Nighttime Impacts Crop Yields
by Larry Reichenberger
Sunlight gets all the credit for crop production, but what happens to plants at night shouldn't be lost in the shadows,  In fact, researchers are learning that nighttime climatic factors, including temperature, humidity and wind speed, can have a big impact on crop yields -- and that impact may explain why those yields sometimes turn out far different than what farmers expect.
  "Most farmers are surprised to learn that what happens at night can have an impact on crop growth and yield," says Suat Irmak, assistant professor of biological systems engineering at the University of Nebraska.  "Plant functions shift when the sun goes down, but plants are still hard at work."
  ALL NIGHT..  Over a two year period, Irmak and his Ph.D. student Denis Mutiibwa have pulled dozens of "all nighters" in corn and soybean fields.  "Our research is designed to help understand the dynamics that occur between crops and the micro-climate at night.  We're measuring plant respiration and transpiration as well as evaporation from the soil.  We think this information will help to assess the impact year-to-year variations in climatic factors have on the variability of crop yields," says Irmak.
  Irmak doesn't have to look far to find an example of this impact.  "Nighttime
air temperature, can be an important factor.  Although the 2007 growing season in many areas of Nebraska was wet and soil moisture was not limited, crop yields didn't reach the record levels many expected.  In part, this was due to higher nighttime temperatures that occurred in 2007 compared to 2006, which was a more normal year," he says.
Researchers measured the nighttime low temperatures during critical two-week periods in July of 2006 and 2007.  "In 2007, the minimum nighttime temperatures were generally higher.  From mid-July until the end of the growing season, they averaged 8 degrees higher than during the same period in 2006.  Higher minimum temperatures cause an increase in plant respiration, so this could have been a factor in the lower yields that surprised some producers and crop experts last season," says Irmak.
  USING STORED ENERGY.  Respiration is roughly the opposite of photosynthesis, it uses the energy that is stored by photosynthesis to allow plants to live and grow.  "Respiration occurs primarily at night and at normal  growth rates it consumes a significant amount of the energy produced by photosynthesis during the day, especially on warm nights," says University of Nebraska agronomist Ken Cassman.
  "Just as humans work harder and breathe heavier (respire) when temperatures are higher, the same is true for plants. Under those conditions, plants have less energy available to produce dry matter which, in turn, reduces crop yields," adds Cassman.
  Cassman says nighttime temperatures is the real sleeper in the growing debate over climate change.  "Global warming is the increase in nighttime temperature, not how high temperatures go in the daytime.  In an on-going 16 year study of rice in Asia, we've found that yields decline by 10% for every 1.8 degree increase in the seasonal average minimum temperature.  Our research is now trying to extend these findings to corn and other crops," he explains.
  OTHER FACTORS.  Evapotranspiration (ET) is a term describing the loss of water that moves to the air from the soil (evaporation).  Both of these factors are strongly related to crop yields and are influenced by nighttime climatic conditions," says Irmak.
  Relative humidity and wind speed are the key climatic factors, Irmak explains.  "In 2007, we observed less wind speed than in 2006, and the relative humidity in the 2007 growing season was 8% to 10% higher.  Higher humidity and less wind speed reduces crop ET and can cause a reduction in crop yields," he says.
  Irmak found that a surprising portion of this total ET occurred at night.  "While solar radiation is one of the major drivers behind evapotranspiration in the irrigated corn, soybean, and pasture sites we've studied, nighttime conditions had a significant impact.  In July of 2007, the monthly total ET for corn in south central Nebraska was 6.3 inches.  Of that total loss, 22% (1.4 inches) occurred during the nighttime while 78% (4.9 inches) occurred during the day," says Irmak.
  "We're finding there's a lot going on in crop fields at night and it's having more impact on yields that we've previously understood, " adds Irmak.