development on the plant come into play?
These are the questions the OFN will look to answer as we expand these trials!
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