Are More Nodes the Key to Higher Soybean Yields?

D.B. Egli

LEXINGTON,  KENTUCKY

   A common belief among soybean producers is that more nodes are the key to higher yields. Since flowers and pods are produced at nodes, its obvious that more nodes will result in more pods and higher yield, or so the story goes. This story, however, is not entirely true. Why doesn’t   this simple relationship, that seems so obvious, always work?    

   This story is appealing because we know that the number of nodes produced by a soybean plant is quite variable. Some of the variation results from the capacity of the soybean plant to add nodes by branching in favorable environments. Early planting will increase the number of nodes while late planting will decrease them. Late maturing varieties produce more nodes than early maturing varieties. Node number in these two examples is related to the length of the vegetative growth period (seedling emergence to growth stage R5); the longer the period, the more nodes are produced. Taller plants usually have more nodes and increasing population will increase nodes per acre. It’s clear that management practices can affect the nodes on a soybean plant or the nodes per acre, making it tempting to postulate a consistent relationship between nodes and yield. Unfortunately, this variation in nodes does not necessarily translate into yield.

   It’s not the number of nodes, it’s the growth capacity of a soybean field that ultimately determines the number of pods, seeds and finally the yield. So the key to understanding the node – flower -  pod – yield relationship lies in the growth of the soybean field. Green plants use energy from the sun to fix carbon dioxide into simple sugars via photosynthesis and these simple sugars are the building blocks for all plant and seed tissues. Adequate supplies of solar radiation,  nutrients and water from the soil, the absence of disease and insect damage and optimum temperatures coupled with enough leaf area to intercept most of the solar radiation ensures rapid photosynthesis and growth, resulting in maximum yield. Any restrictions of these inputs and conditions during reproductive growth will reduce yield. Simply adding more nodes without an increase in photosynthesis will probably not increase yield. 

   We know that the number of pods (and seeds) produced by a soybean field is determined by the supply of simple sugars from the leaves during flowering and pod set (growth stage R1 to R5). Whether or not a flower or small pod will survive or abort is determined by the supply of these simple sugars from flower opening until the pod reaches its maximum length. This mechanism allows the pod load to adjust to environmental conditions so it matches the productivity of the environment resulting in maximum yield for that environment. A highly productive environment (plenty of sunshine, nutrients and water) results in rapid growth (a large supply of sugars) and many pods, while a poor environment (one with drought stress, for example) produces fewer pods because the supply of sugars is limited. 

   Adjusting the pod load to environmental conditions creates a balance between the pod load and the ability of the plant to fill the pods and seeds which usually results in normal sized seeds at maturity. I say usually because seed filling (growth stage R5 to R7), occurs after the pod load is established, and environmental conditions can change after growth stage R5 affecting seed filling. Improving weather conditions after growth stage R5 could result in larger than normal seed, while deteriorating conditions (lack of rain for example) could result in smaller than normal seeds. The soybean plant cannot predict future weather when it’s setting the pod load any better than the National Weather Service, so sometimes it doesn’t get the balance right. 

   Relating the pod load to the number of nodes instead of the growth rate uncouples the pod load from the productivity of the environment. An uncoupled plant could set too many pods without enough sugars to fill them, resulting in smaller seed. On the other hand, the uncoupled plant could set too few pods, increasing seed size and possibly reducing yield. The soybean plant works the best when the pod load matches the capacity of the plant to fill the pods.    

   There are always exceptions to every rule and the exception here occurs when soybean plants are very short when, for example, very early maturing varieties are used, drought stress occurs during early vegetative growth, when the crop is planted late in a double-crop system or the population is too low. In these situations the number of nodes can limit pod set and yield. Higher populations are often recommended for early maturing varieties or double-cropping systems to increase the number of nodes and pods per acre resulting in higher yield. But remember, the exception does not disprove the rule. Managing your soybean crop to simply maximize node numbers is not necessarily the path to high yields.   ∆

D.B. Egli: Professor Emeritus, Dept. of Plant and Soil Sciences, University of Kentucky