Plant Growth Regulators (PGRs) teach us how to move soybean yields to the next level

For several years we have seen several individuals break the 100-bushel mark, and just this year a new world record was set at 171 bu/A. What makes these individuals successful? What do they know that the average farmer doesn’t? More importantly, what are some things these growers do on contest acres that can be used on commercial acres to improve yields and improve profitability?

These growers use many different products and practices to achieve high yields. Whether they realized or not, they are maintaining optimal hormone balance and, in many cases, manipulating plant hormones to elicit specific responses from their plants.

Plant hormones determine growth and affect plant productivity. Today’s growers can buy PGRs and apply them to their crops to improve yield.

To start, let’s focus on two hormones, cytokinin and auxin. First, understand that a plant’s physiological appearance is a direct reflection of the hormonal status of that individual plant. A plant whose natural hormone pool is dominated by cytokinin will tend to have more above-ground branching, wider leaves and a deeper penetrating root system. A plant whose hormone makeup is dominated by auxins will tend to be more singular in stem structure (less above-ground branching) and have a larger, more fibrous root system with increased branching of the roots. Cytokinin and auxin are both involved in cell division and cell differentiation in the plant. They have a somewhat inverse relationship, meaning when one is higher the other is naturally lower in concentration. This relationship determines the type of growth a plant exhibits.

As growers, it’s important to understand this basic relationship between cytokinin and auxin and how it influences plant structure. This understanding can help us choose products to sway this balance and create a more desirable plant for our individual environments. Ultimately, we can create a soybean plant with more branching and flowering sites, leading to potentially more pods and seeds per plant.

Let’s take a closer look at the reproduction crop phase as it relates to hormones. What often goes unrealized is when a plant first flowers (R1 growth stage), it is actually a signal to the plant to produce as much root mass as possible. Cytokinin is produced predominantly in the growing root tips, so what the plant is simultaneously doing is ramping up its cytokinin supply for the reproduction cycle. A large supply of cytokinin is essential as the plant moves through flowering into pod set and seed development.

We have all seen those three bean pods that only produce one seed. Why does this happen? Why does the plant abort so many components of yield and so easily? Pod and seed abortion is caused by ethylene and abscisic acid, two hormones over produced when plants are under excessive stress. Cytokinin has a counteractive effect and can help limit abortion.

Cytokinins are produced in the roots and move throughout the plant via transpiration—or with the water stream. When we have high temperatures, dry soil conditions or high humidity (anything limiting water movement in the plant) cytokinin doesn’t get transported to meet the plant’s needs, leading to a cytokinin deficiency and seed abortions. This would be an opportune time for producers to make a foliar application of a cytokinin product to offset the lack of cytokinin being transported from the roots naturally. Proper seed development then continues, preserving yield even under stressful conditions.

Hormones and hormone balance are essential to producing higher soybean yields, and the above discussion highlights just two of the ways to utilize them. One is to alter plant structure to increase branching, creating more nodes and thereby setting greater yield potential. Second is capitalizing on this set potential by limiting the loss of pods and seeds to abortion. There are more techniques and many ways we can utilize plant hormones to increase soybean productivity to reach even higher yields; these two are a good place to begin.

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About the Author: Rob Jarek