Early planted/early RM soybeans are nearing maturity across our area. Soybean sensitivity to day length speeds up crop development towards physiological maturity. Physiological maturity of soybeans occurs 2-5 days after 95% of the plants have reached growth stage R7 (one normal pod on the main stem has reached a yellow color). Visual or full maturity (R8) is reached when 95% of the pods have attained a yellow color (about 32% seed moisture). Finally, harvest maturity is when the seed reaches a moisture content of 14%. During senescence (yellowing and leaf drop), carbohydrates are converted into oils. Soybean seed moisture changes very little, remaining near 60 percent during the de-greening period. At this time, leaves and stems begin to yellow due to chlorophyll breakdown. When this occurs, yield is fixed and cannot increase. As the pods turn to mature color at the beginning of maturity stage (R7), seed dry matter accumulation is complete and seed moisture rapidly decreases.

In 2017, Iowa State conducted an experiment to quantify the rate of soybean dry down and investigate how planting date and maturity group affect the dry down rate. The experiment contained four planting dates and four maturity groups over three years. Soybean pods were collected to determine soybean grain moisture from September through October. Dry down rate was affected by maturity group selection, planting date, and year (data not shown).

During the first 12 days after maturity, the average dry down rate was 3.2 percent per day, which is about five times faster than that of corn. After that period, the dry down rate significantly slows down or stops completely, stabilizing at about 13 percent moisture. Soybean seeds may absorb moisture from the atmosphere under cold and humid conditions, which can delay dry down. Additionally, in conditions where the atmosphere has a high vapor pressure deficit (warm temperatures with low humidity) soybean grain dry down can reach 9-10 percent grain moisture. Under average weather conditions, soybean will reach 13 percent moisture in about 12 days following maturity. However, if weather conditions are conducive, grain dry down can be achieved as early as nine days after maturity.