Chad Lee, Extension Agronomist, University of Kentucky
So, far, the month of July has been 2 to 4 degrees F warmer than normal in Kentucky, according to the university climate data. The heat could reduce corn yields. Determining how much yield loss is based on temperature, water availability and corn growth stage. Three excellent articles by my colleagues at Iowa State, Illinois and Ohio State address these issues.
As Peter Thomlison at Ohio State reports, corn originated as a tropical grass and can tolerate brief periods of temperatures as high as 112 F, while optimal temperatures are between 77 and 91 F. Emerson Nafziger from Illinois says that if water is adequate, then corn can tolerate temperatures as high as 100 F without damaging leaves.
High temperatures often result in leaf rolling. According to my colleagues at Iowa State, for every 12 hours of leaf rolling, corn will lose about 1% of yield potential. During the week of silking, yields will be reduced by about 1% for every 4 hours of leaf rolling. Assuming adequate soil moisture, the 4th consecutive day with maximum temperatures above 93 F will result in about 1 percent per day yield loss. This yield loss is in addition to the yield loss from leaf rolling. The fifth consectuve day at or above 93 F is an additional 2 percent yield loss and the 6th day is an additional 4 percent yield loss. Looking quickly at weather data from Mayfield, Henderson and Bowling Green, Kentucky, none of these locations had more than three consecutive days with temperatures at or above 93 F in July.
So, to date, we may not have lost yield potential from high daily temperatures, but we have lost yield potential from leaf rolling in some fields across the state.
Dr. Nafziger reports that high night temperatures require the corn plants to lose more sugars to respiration. This results in less sugars available to fuel crop growth. There are no rules of thumbs for night temperatures and yield losses. However, when high night temperatures occur during pollination and early seed development, we can expect fewer seeds per ear and lower yields. In Henderson, Kentucky, night temepratures for July have ranged from 65 to 76 F, with fifteen nights at 70 F or above. We can expect some yield losses from these night temperatures.
High temperatures increase the demand for water. The plant needs more water to help keep leaf surfaces cool and keep the plant functioning. At silking, corn is at peak demand for water, needing as much as 0.4 inches per day (see Nebraska publication on irrigation). That demand decreases to about 0.25 inches per day at the start of dent stage. From silking to maturity, the corn crop will need about 10 inches total of water. Many of our soils in Kentucky will hold about 4 to 8 inches of water. Eroded slopes will hold less.
According to the NASS Progress Reports, the statewide corn crop that reached silking was 17%, 27% and 65% for July 5, July 11 and July 25, respectively. For the same reporting, statwide topsoil that was rated as adequate or surplus 92%, 89% and 70%. The drawdown in soil moisture is evident with rainfall for July. Both Bowling Green and Mayfield have been about 3 inches short on rainfall, while Henderson is about one-half inch above normal for the month.
So, most of the corn crop in Kentucky has gone through pollination under high temperatures but adequate soil moisture. Pollination and initial seed set should have been (or will be) fine, but we are going to need rains, soon, in order to get good seed fill. Even our soils that are at adequate moisture likely do not have enough water to carry a corn crop through to maturity. We still have a good chance to be at normal yield trends for Kentucky, but our odds of getting above normal yields are decreasing with each passing hot day with no rain.
The three excellent summary articles are listed here:
Elmore, R. and E. Taylor. 2011. Corn and “a Big Long Heat Wave on the Way” Iowa Integrated Crop Management Newsletter Iowa State Univ. http://www.extension.iastate.edu/CropNews/2011/0715elmoretaylor.htm
Thomison, P. 2011.High Temperature Effects on Corn. C.O.R.N. Newsletter, Ohio State Univ.
Other Resources Used in this Article:
Krantz, W.L., S. Irmak, S.J. van Donk, C.D. Yonts and D.L. Martin. Irrigation Management for Corn. G1850. NebGuide. Univ. of Nebraska-Lincoln Extension.
USDA Crop & Weather Reports, Kentucky
July 5, 2011
July 11, 2011
July 25, 2011 http://www.nass.usda.gov/Statistics_by_State/Kentucky/Publications/Crop_Progress_&_Condition/cw11/cw0725.pdf
Kentucky Monthly Climate Summary: http://wwwagwx.ca.uky.edu/cgi-public/climsum2.ehtml
Kentucky Daily Climate Summary: http://wwwagwx.ca.uky.edu/cgi-bin/ky_clim_data_www.pl