Understanding Soybean Yield Changes in the Field

Every grower knows that yield varies across the field. We can easily map the differences with yield monitors. But estimating soybean yield is more difficult than estimating corn yield. 

To better understand the reasons for this variation, we collected plants from a 1 meter long strip in the same row in 10 areas of a field. The areas were selected by walking through the field and looking for differences just prior to harvest.

Once the plants were collected, we looked at the number of plants, plant height, number of pods per plant, number of nodes, number of beans per pod, total beans per plant, and seed weight to better understand how each of these affect the yield.

Many factors can change on soybean plants that allow them to adapt to soil moisture and type, weather, nutrient availability, and other field and climate differences and still produce a reasonable yield.

In our pilot project in 2011, we collected plants from 8 different fields. The following figures all show data from a single field in central Iowa. Yield in this field ranged from roughly 40 to 70 bu/a.

There are real differences in grain yield between the different areas of the field. These areas provide an interesting range of conditions from which to make comparisons of different measurements.

Plant height was measured at the highest node with a pod attached. Although there were differences in plant height, these differences did not correlate to the differences in yield.

The difference in plant population does not explain the yield difference. This is not surprising considering 19 plants in a meter is equivalent to 101,000 plants per acre.

For this graph, seed weight is expressed as the number of beans needed to make a pound. The higher number relates to a smaller bean. Area 8 was a wet area in the field, with low plant population, but it produced the heaviest beans.

The total number of nodes tends to closely match the plant height. However, it does not correlate well with the actual grain yield.

A key factor affecting yield is the total number of beans produced per plant. While this is a general trend, this measurement alone does not explain all the yield differences that were documented in this field.

Many people equate the number of pods to grain yield. Across most areas, the number of pods did have some relation to grain yield. But look carefully at areas 2, 8, 9, 10 and compare to the graph of actual yield on the previous page.

While the average number of beans produced per pod fluctuated, this alone did not identify the high and low yield areas. The area with the lowest number of beans/pod was actually one of the highest yielding parts of the field.

In summary, the  yield differences you see on your monitor as you drive across the field are not caused by a single factor. The wet area of the field (plot 8) had a much lower population and shorter plants, yet resulted in a higher yield because bean size was larger there than in the other areas where we sampled.

The lack of similarity across the field explains why getting an accurate yield estimate prior to harvest can be challenging. By better understanding which factors vary in a field we hope to better identify management practices tailored to specific areas that will enable us to increase yield.

The complete report on all 8 fields in this study is available at www.isafamnet.com.

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