Monday, June 28, 2010

Soybeans and Hot Weather

Don't be surprised if soybeans just aren't growing like they should. Hot weather may well be to blame. When temperatures get up to 95 degrees or more, soybeans tend to:
  • Close their stomates to conserve moisture within the leaf. They can't move enough water through the plant to keep up with that high an evapotranspiration demand. Soil moisture levels, therefore, have little to do with it.
  • Stop producing photosynthate, since carbon dioxide can't get in through those closed stomates either.
  • Stop growing vegetatively. No photosynthate, no growth. If the plants make it to 36-inches tall and lapped row middles anyway, losing those days of growth were of little consequence. If they don't get that big by flowering (in determinate varieties), the lost growing days will have been more serious.
  • Abort flowers. Not a big deal, since soybeans can replace those flowers, and they don't seem to care much which flowers become pods.
  • Abort small pods. This is a little more serious, but the plants may well have enough pods left anyway. If a pod was about half full size or bigger, it probably did not abort.
  • Abort seeds within larger pods if the seeds were still pretty small.
  • Produce smaller seeds if the seeds were too big to abort.
Remember that soybeans typically produce way more flowers than the combine will ever find as pods, and will start more pods than will make it to harvest. That's one of the major ways they survive adversity like this. The net result will probably be not much worse than losing a few days of potential production. I wouldn't expect any lasting effect of this response to high temperatures.
Content supplied by:
Dr. Jim Dunphy
NCSU Extension Soybean Specialist

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