The most commonly asked question over the last week is "Why are my soybeans yellow? They looked fine the week before". The first part of the answer to this question is quite simple. Up until the soybean plant starts to initiate tri-foliates, it uses the reserves of the cotyledons for energy to supplement its needs. Now those cotyledons are dried up, as in the picture below, and the plant has to get all its nutrition from it's root system. The roots aren't quite ready to take the full load, so the plant turns yellow.
This yellow condition should only last a few days and if your beans turn dark in a week, there usually is not a big yield impact. Some people would argue that this stress of losing the cotyledons is necessary for the rhozobia bacteria to start colonizing the roots and making nitrogen.
The second part of the answer to why beans turn yellow is more complicated. Look closely at the picture below of a stand of soybeans planted the same day, same drill and same variety.
The left side of the picture is wheat stubble, fall chiseled, spring cultivated. The right side is corn stubble, no fall tillage, 2 passes with a Salford RTS during the last week of April. Why no yellow beans in the wheat stubble? Their cotyledons have dried up the same as the beans in the corn stubble. The picture below is of two plants taken from this field. The plant on the left is bigger with one more tri-foliate. The nodulation on each plant is about the same. Why the growth difference?
If we had to put it down to one thing it would be soil temperature. The soil on the left is warmer because it has less residue covering the surface. A 170 bushel corn crop leaves a lot of residue and keeps soils in place through the winter. But it also keeps soil from warming in the spring. The extra tillage following wheat also leaves more air in the soil, which helps it warm quicker. Will this have a yield impact? We do know it won't be as great a difference as you may think, but we will let you know.
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