University of Nebraska-Lincoln emeritus professor of agronomy Jim Specht answers the following question on what causes a decrease in lower soybean pods.

Q: What would cause soybean plants to pod extremely high off the ground, for example, not producing any pods on the bottom five nodes? It's occurring throughout a rain-fed field planted in 15-inch rows at 180,000 plants per acre the last week of April.

A: High plant populations can result in very intensive plant-to-plant competition within the row. This year, because of high rainfall and little water stress, even rain-fed fields are generating 60-bushel-per-acre yields. However, when you have lush moisture conditions, you will also have lush large leaves on every main stem node at high plant densities.

The amount of leaf area produced in a soybean canopy is huge, particularly by the time you get to early July in a late-April planted field. Upon canopy closure, the huge amount of leaf area at the upper part of the canopy will be intercepting nearly all of the incoming radiation, with little or none filtering down to the leaves in the lower part of the canopy. When those leaves invert from being carbon exporters (the purpose of a leaf), and become carbon importers, the plant sheds those leaves, leaving no leaves at the lower nodes to directly support pod formation or filling there.  

Because pod formation begins in earnest at soybean stage R3 (which is early July in a late-April planted field), nearly all of the solar energy captured by the upper leaves will automatically go to the pods located in the node to which they are attached. Lower nodes without leaves will not form pods at all if all of the reduced carbon captured by the upper leaves is going to support seed-filling in the upper node pods.

In this case, the lack of pods at the lower nodes in fields with excessive plant density is physiologically explainable. The “benefit” of greater distance from the ground surface to the lowest pod-bearing node is that harvest loss with a platform combine is going to be low or nil. However, the “penalty” for excessive plant densities is the extra seed cost per acre, particularly when there is much research evidence that indicates yield is not increased by plant densities much above 100,000 plants per acre at maturity.