The USDA Crop Forecast from August 10, 2018 predicted Kentucky corn at 175 bushels per acre, down 3 bushels from 2017. They forecast a national yield that is about 2 bushels better than 2017.
Predicting corn yield is a challenge each year and perhaps more so for this season. Over the past 60 days, much of the Kentucky corn crop has pollinated, initiated seed fill, progressed through the milk and dough stages and now is in the dent stage. The past 60 days slightly above normal for temperatures and mixed for rainfall. Nighttime temperatures have been higher than the overall average temperatures. Of all the variables, the nighttime temperatures may be the key to expected yields.
During the day, corn uses energy from the sun to produce sugars from water and carbon dioxide. In my opinion, this is the most amazing solar-powered reaction on the planet. At night, the plant uses the sugar through respiration to build plant structures, fill seeds and maintain the plant. Rainfall and temperature are critical to both photosynthesis and respiration working well.
In Indiana, a 1 degree Fahrenheit increase in nighttime temperatures in July/August reduced statewide average yield by 3.7%. Most scientists think that warm nighttime temperatures increase the amount of sugar used in respiration and decrease the amount used to build kernels. Also, warmer nighttime temperatures speed up seed fill. Higher daytime temperatures will increase photosynthetic rate, but not enough to offset the shortened seed fill duration. The lost time for seed fill means lost yields.
The average minimum (nighttime) temperature over the last 60 days has been 3 to 4 degrees Fahrenheit above normal for most of Kentucky. Major corn producing counties that were among the warmest include Allen and Warren counties over to the eastern half of Trigg County. Another band of warm counties includes Shelby County north to the Ohio River and then coming southward along the eastern bluegrass and into the mountains. Part of the Purchase Region was only 2 F above normal at night.
Average temperatures (average of the high and low for each day) was only 1 to 2 F above normal for the last 60 days. The last 30 days has been more mild with average temperature at -1 to 0 F. Corn may be getting a break in temperatures as it nears the end of seed fill. Over the past 60 days rainfall has been about 22% or more below normal in the Purchase and 18% or more above normal for the Central Bluegrass. Much of the rest of the state was near normal in total rainfall.
Total rainfall does not equal timely rainfall. Farmers in Pulaski, Christian and Fulton counties along the Kentucky and Tennessee line have reported symptoms of water stress. Farmers in Larue and Shelby counties have reported water deficiencies in corn. While none of these reports may be cases of severe water deficiency, the observations of symptoms in each of these counties suggests yield reductions.
The combination of water stress and high temperatures makes an overall state average of 175 bushels per acre seem high for Kentucky. This may be a year to examine temperature departures from normal in other corn producing states.