By Peter Thomison, Ohio State University Extension corn specialist
The recent heat wave has generated many questions about the impact of high temperatures on corn yields. The “good news” is that corn originated as a tropical grass and can tolerate exposures to adverse temperatures as high as 112 degrees F for brief periods. Optimal daytime temperatures for corn typically range between 77 degrees and 91 degrees. Growth decreases when temperatures exceed 95 degrees. Fortunately, the high temperatures during the past week have been associated with some much needed rains across the state.
How high is too high for corn? Emerson Nafziger, University of Illinois agronomist, notes that “afternoon temperatures in the mid-90s are not a problem for corn… if they have enough soil water available. In experiments, plant temperatures have been raised to 110 or higher without doing direct damage to photosynthetic capacity. The level required to damage leaves depends on the temperature the leaf has experienced before, but it generally takes temperatures above 100 in field-grown plants.”
According to Iowa State University agronomist Roger Elmore and climatologist Elwynn Taylor, the current heat wave may have a double impact on the Iowa corn crop (that is also applicable to Ohio’s corn although the development of the Ohio crop is well behind that of Iowa’s). “The first is the increase in rolling of corn leaves in response to moisture deficiency. By rule-of-thumb, the yield is diminished by 1% for every 12 hours of leaf rolling — except during the week of silking when the yield is cut 1% per 4 hours of leaf rolling…The second impact is less obvious initially. When soil moisture is sufficient, as it is for the most part this July, the crop does not have a measurable yield response to one day of temperatures between 93 F to 98 F. However, the fourth consecutive day with a maximum temperature of 93 F or above results in a 1% yield loss in addition to that computed from the leaf rolling. The fifth day there is an additional 2% loss; the sixth day an additional 4% loss. Data are not sufficient to make generalizations for a heat wave of more than six days, however firing of leaves then becomes likely and very large yield losses are incurred. Generally a six-day heat wave at silking time is sufficient to assure a yield not to exceed trend (Iowa trend yield is near 174 bushels per acre). Should warmer than usual nights continue for a six-week period the state is assured a below trend harvest.”