As soybeans enter the seed filling stage, how does drought influence seed size? On average, there are 2,500 individual soybean seeds per pound. Soybean seeds produced during drought conditions and at high temperatures tend to be smaller than seeds produced under normal conditions. Smaller seed size means it takes more individual seeds to equal one pound. When soybeans were grown in a greenhouse at 84°F, seeds produced under moderate water stress were 8% smaller than seeds produced with adequate water (Dornbos and Mullen, 1991). When the temperature was raised to 95°F, seeds produced under moderate water stress were 29% smaller than seeds produced with adequate water. This study indicates that seed size is reduced more when water and heat stress occur simultaneously compared to water stress alone.

Soybean seed produced during a drought results in reduced germination rates. What does this mean for next year’s soybean seed? Right now, it’s too early to know for sure. Laboratory seed germination test results are required by law to be shown on seed bags. Make sure to check germination test results before planting next year. If germination is less than 90%, seeding rates will need to be adjusted to achieve desired population.

The effect of drought during the early stages of soybean reproduction was discussed in a previous C.O.R.N. newsletter article (click here).


Dornbos, D.L. and R.E. Mullen.  1991.  Influence of stress during soybean seed fill on seed weight, germination, and seedling growth rate.  Can. J. Plant Sci.  71:373-383.